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Section A
Section B
Section C
Section D
Section E

 

SECTION C:
Plant production and growing techniques

 

Chapter 5. Propagation techniques

Part I. Grafted plants

  • I.1. Seed germination
  • I.2. Sowing
  • I.3. Seedling transplanting and care
  • I.4. Nursery grafting
  • I.4.(a) Scion collection and storage
  • I.4.(b) Nursery grafting techniques
  • I.4.(c) Cultural management of nursery grafts
  • I.4.(d) Nursery scheduling of grafted plant production

Focus (1)

Olive grafting equipment and materials

Focus (2)

Graft histogenesis

Technical flow sheet

(a) Seedling preparation

(b) Production cycle of grafted olives

Part II. Cuttings

  • II.1. Propagation set up
  • II.2. Mist propagation of olive plants
  • II.2.a. Preparation of the rooting medium and cuttings
  • II.2.b. Cutting treatments
  • II.2.c. Technical options for optimising rooting in cuttings
  • II.3 . Micropropagation
  • II.4. Cultural management of rooted cuttings in the nursery
  • II.5. Scheduling of olive plant production from semi-hardwood cuttings

Focus 3

Root formation in olive cuttings ( histological and physiological aspects )

Focus 4

Fog system

Focus 5

Rooting promotion substances ( auxins, plant regulators, polyamides, cyclodextrins, etc. )

Technical fact sheets

(c)Summer production cycle of olive plants from semi-hardwood cuttings

(d)Autumn production cycle of olive plants from semi-hardwood cuttings

Chapter 6. Growing techniques in the nursery

Part I. Shade house management

 

Part II. Growing options in the nursery

  • II.1. Containers
  • II.2. Substrates
  • II.3. Fertilisation and fertigation

Focus 6

Role of nutrients in nursery plant growth

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Plant production and growing techniques

          Civilisation is largely founded on man's ability to propagate, cultivate and preserve plant material in order to supply the products and services necessary for survival. Plant propagation can be defined as the technique used to conserve and multiply selected plants of specific interest. Most plants cultivated nowadays are improved versions of their traditional predecessors. Scientific knowledge, long-standing information, technical know-how and handbooks are essential for efficient propagation of each required cultivar.

 

Chapter 5. Propagation techniques

 

Several approaches exist for disseminating and perpetuating olive growing. Some are very old and of no interest to modern olive nurseries (ovules, suckers, etc.); others, while being based on traditional principles, have been improved by technology and continue to play an important role in the large-scale production of plants (grafting). Yet others, such as mist propagation of semi-hardwood cuttings and, more recently, micropropagation, are constantly changing and provide efficient, concrete responses for olive nurseries, which bolster the continuing expansion of olive growing ( Fig. 6 ).

 

Figure 6 . Olives propagated by traditional and modern methods.

 

 

Part I. Grafted plants

Grafting on seedlings is a traditional propagation method. It is still in use in olive nurseries to optimise production and to meet specific market demands. Technically speaking, it is irreplaceable.

In grafted trees, the rootstock is the portion of tree below the graft union; it will provide the new plant's root system and the lower part of the stem. The scion is the portion above the graft union and will provide the aerial part of the tree ( Photo 28 ). The rootstock is obtained by sexual reproduction and is therefore a seedling (olive trees grafted on seed-propagated rootstock). In some cases, or to meet specific commercial requirements, grafting may be done on self-rooted plants (8–10 months old) (olive trees grafted on clonal rootstock). However, this practice, which is important for morphological uniformity of the nursery plants, is still not widespread in traditional olive nursery production. Clonal rootstocks also have the advantage of being suitable for soils that have difficult characteristics or bear specific diseases.

 

Photo 28 . Young grafted olive showing scion vegetative growth.

 

Nurseries have achieved a high degree of skill in grafting. As a result, it is efficient, with success rates of almost 100%, and very functional since it allows cultivars with low or nil rooting ability (particularly the case of many cultivars intended for table olive production) to be propagated. Furthermore, the heavy market demand for grafted plants demonstrates that this method still has an important part to play in modern nursery production at least until the agronomic/productive peculiarities of grafted olives are recognised.

Compared with plants grown from cuttings, growers believe that grafted plants have a more developed root system including a taproot, which is assumed to make the tree hardier and is thus an advantage, at least during the first period of growth. The taproot grows more quickly and has a greater capacity to explore deeper, moister soil layers. However, it has been shown that taproots stop their development once the seedling is transplanted; thus, the advantages of olive plants grafted on seedling rootstocks are questionable.

Several processes are involved in the production of olives grafted on seedlings. The nursery facilities have to be organised, seedling rootstock and scions have to be provided, grafting has to be done and the plants have to be given the right cultural care to make sure they survive during the first stage of growth.

Section B ( Chapter 3. Layout of the nursery; Chapter 4 Plant material for nursery production ) has already dealt with the organisation and running of the facilities for producing seedlings and scionwood . Seed selection , seed preparation for germination and seedling transplanting or preparation for grafting will be explained below. The chapter ends with a description of grafting and of the cultural care that grafted olive trees require before being transplanted to containers and transferred under shade ( Chapter 6 . Growing techniques in the nursery ).

 

I.1 Seed germination

The choice of variety for obtaining seedling rootstock is important since seed germination capacity is genetically defined, although there are techniques and treatments for enhancing it.

The correlation between seed germination rate and endocarp (stone) size is not clear, but small-stoned cultivars are obviously preferable to make optimal use of the seed germination facility and to obtain as many plantlets as possible. An important issue is the harvest time of the fruits for seed extraction. In most cases the highest germination potential is obtained from fruits harvested at the stage of green maturation. Furthermore, the choice of seeds of specific cultivars for rootstocks is not only based on the seed germination percentage, but also and most significantly on the uniformity of the germinating seedlings. Assessment of the data reported in literature reveals that 72.8% of over 1,500 samples of olive stones belonging to 26 cultivars had a mean unit weight between 0.2 and 0.3 g ( Graph 4 ).

 

Graph 4 . Percentage distribution of olive stone weight in a population of 350 samples of different cultivars ( data compiled from literature ). Values for weight categories refer to 100 stones .

 

It is preferable to choose cultivars whose stones give seedlings with a root system with many lateral roots. This characteristic is essential because it means that when the plants are transplanted, they suffer less damage and their growth in containers is not hindered.

Seed preparation involves removing the stone from the flesh of the fruit and applying some treatments to support germination.

The stones are quickly removed from the olive fruits and placed in a 1% caustic soda solution (NaOH) to strip them of any oil. They are then cleaned thoroughly by rinsing them several times in water and spread out for drying on the floor of a well ventilated shed ( Photo 29 ). For the first ten days, the stones are moved and mixed to make the seeds lose their moisture.

Subsequently, the stones are stored in bags in a cool environment, at a temperature around 4 °C. It is important to prevent excessive drops in temperature during this period to avoid altering germination capacity and to stimulate subsequent germination. In geographical areas where the winter climate is warm, this storage period is not necessary. The seeds can therefore be cleaned immediately to remove any remaining flesh and then immersed in water.

 

Photo 29. Left : olive seeds spread on the ground to facilitate slow moisture loss. Right : preparation of seeds for sowing.

 

Before sowing, the olive stones need to be treated to hydrate the seed and create the conditions for the germination process.

Nurseries have two traditional techniques to choose between: stratifying the stones in sand (moist chilling) or soaking them in water.

In the first case, preparation takes longer although it speeds up subsequent germination. Stratification therefore begins towards the end of July (sowing will be scheduled for about 40 days later).

The stones are soaked every day in water and continually cleaned to remove any traces of oil. This takes 8–10 days if the stones are small and longer (15–18 days) if they are bigger.

When the treatment is completed, the stones are mixed with moist sand and placed in a cool, dark environment for 20–30 days. Careful checks are required during this period to make sure the sand is always sufficiently moist. The stones are also mixed continuously to promote contact with the substrate and to avoid interrupting the seed hydration process. More than 30 days are needed to moist chill heavier stones.

The second method is to soak the stones in water for increasingly longer periods of time. As a rule, 13–15 days are sufficient to stimulate germination of lighter stones (0.2–0.3 g) while 20–22 days are needed for heavier ones (0.3 –0.6 g).

 

I.2 Sowing

The seeds are sown in the greenhouse during the first half of September. This is when the best germination results are usually obtained.

Seeds that have been soaked in water are left to dry for 3–4 hours and then soaked in a fungicidal solution (seed dressing for 24 hours). Moist chilled seeds are sown straight away without separating them from the sand to avoid damaging them.

Sowing is done differently, depending on the seed pre-treatments.

If water immersion was the chosen option, the seeds have to be pressed down lightly with a board ( Photo 30 ) to make them stick to the substrate. Conversely, if they have been moist chilled, this should be omitted to avoid damaging the seeds which are ready to germinate.

 

Photo 30 . Sowing.

 

Next, the stones are covered uniformly to a depth of 1 cm with a mixture of soil and sand (about 35%) which has been sterilised and is perfectly dry.

Quarry sand (granule diameter, 2 mm) should be added to promote water drainage, to loosen the surface layer and to impede the formation of a crust. As soon as the seeds are sown the mixture should be moistened to make the seeds stick to the underlying layer as well. Watering is repeated and frequent checks are carried out to make sure the stones are fully covered ( Photo 31 ).

Photo 31 . Olive seeds being watered after sowing.

 

After this is done, the seedbed frames should be covered with plates of glass, shade nets or felt to stop the seeds from being damaged by the sun's rays and to prevent sudden temperature changes. In favourable climatic temperature and light conditions, seedbed shading can be limited to the first stage of sowing only because it helps seed germination.

After sowing, the care is simple, but it must be done correctly and properly timed. As a rule, the seedbeds must be watered frequently to stop them from drying out too much and the temperature should be checked to protect the young plantlets in wintertime.

Moist chilled seeds start to germinate after 15–20 days. When the seeds have only been soaked beforehand in water, germination begins after 40 days.

Germination is continuous. By the end of December, 70–80% of the seeds have germinated, and the process finishes in January or somewhat later.

Subsequently, during the months before the seedlings are transferred for grafting, it is important to make sure that certain insects and fungi ( Leivellula taurica – Palpita unionalis - Pythium ) cannot damage the young shoots, so making them unfit for grafting ( Photo 32 ).

 

Photo 32 . Shoots growing in the graft house.

 

I.3 Seedling transplanting and care

The seedling rootstock ( Photo 33 ) is lifted and transplanted to the beds in the graft house in early spring (April–May).

If several varieties of seed have been used, it is important to assign separate plots for each variety in the greenhouse before transplanting. In this way, the seedlings in each plot will belong to the same variety and exhibit the same vigour. It also makes the grafters' job easier.

 

Photo 33 . Removal of seedlings.

 

The plantlets, with 4–6 pairs of leaves, are lifted gently from the seedbed ( Photo 34 ). Before transplanting, the roots are lightly pruned to make them all the same length (6–8 cm) and to stimulate the initiation of lateral roots, which contributes to the success of subsequent growth.

 

Photo 34. Olive seedlings ready for transplanting in the graft house.

 

A simple device known as a dibble is used to drill the ground and to transplant the seedlings ( Photo 35 ); the resultant hole is the right size to take the roots of the young rootstock.

 

Photo 35 . Transplanting the seedlings in the graft house.

 

The seedlings are planted in regular rows at a distance of about 8 cm from each other. After transplanting, the ground should be watered to settle the soil around the roots and the plants should be treated carefully (irrigation, fertilisation, plant health protection, etc.) to encourage the formation of well-developed plants ready for grafting.

 

I.4. Nursery grafting

Olive propagation by grafting requires: preparing the seedling beds ( Photo 36 ); collecting scions; assembling the specialised team of grafters and adopting agronomical techniques to encourage the development of the young plants .

The greenhouse where the plants for grafting are located is equipped with hand-operated or automated mechanisms to regulate the environmental conditions. The greenhouse doors should be large to allow the movement of small machinery and equipment .

Photo 36. Olive seedlings in the graft house with grafted plants in the foreground.

 

Particular attention should be paid to preparing the soil, which has to be fine and loose in order to facilitate the incorporation of fertilisers and disinfection with organic fumigants. This last operation is only necessary when the soil has been used for the same purpose the year before and is more effective when the environmental temperature is close to 15 °C. When the effect of the fumigant has worn off (after 15 days) the greenhouse is ready for planting.

Each of the four grafting plots (12 m long and 10 m wide) should be divided into eight uniform beds, 1 m wide, separated by a 30-cm pad for access by staff and grafters. The beds are slightly raised to promote water drainage and covered with plastic sheeting. The goal is to create a layer of mulch to prevent the growth of weeds, which hinder grafting and compete for nutrients with the young plants .

Planting density has to be decided before transplanting the seedlings to the grafting plots. Square layouts are usually chosen (8–10 cm between plants) planted at a density of 130–150 seedlings/m 2 . This works out at around 15,000 plants per plot and 60,000 rootstocks for half of the planned greenhouse.

Traditionally, olive nurseries use seeds from motherstock trees belonging to different varieties. To make the grafters' job easier, it is a good idea to make uniform, easily identified plots before going ahead with transplanting. It is equally important to indicate clearly which cultivar has been grafted .

After transplanting, the graft beds should be given specific cultural care (weeding, fertilisation and irrigation) to facilitate plantlet anchorage to the soil and stimulate vigourous development. In addition, plant protection treatments should be applied to limit the damage caused by insects to the newly formed vegetation.

In Mediterranean environments, the spring of the following year is the best time to start grafting. As a rule, by April, the seedlings have reached a suitable size and the right level of lignification for grafting . Beforehand, the scions are taken from the motherstock trees and stored temporarily in cold rooms. Suitable plant material for grafting is then prepared.

 

I.4.a. Scion collection and storage

The scions are one-year-old, well lignified shoots 4–6 mm in diameter . Avoid drooping shoots, which are often full of flower buds, and feeble shoots. Watersprouts should also be avoided because of their pronounced juvenility characteristics which would be passed on to the new plant .

Generally, grafting is done straight after the scions have been collected, although the plant material can be stored for a short time in cold rooms (2–7 days at temperatures of 2–5 °C) . In this case, bundles of approximately 100 scions are made up, labelled and placed in containers with 2–3 cm of water. To ensure proper storage, which may last longer than 7 days, the storage environment must be kept at a humidity of 80–90%, the scions must be treated preventively with fungicides and the bundles must be sealed in black polythene bags .

 

I.4.b. Nursery grafting techniques

The scions are grafted on the seedlings ( Photo 37 ) in April by a squad of three workers. A specialised grafter does the actual grafting while the other two workers help with specific tasks. The equipment required ( Focus 1 ) is secateurs, sturdy stainless steel knives and various items for binding the rootstock and scion and sealing the graft union .

 

Photo 37. Stages of grafting.

 

 

Focus 1. Olive grafting equipment and materials.

Tapes

Traditional raffia ties are no longer used because they have to be removed when the graft union has joined. Nowadays, synthetic tapes are preferred: they are biodegradable and leave a clean graft union.

Grafting waxes

Several commercial grafting waxes are available . One is a combination of resin–turps ( 2.250 kg), beeswax (0. 350 kg) , crude linseed oil (250 ml), carbon black (30 g) and fish glue (50 g). Plant protection products are usually added. The first stage is to melt the wax in hot water; then all the other ingredients are slowly mixed in, including the fish glue which has also been melted beforehand in water . The carbon black is used to colour the wax (if it is dark, it is easier to make sure the graft union has been fully covered), as well as to make it more malleable and less viscose and stringy. It makes the wax pliable and it sticks well to the bark. The wax is applied warm to facilitate penetration into the cracks in the bark caused by the grafting cut and to prevent the formation of tissue-damaging air bubbles or the entry of micro-organisms .

Grafting waxes (mastics) are needed to seal the graft union, to stop the entry of pathogenic micro-organisms and to prevent the rootstock and scion tissue cells from drying out after they have been cut and failing to produce the callus essential for bonding the graft union. A good grafting wax must stick well to the surface of the grafted plant, it must not wash away, and it must not crack in cold weather or melt in hot weather. Grafters show their skill by preparing grafting waxes with special compositions that are easy to use and which dissolve at low temperatures .

The first stage is for one of the helpers to cut the seedlings to 6–8 cm from ground level with the secateurs, removing the leaves and leaving only the stem, and to start preparing the scions . Seedling suitability is linked to the vegetative conditions of the plant and to the size of the stem. The diameter of the seedling rootstock at the cut surface has to be compatible and slightly bigger than that of the scion. Scion wood should be cut into sections containing two nodes ; the leaves are removed from the base node while the leaf blades are partially removed (between one-half and one-third) from the apical node.

This is designed to limit transpiration while the presence of the leaf is necessary to maintain the viability of the buds and their axils.

A slanted downward notch is then made in the basal node (characteristic quill shape). The cut should be made quickly and cleanly to avoid tearing the bark; it is slanted to increase the area of contact between the rootstock and scion.

Using a sharp knife, the grafter's job is to make a 2-cm lengthwise incision in the stem of the seedling and to quickly insert the scion, making sure the two sets of cambial tissue coincide. The incision should be precise and should reach the woody tissue to allow easy separation of the bark where the base of the scion wood is to be placed .

Lastly, after tightly fastening the cut surfaces together, the third worker applies grafting wax to the graft union and the apical section of the scion wood. This is to seal the cuts and to prevent undesirable dehydration before the meristematic tissue is able to guarantee the histological continuity of the two parts .

High rooting rates will be achieved and vegetative flush will be effective if the cambial tissue heals quickly . Seedlings considered unsuitable are discarded and the grafted plants are cleaned up . It is reckoned that a skilled team will be able to graft approximately 3,000 plants a day .

When done properly, grafting guarantees rooting; as a result, propagation will probably keep to schedule. It is a sound method, although the use of seedlings and scion wood displaying unsuitable vegetative characteristics, or the outbreak of specific diseases, can alter the histological processes ( Focus 2 ).

In nurseries, graft rooting failure is largely caused by methodological factors; only in some cases is it caused by the temperature and humidity conditions of the growing environment. In springtime it is not hard to keep the greenhouse temperature close to 18–22 C° and the relative humidity at 70–80%.

 

Focus 2. Graft histogenesis.

Grafting success depends on histological processes going smoothly at the point of graft union. The rootstock and scion should be joined permanently to establish physiological relations which lead to bonding and make the newly propagated plant functional. Both the rootstock and the scion are involved in the bonding process, which takes place in three stages.

In the first, the outermost cambial and ray parenchyma cells close to the cut start to produce a tissue made of large, undifferentiated cells (callus). The function of the callus is to heal the wound and to join up and support the two parts of the graft (by ensuring tissue continuity) . Optimal temperature and humidity conditions are decisive at this stage . The new tissue produced by the scion and stock fills the empty spaces, so giving rise to the second stage of histogenesis . Meristematic cells form the callus cells and reproduce intensely. The cambium of the stock and the scion very quickly joins up and the cambial activity of the new meristematic tissue ensures the histological continuity between the two portions (third stage) by forming fresh phloem and xylem for tissue union. In specific conditions regulated by hormonal and nutritional balances, the new cambial cells are stimulated into organising vascular cells to recreate the continuity between the scion and the root system .

 

Olives are also grafted on self-rooted, container-grown cuttings (clonal rootstock) to meet specific market demand ( Photo 38 ).

Here too, the plants are grafted when vegetative activity is at a height. It is advisable, though, to place them on raised benches to simplify handling by the grafters . The procedures are similar although obviously work rates are lower when grafting is done on container grown plants.

 

Photo 38 . Stages in cleft grafting olive plants.

 

The production of olive plants from clonal rootstock will probably find more applications in the future because it offers many opportunities for modern olive growing .

In particular, it reduces the variability of plant growth compared to plants grafted on seedling stock. It also simplifies plant growth control ( Troncoso A. et al. 1990; Caballero J.M., Rio C., 1997 ), it makes it possible to use or develop rootstocks for soils that are ‘agriculturally difficult' (for instance, soils with a high total and active lime content) or susceptible to verticillium wilt ( Porras Soriano A. et al. 2003 ) and it has an important effect on fruit ripening ( Fontanazza G. et al. 1992, 1995 ). Scientific research will presumably be able to help significantly in identifying clonal rootstock adapted to farmers' needs. It should be pointed out, however, that the effect of rootstocks on tree development in olive is considerably smaller than in other fruit tree species.

 

I.4.c. Cultural management of nursery grafts

After being grafted ( Photo 39 ), the plantlets are watered and often treated with insecticides to limit damage by plant-eating insects .

 

Photo 39. View of the graft house after grafting.

 

About three weeks after grafting, new shoots 3–5 cm long are clearly visible on each plantlet. Two of them develop from the upper node buds while others may also develop in areas of the seedling below the graft union. The latter should be removed straight away while the weaker of the two apical shoots should be removed and the more vigorous one trained upright because it will be the main stem of the future plant . During these operations, it has to be checked that the grafts are functional; remaining ties have to be removed and any graft failures discarded. Subsequent weed control, fertilisation and irrigation are essential for the grafted plants to grow vigorously .

By autumn, the grafted plants will have completed the first stage of growth and will measure 50–70 cm high. They are now ready for transplanting and transfer to the hardening shed.

The nursery workers use small devices (e.g. spades 10 cm wide and 30 cm long ) to remove the olive plants complete with root ball and then they group them by size and variety.

Using a plant potting machine, the plantlets are swiftly transplanted to containers filled with sterile substrate .

When the olives are transplanted bare-rooted, the root system is pruned lightly to stimulate faster growth after the sapling has been transferred to the container . The olives will continue to grow in the hardening shed until they are sold ( Photos 40 and 41 ).

 

Photo 40 . Young, potted grafted plants in the hardening greenhouse.

 

Photo 41 . Irrigation of young grafted plants.

 

I.4.d Nursery scheduling of grafted plant production

Seed reproduction and the formation of seedling rootstock are the first steps in the nursery propagation of grafted plants. After transplanting to the graft beds, arrangements begin for grafting the plants and for giving them the care they need once the scion has taken on the seedling . Facilities for germination, grafting and hardening need to be organised for this work, which takes about two years from the time the seeds are collected.

The technical flow sheets outline the propagation cycle of olive trees by grafting. They clarify the timing, procedures and cultural care required for the nursery production of seedlings and the stages of grafting the plants, transplanting them to containers and growing them until they are ready for sale.

 

Technical flow sheet (a) Seedling preparation

July

  • Prepare seedbeds
  • Fumigate substrate

August

Sow seeds

September

Give seeds cultural care

October

Check germination

November

Check germination

December

Check germination

January

Check germination

February

  • Treat seeds
  • Prepare greenhouse and grafting plots
  • Fumigate and mulch soil

March

  • Lift seedlings
  • Plant seedlings in graft beds

April

Give seedlings cultural care

May

  • Give seedlings cultural care
  • Check seedling growth

June

Check seedling growth

July

Give seedlings cultural care and plant health treatments

August

  • Give seedlings cultural care and plant health treatments
  • Remove seedlings not suitable for grafting

September

Give seedlings cultural care

October

Give seedlings cultural care

November

Give seedlings cultural care

December

Give seedlings cultural care

January

Give seedlings cultural care

February

Give seedlings cultural care

March

Give seedlings cultural care

 

 

Technical flow sheet (b) Production cycle of grafted olives

April

  • Collect and store scions
  • Graft on seedlings

May

  • Give grafted plants cultural care
  • Clean grafts and remove unwanted shoots

June

Give grafted plants cultural care

July

Give grafted plants cultural care

August

Give grafted plants cultural care

September

Give grafted plants cultural care

October

Transplant grafted olives

November

Transplant grafted olives

December

  • Transplant grafted olives
  • Transfer olives to hardening shed

January

Give saplings cultural care

February

Give saplings cultural care

March

Give saplings cultural care

May

Give saplings cultural care

June

Give saplings cultural care

July

Give saplings cultural care and fertigation

August

Give saplings cultural care and fertigation

September

Sell saplings

 

A glance at these flow sheets shows that nursery economics is linked to production scheduling. Grafting on seedlings is a lengthy process in the olive. All the operations have to be performed on schedule to make sure the nursery is efficient.

 

Part II. Cuttings

Mist propagation is the technique employed in nurseries to obtain self-rooted plants in specific environmental conditions by vegetative propagation of one-year-old cuttings bearing leaves and buds. It is the common practice in olive nurseries because large numbers of olive clones genetically identical to the mother plant can be produced; it is also faster than grafting. The aim is to produce rooted cuttings, which will be transplanted in containers where full plant formation begins. This process of rooting cuttings results in more uniform plants and is considerably cheaper; furthermore it has become the major propagation method employed by the modern, and particularly the intensive, olive industry.

 

Photo 42 . Mist propagation banches.

 

Permanent facilities (greenhouse and heated rooting benches) ( Photo 42 ) and technical equipment (misting equipment, humidifiers, heaters, timers, etc.) are needed to create and control specific environmental conditions inside the greenhouse. Coupled with rationally applied procedures and methodologies, they make it possible to induce rooting in the largest possible number of cuttings.

To ensure efficient plant production, nurseries should have a clear idea from the literature about the complex interactions occurring in root formation. In particular, they should bear in mind that the success of propagation does not depend exclusively on the ‘genotype' but also on the endogenous equilibrium (hormonal and nutritional) of the mother plant when the plant material is collected. They must also monitor the environmental conditions (humidity, temperature and light) inside the mist propagation greenhouse .

The prime goal of nursery growers is to achieve a high, reproductive rooting performance. They should be capable of smoothly running a complex process of operations and should have specific training to organise, run and manage facilities, premises and equipment along efficient business lines aimed at curbing plant production costs.

II.1. Propagation setup

Mist propagation facilities are usually set up in a permanent structure ( greenhouse ), with a rectangular base, arched roof, automated ridge openings and central doors.

The technical specifications of the structure have already been discussed in Section B, Chapter 3, Part I “Nursery setup” .

Assuming that the nursery intends to propagate approximately 100,000 olive plants in two annual cycles, the facility should have an area of around 110 m 2 for setting up four rooting benches (each measuring 1.20 m x 23 m). These should be filled with an inert rooting medium (generally perlite) for planting the cuttings and promoting rooting .

The rooting medium has other specific functions for this purpose; it has to maintain a specific temperature, retain moisture and allow air to circulate throughout root formation. Free space for good air exchange must be present in the rooting medium. This is achieved by shaping the rooting bed in such a way as to facilitate excess water drainage . The bottom of the benches is heated to keep the temperature of the rooting medium at around 20–24 °C while the temperature in the middle or top section of the benches is slightly lower. This ensures the right temperature conditions for the basal, root-forming end of the cuttings. Rooting performance is better at uniform temperatures of around 20–22 °C than when temperatures fluctuate more widely.

Ideal humidity, temperature and light conditions have to be created inside the greenhouse to support the physiological processes necessary to induce rooting in the semi-hardwood cuttings used for this purpose . Mist propagation is still the most extensive automated system ( Section B, Chapter 3, Part I “Nursery setup” ) because it is simple to manage and monitor and it guarantees good results.

 

Photo 43 . Preparation of benches for mist propagation.

 

The mist system increases the environmental humidity by means of a series of nozzles ( Photo 43 ) and helps the leaves of the cuttings to balance their vegetative status to the relative humidity and temperature of the greenhouse. By limiting moisture loss through the stomata, which cannot be recovered until the ‘new' roots form, misting reduces the transpiration of the cuttings, it allows them to carry out photosynthesis and it stops them from withering before or during root formation. Misting too frequently can damage the plants. The reason is that, besides lowering the temperature of the rooting medium, excessive amounts of water can leach the leaves and as a result cause nutrient losses, which may limit root formation in the cuttings.

Lastly, it is important to monitor temperature and light to adapt the conditions of the propagation greenhouse to external climatic variables (temperature ranges and strong solar radiation). In enclosed environments, temperatures can fluctuate and reach values of 30–35 °C, at least during the hottest part of the day. Such high air temperatures increase transpiration and might lead to early leaf drop and reduced root formation. Daytime temperatures inside the greenhouse should not exceed 25–26 °C. At night, they should not drop below 13–14 °C and the structure should be fitted with a shading system to keep the right light intensity to allow photosynthesis to take place in the cuttings as they root.

Several options are available for controlling the environmental conditions: ventilation through mobile side windows or windows near the highest parts of the roof; cooling systems; steam or hot-water burners and permanent or mobile shade netting.

The shading system is particularly decisive because it affects the photoperiod . For root development, the leaves of the olive cuttings need to be exposed to light to manufacture more carbohydrates than what they consume through respiration . Viceversa, low light intensity due to excessive shading does not enhance photosynthetic activity and the cuttings, consuming their nutrient reserves faster than they are able to replace them, suffer from a decreased rooting ability.

Following these guidelines, when organising the propagation facilities, the nursery entrepreneur should adopt standard procedures for operating the greenhouse and for using the equipment that sets the environmental conditions of mist propagation. To manage and monitor misting throughout the production process the entrepreneur can also rely on automated systems which adapt the greenhouse to external climatic variables. Much technological progress has been made in the construction and management of mist propagation facilities. This has helped to modernise the industry and has allowed nurseries to lower production costs and optimise the whole olive propagation process.

 

II.2. Mist propagation of olive plants

One-year-old, semi-hardwood cuttings (12–15 cm long) capable of forming adventitious roots and of producing a new plantlet are used for mist propagation ( Photo 44 ). Only shoots with no differential flower bud will result in good rooting and subsequent new plant development.

The top portion of the shoot has 1–2 pairs of leaves with buds (according to cultivar), the role of which is to promote carbohydrate formation via photosynthesis ; the buds are also essential because they are the main producers of auxins . Both sets of compounds are necessary for root formation.

Two periods are the best for taking cuttings for mist propagation.

 

Photo 44 . Olive cuttings in mist propagation benches.

 

Depending on the climate, t he first period coincides with May–June in the late spring or early summer ( summer cycle ) when vegetative growth is at its peak. This is when the best rooting results are achieved because of the photoperiod (long days and continuous light ) which is more effective in promoting carbohydrate formation. The second period coincides with the interval between October and November ( autumn cycle ), i.e. before the physiological activity of the plant decreases owing to low winter temperatures.

Root formation is a very complex phenomenon in olive cuttings because it depends on histological aspects and physiological phenomena ( Focus 3 ). It is the response of the cuttings to specific levels of endogenous hormones (primarily auxins, but also cytokinins, gibberellins, ethylene, etc.), as well as to the temperature of the rooting medium and the presence of specific enzymes; it is also dependent on their nutritional balance when planted in the rooting bench.

Research has reported extremely high rooting rates in many olive cultivars for which cuttings are therefore a valid method of propagation (including a large number of olive trees whose fruits are used to extract oil). On the other hand, despite research-generated treatments and handling procedures, other cultivars still have an extremely low rooting capacity (mainly cultivars producing table fruits).

Mist propagation is a major example of how scientific research fallout has been applied by the olive industry. It is still used extensively nowadays to produce over 30 million olive plants per year.

The literature provides clear indications on how to minimise the interference between mist propagation and external environmental conditions, how to prepare the rooting medium, how to guarantee adequate auxin levels and how to achieve rooting in as many olive plants as possible. Entrepreneurial skills have made mist propagation cost-effective and have helped nurseries to optimise production standards.

 

Focus 3. Root formation in olive cuttings ( histological and physiological aspects ).

 

Root formation begins with the differentiation of specific meristematic cells present in the cambium, woody parenchyma or medullary area. These evolve and multiply, first pushing through the bark and then outwards near the cut. Internally, they join up with the conducting system (phloem and xylem) to complete the formation of the new self-rooted plantlet (rooted cutting). As the root primordia differentiate and new roots are emitted, tissue (callus) quickly forms at the base of the cutting; this is a natural process to repair the cut. Callus formation is useful because it blocks fungal and bacterial entry, which would compromise rooting. However, it is altogether independent of the success of the process, which is dependent on swift root differentiation to prevent the cuttings from withering. The histological aspects of root formation are linked to specific responses of the genotype. One documented demonstration of this genetic response is the differing anatomical structure of the bark of cuttings from different olive cultivars. Morpho–anatomical study of the origin of adventitious roots has revealed that root emission is correlated with the extension of the pericycle sclerenchyma sheath between the phloem and cortical parenchyma. An almost continuous sheath has been found to be specific to cuttings with a very low rooting ability; in contrast, rooting is better when the sheath is discontinuous, i.e. broken by broad bands of parenchyma tissue. Later studies have demonstrated, however, that morpho–anatomical factors have a less decisive influence on propagation than the endogenous status of the cutting (auxin–nutritional balance) at the time of root formation. The literature reports the identification of high-rooting-ability endogenous complexes in easy-to-root cultivars like ‘ Oblonga' . When extracted, these compounds were capable of stimulating rooting in other plant species; viceversa, the content of the same compounds was lower in the hard-to-root cv. " Merhavia " ( Avidan B., Lavee S., 1978 ). This connection between hormonal and physiological balance and root formation has been extensively documented in the literature ( Caballero J.M., Río C. 1997; Cimato A, Fiorino P. 1980; Fiorino P., Mancuso S., 2003; Fontanazza G., et al., 1996; Garcia J.L., et al., 1998; Gautam D. R., Chauhan J.S., 1991; Epstein E., Lavee S., 1984; Khabou W., 2002a; Koukourikou M., et al., 1999; Petridou M., et al., 2002; Porlingis I.C. et al., 1999; Qrunfleh M. M., et al., 1994; Troncoso A. et al. 1976).

Research has revealed that the internal hormone levels of the cutting vary throughout the year, which is why root formation is not always optimal. Hormone content is higher in the springtime, the period of active vegetative growth of the plant, when the balance differs between the apical portion of the shoot, which has a higher content because it is more exposed to light, and the basal portion. Conversely, in winter, the overall hormone content is lower; as a result, the endogenous conditions of the cutting do not permit correct root formation. It has also been demonstrated that most olive cultivars do not have adequate levels of auxins to stimulate new root formation; therefore, it is common practice for nurseries to treat the cuttings with commercial auxins to speed up and enhance rooting. This causes big changes in the physiological and nutritional functions of the treated cuttings. The initiation of adventitious roots is, in fact, a response of the cutting to specific levels of hormones and to the temperature of the bench. When stimulated into root formation, the cutting increases its respiration rate and consumes more of the reserve carbohydrates produced by photosynthesis (saccharose especially), which provide nutritional sustenance, maintain the fast respiration rate, create new cells and promote root elongation and expansion ( Rallo L., et al., 1990; Río C. et al., 1991; Wiesman Z., Lavee S., 1995a, 1995b ). Lastly, physiological studies have shed light on the procedures for optimising auxin treatments ( Fernandes Serrano, J. M. et al., 2002 ), the action of specific enzymes ( Sebastiani L. et al., 2004) and the close relationship between root formation and the photosynthetic activity of cuttings ( Proietti, P. et al., 2003 ).

 

II.2.a. Preparation of the rooting medium and cuttings

The plant material chosen for cuttings consists of one-year-old shoots, which are therefore partly lignified (semi-hardwood cuttings). The shoots should have leaves and whole healthy buds to make sure the cuttings have adequate levels of carbohydrates and auxins. Rooting success depends on the replenishment of auxins (which are produced in larger amounts by the buds) and on the relations between these hormones, enzymes and the production of carbohydrates, the prime source of which is leaf photosynthesis.

The best time for gathering the shoots from the motherstock trees is early in the morning, when the sunlight is not yet intense and the leaves and shoots are turgid. As rooting depends on the water balance of the cutting, amongst other things, the plant material should be handled with care to minimise physiological stress and shoot desiccation.

When transporting the shoots from the motherstock collection to the propagation site, it is good practice to prevent dehydration by soaking the basal portion in water (2–3 cm) and protecting it from light by covering it (in dampened, sealed bags). Sometimes it may be necessary to store the shoots for a period lasting from a few days to a fortnight. In every case, they should be placed in sealed bags and kept in a controlled environment at a relative humidity of 80–90% and a temperature around 2–5 °C.

When the plant material is being collected, the shoots should be checked for any signs of disease that might modify propagation performance. Even so, before preparing the cuttings, it is common practice to treat the plant material with suitable solutions to preclude the development of diseases on the leaves and buds ( Spilocaea oleagina , Sphaerotheca pannosa , Botrytis cinerea) that could damage the functionality of these organs and compromise root formation. Alongside the choice of plant material, successful propagation is achieved by implementing the procedures for preparing and treating the cuttings inside the propagation facility.

The cuttings are prepared by dividing the plant material into portions, each with four nodes , and removing the leaves from the two basal ones. One effective way of ensuring auxin absorption prior to treatment is to store the cuttings (for a few hours) in shallow crates which should be covered to stop the leaves from losing moisture and drying out . As soon as the cuttings have been treated, they should be planted as quickly as possible in the rooting benches.

The rooting benches are filled with perlite to a depth of approximately 20 cm and prepared a few days before planting the cuttings . The perlite should be levelled, pressed down lightly and watered repeatedly to make it uniform and give it the right consistency to hold the cuttings . The medium has to stick to the cuttings during rooting to prevent large air pockets which inhibit root formation in general and particularly in the basal portion of the cuttings .

Perlite is an ideal rooting medium: it is light, inert and sterile. Given its ability to maintain high moisture rates and its porosity, which ensures sufficient drainage and good aeration during root formation, it is considered better than other substrates (i.e. vermiculite, quartz sand, etc.) . Commercial perlite is available in various sizes (diameter 1–5 mm), at a specific weight between 95 and 140 kg/m 3 .

After auxin treatment, the cuttings are planted to a depth of at least half their length (8–10 cm), taking care to keep the basal portion 3–4 cm from the bottom of the rooting medium ( Photo 45 ) . The cuttings should not be planted too densely to make sure the leaves are exposed to light and to prevent the onset of diseases. At a density of 650–700 cuttings/m 2 , approximately 15,000 cuttings can be planted in a 23 m 2 rooting bench in each production cycle.

 

Photo 45 . C uttings in rooting benches. Rooted olive cuttings lifted after propagation.

 

After planting, the cuttings should be watered abundantly to make them hold properly in the rooting medium.

Monitoring at this stage is to make sure there is adequate aeration and that the temperature is between 20 and 24 °C. The base of the cuttings has to be kept at a higher temperature than the axillary leaf buds by heating the rooting medium in order to support the biochemical processes needed to initiate new root formation before the buds open. If the temperatures are too high, even for a short time, the cuttings may die. Small temperature variations do not so much modify rooting ability as affect the length of time it takes for root emission.

During root formation, it is important to manage and monitor carefully the environmental conditions of mist propagation to minimise any interferences and possible interruptions in the physiological processes of the cuttings. In particular, the cuttings need a low level of transpiration and high light intensity to allow adequate leaf photosynthesis . During this stage, the cuttings manufacture more substances than they consume through respiration and the synthesis products (carbohydrates) are used primarily for the formation and development of the new roots. Prior to root formation, the vegetative growth of the cuttings should be kept low so that the buds do not open and cause excessive transpiration .

To achieve both these objectives, the environmental humidity should be kept close to 85–90%. Watering (types of nozzles, pressure, unit volume of water, frequency and length of watering) and evaporation (presence of tunnels on the bench, cooling systems, etc) should help to maintain the biochemical equilibrium of the cuttings and allow rooting processes to take place.

On average, the cuttings are kept in the rooting benches for around 60 days, although some cultivars may need a longer time (70–80 days). Regular inspections should be carried out during this period to check for the outbreak of diseases causing anomalous leaf abscission ( Spilocaea oleagina , Sphaerotheca pannosa , Botrytis cinerea) and to make sure that root formation is underway.

Another innovation introduced in the propagation of olive plants by cuttings is known as the fog system ( Focus 4 ).

 

Focus 4. Fog system.

 

The fog system is a sophisticated alternative to mist propagation. It produces much smaller droplets of water, creates almost 100% humidity conditions, controls air temperature more effectively, prevents overheating of the leaf surface and reduces undesirable waterlogging in the rooting medium. Generally, the best performance is achieved by placing covers or plastic tunnels over the rooting benches; however, it is important to make sure that the air temperature, which is usually higher than in mist propagation, does not reach critical levels conducive to the development of pathogens and to excessive leaf transpiration, which hinders normal physiological processes. One critical aspect of fog system efficiency is that the water has to have lower salt and impurity levels than that used for mist propagation. Propagation under fog is more sophisticated than under mist and its success depends on whether it has been done properly and according to high standards; however, the use of the fog system rather than mist propagation is often determined by financial considerations.

 

II.2.b. Cutting treatments

The identification of the relationships between endogenous plant regulators and root formation in mist propagated olive cuttings led to the possibility of stimulating the process through root-promoting treatments. Scientific research has provided extensive, explicit, unquestionable evidence that the variability of propagation performance is due to the differing natural cultivar-related ability of cuttings to form new roots.

Studies have reported natural rooting ability to be quite high in some cultivars but poor or almost nil in others (the latter group is bigger and includes many of the table olive varieties). Hence, mist propagation is a feasible, cost-effective method for propagating olive cuttings in the first case; in the second, the plant material has to be treated with commercial auxins to achieve higher rooting percentages. The variability of mist propagation performance, which is linked to the genetic characteristics of the mother plant, is well documented in the scientific literature.

Experimental data contained in almost 500 scientific publications (284 published between 1950 and 1987 and 213 subsequently) provide information on rooting performance which is summarised in Graph 5 .

Of the 426 cultivars tested, 59 had mean rooting rates of around 1.5% (values between 0 and 5%), 213 recorded average rates of 21.3% (5–40%), 86 cultivars achieved rates of close to 54% (40–70%) and only 68 had rates of between 70 and 100% (84.5% on average).

 

Graph 5 . Mean rooting rate of 426 olive cultivars (compiled from experimental data reported in the literature).

 

Comparison of Graph 5 with Graphs 6 and 7 sheds further light on the rooting ability of the olive and on the role of scientific research in this area. These graphs plot the best rooting values of 426 cultivars which did not receive auxin treatment (control cuttings). They confirm ( Graph 3 ) the variability in performance, linked to the genetic characteristics of the mother plant, and the natural difficulty of the cuttings to grow roots; only 10% of this varietal population was capable of achieving acceptable rooting rates above 50%.

 

Graph 6 . Percentage distribution of 426 olive genotypes by natural rooting ability (cuttings not treated with auxins) (compiled from data in literature).

 

However, rooting rates in the same population of varieties differed considerably when the cuttings were treated with auxins . The results plotted in Graph 7 show that approximately 50% of the samples had rooting rates of more than 50%, which is what nurseries are looking for . These important data show how research has helped to improve mist propagation technology and has provided olive nurseries with concrete results for optimising the efficiency of facilities and lowering plant production costs.

Graph 7 . Percentage distribution of 426 olive genotypes treated with IBA auxin to stimulate rooting (compiled from data in literature).

 

Consequently, cuttings are treated with auxins ( Photo 46 ) to enhance their natural rooting ability and indolebutyric acid (IBA) is the auxin used most extensively for this purpose.

 

Photo 46 . Preparation and hormone treatment of bundles of cuttings.

 

A small insert ( Focus 5 ) in this section gives some brief details about the substances which have a positive effect on olive rooting.

Besides the commercial compound chosen, the method of auxin application is also important to achieve fast, abundant root formation and to guarantee the repeatability of the propagation results.

Before the auxin is applied to the cuttings, they are treated to prevent infections and/or the spread of diseases during rooting . Specific fungicides are generally used for this purpose at low concentrations (0.5–1%). Next, the base of the cuttings (1–3 cm) is dipped for a few seconds in a water–and–alcohol solution (30% ethanol and 70% de-ionised water) containing 2–4 g/L of indolebutyric acid (IBA 2,000–4,000 ppm). The length of treatment (5–10 seconds) should be inversely proportional to the molecular concentration of the auxin; no more than a few seconds are needed at higher concentrations.

According to the literature, rooting stimulus generally increases as IBA concentration increases, but not indefinitely. Above a certain “optimal” dose, toxic effects appear at the base of the cutting.

Obviously, the “optimal” concentration varies according to the genetic predisposition of the plant and to technical factors ( mother plant , juvenile and nutritional status of the cutting , propagation season, etc.) which determine the success of propagation.

The next section deals in more detail with this topic.

The auxin solution should be prepared fresh on a daily basis. It has to be prepared with special care because the hormone is not readily soluble in water and first has to be dissolved in pure ethanol. Not more than 30% alcohol should be used to avoid having a caustic effect on the base of the cutting.

One effective way of overcoming the insolubility limit is to dissolve IBA in a solution of potassium salts ( Avidan B., Lavee S., 1978 ) or other commercial powders which are easily soluble in water .

Another, more recent solution for ensuring effective auxin absorption is to bind the hormone to relatively simple organic molecules, i.e. cyclodextrins ( Mura P. et al., 1995; Mancuso S. et al., 1997 ). These are cyclic oligosaccharides with a -1,4 bonds of D-glucopyranose, which are produced by enzymatic degradation of starch. The result of the inclusion complex ( cyclodextrin-auxin ) is an interesting improvement on the recognised specific properties of auxins because it increases the solubility of the root-forming hormones, ensures greater stability, improves the transportation and availability of the active ingredient through the cell membranes and reduces the toxic effects of high hormone concentrations .

 

Focus 5. Rooting promotion substances ( auxins, plant regulators, polyamines, cyclodextrin, etc. ).

 

Scientific confirmation of the relation between rooting in olive cuttings and endogenous root-promoting substances has clearly shown that this phenomenon needs to be stimulated by specific rooting promotion treatments. This subject area has generated intense research. A large number of auxins [IAA, ( indolylacetic acid ), IBA ( indolebutyric acid ), NAA ( naftalenacetic acid ), 2,4-D ( 2-4 diclorophenoxyacetic acid ), 2,4,5-T ( 2,4,5 triclorophenoxyacetic acid ) and 2,4,5-TP ( triclorophenoxypropionic acid )] have been studied at length, individually and combined, to check their root-inducing effect. However, the “optimal hormone” level has not been set because the values can differ for the same cultivar depending on the time of the year, the physiological period when the cuttings are used and treated and other factors which are currently the focus of research attention and which are discussed in the next section ( nutritional status, type of cutting, etc .). This explains why the literature reports ‘optimal' responses at IBA concentrations ranging from 1,000 to 10,000 ppm. The best performance has been reported for IBA at rates of 2–4 g/L (2,000–4,000 ppm).

The literature also documents interesting applications of other auxin compounds ( NAA, IAA, etc .) or plant regulators ( ethylene, ACC etc. ). It is also clear from the available bibliography that these two groups of compounds are not the only active substances. Good rooting results have also been obtained using vitamins B1 and B6, boron and polyamines ( Rugini E., et al., 1990 ).

More recently, an interesting application of auxins with cyclodextrins has been identified to improve olive rooting ( Ozkaya M. T., G elik M., 1994 ; Mura P. et al., 1995; Mancuso S. et al., 1997 ). The effect of this complex, described earlier, is to protect the auxins from bacterial ( acetobacter ) or enzymatic attack (enzymatic degradation of the auxin by IAA-oxidase) by guaranteeing their light and temperature stability.

 

II .2.c. Technical options for optimising rooting of cuttings

During root formation it is important to check that the equipment controlling the greenhouse environmental conditions is working properly and that rooting is actually underway. The appearance of the first rootlets and the presence of buds and leaves on the cutting within the first 30 days are indications that the cutting is functional and that root formation is active.

Below are presented some working and technical options and suggestions that can interact positively with this phenomenon. The indications provided clarify how the success of the rooting process depends on ‘factors intrinsic to the mother plant' ( juvenile and nutritional status of the cutting, timing of collection of plant material, shoot area, presence of leaves and buds, presence of viruses, etc. ) and on ‘working options' reported in the literature or learned from everyday experience in the nursery. The latter group includes important parameters ( bench temperature, humidity, light, presence of CO 2 ) which optimise greenhouse environmental conditions ( Porras Piedra A., et al., 1998, 1999) , the type of rooting medium ( perlite, sand, mixes, peat, etc. ), the length of time the cuttings are kept in the mist propagation unit, the type and concentration of the rooting promotion substances ( IBA, NAA, IAA, MH, B 1 , B 6 , boron, ethylene, ACC, AVG, polyamines, etc. ) and the methods of auxin treatment of the cuttings ( basal or leaf treatments, notching, etc. ) .

 

Mist and fog systems. These systems are equivalent and the choice of one or the other is often linked to the environmental conditions of the nursery site and to economic considerations. In conditions of excessive external light and temperatures, the fog system gives better rooting percentages because it creates almost 100% humidity conditions inside the rooting bench and it reduces the amount of water in the rooting medium. As a rule, the rooting benches are covered with plastic when the fog system is applied.

Greenhouse temperature. Temperature conditions are regulated between 18 and 26 °C.

Humidity. The relative air humidity should be kept above 85%.

Misting frequency and duration. Misting usually lasts between 10 and 20 seconds. Obviously, the duration can oscillate depending on the desired ‘optimal' environmental temperature. Misting frequency varies according to external seasonal conditions.

Quality of water for mist propagation. Water characteristics ( electrical conductivity and hardness, expressed in French degrees ) are important because excessive environmental evaporation can induce salinity phenomena that slow down or inhibit root production . Salinity should not generally exceed 100 mg/L.

Photosynthesis. When the new adventitious roots are forming, cuttings need adequate levels of light to promote photosynthesis and hence the production of the carbohydrates needed for cell division and root elongation.

Photoperiod. The photoperiod (period of daily exposure to light) has been reported to be more effective in rooting when it is long. However, in most cases tested, this has only a minor effect . In other cases, the effect is more limited.

Leaf temperature. Leaf temperature generally stabilises at around 18–22 °C during the day and is slightly lower at night. The air temperature must not be too high in the rooting environment; otherwise, the buds develop faster than the roots and leaf evaporation increases too much.

Types and characteristics of rooting medium. Perlite, vermiculite, sand, peat or mixes of these compounds are usually used for mist propagation of olive trees . The characteristics of the rooting medium play an important part in the development of the root system of the cuttings. The ideal substrate should be sufficiently porous to permit good aeration and drainage, it should be able to maintain high moisture levels and adequate temperature levels, and above all it should be free from fungi and bacteria.

Oxygen. This is essential for root production and growth. Industrial rooting media are very porous and guarantee good aeration ; however, to keep the oxygen level at optimal levels, the rooting bench should be shaped in such a way as to promote the drainage of any excessive water.

Temperature of rooting medium. This may range between 18 and 26 °C; obviously, the optimal temperature lies inside this interval (20 °C and 22 °C). The temperature should be slightly lower in the middle and at the top of the rooting medium than at the bottom. At some times of year the cooling caused by the mist or fog system can lead to a decrease in the temperature of the rooting medium; therefore, it may be preferable to provide extra heat to the benches.

Length of time cuttings are kept in rooting benches. The cuttings are kept in the heated benches for approximately 60 days to stimulate rooting. Difficult-to-root cultivars need to be kept longer (70–80 days).

Type of semi-hardwood cutting . One-year-old shoots, 12–15 cm long are used, the terminal portion of which has two pairs of leaves with buds .

Leaves and buds. The presence of leaves and buds on the cutting is essential for photosynthetic activity, and hence carbohydrate production (source of energy), and auxin replenishment (produced largely by the buds). The success of rooting depends on the relations inside the cuttings between auxins, enzymes and carbohydrates. This is why it is essential to ensure that when the cuttings are in the rooting benches they have leaves and buds, at least in the first 30 days, which is when root-forming processes are intensely underway.

Water balance of cuttings. The cuttings need high humidity levels to keep the cells turgid, to facilitate nutrient mobility, to lower transpiration levels and to encourage photosynthesis. When preparing the cuttings and keeping them in the rooting benches, it is important to minimise water stress to stop them from drying out.

Cultivar. Natural rooting ability is a genotype-linked potential. Some researchers associate this ability with the capacity of the cultivar to synthesise indolylacetic acid , which is a co-factor in the rooting process.

Anatomical structure of cuttings. This option may possibly play a part in the phenomenon of root formation, specifically the structure of the cortex, the tissue lying just under the epidermis (outermost cell layer). It has been observed that when the sclerenchyma bundle, which is formed between the phloem and the external cortex and performs mechanical support functions, is continuous and heavily lignified, the cutting has a low rooting ability. Conversely, when the bundle is punctured with parenchyma-like cells (live cells), the rooting ability is higher. More recent studies have concluded that morpho–anatomical characteristics have a less decisive influence on propagation and that what is at play is merely an interaction with the endogenous status of the cutting (auxin–nutritional balance) at the time of root formation.

Motherstock tree or mother plant. The literature has documented that the key to the successful rooting of olive cuttings lies in the physiological (age, nutritional status, etc.) and health conditions of the mother plant when the plant material is collected. The presence of viruses (SLRV and CMV) in the motherstock trees has a negative effect on root formation ( Fernandes-Serrano J.M. et al., 1995 ).

Age of mother plant. Strong vegetative development (vigour) and high contents of endogenous substances with high root-forming power enhance the rooting capacity of olive cuttings. Such characteristics are typical of younger motherstock trees.

Nutritional status of cuttings. Optimal physiological status is observed when the olive cuttings have high carbohydrate levels due to the fact that the plant material was collected when there was no fruit on the motherstock trees . Competition for photosynthetic assimilates between growing fruits and shoots on the mother plant in summertime lowers saccharose levels in the cuttings and jeopardises successful rooting. It has also been demonstrated that the presence of potassium in the leaves has a positive effect on root emission and even future fruiting initiation in the orchard.

Auxin levels of cuttings. Auxin levels vary through the year in the cuttings and are at their best during the period of maximum vegetative growth of the motherstock trees . Auxins are largely produced by the buds and to a lesser extent by the nodes and leaves.

Shoot zone. Cuttings taken from the subapical section of the shoots show greater rooting potential and increased sensitivity to auxin treatments than cuttings taken from the basal section, which are more lignified and characterised by more inhibitors.

Shoot collection period. The rooting capacity of cuttings varies according to the phenological stage of the mother plant. Two periods in the year are conducive to rooting owing to the favourable endogenous balances (auxin and nutritional) . The first is in late spring or early summer, when vegetative shoot growth is at its height and cuttings have higher contents of endogenous, root-promoting complexes . The second is in the autumn, but before the drop in the environmental temperature makes the plants decrease their physiological activity . This is when the cuttings have maximum levels of carbohydrates, the active compounds which support respiration and which are important for root development and growth. During both of these phenological periods, it is important for the motherstock trees not to have flowers or fruit when the plant material is collected. This is to stop competition between flowers/fruit and shoot growth from limiting the concentration of carbohydrates and levels of endogenous auxins and/or rooting co-factors .

Treatments of cuttings . Both groups of cultivars (easy and difficult-to-root) need auxin treatment. When effective, this leads to higher rooting percentages and increased repeatability of propagation performance. The cuts should be made at the base of the cuttings just before auxin treatment.

Type of root-inducing substances. The literature reports a large number of natural or synthetic products which promote rooting in semi-hardwood olive cuttings. In the nursery industry it is necessary not only to optimise propagation performance but to make it repeatable. Positive effects have been obtained on root formation by treating cuttings with many auxins [ IBA ( indolebutyric acid ), IAA ( indolylacetic acid ), NAA ( naftalenacetic acid ) ], either singly or synergically combined with other plant regulators ( ethylene, ACC ). To overcome the limitations of higher auxin levels (6,000–10,000 ppm), more effective means have been identified such as using potassium salts which are easily dissolved in water, combining IBA with putrescine or combining the auxins into a complex with cy clodextrins. Gibberellins and cytokinins have not exhibited interesting effects on rooting performance in olive cuttings. The standard growth regulator used commercially for most cultivars is still IBA. Commercial formulations are commonly used in the nursery sector for most cultivars.

Solutions for administering auxins to cuttings. A mixture of water and alcohol (30% ethyl alcohol and 70% water) is the solution used most frequently for applying auxins to cuttings.

Water. The water used for the water/alcohol solution should be pure and free from bacteria and salts (de-ionised water).

Anti-cryptogamic preparations and hormone solution. As a rule, it is wise to avoid applying root-inducing treatments together with anti-cryptogamic preparations in order to avoid affecting propagation performance . Such preparations should be applied to the shoots before preparing the cuttings.

Preparation of hormone solution. Most auxins are not readily soluble in water . The solution has to be prepared by first dissolving the hormone in ethyl alcohol (30%) and then adding de-ionised water (70%).

Storage of auxin solutions. Auxins are sensitive to sunlight and to temperature. This makes it advisable to use hormone solutions straight away or to store them in sealed containers in the fridge at temperatures of 4–5 °C .

Optimal concentration of root-inducing substances. The great variability in olive cutting response to root-inducing substances is a clear indication that the optimal concentration is specific to the cultivar, to the environmental conditions in the mist propagation greenhouse and to the technical options chosen. Olive cultivars can be divided into different categories according to their natural rooting ability. Obviously, the ‘optimal IBA concentration' for each one is based on first-hand experience . Research data report that the variability in concentration is optimal between 1,000 and 10,000 ppm.

Duration of treatment. It has been confirmed that the duration of treatment should be inversely proportional to the concentration of the auxin and its formulation. Basal treatments of the cuttings with a water-and-alcohol solution and concentrations ranging between 1,000 and 5,000 ppm require only a few seconds (3–5) ; treatments at higher concentrations which use talcum powder take longer (1–5 minutes) .

Basal treatment method . To speed up treatment, the cuttings are gathered in bundles and 1–2 cm of the base is dipped in the treatment. During dipping, they should be kept at ambient temperature out of sunlight .

Use of commercial products ( powders ). This is an easy way to supply the required auxins for rooting . Soaking in water is preferable if the base of the cutting is not sufficiently moistened in order to help the powder to stick to the base of the cutting. The advantage of commercial formulations is that they are ready and easy to use and available at a wide range of concentrations. The use of powder formulations of IBA is widespread and usually most effective due to slower, longer contact of the cutting with the hormone. In some cases, however, it is hard to obtain uniform results, probably because variable amounts of the substance adhere to the cutting.

Notching. This entails making a lengthwise incision a few centimetres long at the base of the cutting to stimulate rooting; however, it adds to labour costs.

Leaf treatments. Some attempts have been made to apply low-concentration doses of auxins to cuttings by leaf application through the misting water . The results have been promising at experimental level but have not been easy to apply. Low auxin concentrations in the mist were shown in various cases to reduce or delay leaf drop and thus increase the rooting percentage of the cuttings.

Inspections during root formation. Both the mist propagation facilities and cuttings must be inspected meticulously during root formation. It is particularly necessary to check the temperature of the rooting benches , to make sure the mist system is working properly and to check that the sensors and automatic devices which measure the environmental parameters are operating efficiently. The cuttings should be inspected regularly to prevent the development of diseases on the leaves and buds that could damage their functioning and compromise root formation.

 

Carefully choosing working options, managing the motherstock trees and treating the cuttings with a variety of synthetic auxins (active ingredient and concentration) have brought definite improvements in the rooting of olive cuttings .

After more than 50 years since the first research on olive propagation by semi-hardwood cuttings, scientific research can be considered to have made a notable contribution to the advancement of knowledge in the nursey sector.

Comparison ( Graph 8 ) of the best rooting results obtained up to the late 1980's (green line) with more recent results (red line) shows that the mean percentages have shifted to higher values.

 

Graph 8 . Comparison of the best rooting percentages ‘induced' in olive cuttings (mean rooting percentages are plotted on the x-axis and the frequency of the cultivar samples at the respective rooting rates is plotted on the y-axis). (Compiled from bibliographical data published before 1980 and updated in 2006).

 

Graph 8 shows that prior to the 1980's, 15% of the cultivars tested were not capable of rooting, not even when treated with hormones. Fifty-eight percent of the population was made up of cultivars with mean rooting rates of between 1 and 50%, which left only 27% with optimal rooting percentages.

Conversely, the line plotting the most recent results reported in the literature shows, firstly, that in only 5% of cases research has not been able to find a solution to stimulate rooting in cuttings; secondly, that 56% of the varieties has optimal rooting rates; and lastly that 16.8% achieves mean rooting rates of 100%. The table below gives a list of some of the olive cultivars displaying the best rooting rates ( Table 1 ).

 

Rooting rate (%)

Cultivar

> 90 %

Aglandau, Ashrasi, Carasquena, Coratina, Crnica, Ecijano, Jaropo de Lucena, Kilis Yaglik, Kothreiki, Manzanilla, Manzanilla de Tortosa, Moraiolo, Morsica, Negrillo de Arjona, Picholine Marocaine, Picual, Redondilla de Logroño, Romanella, Verdial de Jaén, Wetaken, Zorzaleño

100%

Bashika I, Frantoio, Gemlik, Oblonga, Pendolino

 

Table 1. Groups of olive varieties characterised by high rooting ability.

 

II.3. Micropropagation

Micropropagation is a clonal multiplication technique. It employs aseptic cultural media and specific light and temperature conditions to reproduce clones by using apical meristems, shoot apices and micro-cuttings as initial explants ( Photo 47 ) .

 

Photo 47 . Olive micropropagation.

 

Suitable technical facilities and specialist staff are required for this technique, which could help nursery entrepreneurs to improve the management of their business.

As a matter of fact, m icropropagation enables the reproduction of the genetic characteristics of the motherstock trees and the obtention of a large number of progeny from a few initial explants in a small space and in a short time-frame. It is also a way of obtaining virus-free olive cultivars required for genetic–phytosanitary certification (by using healthy material or material that has been restored to health).

Nurseries are thus able to respond quickly to market demand. They no longer have to depend on seasonal production cycles and they apply highly standardised techniques compatible with substantial economies of scale.

The working protocol for the micropropagation of olive trees is described below:

 

Preparation and sterilisation of culture medium : The medium is made of macro- and micro-elements, vitamins, aminoacids, sugars and growth regulators (cytokinins, auxins, gibberellins). The media generally used for olive are variations of the Murashige and Skoog medium (MS). The chief media proposed in the literature have been capable of inducing good shoot growth in the cultivars tested without causing mutations or vigour loss.

 

Explant collection and sterilisation : Motherstock trees raised in a controlled environment give more guarantees from the phytosanitary point of view. Container-grown, 1–2 year old plants kept in the greenhouse are the best for supplying explants. However, good results have also been obtained using shoots taken from motherstock trees grown in the field. The starting material is generally collected at vegetative flush, when cell activity is high.

Solely the ‘axillary bud' technique is applied in the micropropagation of olive trees: the initial explant, which has to be sterilised, is made up of single-node portions partly stripped of their leaves, with the axillary buds intact. Sterility is a prerequisite for any type of in vitro culture because the media are an ideal habitat for the development of fungi and bacteria which colonise plant surfaces.

 

Explant culturing: Culturing is performed under a laminar airflow hood. The treated microcuttings are placed in the containers with the initial medium. This stage, known as pre-conditioning, is to identify and discard material still contaminated with fungi and bacteria even after disinfection. It is important for the medium not to have a high hormone concentration; hormone use should be limited to cytokinins (zeatin) (Standardi A. et al., 1998) . Next, the containers are placed in a thermostat-controlled growing room at a temperature of 23–25 °C with overhead fluorescent lighting at 40 µ Em -2 S -1 for 16 hours a day. Explant culturing is completed when, after about 3–6 weeks, the initial explants have developed into shoots a few centimetres long ( Photo 48 ).

Photo 48 . Olive explants .

 

Explant proliferation or multiplication: Multiplication entails excising small, single-node cuttings from the healthy shoots which have been formed in order to obtain new explants. The cycle is thus perpetuated (subculturing) until the desired number of progeny is obtained. The number of subcultures should be limited to stop lengthy in vitro handling from inducing modifications in varietal characteristics. Assays using liquid cultural media are currently underway to improve explant multiplication ( temporary immersion system ) ( Grigoriadou K., et al., 2005 ).

 

The findings of research into olive performance in vitro have revealed that the plant proliferates primarily through elongation of the main axis, it does not send out shoots from the base of the explant, and it rarely produces lateral shoots from axillary shoots in the presence of the apical bud. The proliferation rate is calculated from the number of internodes produced, which varies according to the cultivar and to its adaptability to in vitro conditions.

The functions of various compounds have been tested for preparing the proliferation medium. Amongst the micronutrients, boron induced the greatest linear growth expressed as number of nodes ( Leva A.R., et al., 1995 ). Saccharose ( Garcia J.L., et al., 2002 ) and mannitol ( Leva A.R., et al., 1994 ) have been compared as sources of energy; the latter, in particular, was found to be better suited to performing an energy-providing, osmotic function as well as to improving growth and reducing the basal callus of the shoots. Obviously, besides the source of carbon, the role of growth regulators has also been tested ( auxins and cytokinins ) to streamline this technique and prevent vitrification phenomena.

Zeatin is the most effective cytokinin for stimulating growth of the primary shoot, but it is also the most expensive. Recent attempts have been made to replace zeatin by TDZ ( thidiazuron ) or by a mix of cytokinins including lower concentrations of zeatin.

When the medium contains the gibberellin GA 3 (20–40mg/L) this appears to induce rapid shoot elongation (Rugini E. 1997; Grigoriadou K., et al., 2002 ) , which helps to shorten subculture preparation time.

 

Rooting : Difficulties do not usually arise during this stage. The explants, which have 3–4 nodes, are transferred to a cultural medium ( MS/2 ) that does not contain cytokinin but is auxin-enriched ( IBA or NAA ) at a concentration of 1 mg/L. Alternatively, the literature reports that a root-inducing treatment can be applied at the base of the shoots with IBA or NAA, or with antioxidants like GSH or H 2 O 2 prior to auxin treatment, in order to stimulate earlier, enhanced root emission.

Temperature and photoperiod conditions are the same as those described for proliferation, although explant rooting is improved considerably by basal etiolation. For etiolation to occur, it is necessary to paint the bottom of the container black in order to cover the medium or to enrich the medium with substances capable of causing etiolation at the base of the induced shoots (Rugini E. and Lavee S., 1993) . The addition of 1mM of putrescine (polyamine) to the medium substantially increases the rooting percentage and the number of roots per explant.

 

Acclimatisation and in vivo transplanting of self-rooted plantlets : This is a very tricky stage and has to be managed carefully when the in vitro plantlet is transplanted to typical greenhouse conditions where it turns from a heterotroph into an autotroph, the light, humidity and temperature conditions change and, above all, the surrounding environment is no longer aseptic. Relative humidity is definitely the most delicate factor because in vitro leaves are morphologically different and more delicate. A period of acclimatisation is therefore necessary in conditions of high relative humidity and low light intensity.

 

At the greenhouse, the rooted explants are placed in containers (approx. capacity, 150 cc) in a substrate made up of beech leaves, sand and peat (1:1:0.5).

After acclimatisation, they are hardened off in containers filled with a compost mix of beech leaves–sand–peat (1:1:1) until they are one year old, at which age the plants are ready to be sold.

The application of micropropagation techniques has been slower in the olive sector than in other fruit plants. This is due not so much to the lack of an in vitro multiplication protocol, which the research community has refined over the years, but to the cost of the basic equipment and of the products used to prepare the culture media.

At present ( Table 2 ) some 33 olive varieties have been micropropagated. The scientific insight gained into the field performance of these plants (start of bearing, possible phenotypical and/or genotypical variations), is encouraging ( Briccoli Bati C. 2001; Briccoli Bati C., et al., 2006; Leva A.R., et al., 2002, 2003; Mencuccini M., et al., 2003; Santos C.V., et al., 2003) although limited for viewing olive micropropagation as a technique for large-scale application in the olive nursery industry.

 

Americano, Ascolana, Cailletier, Canino, Carolea, Chondrolia chalkidikis, Cipressino, Cordovil de Serpa, Cornezuelo, Dolce agogia, Frangivento, Frantoio, Galega vulgar, Giarraffa, Gordal, Kalamon, Leccino, Lechin, Lucques, Manzanilla, Marteño, Maurino, Moraiolo, Morziolo, Nocellara del Belice, Nocellara etnea, Oblonga, Picholine, Picholine marocaine, Picual, Tanche, Verdeal alentejana

Table 2. Olive cultivars that have been micropropagated.

 

II. 4. Cultural management of rooted cuttings in the nursery

When root formation is completed, the rooted cuttings cannot be transferred straight from the protected environment of the rooting benches to the field; they have to be gradually acclimatised beforehand. This entails a first transplanting stage ( Photo 49 ) and subsequent transfer to a greenhouse.

The propagated material is transplanted to 0.25–0.50 L containers (7x7x8 cm or 7x7x10 cm) filled with a sterile artificial substrate (peat /pumice 1:1 v/v) and enriched with 1.5 g/L of controlled-release fertiliser (N-P-K + micro-elements) to satisfy the growth requirements of the plants. At this stage, it is possible, and to a certain extent also advantageous in economic terms, to transplant the rooted cuttings to somewhat larger containers (7x7x15 or 8x8x20 cm) with grooves for root leading.

As soon as they are potted, the olives are transferred to the hardening greenhouse to encourage the development of the newly formed roots and the first stage of growth . During this period it is necessary to remove the lateral shoots and to apply the right amount of water to keep the moisture content of the substrate as close as possible to field capacity.

 

Photo 49 . Rooted cuttings after transplanting and in the hardening greenhouse .

 

The hardening greenhouse is equipped with microsprinkler irrigation facilities ( Photo 49 ) . The plants are also checked to prevent insects or fungal or bacterial pathogens from attacking the root system or vegetative organs .

Galvanised metal, tunnel greenhouses fitted with openings to regulate internal humidity and temperature conditions are usually used to harden off the cuttings . The ground is covered with concrete or mulching material to limit the emergence of weeds; alternatively, it may be covered with a layer of chippings (about 5 cm deep) to facilitate pot drainage. The photoperiod is usually controlled to encourage the growth of the rooted cuttings. This is done by covering the greenhouse roof and side walls with shade nets of varying light intensity (max. values 50–60%). The environmental light intensity must not be decreased too much because the rooted cuttings must always have sufficient light for normal photosynthesis.

 

II.5. Scheduling of olive plant production from semi-hardwood cuttings

The aim of propagating olives from semi-hardwood cuttings is to produce rooted cuttings which, after being transplanted to suitable containers, begin the first stages of vegetative growth and development of the newly formed roots in the hardening greenhouse.

The process begins by preparing the facilities (greenhouse and heated rooting benches) and the technical equipment (mist propagation units, humidifiers, heaters, timers, etc.) controlling the environmental conditions in the greenhouse (humidity, temperature, light). The next steps are to prepare and treat the cuttings and to optimise the production process. The technical fact sheets summarise all the stages in this process.

 

Technical fact sheet C

 

Summer production cycle of olive plants from semi-hardwood cuttings

May

  • Preparation of greenhouse and facilities.
  • Collection and storage of shoots. Storage is not necessary in nurseries where a small number of olive plants are produced.

June

Preparation and treatment of cuttings and placement in rooting benches.

July

  • Rooting; start of checks.
  • Rooting control.

August

Rooting control.

September

Production of rooted cuttings, transplanting and transfer of plants to acclimatisation greenhouse. When using strongly rooted cuttings it is possible to transplant directly to final suitable containers with a volume of about 1 L.

October

Acclimatisation and growing of rooted cuttings.

November

Acclimatisation and growing of rooted cuttings .

December

Acclimatisation and growing of rooted cuttings.

January

Acclimatisation and growing of rooted cuttings.

February

Acclimatisation and growing of rooted cuttings.

March

  • Transplanting of olive plants to larger containers.
  • Transfer to shade house; fertigation.

 

Technical fact sheet D

 

Autumn production cycle of olive plants from semi-hardwood cuttings

September

  • Preparation of the greenhouse and replacement of perlite.
  • Collection and storage of shoots. Storage is not required for small-scale production.

October

Preparation and treatment of cuttings and placement in rooting benches.

November

  • Rooting; start of checks.
  • Control of root formation.

December

  • Control of root formation.
  • Production of rooted cuttings, transplanting and transfer of plants to acclimatisation greenhouse. When using strongly rooted cuttings it is possible to transplant directly to final suitable containers with a volume of about 1 L.

January

Acclimatisation and growing of rooted cuttings.

February

Acclimatisation and growing of rooted cuttings.

March

Transplanting of olive plants to larger containers.

April

Transfer to shade house; fertigation.

 

To conclude, while specific items of the information provided will help nurseries to improve their propagation performance, many of the choices made and technical devices and methods used are the result of irreplaceable personal experience. Experience of this kind cannot be found in the literature but is based on a wealth of professional knowledge acquired from continuous work in this area of olive production.

Chapter 6. Growing techniques in the nursery

 

Before being released for sale, the olive plants complete their growth in another suitably prepared section of the nursery known as the shade house. The following schedule is suggested as a guideline but may vary according to the local climatic and physical conditions.

In March, and in any event when the environmental conditions are favourable, all the plants, both those grafted the preceding spring and those self-rooted (cuttings produced in the summer), are placed in the shade house. It is advisable to transfer the grafted olive plants first, as they are already in their definitive container (3.5 L), and to deal with the rooted cuttings later. However, transplanting is done quickly because they are mechanically potted (2.5–3.5 L containers).

The potted olives are tied to canes (stiff plastic stakes, bamboo canes etc. which can be bought for this purpose) to stop them from bending over, which would modify the growth of the stem ( Photo 50 ).

 

Photo 50. Rooted olive cuttings after being transplanted to containers (note the different types of canes used).

 

When the plants are transferred to the shade house, each variety should be distinguished from the others and the method of propagation should be indicated. The plants are lined along two adjacent rows separated by a small aisle to allow the passage of staff and small equipment. To stop the containers and plants from toppling over, the canes are fastened to stiff, plastic-coated wires attached to iron rods positioned every 8–10 m along the row ( Photo 51 ) .

 

Photo 51 . Plants growing in the field and in the shade house (note the metal posts supporting the wires for fastening the canes).

 

During this stage of production it is important to choose the best materials (containers, substrates) and cultural care (fertilisation, fertigation, phytosanitary control) to achieve the balanced growth of the olives until they meet the planned commercial standards.

 

Part I . Shade house management

The shade house is a very simple structure ( Section B. Chapter 3, Part I ) which allows the plants, protected until then in the greenhouse, to grow in a better environment while sheltered from harmful weather events (hailstorms, wind, etc.) and solar radiation.

The rows should not be lined too closely together to avoid concentrating the new canopy solely in the upper part of the tree, making it unbalanced ( Photo 52 ).

 

Photo 52 . The space between the rows must not be too narrow to stop the olive trees from concentrating vegetation solely in the upper part of the plant (photo on the right).

 

During the time the plants are kept in the shade house, it is important for them to be fertigated and treated against noxious insects that could damage plant quality ( Palpita unionalis, etc .) and for the environment to be properly ventilated to prevent the outbreak of fungal diseases ( Spilocaea oleagina, etc. ). In very hot weather it is important to open the windows if the shade house is set on rigid supports or to raise the lateral side covers ( Photo 53 ) if the structure is simpler in construction.

However, the best conditions for bringing on the olive plants are determined by the choice of container, the formulation of the substrate and the regular application of fertilisers and water, which is done optimally through fertigation.

 

Photo 53 . Plants under shade.

 

Part II . Growing options in the nursery

Nowadays, nurseries have a range of solutions open to them for this stage of production. By applying modern growing techniques, they can make the quality standards of their plants reproducible while lowering production costs.

The following sections are designed to give clear information on innovative aspects of modern olive nursery production.

 

II.1. Containers

In the past, nursery plants were grown directly in the field. When they were to be sold, the olives were lifted from the ground, complete with the ball of earth around the roots, and they were tied or bagged for direct delivery to buyers ( Photo 54 ). The advent of containers brought the first innovation in nursery olive production.

Containers have several advantages. First, they improve plant growth control, they keep the olive root system intact and they make it easier to apply nutrients to the plants at the right rates. Second, they make it simpler to move the plants inside the nursery and to deliver them to buyers. Lastly, they allow nurseries to schedule their activities better and to make savings, and they give farmers the chance to plant the trees when it best suits them.

 

 

Photo 54 . Left to right, from top to bottom: olive trees grown in the field and prepared for delivery; olive trees ready for sale in planter bags and modern containers.

 

The containers usually used nowadays are made of black polythene: they are cheap, can be re-used, are easy to handle and to transport and they reduce water loss, so limiting nutrient loss in the substrate.

Owing to the type of material and colour, they keep the temperature of the substrate above the mean environmental temperature in winter. This effect should not be overlooked because it results in greater plant growth. In some nurseries differently coloured pots are used for each cultivar.

Container shape ( Photo 55 ) is of fundamental importance. The ones used most extensively are cylindrical or have a truncated cone shape to promote water drainage. The second type is used more often nowadays because they take up limited space and are steady. These are two important characteristics that simplify plant transportation.

Photo 55 . Different types of commercial containers available.

 

However, there are limitations to the use of these shapes.

The container walls can intensify the deformation of the root system by making them spiral. Most of the roots in the container move in a centripetal direction which delays or hinders root elongation when the olive trees are established in the field after transplanting. In such cases, the root system develops slowly and the plant delays resuming vegetative growth. If deformation is more pronounced, it may compromise the survival of the saplings because of their poor anchorage.

Similar problems occur when the olives are kept for a long time in the containers. In this case the roots straggle out of the container and continue growing, which causes problems when it is time to remove them from the pots.

Alternative containers have recently been designed to overcome these drawbacks ( Prado M.A., et al., 2002; Cimato A., Petruccelli R., 2006 ) by promoting a more rational, internal root distribution. These have a pronounced truncated cone shape; the inner walls are ridged lengthwise and have large apertures at the bottom.

These two design features avoid spiralling, because the ridges encourage root growth towards the outermost parts of the substrate, and prevent waterlogging and the roots from straggling out of the container because of the improved aeration at the bottom (usually called air pruning) ( Photo 56 ). As a result, the olive trees have a better root system, which grows in balance with canopy development.

 

Photo 56 . Close-up of the effect of air pruning containers on root growth.

 

II.2. Substrates

A mix of peat and another inorganic material is generally used as the substrate or potting mix for container-grown olives. The mix satisfies plant requirements because it is an excellent nutrient reserve and it retains water and makes it readily available. Owing to its porosity and permeability, it ensures gas exchange between the roots and the atmosphere. If the right formulation is used, it should also be sufficiently compact to stick to the roots during transplanting and to support plant anchorage in the ground.

A vast range of information is available to nurseries to improve substrate characteristics and to decide which solutions are best for making them more efficient and economical. They should also look into partly replacing peat by waste of different origins ( compost ) to find an answer to the environmental problem of organic material that otherwise has little use ( Tattini M. et al., 1990b, 1992; Cimato A., et al., 2001; Khabou, W., 2002b ) .

Nowadays, many commercial inorganic products (sand, pumice, expanded clay, quarry residue, etc.) and organic matrices (peat, manure, compost , etc.) are available for preparing artificial substrates. In general, they all have advantages, but it is important to know the characteristics of each component in the intended mix in order to find the most suitable solution.

Some of the chief matrices used to prepare substrates are now listed and their characteristics are described.

 

Inorganic materials

The main function of the inorganic part of the mix is to improve the physical properties (porosity) of the substrate and to make it lighter in weight and cheaper.

 

Sand : This is used in mixes with other compounds because of its low water retention capacity. Silica sand is usually used to avoid altering the pH. This material can be found at little cost .

Pumice : This lava rock froth is very light. Granules of different weight and diameter (1–5 mm) are used. It is also useful because it has a good cation exchange capacity.

 

Organic materials

Peat : Sphagnum peat moss keeps stable over time, has a high porosity (88–97% in volume), and a high water and cation exchange capacity; it is free from animal and plant pests and from phytotoxic substances. Initially, it is characterised by quite a low pH, which means that before use it has to be buffered with calcium carbonate CaCO 3 (about 1 kg/m 3 per unit of pH).

Coconut fibre : This is obtained from the fibrous husk of the coconut and has good physico–chemical characteristics, that is to say low cation exchange capacity, high air retention, low waterlogging power, water retention capacity, expressed in weight, equal to 300% (the water retained is basically capillary and is therefore easily absorbed by the plants), lengthy resistance because of its high lignin content, slow degradation and a pH of 5.3–5.5. Its low nutrient content can also be an asset because the addition of specific nutritional formulations for olives makes for improved control of plant growth in the nursery. It has a better pH, drainage capacity and stability than peat. Experimental trials ( Cimato et al., 2001 ) have demonstrated that coconut fibre can be an excellent replacement for peat if it accounts for 50% of the mix at the most.

Composted organic materials

This category covers industrial or agricultural waste products (animal and sawmill waste, agri-food waste, pruning residues, sludge, etc.) which are composted before being used as organic matrices for the preparation of substrates.

Good composts should have the appearance, colour and consistency of overturned earth; they should have no unpleasant smell and should be stabilised to stop them from causing problems of plant toxicity. They should contain a high percentage of organic matter and their content of fertiliser elements should be known.

When used in the substrate mix, compost optimises fertility and has a beneficial effect on its physical, chemical and microbiological characteristics. In the first case, it increases substrate porosity (78–85%) and permeability by forming more stable aggregates and it enhances water-holding and drainage capacity. In the second, it improves the pH and supplies organic substances and nutrients which increase cation exchange capacity. Lastly, from the biological standpoint, it activates the microbiological changes which generate final (humic acids, fulvic acids, etc.) and intermediate products (aminoacids, vitamins, auxins, etc.) that are fundamental to soil fertility.

The starting percentage of compost for preparing substrates is around 20–30%. Inoculation with suitable mycorrhizae is now used more and more to enhance the growth of the nursery plants.

 

II.3. Fertilisation and fertigation

Container growing has considerably modified the procedures for applying fertilisers to growing substrates. The fact is that the plant is forced to live in a small volume where the root system has limited possibilities of exploration. Growth is maximised by controlling the development of the main stem and lateral shoots, which are fundamental parameters for commercial standards. There are three ways of fertilising container-grown plants: by applying mineral salts to the initial substrate; by adding slow-release fertilisers; or via fertigation (application of soluble mineral salts in irrigation water).

Fertilisers are applied to complement the nutrients provided in the substrate mix. They are designed to support the different stages of olive growth and to guarantee that the development of the root system is suited to container size and balanced canopy growth.

When the young plantlets are transferred to containers, the root system is not very efficient in absorbing the available nutrients; hence, they would not benefit from large amounts of mineral elements. High nutrient concentrations cause high energy expenditure by the roots, in addition to toxicity phenomena and environmental pollution.

To avoid these drawbacks, fertilisation has been redesigned by adding slow-release or controlled-release fertilisers to the initial substrate. The first group includes traditional products ( urea, dicalcium phosphate, etc. ) while the second includes formulations of macro- and micro-elements (held together by acrylic resins, polythene and other materials) which interact with the organic matter in the substrate and are capable of releasing nutrients essential to roots for 4–6 months.

A later technological innovation proposed fertigation ( Photo 57 ) as a more advantageous method of providing mineral elements for supporting plant growth. This was prompted by the observation that plant nutritional requirements in ideal growing conditions are closely correlated to plant development. Clearly, with time, the fertilisation dose has to be modified to make it more efficient and cost-effective ( Barberis R., et al., 1988; Tattini M., 1990; Tattini M., et al., 1990a, 1990b ).

Having established that fertigation does not exclude the initial addition of slow-release fertilisers to the substrate and that it is advisable to adapt fertigation to container-grown plant status, nursery growers have to decide which business plan responds to their production needs . In particular, they have to examine water characteristics and the procedures for monitoring water distribution in the containers, as well as to establish the composition of the inorganic nutrients and the preparation of the nutrient solutions.

The water and added nutrients are applied through localised drip irrigation facilities. The water should not have a sodium content of more than 100 mg/L or a calcium content of more than 200 mg/L.

Experience is essential when monitoring fertigation. The chief objective is to identify an irrigation system that distributes the water very efficiently, reduces losses through seepage and surface runoff and makes optimal use of available water resources. Therefore, nurseries should also consider the possibility of introducing computerised aids which help to ease the management and maintenance of the installations and to save a considerable amount of water.

 

Photo 57 . Fertigation of growing olives.

 

As has already been said, fertigation is a cultural practice aimed at supplying fertilisers to the olive plants rather than to the substrate .

Before explaining the role of mineral elements, specifically in the control of plant growth ( Focus 6 ), it should be made clear that no standard fertigation formula can be identified for olive plants owing to the great variability in environmental conditions and substrates and to the genetic vigour of cultivars. It is better for nursery growers and/or their technical advisers to choose their own quantities and duration of nutrient applications.

 

Focus 6 . Role of nutrients in nursery plant growth.

 

Nitrogen

The olive responds quickest to this inorganic nutrient because it improves vegetative activity. Nitrogen stimulates growth by supporting the production of new shoots; together with phosphorus it controls apical dominance; it is a component of chlorophyll and increases the amount in leaves, so promoting the assimilation of other elements . The ammonium form (NH 4+ ) is assimilated by passive diffusion, which involves less energy demand. Nitrogen absorption in the form of nitrate (NO 3- ) is an active process and depends on the calcium and/or potassium ion concentration of the solution.

Potassium

This is the element behind productivity. It is present in the centres of greatest biological activity and is important in phenomena connected with plant water metabolism. It regulates photosynthesis and assimilates transportation; it encourages sugar synthesis and it accentuates drought resistance. Absorption is by diffusion and is closely connected with the water regime, the concentration of the nutrient solution and the genotype. Absorption is jeopardised by water deficit, even if moderate, and is more active when the nutrient is at lower concentrations in the soil solution .

Phosphorus

This compound is essential for enzymes and proteins. It plays a leading role in cell division and in the development of meristematic tissue. It is absorbed as phosphate ion (HPO 3- ; HPO 4- ). Phosphorus absorption is promoted by the presence of ammonium nitrogen in the soil solution.

Calcium

This is the chief nutrient because its presence in the tissues lends mechanical resistance to the cells. The olive is the species of fruit tree most sensitive to calcium deficiency. A constant water regime is essential to ensure good absorption of the Ca 2+ ion.

Magnesium

This is a constituent of chlorophyll. It also activates many enzymes. The rare cases seen of magnesium deficiency occur at subacid pH values or when potassium concentrations are high.

Sulfur

This is the specific constituent of some aminoacids, and hence of proteins and biologically active compounds (biotin, diamine, coenzyme A).

Boron

This element is involved in carbohydrate metabolism. Boron activates the enzyme systems and hormonal functions; it promotes flavonoid synthesis and sugar transportation through the phloem. It is absorbed as boric acid (H 3 BO 3 ) and benefits from a constant water regime.

Iron

This element stimulates plant photosynthesis. It is absorbed in the ferrous (Fe 2+ ) or ferric form (Fe 3+ ).

Copper

Copper has an exclusively catalytic role in plants and is part of numerous enzymes such as cytochrome oxidase, polyphenoloxidases, etc. Copper deficiencies are not usually observed in plants, but excessive phosphate fertilisation can reduce copper availability and give rise to insoluble precipitates.

 

More recently, a way has been identified of enhancing the morphological development of the root system and so improving plant characteristics. This involves creating a symbiotic complex (mycorrhizae) between the container-grown olive roots and arbuscular mycorrizhae (AM : Glomus mosseae ). The primary effect of the mycorrhizae is to increase nutrient absorption by the mineral elements present at low concentrations or those whose ion form is not mobile ( PO 3 2- , Zn + and Cu + ) and to promote growth of the 1st and 2nd-order lateral roots and the number of 2nd-order lateral branchings ( Citernesi A.S. et al., 1998; Marín Zamora M., et al., 2002 ).

Clear technological know-how is available to nursery producers to help them optimise the growth of container-raised olive trees. They have the tools to choose solutions capable of supplying plants which meet high quality standards at competitive production costs. Technological innovation has helped to create an energy-saving process where components (nutrients, water) pass from one system (the container) to another (biomass production/olive growth) with maximum efficiency.

Process optimisation is simple if three points are understood: the plants are forced to live in a small volume of area, their root system has limited room for exploration, and growth has to be supported by suitable amounts of fertilisers applied through localised drip irrigation. Nursery producers can thus reproduce the quality standards of their plants, lower production costs and sell plants meeting high commercial standards ( Photo 58 ).

 

Photo 58 . Olives ready for sale.

 

 

 

 

 

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