PISTACHIO: PRECISION IRRIGATION FOR WATERING THE GREEN GOLD

CHARACTERISTICS OF THE CROP

Originally from the Middle East, the pistachio tree (Pistaciavera L) was introduced to the Mediterranean in the early Christian era. Nowadays, this crop has
become extremely popular on account of its enhanced taste and nutritional qualities, not to mention its hardiness. It is able to withstand drought and extreme temperature variations, and is tolerant to poor and saline soils, making it the crop of choice in the face of the climate change.
Certain species of pistachio plants are found among the shrubs growing wild in the Mediterranean region, such as the Mastic Bush (PistacialentiscusL) and the turpentine tree (PistaciaterebinthusL). The latter is mainly used as root stock.
The aim of the brief description that follows is to emphasise the plant-water relationship of this crop.

• Soil and climatic conditions:

This small tree from the cashew (Anacardiaceae) family, which can grow to a height of 3 to 10 metres, requires a cold winter and a hot, dry summer in order to reach its full potential.
The winter chill is a decisive factor in achieving optimum flowering potential as it emerges from bud dormancy. On average, it requires 800 to 1,000 hours
at a temperature lower than 7°C to break out of this dormancy. Thus, mild winters could have a significant impact on the year’s production potential by reducing flowering.
It is susceptible to fungal diseases, such as Alternaria Late Blight (Alternaria Alternata), which affects the leaves and fruit, and Verticillium wilt (Verticillium
Dahliae), which invades the tree through the rooting system, so the high temperatures and low moisture content of the summer air are features that help control the spread of these diseases. The pistachio plant’s rooting system also has a low tolerance to excess moisture. Thus, it is for these reasons that it is particularly fond of poor and well-draining soils and a hot summer.

It is also worth noting that the pistachio plant has a high tolerance of saline soils. Although the development of its rooting system is determined by the soil type and root stock, it is characterised by a taproot system, which acts as an anchor, and a creeping root system at a depth of 60 cm, which supplies water and minerals to the plant.

• Flowering and pollination:

Being a dioceous shrub, the female and male blossoms appear on separate plants. The result of this feature is that the correct choice of the female and male variety becomes essential to ensure that the flowering procedure is synchronised. Generally-speaking, flowering occurs from mid-April to early-May for the very late varieties.
Being later than the almond tree, the flowering of the pistachio is less sensitive to spring frosts, but a later frost could seriously affect production. The pollination process itself is anemophilous, i.e. the wind carries the pollen from the male trees to the female trees. Although the positioning of the male trees is crucial, the rains and high moisture content of the air during this period restrict the propagation of the pollen and, consequently, the fertilization and future harvest.

• This kernel which is the pistachio:
The fruit of the pistachio tree is a drupe, just like the almond, peach, apricot, etc. A drupe consists of three parts: a pericarp (which is eaten in the case of the apricot, for example), an endocarp (the shell) and a kernel (the pistachio). Unlike the almond, the pistachio has a dehiscent shell which opens upon
ripening. In order to avoid the development of toxins and stains on the shell, peeling and drying must be carried out rapidly after the harvest.
Although the fruit has three growth stages (pericarp growth, shell hardening and kernel development), only the hardening stage can withstand water stress.
The other stages are tributaries of the plant’s water supply system. Therefore, a water deficit during these stages will lead to a drop in yield and reduced fruit size and quality. Generally, the fruits do not all ripen at the same time. This staggering of the plant’s becomes even more apparent when the pistachio trees are grown dry. In the case of the crop being harvested manually, then this is carried out in 2 or 3 stages but nowadays the process is more mechanised, which means that the ripening of the fruit has to be less staggered.
For a producer, apart from the yield, the quality of the pistachio is essential for obtaining a higher price for the product. As far as the pistachio is concerned, there are three main criteria:

– Fruit size
– The absence of stains or markings so they must be peeled very quickly after harvest
– The speed at which the shells open. In fact, it is the growing of the kernel that enables the shell to open.
Therefore, an open shell is evidence of the optimum growth of the kernel.

These are three very important factors, which will allow for an optimum price to be achieved for the product.
It should be pointed out that there are two types of product: The dry pistachios, which are harvested after ripening, and the green pistachios, i.e. which are
harvested just before the opening of the shell. Being less sensitive to outside conditions, the green pistachio has a higher value but it requires more handling.

• Biennial production (alternate bearing cycle)

The production potential is found at the end of the branches appearing during the current year. It is during year N-1 that the shoots distinguish the latent buds for the following year. Under dry conditions, the pistachio tree only produces fruit every other year, which is called biennial- or alternate-bearing. This alternating pattern is explained by the competition between vegetative growth and fruit development. This competitiveness is accentuated by the fact that the pistachio tree grows under conditions of extreme drought and temperature variations. This is the case with the pistachio trees of Bronte, in Sicily, which only produce biennially.
Although other irrigation methods do exist, precision irrigation is the one that is most suitable for the production of pistachio nuts.

PRECISION IRRIGATION

Even though it is a hardy crop that consumes little water, the pistachio tree does need water to achieve its production potential. This is something that we have observed not only with the fruit development but also with the alternate fruit bearing aspect. Nevertheless, this water should be applied rationally in consideration of the above-mentioned characteristics. Nowadays, irrigation is an essential tool for safeguarding production and its market status.
Even though other types of irrigation exist, here we are only dealing with precision irrigation or micro-irrigation. This is certainly the most efficient system for a demanding crop such as the pistachio tree, allowing for optimum control of the use of the water resources.

• Impact of irrigation on this crop:

Many studies have shown the benefits of irrigating pistachio orchards. If it is well-managed and well-controlled, the use of irrigation has  several advantages:

– By reducing the two-year alternate bearing cycle. By controlling the supply of water and minerals during the cycle, a balance is maintained between leaf growth and the development of the fruits, allowing for production to be more even, year on year.
– By increasing the rate at which the shells are opened. With irrigation, the kernel grows better and more uniformly across all the fruit on the plant. As a result, the fruits ripen earlier and the shells open at a faster rate.
– By encouraging the production potential to be realised through the development of the fruit and the yield. In the end, with a controlled irrigation technique, yield and quality is increased while reducing the competition between the fruits and the development of the pistachio tree.
The benefits do not end there. Irrigation is also very useful when planting the trees. In fact, the rooting system grows slowly, whatever the root stock, making the plantation more sensitive to the vagaries of the weather at the beginning of its life. With irrigation, the orchard is more uniform and it starts producing earlier(within one to two years, on average).

It should be pointed out that the pistachio tree takes a long time to come into production. The first harvest is gathered after 6 years and full production is achieved after 12 to 15 years. This slow development is counterbalanced by the long life span of these shrubs which can live for more than 100 years.
The orchard’s water requirements are substantial. In the case of a mature orchard, i.e. of more than 12 years old, we have a maximum crop coefficient of around 1, ranging from 1.19 to 0.9, for the month of July. This represents, under the current conditions of the Mediterranean region, a maximum requirement of around 6 mm/day. However, this figure needs to be put on hold in the light of the predicted climate changes.
As with other crops, it is possible to induce water stress at certain stages without negatively affecting neither the production of the current year nor that of the year N + 1. This is something that we have seen above with stage II of the fruit development. In this case, it would be possible to reduce the water requirements by 50% without causing any notable change in the yield or the quality of the harvest.

• Precision irrigation:

When we refer to precision irrigation, this encompasses micro-irrigation and drip irrigation. These two techniques involve the localised application of water at a low flow rate and low pressure and they have an irrigation efficiency of up to 95%. The other side of the coin is their susceptibility to clogging: biological, physical and chemical. To preserve these networks, the filtration, which is crucial for the system, and the maintenance schedules are essential.

For orchards such as these, the solutions put forward are self-adjusting, i.e. the device applies the same flow rate at a range of given pressures. This point is very important for having a perfect uniformity of distribution at each point in the field. And it is even more important for a crop such as the pistachio, which needs the water but it does not tolerate excess wetting. To guarantee this uniformity, it is essential that professionals are contracted for the hydraulic design of the system.

For pistachio orchards there are three main solutions:

– Micro-sprinklers: This system, which is less susceptible to clogging, is suitable for a wide range of water qualities and it can irrigate a larger area of soil.
For pistachio orchards, it is advisable to place one micro-sprinkler between two pistachio trees and to ensure that the spray does not wet the trunk or the foliage. This system can be suspended above ground or laid on the soil surface.
You must be aware that this system could maintain a high moisture content which could create a risk of fungal diseases.
– Drip irrigation: Generally-speaking, the pistachio orchards are laid out in a pattern of 7×7 or even 6×4 but rarely denser. In these configurations and given the peak water requirements of the crop, it would be advisable to lay two drip lines per row. The dripper spacing will be determined by the soil type.
The better draining the soil, the more there will be a tendency to have the drippers closer together with a spacing of 50 cm.
As far as the dripper’s flow rate is concerned, we will opt for flow rates of less than 2 l/h. The aim of having these low flow rates is to facilitate the movement of water in the soil while preserving its structure and water/O2 balance.
To encourage the development of the roots in the orchard, we will gradually move the lines away from the rows as the tree develops, eventually being located at 60 – 80 cm from the row, on average.
The double attraction of this system is the possibility of using it to deliver fertilizer supplements closer to your plant’s rooting zone. In this case, the water is used as a vector for absorbing nutrients into the soil.

– Subsurface drip irrigation: This is the most efficient system for delivering water and nutrients right to the heart of the rooting system and avoiding losses through evaporation. Numerous studies have shown that it has the advantage of reducing the pressure to use phytosanitary products on the orchards. It should be noted that we will be using a system equipped with an additional feature that inhibits the risk of root invasion. If possible, it is preferable only to bury the lines at the end of the fourth leaf (stage) in order to support root  development. When burying the tubes, lay them at a depth
of 30 cm in the soil. Always ensure that this work is carried out in the winter and think about starting irrigation in order to reduce the impact on the orchard during the year that the system is installed.

• The choice of system:
It is never easy to choose the most suitable system for the orchard. To assist you in this, it is important to have an overall perspective of the characteristics of the orchard. The first criterion that needs to be understood is the soil. This is the base and support of your orchard. To achieve this, an analysis of the soil is essential but not enough on its own. If you want to be really efficient and fine-tune the management of your system, it is important to dig one or
more soil sampling pits in order to understand your soils and visualise the movement of water therein. With this knowledge of the soil, you will be in a position to define rational irrigation cycles so as to maximise the root volume in question, while avoiding losses through percolation and over-watering.
The second most important criterion is the water resource: water source, its availability, localisation and quality. Water quality is a major factor because this will allow you to determine the extent of the risk of clogging and assess the feasibility of a drip irrigation scheme. Let us take the example of iron or manganese, whose presence in the water could be a restrictive factor in the use of a drip system.
Finally we come to the characteristics of the orchard: type of production, density, topography, shape and cultivation techniques. In fact, the aspect of the crop management technique and protocols is an extremely important element to be considered and planned: tillage, crop (weed) management, harvest, environmental certificates, etc.
In short, every project is unique. That is why it is important to be advised and supported in these projects in order to determine the most sustainable and efficient solution under our conditions.

CONCLUSION

Even though they are very hardy, the market requirements demand that the pistachio orchards must be irrigated. Irrigation, particularly precision irrigation,
allows for this green gold to be produced year after year and to enhance the value of those land surfaces where no other crops could exist.
With the use of precision irrigation, the pistachio orchards will be productive while conserving resources (water, nutrients, energy) and reducing their susceptibility to fungal diseases.