DESCRIPTION OF THE PISTACHIO TREE
Dipartimento di Arboricoltura, Botanica e Patologia Vegetale, Università Federico II, Napoli Italy
BOTANICAL SPECIES: Pistacia vera L.
Origin and diffusion: Native of arid zones of Central and West Asia, the pistachio tree was distributed throughout the Mediterranean basin by the Romans. Only in the current century has the crop been introduced away from its centre of natural distribution and domestication, such as in Australia
Main growing countries: Iran, Iraq, Syria, Turkey, Greece, Tunisia, Italy, California, Arizona.
Climatic and soil requirements: Long and dry summer seasons. Depending on the cultivar, 500-1000 chill units are required during winter to assure regular dormancy development. Areas with high levels of relative humidity are not suitable. Well-drained soils are preferred to water logged conditions. It is quite tolerant to alkalinity and salinity.
Tree: Xerophitic, deciduous and dioecious (separate male and female trees), up to 8-10 m. high,
Leaves: Pinnate compound leaf. Three to-five broad-elliptical to round-ovate leaflets. The lamina presents a thick texture due to the presence of cuticule epidermis on both sides. In some cultivars the rachis and petiole present a small wing–like expansion.
Buds: Single apical, usually vegetative, at the end of shoots and branches; single lateral, both vegetative and reproductive (inflorescence bud), axillary with respect to the leaf; along the current year’s shoot or the one-year-old wood. Inflorescence buds differentiate throughout spring, stop in summer, and resume in autumn. Flower morphology complete in the following spring, just before anthesis.
Flowers: In male and female trees, flowers are borne in compound racemes and paniculate inflorescences, respectively. The perigonium of the male flower and the perianth of the female flower contain 3-5 sepals. The female flower has no nectaries. Pollen is distributed by the wind.
Blooming time: April. In female trees blooming lasts 3-4 weeks, whereas males shed pollen for a couple of weeks. Most of the male cultivars bloom earlier than the female. Thus, to guarantee an extended pollination period it is necessary to introduce into an orchard early, medium and late blooming males. To increase nut set, some authors recommend "supplementary" artificial pollination
Fruit growth and development: The fruit is a drupe. Exocarp and mesocarp (hull), endocarp (shell) embryo and cotyledons (kernel) do not grow and develop simultaneously. Soon after fruit set, the hull and shell start to enlarge but the embryo becomes evident only 6-8 weeks later, when the cotyledons start to fill the nut. Six – seven weeks before ripening, the suture along the two shells that form the endocarp starts to separate. This phenomenon, known as endocarp dehisence or splitting, affects the commercial value of the crop: for direct consumption, splitting nuts are usually preferred to the non-splitting ones.
Two weeks before ripening, the hull changes its ground colour and overcolour, and easily separates from the endocarp. Two weeks later, ripened fruits abscind by the infructescence.
Fruit drop There are two peaks of fruitlets drop, respectively 2 and 4 weeks after full bloom. Only a small proportion of the flowers develop fruits (about 3%).
Empty fruit. "Blank" nuts develop the hull and shell but not the nutmeat, as a consequence of;
Endocarp dehiscence (splitting). The degree of splitting along the shell suture and the percentage of nuts that split depend on the cultivar , the pollen source (pollens of P. vera males increase splitting with respect to other Pistacia spp pollens). Furthermore splitting is also affected by ripening (harvest time), tree water status (irrigation), tree nutritional status and the relative crop load.
Alternate bearing The mechanism underlying this phenomenon differs from that in other fruit tree species and the details remain unclear . A pistillate tree differentiates axillary inflorescence buds every year, but in the "on year" (heavy production) they abscind. Because inflorescence bud abscission occurs soon after the cotyledons start to enlarge actively and the kernel becomes evident inside the shell (July-August), the phenomenon has been related to a competition for carbohydrate between the current and future production: i.e. fruits on the previous year’s wood and inflorescence buds on the current year’s shoots.
Cultivars Only about 100 pistachio cultivars have been described world-wide. It is of very low value when compared with other temperate zone fruits with a similar ancient cultural history. The reasons for this paucity of cultivars are thought to be the long juvenility of pistachios, the long life-span of the trees and the hybridization phenomenon that is very common among Pistacia species.
Some authors hypothesise the existence of two major clusters of pistachio varieties, one including cultivars of the Iranian-Caspian region and another comprising cultivars of the Mediterranean area. The cultivars originated in Central and West Asia are generally characterized by large, rounded fruit, showing a clearly split endocarp containing a yellow-green kernel. Under the climatic conditions of the inland areas of Sicily, this group of cultivars complete the phenological phases generally later than cultivars originating in the Mediterranean. These latter cultivars usually bear small unsplit fruits enclosing a green kernel. Among this second group, the Italian varieties can be distinguished from the others by their deep green colour and aromatic taste of the kernel.
Grouping the cultivars by origin and fruit characteristics, the nut size ranges from 0.82g (Tunisian varieties) to a maximum of 1.17 g (Iranian and Turkish varieties). The longest nuts (2.23 cm) are produced from the Italian cultivars, the shortest (1.95 cm) among the Iranian. Nut shape is elongated (length/width >1.8) in the Italian, Greek and Tunisian varieties and ovoid (l/w > 1.5 and < 1.8) for the Iranian and Turkish cultivars and, to a minor extent, for the Syrian.
Use: There are many uses of the fruits, both in- shell and shelled, natural or processed: they can be naturally dried in- shell for direct consumption; in- shell salted and toasted for snack consumption; shelled and peeled for industrial use (sausages, confectionery, cosmetics); or ground for ice-cream.
Fruit composition: Proteins 20%; Fats 50 %; Carbohydrate 20 %; Fiber 2%; Ash 3%; Sugars 6%; Minerals (Ca, P, Fe K, Mg) 2 %; unsaturated/saturated fatty acid 8;
Nutritive value: 600 calories per 100 g. of kernel.