About Pistachios
History
The modern pistachio nut was first selected from wild Pistacia vera trees native to central Asia. The initial selection and improvement was first undertaken in the era of the Persian Empire which spanned from the Eastern Mediterranean to central Asia. Ever since, pistachios have been an important crop in cooler parts of the Iranian plateau.
Pistachio cultivation spread into the Mediterranean world where it has continuously prospered in Syria, Turkey, Greece and Sicily. Pistachios were already well known in Late Antiquity. Since 1970s, pistachios have become a commercial crop in many countries which fall around the 30th parallel north and south of the equator. These regions include: California, North Africa, New South Wales in Australia, South Africa, Argentina and Chile.
Since 1970s, pistachios have developed from a luxury nut appreciated only by the rich into a supermarket item enjoyed by all.
Botany
The pistachio of commerce is the only edible species among the 11 species in the genus Pistacia; all are characterized by their ability to exude turpentine or mastic. Several are referred to as pistachios, but the name is generally reserved for the edible nut of commerce. Its Latin name is Pistacia vera L. A member of the family Anacardiaceae, it is related to the cashew, mango, poison ivy and oak, pepper tree and sumac.
The Pistachio is a relatively small tree mostly grown in the arid parts of the world close to the 30th parallel latitude. It grows from sea level (e.g. Attica, Greece) up to an altitude of 1800 meter above sea level (e.g. Kerman, Iran). The combination of latitude and altitude should provide enough chilling hours in winter and long hot and sunny summer conditions to produce a viable crop.
Pistachios bear laterally on one-year-old wood. This causes an alternate bearing habit, very prominent in pistachio production with extensive commercial consequences.
Pistachio tree is deciduous, i.e. it loses its leaves in the fall and remains dormant through the winter. The tree is dioecious (i.e. two houses), meaning the male flowers are borne on one tree and the female flowers on another. Therefore, both male and female trees are needed for nut production. The pollen is spread by wind.
The trees have extensive root systems allowing them to mine the soil deeply. Thus, pistachios are adapted to survive long periods of drought. They need a well-drained soil. In such soils, the tree can tolerate relatively high levels of salinity in soil and/or irrigation water.
Pistachios are characterized by a long juvenile period, typically bearing few nuts before six years of age. They achieve full bearing between 15 to 20 years of age.
http://fruitsandnuts.ucdavis.edu/dsadditions/Pistachio_Manual_2005/
Nut Morphology and Maturation
The individual pistachio nut grows in fruit clusters of multiple nuts, much like grapes. Botanically, they are drupes, the same classification for almonds, peaches, apricots, cherries and plums. All drupes consist of three parts; an exocarp, a fleshy mesocarp (together called hull) and an endocarp (shell) that encloses a seed (kernel). The difference between nuts and fruits lies in the edible portion. In pistachios and almonds the seed is consumed, rather than the mesocarp as in stone fruit.
Ripening of the fruit, late in summer, is signaled by the loosening and separation of hull from the shell and consequent splitting of the shell inside the loosened hull. In reality, not all nuts split open upon ripening. This depends on the cultivar (variety), weather conditions and irrigation schedule.
In all cultivars there exists a genetic aberration that causes shell splitting before hull separation (early splitting). This causes the splitting of the unripe hull which creates access for Aspergillus Flavus spores to the kernel. Such nuts, if not harvested quickly, may provide a good medium for the growth of Aspergillus and the creation of aflatoxin which is a secondary metabolite in the growth of the fungus. The percentage of early splits produced by the tree at the time of the harvest will differ from one variety to the other; it also varies from year to year depending on the weather and irrigation schedule. Early and quick harvest is the best practice in avoiding high levels of contamination by aflatoxin in the harvested crop.
http://fruitsandnuts.ucdavis.edu/dsadditions/Pistachio_Manual_2005/