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Pistacia vera L. is a member of the genus Pistacia, in the family Anacardiaceae, order Sapindales (Stevens, 2008), tribe Rhoae. The Latin word pistachio is derived from ‘pissa’ = resin and ‘aklomai’ = to heal according to Dioskurides, although Davatchi (1958) believed it originated from a Persian loanword, coming into English through Italian, and is a cognate to the Modern Persian word پسته Peste. There are three main taxonomic classifications of the genus, based on flower morphology and leaf phenology by Engler (1883) and Zohary (1952), and on molecular analyses by Parfitt and Badenes (1997), Yi et al. (2008) and Al-Saghir and Porter (2012). Linnaeus (1753) defined three species (P. vera, P. lentiscus, P. terebinthus) in the genus, and Engler (1881) based his classification on eight species. Zohary (1952) divided Pistacia into four distinct sections, with at least nine species and five subspecies with a geographical distribution from Western Asia, Asia Minor, and the Mediterranean region to the Americas, confirmed by the most recent taxonomic revision (Al-Saghir and Porter, 2012). Fifty-five binomials for Pistacia appear in the International Plant Names Index; however, only 14 species are classified in two sections: section Pistacia and section Lentiscella. The recognized species are P. khinjuk Stocks, P. chinensis Bunge subsp. chinensis, P. eurycarpa Yalt., P. atlantica Desf., P. chinensis subsp. falcata (Bess. ex Martelli) Rech. f., P. chinensis subsp. integerrima (J.L. Stew. ex Brandis) Rech. f., P. lentiscus L. subsp. lentiscus, P. lentiscus subsp. emarginata (Engl.) Al-Saghir, P. mexicana Humb., Bonpl., & Kunth, P. × saportae Burnat, P. terebinthus L., P. vera L. and P. weinmannifolia Poiss. ex Franch.

Although morphological and molecular markers have been used to study phylogenetic relationships of Pistacia species, the taxonomy remains unclear (Zohary, 1952; Morgan et al., 1992). Zohary (1952) considered the genus had 11 species in four sections: Lentiscella (P. mexicana and P. texana), EuLentiscus (P. lentiscus, P. saportae and P. weinmannifolia), Butmela (P. atlantica) and Eu-Terebinthus (P. chinensis, P. khinjuk, P. palaestina, P. terebinthus and P. vera) based on morphology. Whitehouse (1957) identified 15 Pistacia species. Recently, sequencing of plastid and intron fragments has provided information for resolving putative hybrids and geographic distribution of Pistacia species (Yi et al., 2008; Xie et al., 2014). Using restriction fragment length polymorphism (RFLP) (Parfitt and Badenes, 1997), random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) (Katsiotis et al., 2003; Golan-Goldhirsh et al., 2004) markers, Pistacia has been divided into two sections: (i) Eu-Terebinthus and Butmela sections are merged into Terebinthus; (ii) Lentiscella and Eu-Lentiscus sections are merged into Lentiscus. The Butmela section could be merged with Terebinthus since the geographical origin and morphological characters of P. atlantica and P. mutica overlap (Parfitt and Badenes, 1997; Golan-Goldhirsh et al., 2004; Kafkas, 2006; Yi et al., 2008; Kirdok and Özden Çiftçi, 2012). With sequencing data, the Terebinthus section is monophyletic and a close relationship existed between some species, e.g. P. vera, P. khinjuk, P. atlantica, P. mutica, P. palaestina and P. terebinthus.

The taxonomic relationship of P. palaestina and P. terebinthus has been addressed using molecular markers (Parfitt and Badenes, 1997; Kafkas and Perl-Treves, 2002; Katsiotis et al., 2003; Golan-Goldhirsh et al., 2004; Yi et al., 2008). Engler (1881) believed that P. palaestina might be a subspecies of P. terebinthus. According to sequencing, Lentiscus seems to have evolved in parallel since it contains sister clades (Yi et al., 2008). Based on nuclear and plastid markers, P. saportae has at least two different types and is a probable hybrid between P. lentiscus and P. terebinthus. There could have been two separation and dispersal events resulting from the separation of old and New World species (nearly 19 million years ago (mya)), separation of Terebinthus species (nearly 15 mya) and long dispersal (6 mya).

P. vera is the most ancestral species in the genus based on morphology (Zohary, 1952) together with plastid restriction site analysis (Parfitt and Badenes, 1997) and from P. vera it is assumed that P. khinjuk and other species of the Terebinthus section have evolved (Zohary, 1952).

Inbar (2008) differentiated New World Pistacia species (P. mexicana and P. texana) from other species; P. vera, P. atlantica and P. mutica were classified as ‘Vera’ while P. khinjuk, P. chinensis, P. integerrima, P. palaestina and P. terebinthus were grouped as ‘Khinjuk’ based upon close relationships between gall-forming aphids and Pistacia species.

Pistacia species are dioecious and deciduous trees with a long juvenile period, e.g. 5–8 years. Dioecy requires the interplanting of male and female trees in commercial pistachio orchards, thereby reducing the profitability of the crop; determining the genetic basis for monoecy is important. Deciduous trees are more tolerant of extreme climate conditions (−26 to 46°C) than the evergreen species (−8 to 41°C), except P. lentiscus, which can tolerate 45°C (Kozhoridze et al., 2015).

Leaves are alternate or persistent, pinnately compound, but sometimes trifoliolate or unifoliolate, pari- or imparipinnate. Leaflets are large, 1-2 (-3)-double, ovate or broadly lanceolate, acute or obtuse, mucronulat to 5-10 (-12) × 3–6 cm, often hairy; the rachis is wingless and hairy (Kozhoridze et al., 2015). Dioecy appears to be common in the Anacardiaceae (Bawa, 1980). Pistachio trees can grow to a height of 10 m although considerable variability can occur. The canopy size can be up to 12 m width, depending on growing conditions. The root system is deep and widespread, depending on planting material, soil type, nutrition, irrigation and age. Both staminate and pistillate inflorescences are panicles that can have a single to several hundred individual flowers (Crane and Iwakiri, 1981). Flowers are almost always unisexual, radial, small, with well-developed staminodes or carpellodes, and apetalous; sepals are usually five, distinct to slightly connate. The male flowers usually have 4–5 stamens with short filaments, usually glabrous, and producing tricolporate or triporate pollen grains; a nectariferous disk is present, usually intrastaminal (Al-Saghir and Porter, 2012). Female flowers usually have three carpels with superior ovaries and only one carpel is fertile and well developed with a single ovule. Generally, one or two axillary buds located distally on new growth are vegetative. They are considerably smaller than the inflorescence buds and may give rise to lateral branches in the following year or remain dormant. Pistachio bears its fruit laterally on wood produced during the previous season (Ayfer, 1963; Crane, 1984). In the northern hemisphere, inflorescence buds begin to elongate at the end of March, and anthesis occurs generally in late May and for about 3 weeks (Ayfer, 1963). Fruits of cultivars are paniculate, ovate-oblong, 16–29 × 9–12 mm. The pistachio nut is a green seed with a hard shell, enclosed within a fleshy fruit, a drupe. Fruit maturity occurs with a change in the epicarp (skin) from translucent to opaque, and a softening and loosening of the epicarp and mesocarp (hull) from the endocarp (shell) which encloses the embryo (kernel) (Crane and Iwakiri, 1981). The nuts are borne in clusters. Although seeds of most Pistacia species are consumed by local populations, P. vera is the only species that is cultivated commercially (Whitehouse, 1957). Pistachio nuts of commerce comprise the endocarp and the large edible kernels. There are few cultivars of P. vera, probably <100 worldwide (Maggs, 1973). Pistachio production in 2016–2017 was 1.115066,00 t (in shell basis), that represents a 55.3% increase since 2010 (FAOSTAT, 2017). Iran leads world production (574.987,00 t), followed by the USA (272.291 t), China (95.294 t) and Turkey (78.000 t).

Although Syria and Mesopotamia were thought to have been the centre of origin of pistachio (Bailey, 1947), it is now believed to have had two centres of origin and domestication: (i) West and Central Asia and the Mediterranean region (Zohary, 1952; Whitehouse, 1957); and (ii) Central Asia and the Near East (Vavilov, 1951). Most wild pistachio species are distributed within Western and Central Asia, the Middle East and the Mediterranean region. According to Zohary (1952) and Whitehouse (1957), P. vera grows wild in the low mountains and foothills of the semi-desert zone of south-central Asia. The range extends from northern Iran and northern Afghanistan through Turkmenistan, Uzbekistan, Tajikistan, Kyrgyzstan, western Tien-Shan and the mountains of Karatau (Hormaza et al., 1994). The global distribution of the genus Pistacia is restricted to latitudes between 10° and 45°N (Kozhoridze et al., 2015). The presence of pistachio nuts in archeological excavations provides evidence that pistachio has long been associated with human activities, e.g. Neolithic settlements in the Mid-East. It was introduced into Mediterranean regions of Southern Europe and North Africa from Italy and into the USA in 1853–1854 (Lemaister, 1959).