1 Introduction

The genus Pistacia L. is a member of the Anacardiaceae family that also includes cashew, mango, poison ivy, poison oak, pepper tree and sumac. The genus consists of eleven or more species (Zohary 1952; Whitehouse 1957; Kokwaro and Gillett 1980; Kafkas and Perl-Treves 2001; Parfitt and Badenes 1997). Pistacia vera L. has edible nuts and is the only commercially important species. The other species have been used for many years as rootstock sources for P. vera. P. vera is believed to be the most ancestral species and the other species are probably its derivatives (Zohary 1952). Wannan and Quinn (1991) compared the floral morphology of the genus with that of sister groups. All members of the genus are dioecious (note exceptions below), diploid dicots. Ila et al. (2003) reported a diploid chromosome number of 30 for P. atlantica Desf., P. eurycarpa Yalt., P. terebinthus L., and P. vera. These results were consistent with an analysis from Parfitt and Lin unpublished, of 11 species in which 2n  =  28 or 30. Prior reports (Ghaffari and Harandi 2001) of 2n  =  24 for Pistacia khinjuk Stocks appear to be incorrect. Molecular taxonomic descriptions of the genus have been reported by Parfitt and Badenes (1997), and Yi et al. (2008). Major differences from the 1952 Zohary classification include the definition of P. integerrima as a separate sister species to P. chinensis and classification of the species into only 2 sections.

The cultivated pistachio of commerce is the species P. vera L. It is native to the Middle East and Central Asia. There are two centers of diversity of cultivated pistachio: one comprises the Mediterranean region of Europe, Northern Africa, and the Middle East. The second comprises the Eastern part of Zagros Mountains from Crimea to the Caspian Sea (Maggs 1973; Hormaza et al. 1998). The Vavilov center of diversity for wild P. vera is located in Northern Iran and Southern Turkmenistan as well as parts of Afghanistan. The region straddling the border of Turkmenistan and Iran is referred to as the Baghtis region and is an area of rolling hills covered by grasslands and scattered stands of P. vera trees. This is the area with the greatest present natural diversity, primarily because other areas of Iran with native pistachio stands have been topgrafted with clones of improved cultivars (Maggs 1972, 1973) (Fig. 21.1).

Fig. 21.1
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Pistachio distribution in Europe and Asia. Textured area is the Badgtis region of wild pistachio savannah (Maggs 1973)

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Pistacia species are dioecious with several isolated reports of monoecious individuals (Ozbek and Ayfer 1958; Crane 1974; Kafkas et al. 2000; İsfendiyaroğlu and Özeker 2009) and wind pollinated. Male and female apetalous flowers are borne in panicles on separate trees. Each panicle can have more than 100 flowers (typically 100–300 flowers per inflorescence, Fig. 21.2). P. vera produces a nut (classified as a semidry drupe) which is marketed as a dried in-shell product after removal of the husk. Pistachio nuts are drupes, the same classification as for almonds and stone fruits. All drupes consist of three parts: an exocarp, a fleshy mesocarp, and an endocarp that encloses a seed. The pistachio endocarp or shell encloses a single oil-rich seed and usually splits along its lateral suture, when the nut is ripe. The exocarp changes from green to white or white-purple color at maturity (Fig. 21.3). The ­kernel has a papery seed coat and two cotyledons.

Fig. 21.2
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Male and female P. vera inflorescences

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Fig. 21.3
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‘Golden Hills’ pistachio, showing typical clusters

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The tree has a growth habit characterized by a strong apical dominance and lack of vegetative buds in old trees. Pistachios have an extensive root system. Under natural conditions, P. vera does not develop a central tap root, but produces a highly branched root system with many fine roots that allows the tree to efficiently extract water and nutrients. The tree has a pinnately compound leaf. Each leaf subtends a single axillary bud. Most of these lateral buds differentiate into inflorescences and produce female or male flower bearing rachi.

Currently, Iran, the USA, Turkey, and Syria are the main pistachio producers in the world, contributing over 90% of the world production (FAOSTAT 2007) (Fig. 21.1). US production and planted areas have continued to expand, planted primarily with ‘Kerman’ and recently with a new University of California release, ‘Golden Hills.’ In 2006, 45,527 ha of pistachio were harvested in California and another 16,228 nonbearing ha were in the ground (CPC 2007; Pollack and Perez 2007). The average value of the California crop in 2005 and 2006 was approximately 518 million dollars (CPC 2007). Production is located primarily in southern San Joaquin valley of California (Fig. 21.4). The California industry is highly mechanized and is characterized by a few large, well-funded growers.

Fig. 21.4
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Pistachio production areas in California

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2 Origin and Domestication of Scion Cultivars

The natural distribution of wild P. vera is centered in Takzhikistan, Kirgizia, and north Afghanistan, and extends westward to the northern part of Khorasan district in Iran, and the Kopet mountain range of southern Turkmenistan (Zohary 1996). The Badgtis region of southeastern Turkmenistan and northeastern Afghanistan is an area where significant undomesticated P. vera forests remain (Popov 1994; Maggs 1973) and appears to be the center of diversity for this species (Whitehouse 1957). These wild pistachio nuts are typically much smaller than the cultivated pistachio, usually about 1 cm long and do not split. The trees are widely scattered among grass covered hills (Parfitt personal observation) in an area that is nominally protected as a nature preserve. Thus, geographically, wild P. vera represents the most northern wild pistachio taxon in Central Asia and it is spatially almost fully separated from the two other wild pistachio species that occur in this region (P. atlantica and P. khinjuk). The later two grow farther south, and overlap with wild P. vera only at the fringe of their distribution range in Khorasan and probably in the north of Afghanistan. In the middle part of Central Asia (Uzbekistan, Tadzhikistan, Kirgizia, and the southern most parts of Turkmenistan and Kazakhstan) P. vera forest extends over some 300,000 ha. Wild P. vera has been and is used as a source of nuts. The distribution area of the wild forms of P. vera and archaeological evidence indicate that this species was first brought into cultivation in Central Asia (Zohary and Hopf 2000; Zohary 2006). The species has been long propagated for nuts throughout the Mediterranean and Middle East. Several reports suggest that the Romans were responsible for the spread of P. vera within the Mediterranean basin. The total dependence of pistachio on grafting today suggests relatively late domestication.

About 100 cultivars from several regionally distinct groups have been described worldwide (Maggs 1973; Parfitt 1995). Pistachio cultivars from the middle East had good split shells and from Iran generally had large nut size and split shells, whereas those from Italy had small nuts with many unsplit shells and dark green kernels. Iranian pistachios are apparently more diverse than cultivar populations from other regions, probably because they have been selected from wild materials near the center of diversity.

3 Genetic Resources

3.1 Female Cultivars

The following descriptions are a partial list of the main cultivars grown in various regions of the world. In Europe and especially in Turkey and Iran, a substantial number of additional named cultivars exist, which may be synonymous with the cultivars described below or they may be distinct local varieties, analogous to land races of wheat. A single cultivar may have several local names; however, different discrete selections may be given the same name such as the name of a region or municipality. Therefore, reliance on cultivar names as the basis for accessing “trueness to type” may not be reliable. Patented or recently released cultivars are more likely to be correctly identified than old cultivars, especially those of Eurasian origin.

Old European cultivars are found distributed around the Mediterranian basin. ‘Napoletana’ is the predominant variety in Sicily and is considered to be synonymous with the ‘Bianca’ cultivar grown in Italy. Less often planted are ‘Agostana,’ ‘Girasola,’ ‘Notaloro,’ ‘Cappuccia,’ and ‘Femminello.’ ‘Trabonella’ and ‘Bronte’ are Sicilian cultivars with similar characteristics. Nut color is greenish and nut shape is longer and thinner than for ‘Kerman’ or Iranian cultivars. Nut size is considerably smaller than ‘Kerman’ and nut quality under California conditions is poor, with a high level of nonsplits in some seasons and significant disease and pest problems. The hulled nuts have a tendency to stain. ‘Sfax,’ ‘Mateur,’ and ‘El Guettar’ are grown in Tunisia. ‘Mateur’ may be the best of the Tunisian selections with the nut size and appearance similar ‘Kerman.’ ‘Sfax’ produces large tight nut clusters, but nut size, yield, and percent splits are inferior to ‘Kerman’ nuts. ‘Aeginea’ (‘Aegenes’) and the more recently released ‘Pontikis’ (Pontikis 1986) are grown in Greece. ‘Aeginea’ appears to be very susceptible to Botryosphaeria dothidea, perhaps due to its very early flowering in the spring. ‘Pontikis’ has a moderately large fruit, high kernel weight (55% of fruit weight) and an oblong-ovate shaped nut and kernel. It splits much better than ‘Aegenes,’ with 90–98% splits. Blank nut percentage is about 5–10%, the same as ‘Aegenes’ and has yields similar to ‘Aegenes.’ However, the adaptation of ‘Pontikis’ outside of the Athens area has not been determined. ‘Lamarka’ is the main cultivar in Cyprus.

A large number of named cultivars are grown in Turkey. ‘Uzun,’ ‘Kirmizi,’ ‘Siirt,’ ‘Halebi’ are favored. Less favored is the Turkish cultivar ‘Red Aleppo’ which is commonly grown in Syria. ‘Achoury’ is a major Syrian cultivar. ‘Achoury,’ ‘Alemi,’ ‘El Bataury,’ ‘Obiad,’ and ‘Ayimi’ are also grown in Turkey. ‘Red Aleppo’ was used as a cultivar during the early development of the California industry and produces a good quality nut with reasonable yield under California conditions. It has a high percentage of split nuts but nut size tends to be somewhat smaller than ‘Kerman.’

Many of the best pistachio cultivars are found in Iran. Large nuts are preferred and many of the Iranian cultivars have relatively large nuts with good split percentages. Major pistachio cultivars grown in Iran are ‘Ohadi,’ ‘Akbari’ and ‘Ahmad Aghaii.’ Other cultivars are ‘Momtaz,’ ‘Kalehghouchi,’ ‘Ghermeza,’ ‘Tbeahimi,’ ‘Ogah,’ and ‘Wahidi.’ Another Iranian cultivar ‘Rafsanjani,’ considered to be a promising cultivar in Iran, is being tested for adaptation in Azerbaijan. In addition to these cultivars, many other named cultivars are grown in Iran, either locally or across the country. Some of these items may be the same as the aforementioned cultivars, but with different local names. ‘Ohadi’ produces attractive nuts that are slightly smaller than ‘Kerman’ nuts. ‘Kalehghouchi’ has very large nuts as well as a good yield and has attracted some interest in California because of its nut size and good split percentage, which may be better than ‘Kerman’ under California conditions. In a replicated trials budded in 1998, located on the west side of Kern County, ‘Kalehghouchi’ and ‘Aria,’ another Iranian variety were tested against ‘Kerman.’ Seventh to tenth leaf ‘Kalehghouchi’ trees yielded similarly to ‘Kerman,’ while ‘Aria’ yields were lower. Nut split percentages and weights were higher for both of these cultivars than for ‘Kerman.’ Both ‘Kalehghouchi’ and ‘Aria’ flower 5–10 days earlier than ‘Kerman’. ‘Kalehghouchi’ matures at about the same time as ‘Kerman’ while ‘Aria’ matures about 2 weeks earlier than ‘Kerman.’ ‘Kalehghouchi’ produces excessive vegetative growth on mature trees under California management conditions. It has a tendency to produce many long ‘whips’ and ‘hanger’ branches on which a considerable fraction of the fruit is borne. This makes shaking difficult or requires considerable additional pruning to maintain tree structure. ‘Aria,’ when tested in the same trials, produced nuts with poor shell-hinge strength resulting in excessive loss of shells and kernels during hulling. The location of flower buds on new branches has made training this cultivar difficult and has resulted in sunburned nut clusters. These limited replicated trials have demonstrated that a particular cultivar’s success in one region does not necessarily translate to successful performance in another.

Extensive collections of pistachio cultivars and germplasm resources were assembled at several experiment stations in the southern former Soviet republics during the 1950s and 1960s. With the dissolution of the Soviet Union, the independent republics have had difficulty supporting these stations and collections, so most of the materials have either disappeared or will disappear within the near term. The collections are in poor conditions and records for some of them are no longer available (Parfitt—personal observation-Kara Kala, Turkmenistan). Some of the selections maintained at these sites are of commercial quality, some are not. Several promising selections have been made at the Genetic Resources Institute of the National Academy of Sciences in Baku, Azerbaijan.

‘Sirora’ was developed in Australia (Maggs 1990) from a formal breeding program. The industry has remained small in Australia, so this cultivar has not been widely planted. ‘Kastel’ and ‘Rashti,’ grown in Israel, are similar in some aspects to the ‘Kerman’ variety. Neither ‘Kastel’ nor ‘Rashti’ have been tested directly against ‘Kerman’ in California yield trials. ‘Rashti’ has large nuts, high split percentage, and a good flavor. Tree structure is similar to ‘Kerman’ as is its alternate bearing characteristic. It is a late maturing cultivar, probably several weeks after ‘Kerman.’ This has been an issue during years when late summer and fall have been very cool. Under these conditions it may not ripen before the start of winter rains. ‘Kastel’ seems to be very similar to ‘Kerman’ in most characteristics. Nut size may be slightly larger.

Less than 20 named cultivars have been imported into the USA. Some of the female cultivars that were introduced into California by the USDA in the early 1900s were: ‘Red Aleppo’ from Syria, ‘Bronte’ and ‘Trabonella’ from Sicily, ‘Sfax’ from Tunisia, ‘Kastel’ and ‘Rashti’ from Israel and some from other countries. Additional unnamed P. vera germplasm was introduced through the USDA Plant Introduction Gardens at Chico which was closed in 1967. However, very little germplasm has been imported into the USA since the station was closed. A number of the introductions, both P. vera and other species, at the former Plant Introduction Gardens were collected and transferred to the National Clonal Germplasm Repository at Davis during the 1980s.

‘Kerman,’ a California cultivar, was collected in 1929 by W. E. Whitehouse in either Iran or Turkmenistan (Joley 1979), selected as a seedling in 1936, and released for trial by the USDA Plant Introduction Station, Chico CA in 1957 (Whitehouse 1957). The release of ‘Kerman’ occurred just prior to the major growth of the industry in California. ‘Kerman’ which is now the primary female cultivar commercially grown in California, produces large yields of attractive nuts with superior size (>1.2 g), especially in the primary growing areas of the southern San Joaquin valley. It is not a perfect cultivar. It has a strong alternate bearing tendency, a high percentage of blank nuts in some years, a relatively high level of nonsplit nuts, and a light greenish-yellow kernel with almost minimal flavor when dried at commercial temperatures. Kernel color and flavor has not been an issue with American consumers, who have not been exposed to European cultivars with different flavor characteristics. ‘Kerman’ is a relatively late maturing cultivar, which is not usually a problem in the San Joaquin valley with a large number of heat units. However, it’s later maturity means that it can be exposed to a third flight of navel orangeworm (Amyelois transitella Walker). Beyond the direct losses from damaged nuts, navel orangeworm infestations have been implicated in the development of higher levels of Aspergillus flavus var. flavus (Klich and Pitt 1988) contamination and consequent aflatoxin contamination. In the Sacramento valley during years with low heat unit accumulation, ‘Kerman’ may not mature until after the first fall rains, with a concomitant increase in disease problems. ‘Kerman’ is relatively susceptible to Alternaria alternata (tenuissima, and arborescens species groups; Pryor and Michailides 2002), but under dry California conditions this is not usually a problem for growers, and can be controlled with fungicides where it is a problem.

In 1980, another open-pollinated seedling introduced as a seed from Damghan, Iran, was selected at the University of California, Davis, by Dr. J. Crane and named ‘Joley,’ in honor of the former director of the Chico USDA Plant Introduction Station (Gardens). ‘Joley’ has been planted in a few orchards in California and in the state of New Mexico, where there is a limited acreage of pistachios. The cultivar is considered by some to be one of the best tasting pistachios developed in California. The tree has moderate vigor and blooms and matures about 10 days earlier than ‘Kerman.’ It has almond-shaped nuts similar to ‘Trabonella’ or ‘Bronte’ (Sicilian varieties). The kernel color is greener than ‘Kerman,’ but the nut size is significantly smaller and the nonsplit percentage can be high in some years. There are few blanks; however, in some instances shell removal by consumers is not as easy as for ‘Kerman.’ On a few young trees grown in Kern County, the yield has never been high. It has a tendency to stain and does not appear to be a commercially viable cultivar.

‘Lassen’ was developed by Whitehouse from the same seed lot as ‘Kerman’ and released in 1962 from the USDA Plant Introduction Station at Chico CA. It is very similar to ‘Kerman’ with respect to nut characteristics. It has never been tested in replicated trials, but has good potential as a cultivar. Data from yield trials with ‘Kerman’ is not available. ‘Damghan’ was also developed from this seed collection. It has very large nuts but appears to be very low yielding under California conditions.

3.2 Male Cultivars

The California pistachio industry is based on one male cultivar, ‘Peters.’ ‘Peters’ is a good pollinizer and was found in the early 1900s by A. B. Peters at Fresno, California. However, the parentage is unknown. It produces abundant pollen, shed over a relatively long period (ca. 2+ weeks) and pollen viability over time (durability) is very good. In recent years, under low chill conditions it has shed pollen at the end of the receptive period for ‘Kerman,’ resulting in poor and irregular pollination. In addition to ‘Peters,’ the selections at Chico of ‘02-16’ and ‘02-18’ imported from Russia are early and late blooming compared to ‘Peters.’ Pollen from ‘2-18’ is somewhat less durable than pollen from ‘Peters.’ ‘Nazareth,’ ‘Ask,’ and ‘Chico’ males are also grown sporadically in some locations in California. ‘Chico’ was introduced from the Chico, Calif., USDA Plant Introduction Station in 1962 as PI150646. It is reported to be a male seedling of P. vera selected from seed introduced under PI73396 from Aleppo, Syria; probably of species hybrid origin and tested as Chico 23. Leaf characters and bloom period observations suggest that it is probably an interspecific hybrid between P. vera L. and Pistacia integerrima L. (Parfitt personal observation). It is a prolific pollen producer and blooms early, courtesy of its P. integerrima parentage. ‘Chico’ sheds pollen prior to and during the earliest part of the ‘Kerman’ bloom period but often flowers too early to pollenize ‘Kerman.’ Some growers have used ‘02-16’ to match the early part of the ‘Kerman’ bloom period. If there are any xenia effects on nut size (male contribution), then this cultivar should not be used. ‘Ask’ and ‘Gazvin’ were introduced from Israel a few years ago and may have some value as pollenizers. They flower earlier than ‘Peters,’ but have poor pollen durability.

3.3 Rootstocks

California pistachios are grown primarily on three rootstocks, two species and one interspecific hybrid, all members of the genus Pistacia. They include Atlantica (P. atlantica) Pioneer Gold I (P. integerrima), and UCBI, which is a hybrid between a P. atlantica female crossed with a P. integerrima male. Two other rootstocks have occasionally been grown in California; they include P. terebinthus and Pioneer Gold II (a P. atlantica female crossed with a P. integerrima male). All of these rootstocks are produced from seed. Pioneer Gold I (PG1) is distributed as seedling plants produced from a population of P. integerrima parents selected for resistance to Verticillium wilt (Verticillium dahliae, http://broad.harvard.edu/annotation/genome/verticillium_dahliae/MultiHome.html). Consequently, these plants are expected to be genetically variable, although they appear to be quite uniform in the field. During the last 15 years, Pioneer nursery has continued to improve and select the best parent stock plants, so what is being released as Pioneer Gold I today may be genetically different than the material released in the past. They are all P. integerrima. Pioneer Gold II was released at about the same time as UCB1, but was not widely accepted by growers. UCB1 was released by L. Ashworth at the University of California Berkeley (Morgan et al. 1992). This rootstock is also distributed as seed or seedlings, but is the result of a closed cross between a P. atlantica female and a P. integerrima male. Originally two different P. atlantica females were used, but after incompatibility with ‘Kerman’ was detected in the seed lot derived from one of the females, UCB1 was refined to be the result of only one P. altantica female and one P. integerrima male.

4 Major Breeding Achievements

4.1 Cultivars

The only organized breeding programs at present are located in Spain, Turkey, and Israel. There is probably active work being conducted in Iran as well, but it is less well documented. California and Australia had breeding programs in the past, but these have been discontinued due to loss of funding.

The California program was a conventional breeding and genetics program using crosses among all possible combinations of 30 female and 45 male genotypes. Materials from the former Chico Plant Introduction Gardens were used as well as Iranian and Italian selections. Males from J. Cranes selection program of the 1970s were also used to make some of the crosses. Approximately 8,000 seedlings were produced and were evaluated initially at three locations on their own roots. Superior plants were selected from these seedlings and were tested in replicated trials on the two rootstocks used in California. Three cultivars, described below, were released from this program. A detailed description of the program is given in Chao et al. (1998). Some potential cultivar materials from the California program continue to be tested and several superior cultivars may be selected from among them.

Two new female cultivars ‘Golden Hills’ and ‘Lost Hills’, were released in 2005 from the pistachio breeding program of Parfitt et al. (Kallsen et al. 2009). Five years (6th through 11th leaf) of production and phenological data have been taken on these cultivars. ‘Golden Hills’ (Parfitt et al. 2007) is a new female cultivar that flowers 5–7 days before ‘Kerman’ and matures about 2 weeks earlier than ‘Kerman’; permitting efficient use of harvesting and processing equipment in much the same way that cultivar maturity series for peaches facilitate limited resources for harvesting and marketing. Earlier harvest also allowed this cultivar to miss infestation by the (most damaging) September flight of navel orangeworm. This cultivar has produced 35% higher yield than ‘Kerman’ during the first 5 years of harvested yield trials in the southern San Joaquin valley. Nut size and weight were similar to ‘Kerman,’ but percentage of blank nuts was lower and split nut percentage about 25% greater than for ‘Kerman.’ ‘Golden Hills’ has more but smaller scaffold branches than ‘Kerman,’ producing a smaller more bushy tree after 3–4 years of training. ‘Lost Hills’ (Parfitt et al. 2008) is a new female cultivar from the breeding program of Parfitt et al. that flowers 4–7 days before ‘Kerman’ and matures about 2 weeks earlier than ‘Kerman.’ As with ‘Golden Hills,’ earlier maturity results in much less navel orangeworm damage at harvest (<0.2%) This cultivar produced 28% higher yield than ‘Kerman’ over the first five bearing years. Nut size and weight and percent split nuts were about 26% higher than for ‘Kerman.’ ‘Lost Hills’ produced more lost kernels and loose kernels than ‘Kerman’ during hulling as well (3.2%). Flowering was more uniform than ‘Kerman’ for both this cultivar and ‘Golden Hills’ during low chilling seasons. This translated into a more uniform maturity and less difficulty in determining the correct time to harvest for maximum splits and minimum staining. Evaluation of ‘Lost Hills’ and ‘Golden Hills’ is continuing.

A number of new pollen sources were evaluated as part of the pistachio breeding program conducted by Parfitt et al. They were evaluated for quantity of pollen produced, the period over which the pollen was shed, pollen viability percentage at pollen shed, and pollen durability (the length of time during which pollen viability remained high). Two selections were made to complement ‘Peters,’ flowering earlier and later than ‘Peters.’ The early flowering selection, ‘Randy,’ released in 2005, flowers 1–3 weeks earlier than ‘Peters.’ It is characterized by a long bloom period, in excess of 2 weeks which is twice that of most male pistachios, a characteristic that ‘Peters’ shares. Peak flowering is 1–2 weeks earlier than peak flowering for ‘Kerman.’ The pollen is more durable than ‘Peters’ pollen (75% viable declining to 35% viable after 29 days of storage vs. ‘Peters’ initial viability of 45%, declining to 15% to 5% after 27 days). Although it was selected as a pollenizer for early flowering cultivars from the breeding program of Parfitt et al., and blooms too early to serve as the primary pollenizer for ‘Kerman,’ ‘Randy’ may be useful as a ‘Kerman’ pollenizer during low chill seasons, when there is less overlap in flowering period between ‘Kerman’ and the later flowering ‘Peters.’ The bloom period for ‘Randy’ closely matches that of ‘Kalehghouchi’ and would be a better pollenizer choice for this cultivar than ‘Peters.’

Small breeding and/or selection programs in Australia (Maggs 1990) and Greece (Pontikis 1986) have released the cultivars ‘Sirora’ and ‘Pontikis.’

In Spain a pistachio scion breeding program has been conducted at IRTA Mas de Bover since 1989. A total of 31 controlled crosses among 10 female and 12 male parents were made between 1989 and 1990. Selections have been made from about 2000 seedlings to date (Vargas et al. 1996). Currently, 9 female selections and 7 male selections are under trial. Cross combinations were planned considering that they could not freely occur in nature due to their different geographical origins. The main female parents used were ‘Aegina’ (Greece), ‘Batoury’ and ‘White Ouleimy’ (Syria), ‘Kerman’ (The USA), ‘Larnaka’ (Cyprus), ‘Mateur’ and ‘Sfax’ (Tunisia), and the main male parents chosen were: ‘B’ and ‘C’ (Greece), ‘M-36’ and ‘M-38’ (Syria), M-502 (Italy), ‘Nazar’ and ‘Enk’ (Israel) (Vargas et al. 1996). A number of traits have been studied in the progenies like vigor (Vargas et al. 1996; Vargas and Romero 1998a, b), blooming and leafing time (Vargas et al. 2001), flowering precocity (Vargas et al. 2002), and nut traits (Vargas and Romero 2005).

Even though P. vera was introduced into Italy from Syria by the Romans from selections developed by the Arabs, only 10+ female cultivars are grown together with even a more limited number of male selections (Barone and Caruso 1996). Currently, ‘Bianca’ (syn. ‘Napolitana’) is the main cultivar grown commercially in Bronte, Sicily which is the main Italian growing area. In 1984, the University of Palermo established a germplasm collection of 10 cultivars including ‘Bianca’ and ‘Kerman’ and 8 male pistachio selections (M1, M3, M4, M5, M7, M8, M9, and M10). M9, locally called ‘Santagilisi,’ is a putative hybrid between P. vera and P. terebinthus.

4.2 Rootstocks

Lloyd Joley identified several P. atlantica selections with nematode resistance at the former Plant Introduction Station in Chico CA (Joley 1979). Later J. Crane selected several P. atlantica seedlings that showed high levels of vigor. These selections were not used in California production, primarily because of the absence of a good clonal propagation system for P. atlantica. Seedling P. atlantica was used as the primary rootstock in the California production system until the development of UCB1, a P. atlantica × P. integerrima hybrid rootstock. This cross was identified by Ashworth (Morgan et al. 1992) as being resistant to Verticillium wilt, which was a major problem in California when using P. atlantica as a rootstock. Not only was this rootstock Verticillium sp. resistant, but it had exceptional vigor, and produced much improved yields of ‘Kerman’ at a relatively early point in the orchard development. Production could begin at 4–5 years, rather than 8–10 years, in the San Joaquin valley of California.

Pistachio rootstocks are produced from seeds except for a clonal selection of UCB1, being produced via micropropagation by Duarte nursery. Originally, the seed was produced from open pollinated tress, with control of the genetic composition of the seed being limited to the source tree. More recently closed crosses (control of both male pollen sources and female source trees) has become standard practice. PG1 was originally produced from open pollinated trees, but more recently has been reported to be produced from selected male and female trees. Both of these rootstocks were selected for resistance to Verticillium wilt. PG1 does not denote the specific genetic composition of the progeny, unlike UCB1, which is produced from an individual female clone and male clone. Thus, there is more potential for variability among the non-UCB1 hybrids. In the San Joaquin Valley, PG1 and UCB1 are the most commonly used rootstocks. In colder areas outside the San Joaquin Valley, P. atlantica and UCB1 are the most commonly planted rootstocks.

After a survey of native P. terebinthus located on the Spanish central plateau, a clonal selection from the best genotypes was made available to the nursery sector and used to produce seedling rootstocks (Guerrero et al. 2002). P. atlantica is thriving in the Canary Islands (Batlle et al. 2006).

5 Current Goals of Breeding

5.1 Scion Genotypes

The California breeding program was formerly focused on precocity (early bearing), nut size, yield, split percentage, and early season harvest. The program is currently limited to selection of additional clones from the original crossing programs and evaluation of progeny from several parents selected from that program. The primary objectives continue to be early season maturity, yield, nut size, and high split nut percentage (Kallsen et al. 2009). Early season maturity is important to avoid navel orangeworm damage and to maximize the efficiency of harvest and processing facilities. Disease resistance, especially resistance to A. alternata was a secondary objective in the program but is not being actively pursued at present. Several selections with high levels of resistance to A. alternata as well as nut size and potential yield have been retained for use in future breeding efforts.

The Spanish program has emphasized early bearing, nut quality, high productivity and strong vigor. Major traits of the female parents were early bearing, nut quality and productivity and, for the male parents the features were flowering precocity and trueness to type.

6 Breeding Methods and Techniques

Most of the genetic information that has been developed to date for pistachio has been derived from traditional qualitative and quantitative genetic analysis. The only simply inherited traits that have been described for pistachio are (a) a dwarfing gene that produces a genetic dwarf in a 1:3 ratio in the progeny of two specific parents from a P. chinensis × P. integerrima cross (Parfitt 2003) and (b) sex expression in the genus for which all progeny segregate 1:1. The dwarfing gene is a simple recessive, and probably acts through interruption of the gibberellic acid pathway, since application of gibberellin induces a normal phenotype in dwarfed progeny plants. Sex expression should be conditioned by a dominant gene in heterozygous condition in one of the sexes and a recessive condition in the other (Hormaza 1994). Interesting exceptions have been reported by Kafkas et al. (2004), who have described the occurrence of monoecy in an otherwise dioecious crop. Hormaza (1994) and Hormaza et al. (1994b) have described the characterization of a RAPD molecular marker for sex expression in P. vera from a bulked segregant analysis. RAPD primer OP008 was used to generate a band of 945 bp that was present in female but not male progeny from ‘Kerman.’ The marker was tested and confirmed against 14 other P. vera cultivars. This marker was subsequently tested by Kafkas et al. (2001) and was not found to be diagnostic for determination of sex in other Pistacia species. Kafkas et al. (2001) subsequently developed additional sex linked RAPD markers for several Pistacia species. A 1,300 bp sex linked band was identified in P. eurycarpa and cloned, but the cloned region was found to hybridize to both male and female plants. A 700 bp marker (amplified with primer BC346) and an 850 bp band (amplified with primer OPAK09) were also characterized in P. atlantica. Yakubov et al. (2005a) used the RAPD marker described by Hormaza (1994) as the basis for an improved RAPD marker which they converted to a SCAR marker. Yakubov et al. (2005b) have described the identification and cloning of a gene for a dehydrin-like protein from pistachio.

Several quantitatively inherited characters have been described for P. vera. Half sib family and parent–offspring regression analyses of flowering and leafing date (Table 21.1) showed that both are highly heritable (Chao et al. 2003). Leafing and flowering dates were strongly correlated (0.59–0.78), suggesting that early flowering could be selected for at the seedling stage. Resistance to A. alternata (Fries) Keissler was also shown to be heritable (Chao et al. 2001). Narrow sense heritability for precocity (early fruit production) was 0.54 and 0.93 at two locations, respectively (Parfitt and Chao unpublished). Other traits such as kernel weight, nut splitting, Alternaria resistance, and vigor (trunk cross-sectional area) have been shown to have low to medium (0.20–0.50) heritability (Table 21.1) (Chao et al. 1998; Parfitt et al. 1996).

Table 21.1 Heritability estimates and estimates for nut weight, Alternaria resistance, and trunk cross sectional area
Full size table

Pistachio is a long generation tree crop. In the past, a generation time of 8–10 years was given for making crosses and obtaining progeny plant for grow out and evaluation. However, this can be shortened to a generation cycle of 4–6 years (Parfitt and Kallsen unpublished) by growing the plants on their own roots under ideal growing conditions with adequate water and selecting for early flowering plants.

Since males and females do not necessarily flower at the same time, so pollen may need to be stored. As is typical of many wind pollinated crops, pistachio pollen is not especially durable and is only useful for less than 4 days at room temperature, about 3 weeks at 4°C and stored under desiccation, and up to 8 months at −20°C (Polito and Luza 1988), although it should be noted that pollen germination and durability varies considerably among male genotypes (Parfitt unpublished).

Pollen is collected from male panicles brought from the field as soon as pollen shed is observed from the first open flowers. These are dried on a sheet of paper in the lab and the pollen is separated from the flowers and then stored at 4°C in a desiccator until used. Female inflorescences should be bagged in pollen and waterproof paper bags, tied at the base with cotton batting fitted around the branch to seal the base of the bags. Pollen may be applied by collecting pollen on a camel’s hair brush and blowing the pollen off of the brush into the temporarily open mouth of the bag. Application of large amounts of pollen should be avoided. Once the receptive period of the last flowers in the bagged inflorescences has passed (usually about 4 weeks after pollination), these bags are removed and then replaced with breathable mesh bags to protect the developing fruit from birds and animals. Fruits should be collected in the fall after shell split when the husks are usually purplish-red or for some genotypes, white and easily separated (slipping) from the shells (Fig. 21.3).

An alternative mass selection strategy where only two or a few parents are to be used, is to set up a seed orchard with the female(s) and selected males or with only selected females and introduce the male pollen mechanically (broadcast across the orchard). Seed are harvested, planted, and evaluated. While this approach is more labor and cost efficient at the initial seed production stage, this efficiency is likely to be lost later in the grow out and evaluation process, which is the most expensive part of the breeding process, since this approach will result in a high level of redundancy and is ½ as efficient as a pedigree program for maximizing additive genetic variance.

Seed may be stored at 4°C under desiccated conditions for up to 1 year. After that time, viability will decrease but viable seed has been obtained after more than 3 years of storage. Frozen storage has not been used by the authors and may not be successful unless seed moisture can be carefully adjusted.

For germination the seed is hydrated (water soak) for 12–4 h at 4°C prior to planting into forestry pots (Jiffy 7 s were also used successfully). Once the plants are 15–20 cm tall, they are transferred to the field in the summer (if water can be applied immediately following planting), or the following spring when soil moisture is good. If resources permit, seedling scion wood can be grafted to rootstocks in the field. This approach has the advantage of producing faster growing and fruiting plants (due to rootstock vigor), and permitting an evaluation of scion–rootstock compatibility at an early stage.

All breeding programs should include an advanced selection yield/performance trial using commercial production conditions with replicated clonal plants on rootstocks. Specific numbers of plants/clone and arrangement of replicates will be a function of the number of selections to be tested and the financial and/or human resources available to manage the trial. This is likely to be the part of the program requiring the most years to complete since multiple seasons with harvestable yields are needed for a robust evaluation of superior genotypes.

7 Integration of Biotechnologies into Breeding Programs

7.1 Molecular Markers

Isozyme markers (PGI, MDH, PGM, AAT, PER, and EST) in pistachio have been studied in California (Arulsekar and Parfitt 1986), Sicily (Dollo 1996; Barone et al. 1996) and Iran (Aalami and Nayeb 1996). Dollo (1996) studied isozyme polymorphism of Pistacia species and varieties growing in Sicily and analyzed the offspring obtained by controlled pollination of P. vera cv Bianca with pollen of P. atlantica, P. vera, P. terebinthus, and ‘Santangilisi’, a Sicilian pollinizer (previously published as a hybrid between P. vera and P. terebinthus). The Sicilian Pistacia species and P. vera cultivars showed high levels of polymorphism in the two systems studied. The results suggested that ‘Santangilisi’ is a hybrid between P. vera cultivars, and that the P. vera cv Insolia is a hybrid of P. vera × P. terebinthus. Isozyme analysis of eight male pistachio selections and 10 female pistachio cultivars (Barone et al. 1996). indicated that the male germplasm had a higher degree of polymorphism as compared to the female germplasm. Hence, using only three enzymes, it was possible to identify all of the male selections, but only 50% of the females.

Morphological descriptions and RAPD fingerprinting analysis were conducted on 24 cultivars of P. vera (8 male and 16 female) collected from Italy, Greece, Morocco, Spain and Turkey (Caruso et al. 1998). A high degree of polymorphism was detected both at the phenotypic and molecular levels. Among the female accessions, cluster analyses of both morphological and bio-molecular characters did not separate the Mediterranean from the Iranian–Caspian genotypes, in contrast to a study conducted by Hormaza et al. (1994a) which revealed two major clusters of P. vera germplasm: a Mediterranean cluster, which includes cultivars originating from the Mediterranean region of Europe, North Africa and the Middle East; and an Iranian–Caspian cluster, comprising germplasm originating from locations east of the Zagros Mountains. ‘Kerman’ is associated with Iranian cultivars, which is consistent with its selection from Iranian germplasm imported through the USDA Plant Introduction Garden at Chico California. The molecular data in combination with historical and geographical records, support the hypothesis that pistachio cultivation originated within, or near the present natural range of the species and was spread by cultivation to the Mediterranean region of the Middle East (Hormaza et al. 1994a, 1998).

Seedlings of P. vera developed from seeds of two separate populations in Turkmenistan, Kepele and Agachli, were characterized by Barazani et al. (2003) using RAPD markers. Genetic and morphological results showed some differences between ‘Agachli’ and ‘Kepele’ P. vera L. accessions. UPGMA cluster analysis divided 24 of the 27 accessions into two main groups according to their origins.

A genomic DNA library enriched for dinucleotide (CT)n and (CA)n, and trinucleotide (CTT)n microsatellite motifs was developed from ‘Kerman’ by Ahmad et al. (2003), who generated 14 polymorphic SSR primer pairs in pistachio with the objective of distinguishing US pistachios (‘Kerman’) from Iranian cultivars. The authors used them to characterize 25 commercially cultivated pistachio cultivars from Iran, Turkey, Syria, and the USA. Cluster analysis placed most of the Iranian samples in one group, while the Syrian samples were the most diverse and did not constitute a single distinct group.

Kafkas et al. (2006) characterized 69 pistachio cultivars and genotypes cultivated in Iran, Turkey, the USA, Syria, Greece, Italy, Israel, Cyprus and Tunisia by AFLP, ISSR, and RAPD analysis. Cluster analysis of the combined data formed two main groups correlated with the geographic origin of the pistachio genotypes. One group contained the cultivars originating from Iran, while the second group included cultivars originating from Turkey, Syria, Greece, Italy, Cyprus, and Tunisia. ‘Siirt’ (origin is the southeast part of Turkey) and its variants were placed between the two main groups. Turkish cultivars and the rest of the cultivars in the Mediterranean group were separated into two subgroups. One subgroup consisted of Turkish cultivars and the other subgroup contained Syrian, Italian and Tunisian cultivars. A recent paper by Afzadi et al. (2007) reports the use of AFLPs with UPGMA and PCA to characterize a number of Iranian pistachio cultivars.

Kafkas and Perl-Treves (2001) used RAPD markers and UPGMA cluster analysis of 41 accessions to show that the new species P. eurycarpa, formerly considered P. khinjuk in Turkey, and P. altantica accessions form separate clusters but were closely related when compared to P. vera and P. terebinthus, which was placed as the most distant of the four species studied. A considerable amount of morphological data was collected to support the analysis. A follow-up paper using RAPD markers and parsimony analysis supported the placement of P. eurycarpa with P. atlanica and separate from P. khinjuk (Kafkas and Perl-Treves 2002). The placement of P. terebinthus, distant from P. atlantica, did not agree with other phylogenetic analyses. An analysis of Pistacia species by Katsiotis et al. (2003) with RAPD and AFLP markers and grouped by UPGMA cluster and principle component analyses supported the conclusion of Kafkas and Perl-Treves (2002) associating P. terebinthus with P. palaestina. Male and female cultivars of P. vera were clearly separated into distinct clusters, suggesting that a number of the markers that were evaluated were closely associated with the gene (or genes) associated with sex expression. A limitation of all of the preceding studies is that a high level of polymorphism ­usually exists for these markers within species, such that samples to represent each species must be of sufficient number and composition (sampled from the full geographic species range) to correctly represent the genetic composition of that species.

Werner et al. (2001) used six RAPD primers and morphological analysis to show that Pistacia × saporte Burnat is a hybrid between Pistacia lentiscus and Pistacia terebinthus. This conclusion differs from Zohary’s ((1972) evaluation of P. × saporte as being a hybrid with one of the parents consisting of Pistacia palaestina Boiss.

A combination of PCR amplification of chloroplast DNA, followed by RFLP analysis was used by Parfitt and Badenes (1997) to characterize species relationships among pistachio species when evaluated with distance and parsimony analyses. These results suggested that (a) Pistacia should be divided into two sections, Lentiscus and Terebinthus, rather than the four sections described by Zohary (1952), (b) P. integerrima and P. chinensis should probably be classified as separate species based both on molecular analysis, crossing behavior, and karyotype, and (c) the evolutionary rate for Pistacia is slow compared to annual crops. These conclusions were also supported by work using ITS and cpDNA sequence (Yi et al. 2008) and AFLP analysis Kafkas (2006). They also found evidence for reticulate evolution in the genus.

While a number of molecular marker studies have been conducted, a molecular genetic marker map has not been constructed. This is due to the need for well constructed crosses and the funding needed to place the markers on the map (e.g., by analysis of appropriate progeny populations). Funding for map development must be coupled with an effective breeding program from which relevant progeny populations can be obtained. Consequently, Marker Assisted Selection has not been practiced in pistachio improvement efforts. A potential MAS trait is the character of sex expression for which markers have been developed. Future directions for molecular genetic research should be focused on the identification of useful genes to permit the development of linked markers, development of a map with robust markers in P. vera for QTL or other applications, and more fundamental genetic analysis. Some areas for study are functional genomic analysis, identification and cloning of directly useful genes, and continued application of molecular markers to answer questions related to insect and disease control (characterization of specific genes or gene combinations for tolerance and/or resistance).

7.2 Micropropagation and Transformation Technology

Several research groups have developed micropropagation protocols for shoot tips of P. vera and rootstock species from shoot tips from seedlings (Barghchi 1982; Barghchi and Alderson 1983) and from clonal sources for several Pistacia species (Martinelli and Loretti 1988), Pistacia terebinthus (Pontikis 1984), P. vera ‘Mateur’ using methyl jasmonate (Dolcet-Sanjuan and Claveria 1995), the male P. vera ‘Atli’ (Tilkat et al. 2008) and P. vera, P. integerrima and hybrids (Parfitt and Almehdi 1994). Variants of the medium developed by Parfitt and Almehdi (1994) are being used commercially to produce clonal pistachio rootstocks. Onay (2000a) reported a protocol for in vitro micropropagation of pistachio from mature trees. In vitro micrografting may be used in combination with in vitro propagation techniques to produce in vitro derived clonal trees from difficult to root pistachio genotypes (Onay et al. 2004 b).

The development of protocols for regeneration of pistachio from somatic embryos and/or callus is a necessary first step for genetic engineering of pistachio, as the plants resulting from the protocol are generated from single cells, so that issues of generation of chimeric plants can be avoided. Genes for selection of single transformed cells in culture can be included in the transformation cassette, providing an efficient mechanism for selection of mutants. Onay (Onay et al. 1995; Onay 1996) developed a procedure for developing somatic embryos from pistachio kernels. The fact that preseedling materials were used as the source limited the value of these observations, however. Subsequently, the regeneration of plants (2n  =  30) from somatic embryos derived from callus of P. vera ‘Siirt’ pistachio flowers (Onay et al. 2004a) and from leaf explants (of the cv. Antep) (Onay 2000b) have been reported. Production of clonal plants via somatic embryogenesis can potentially be used to propagate other valuable cultivars, providing an opportunity to use genetic engineering approaches as well as conventional breeding strategies for cultivar improvement. Two major issues will limit genetic advance through this pathway.

  1. 1.

    Consumer acceptance: Lack of public acceptance and/or regulatory restrictions for genetically engineered food products may inhibit the introduction of new or modified genetically engineered cultivars. This is probably most likely to be an issue in Europe and since Europeans are major consumers of both Middle Eastern and US sourced pistachios, genetically engineered cultivars will be approached with caution by both US and Middle Eastern growers.

  2. 2.

    Control of Gene Expression: Introduction of a new trait via a genetic engineering approach requires stable expression of the introduced gene as well as expression during the appropriate developmental and/or seasonal stages of plant growth. In addition, expression of an introduced gene should not result in undesirable gene expression changes for nontransformed genes. Extensive testing of transformed products, probably to a greater extent and over a longer time period than for conventionally derived cultivars, will be needed prior to commercial utilization.

8 Conclusions: Status of Pistachio Improvement

Continued improvement of pistachio will require sources of stable, secure funding, since conventional programs require long term evaluations of selected materials. Costs for maintenance of collections and evaluation plots are significant due to the relatively large size of the plants (trees) and the need to maintain them for many years. Development of molecular markers and linkage of high resolution markers with important traits may help reduce the number of progeny that have to be evaluated in the field. Much more advanced and economical molecular marker technology is being developed in other fruit crops, and if resources become available, these technologies may change the way that researchers breed pistachio.

General information on pistachio production can be found in Joley (1979), Hormaza and Wünsch (2007), and Westwood (1993), as well as the “Pistachio Production Manual 4th edition (2005),” and the University of California Fruit and Nut Information Center (http://fruitsandnuts.ucdavis.edu).