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Iranian pistachios are grown on five seedling rootstocks: two species (P. mutica F. & M. and P. khinjuk), one wild P. vera (‘Sarakhs’) and two hybrids (‘Badami-Riz’ and ‘Ghazvini’) (Rahemi and Tavallali, 2007). California relies on P. atlantica subspecies atlantica, P. integerrima (‘Pioneer Gold I’) and an interspecific hybrid (UCB I) obtained from a P. atlantica female crossed with a P. integerrima male. Two other rootstocks are also used in California: P. terebinthus and Pioneer Gold II (PG II) obtained from a P. atlantica female crossed with a P. integerrima male (Ferguson et al., 2001). In Turkey, seedlings of P. terebinthus, P. khinjuk, P. atlantica and P. vera ‘Siirt’, ‘Ohady’, ‘Kerman’ and ‘Kalehghochi’ are used as rootstocks; the primary rootstocks are P. vera seedlings (Kaşka et al., 2002). In Italy, ‘Integerrima’ clone 2 and ‘Terebinthus’ are used (Caruso et al., 2006).

MAJOR BREEDING OBJECTIVES. Pistachio seedling rootstocks from different species or hybrids significantly affect nutrient status, tree vigour, habit, fruit quality, blooming time, alternate bearing, the non-bearing period, early nut production, cold and salt tolerance, soilborne diseases, degree of shell splitting and blank nut production (Crane and Forde, 1976; Ashworth, 1985; Crane and Iwakiri, 1986; Kaşka et al., 2002). These traits are important for selecting suitable rootstocks for each production area (Tasias i Valls, 1990).

BREEDING ACCOMPLISHMENTS. Although several breeding programmes have been developing rootstocks, their number is rather limited, and they are not used on a large scale except in California USA, which is the only producing area that is utilizing cloned rootstocks for the establishment of commercial orchards. Currently and traditionally, pistachio rootstocks are from seedlings except for clone UCB1, which is available as micropropagules from private companies in the USA. California has the only breeding programme for rootstocks. This programme focuses on vigour and resistance to Verticillium wilt, which is a major problem in California (Parfitt et al., 2012). Originally, five rootstocks were tested for the California pistachio industry (Ferguson et al., 2005): P. terebinthus, P. atlantica, P. integerrima (Pioneer Gold I (PG I)), and two hybrids of P. atlantica (Pioneer Gold II (PG II) and UC Berkeley I (UCB I)) pollinated by P. integerrima. Seedlings produced from seeds of P. vera ‘Kerman’ had no resistance to nematodes or Phytophthora. Therefore, P. terebinthus and P. atlantica were used as rootstocks during the early years of the pistachio industry. Both rootstocks are very susceptible to the soilborne fungal disease, Verticillium wilt. PG I is the dominant rootstock of the California pistachio industry, but seedlings of this genotype (produced from selected male and female trees) are variable (Parfitt et al., 2012) under different climatic conditions. Therefore, interest in increased vigour and early production, and research of the effects of rootstock on yield, Verticillium wilt and salinity tolerance led to the increased use of the hybrid rootstock, UCB I (Ferguson et al., 2005). PG II is no longer commercially available, having been demonstrated to have poor Verticillium tolerance. UCB I is now the dominant rootstock in California (Parfitt et al., 2012), but the reliance on a single rootstock cultivar may cause problems later; a rootstock breeding programme was therefore initiated to address this issue. The first trials were conducted to evaluate new varieties, but there have been no releases (Kallsen et al., 2009). Karimi et al. (2009) reported that six species and subspecies of Pistacia are used as rootstocks in Iran which provide an enlarged pool of available germplasm with resistance to drought, salinity and diseases. Effects of rootstocks on morphological characteristics were investigated during 1991–2001 at the Iranian Pistachio Research Institute (IPRI) (Ghazvini Foutouhi et al., 2007). Suitable rootstocks for commercial pistachio cultivars were introduced: P. vera cv. Badami riz; P. atlantica subsp. mutica with resistance to nematodes and a good rootstock for sandy soils; P. khinjuk rootstock with resistance to drought stress and suitable for arid regions. In another breeding programme (Rahemi and Tavallali, 2007), 20-year-old trees of nine rootstock/scion combinations were used in a 2-year study to evaluate the effects of ‘Badami’ (P. vera), ‘Sarakhs’ (wild P. vera) and ‘Beneh’ (P. mutica) seedling rootstocks on the nut quality of ‘Owhadi’, ‘Kalleh-Ghuochi’ and ‘Ahmad-Aghaie’. ‘Badami’ resulted in the greatest cumulative production and yield efficiency, the lowest blankness of pistachio nuts, and the highest percentage of splitting nuts. ‘Sarakhs’ was the least vigorous among the tested rootstocks and gave the lowest leaf area of scion cultivars. ‘Sarakhs’ rootstock consistently influenced the formation of the highest number of suckers. Genetic variability among pistachio rootstocks therefore influences scion vigour, yield, extent of shell splitting, blankness and nut weight (Rahemi and Tavallali, 2007). In Iran, ‘Sarakhs’ and ‘Badami Zarand’ rootstocks are used in water-deficient areas because they are resistant to drought although they are less tolerant of salinity (Mohammady and Sepas-Khah, 1996). ‘Sarakhs’ is also important in forest regions of Iran because it provides protection from water erosion and wind and contributes to soil stability, although it is very susceptible to Phytophthora spp. (Banihashemi, 1995). Tolerance of salinity in Iranian rootstocks is in ‘Badami Riz’ and ‘Ghazvini’, ‘Sarakhs’ and P. mutica (Hossein-Khan and Farhang, 1996).

In Turkey, the performance of hybrids of P. vera and other Pistacia species was evaluated as rootstocks (Atlı and Kaşka, 2002) to obtain fast-growing, vigorous and budding-compatible rootstocks. The parent plants were from the Gaziantep area in 1996–1997. Seeds of 14 P. vera cultivars and 44 P. khinjuk types were evaluated. These parents were re-selected according to the growth rate of their seedlings. Five different types of P. khinjuk and of P. vera were used as female parents, and six different P. vera and of P. khinjuk were used as male parents; 120 hybrid combinations were obtained. The performance of different genotypes of P. vera, P. atlantica, P. terebinthus and P. eurycarpa were also evaluated as P. vera rootstocks (Kafkas et al., 2006b). The rootstock candidates from these two programmes require several years of evaluation, and data from yield trials are not yet available.

The effects of rootstocks on growth characteristics, quality and yield of Iranian and Turkish pistachio cultivars have not been fully evaluated and new rootstocks have not been released. In Spain, Australia and Israel, pistachio rootstock improvement programmes involve evaluating local genotypes, local seedling populations and species. A breeding programme at El Chaparrillo Research Station, Ciudad Real, Spain is using two techniques: (i) crosses have been made between traditional rootstocks; and (ii) germplasm from different areas of Castilla-La Mancha (central Spain) and northern Andalucia (southern Spain) has been collected for use in the breeding programme (Guerrero et al., 2003). One of the best rootstocks is the second-generation, open-pollinated P. atlantica × P. vera, which exhibits high vigour (Guerrero et al., 2007). The hybrid rootstock was associated with greater stomatal control and reduced leaf senescence in comparison with P. atlantica and P. terebinthus, despite being associated with the most vigorous shoot growth. P. terebinthus had very effective stomatal control but was also associated with the most rapid leaf senescence. P. atlantica was associated with less vigorous shoot growth and similar levels of water stress like the other rootstocks under conditions of high evaporative demand. Currently, the most promising individuals are being evaluated to select the most effective rootstock for different environmental conditions (Gijon et al., 2010).