Iran is located in the midlatitude belt of arid and semiarid regions of the world (Modarres and Da Silva, 2007). Approximately 42 M ha of land in these regions in South Asia are affected by salinization (FAO, 1994). Approximately 33 M ha of these areas are in Iran. Walnut trees need large amounts of water for optimum growth and productivity and are among the more sensitive plants to abiotic stresses (Fulton and Buchner, 2006). The majority of walnut trees in the world are propagated by seed or by grafting on rootstocks (Vahdati, 2003). Hence, there is huge genetic diversity among rootstock traits. For example, there are many old Persian walnut (Juglans regia L.) trees in Iran that have been planted on the banks of rivers. The long survival of these trees may indicate possession of valuable stress resistance genes that help them cope with unfavorable environmental conditions (Vahdati, 2003).
Finding genetic resources tolerant to drought stress at different growth stages is important for semiarid regions. In studies of fruit trees, half-sib progeny of several species (for example, ‘Serr’ walnut, ‘Texas’ almond, ‘Lovell’ and ‘Missouri’ peach) have been used for producing rootstocks (Cummins and Aldwinckle, 1983). Because half-sibs are individuals that have one parent in common and differ in the other parent, the mean genotypic value of the group of half-sibs is by definition half the breeding value of the common parent. This reduces the number of seedlings of half-sib families needed as replicates for studying tolerance genes in rootstock breeding programs (Falconer and Mackay, 1996).
Germination is an important stage in the life cycle of species growing under stress conditions, because this stage determines the soil conditions to which plants will be exposed in later stages of their life cycles (Mohammad and Sen, 1990). Soil water supply is an important environmental factor controlling seed germination (Kramer and Kozlowski, 1979).
Osmotic solutions are often used to simulate drought stress in vitro (Pandey and Agarwal, 1998). Polyethylene glycol molecules (PEG 6000) are osmotically active, inert, nonionic, and virtually impermeable to plant cells and are frequently used to induce water stress and maintain uniform water potential throughout the experimental period (Lu and Neumann, 1998).
Although walnut is considered to be sensitive to drought stress, the degree of its tolerance has not been determined at different stages of growth and in different varieties. The objectives of this study were development of a simplified method for determining drought resistance in Persian walnut and evaluation of both commercial varieties and new selections of this species to drought stress.
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