The study of physiological responses of different wheat varieties to water stress could be a useful tool to understanding of the mechanisms of drought resistance. In order to evaluate some physiological responses in four wheat cultivars commonly grown under contrasting climatic conditions of the country, a 3-replicate RCBD field experiment was carried out during 2000-2001 growing season at the experimental farm of the agricultural faculty, University of Tehran with two watering regimes. Samples of flag leaves were taken at ear emergence stage as well as 20 days after anthesis, immediately before rewatering of stressed plants. The soil water potential of stress treatments at the two sampling stages were –2.26 and –5.15 MPa, respectively. Water stress reduced final grain yield of Chamran, M-75-7, Alvand and Tajan by 2.8, 22 and 34% respectively. In contrast to other cultivars, chlorophyll a and b contents of Tajan cultivar, measured at first sampling stage, was not affected by water stress. Increase in chlorophyll a/b ratio under stress conditions was higher in Chamran as compared to other cultivars. A considrable reduction in total chlorophyll content in both moisture regimes was evident at second sampling stage, the highest reduction being observed in Chamran cultivar. In control and stress treatments, the concentration of soluble carbohydrates (SC) at 20 days after anthesis was higher than that measured at ear emergence stage. Increase in SC due to water stress was pronounced only in Chamran and Tajan, possibly implying the role of these compounds in osmotic adjustment or limited capacity of sink (developing grains) to metabolize photoassimilates. In contrast to other cultivars, SC concentration in Alvand decreased under stress condition at earing stage. This reduction was accompanied with the highest reduction in chlorophyl content in this cultivar. Water stress increased proline concentration at both sampling stages. The greatest increase in proline content, at both sampling stages, was observed in susceptible cultivar Tajan, whillst lowest increase was found in drought resistance cultivar Chamran, implying the possible role of proline in osmotic adjustment in susceptible cultivars. Thus, low proline concentration could be used as a selection criterion in screening for drought tolerance in wheat cultivars.
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