The metabolic reasons associated with differential sensitivity of soybean cultivars to water deficit stress are not well understood. Therefore, field experiments were conducted in 2008 and 2009 to determine some agronomic traits and activity of antioxidants under different levels of irrigation. The experimental design was randomized complete block in a split plot arrangement with four replications at both years. Irrigation treatments were (S1, 50; S2, 100 and S3, 150 mm evaporation from the Class "A pan" evaporation) and cultivars were (L17, Clean, TMS, Williams*Chippewa and M9). Results showed that, water deficit stress increased antioxidants content [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX)] significantly, but content of them were more at mild than high water deficit stress (S2>S3>S1). Furthermore, water deficit stress, decreased total chlorophyll content, number of pods per plant, thousand seed weight, seed yield and harvest index in all of cultivars. Among cultivars, L17 and Williams*Chippewa produced the highest seed yield at the optimum condition of irrigation and both water deficit stress levels, respectively. Assessment of correlation results indicated that, there was a positive and significant correlation among SOD and seed yield in both water deficit stress levels, too.
Key words: Soybean, water deficit, antioxidant enzymes.
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