Low dose of gamma irradiation enhanced drought toLerance in soybean

MOUSSA, H. R., 2011. Low dose of gamma irradiation enhanced drought tolerance in soybean. Bulg. J. Agric. Sci., 17: 63-72 Drought stress is the main limiting factor of soybean production. However, no work has been done on how application of low-dose of gamma rays could help to overcome water deficits during critical stages of soybean development. Gamma rays at doses 0.0 and 20 Gray (Gy), from a cobalt source (60Co) with strength of 500 Ci and the dose rate of 0.54 Gy/min-1, were applied to dry seeds of soybean before planting. Two levels of soil moisture (80% field capacity for well-watered control and 35% for drought-stressed treatment) were applied at pod initiation. Thereafter, the interaction effects of low dose of gamma irradiation and water stress on some growth, biochemical, anatomical and antioxidative parameters of soybean plants were investigated. Low dose of gamma irradiation increased biomass accumulation and seed yield for both treatments. Drought stress depressed chlorophyll content and photosynthetic activity (CO2-fixation), while chlorophyll content, leaf water potential and photosynthetic activity of plants irradiated with gamma rays at a dose 20 Gy were greater than that of drought-stressed plants. Water deficit decreased the enzyme activities of phosphoenol pyruvate carboxylase and ribulose-1,5-bisphosphate carboxylase/oxygenase. However, application of low dose of gamma irradiation (20 Gy) increased the activities of these enzymes, except for phosphoenol pyruvate carboxylase under drought stress. Gamma irradiation dose at 20 Gy increased the concentration of soluble sugars, protein and proline content and the activities of peroxidase and superoxide dismutase of soybean leaves when drought-stressed. However, it decreased the malondialdehyde concentration and electrical conductivity of leaves under drought stress. The following physicochemical characteristics of chloroplasts were chosen as indicators of drought-stressed effects: average size, and ultrastructure. The results suggest that gamma irradiation at dose 20 Gy can partly counterbalance the destructive effects of water deficits. This protective action led to an increase of chloroplast size reduced by drought treatment and rebuilt, to some extent, the chloroplast ultrastructure. Overall, the results indicated that pre-treatment with low dose of gamma rays (20 Gy) to dry seeds of soybean before planting can be used to enhance drought tolerance and minimize the yield loss caused by water deficits. Thus, it may be a useful management tool in afforestation projects in arid and semiarid areas as a promising technique for agricultural