RESPONSE OF SOYBEAN TO WATER STRESS CONDITIONS AND FOLIAR APPLICATION WITH SALICYLIC AND ASCORBIC ACIDS

A field experiment was conducted at Etay El-Baroud Agricultural Research station during the two successive seasons 2012 and 2013 to study the effect of three levels of available soil moisture depletion (ASMD) namely wet (25-30%), medium (45-50%) and dry (65-70%) and foliar spray of salicylic and ascorbic acids singly or in combination at the concentration of 100 and 200 ppm on vegetative growth, yield components and some biochemical constituents of soybean (Glycine max L.). Increasing soil moisture stress up to 65-70% ASMD significantly decreased shoot and leaf dry weight, leaf area and leaf area index as well as, chlorophyll a, b and chl. (a+ b) content, relative water content in leaves and seasonal water consumptive. Whereas, peroxidase and polyphenol oxidase activities, proline content and water use efficiency were significantly increased when, the plants were received medium treatment compared with wet or dry treatment. Dry treatment significantly decreased plant height, number of branches/ plant, number of pods/ plant, weight of seeds/plant, 100-seed weight, harvest index and seed yield/faddan as well as total carbohydrates, protein accumulation and oil content in seeds. Foliar application of either salicylic or ascorbic acid and their combination had significant effect on growth parameters and yield components in both studied seasons, except, shoot dry weight at 90 days after sowing in the first season, leaf dry weight and number of branches/ plant in the second season as well as, number of pods/ plant which were insignificant affected in both seasons. Foliar application with salicylic or ascorbic acid and their combination induced significant increase for chlorophyll a, b content, peroxidase and polyphenol oxidase activities, proline, total carbohydrates, protein accumulation and oil content in seeds as well as, seasonal water consumptive use and water use efficiency compared with untreated plants. The interaction effect between water stress and foliar application of salicylic and ascorbic acids was found to be significant on leaf area, leaf area index, peroxidase and polyphenol oxidase activities and proline in leaves at 75 days after sowing in the second season. The maximum values of water use efficiency were obtained when plants were irrigated at 45-50% ASMD and sprayed with 200 ppm of salicylic or ascorbic acid and their combination at 100 ppm for each.