The effect of drought stress on the activity of antioxidant enzymes, malondialdehyde, soluble protein and leaf total nitrogen contents of soybean (Glycine max L.)

  Environmental stresses, in addition to their straight effects on plants growth and development, induce oxidative stresses on plants. These secondary stresses cause degradations on cell wall lipids and cell organelles proteins. In response to overflow of reactive oxygen species (ROS) enzymatic and non-enzymatic antioxidants productionscould be induced in plants. Due to the kind and intensity of environmental stressesand plant physiological pathways the activity responses of antioxidant enzymes could be different. Therefore, the aim of this research was to evaluate the effect of water stress on soybean antioxidants induction. This trial was done on spring and summer 2012 in Sari Agricultural Sciences and Natural Resources University (SANRU) research station, Sari, Iran. The JK variety of soybean (Glycine max L.), which is widely cultivated in the region was selected for this purpose. A completely randomized design with 4 soil available water (AW) treatments in a pot experiment was employed. After 4-leaf stage soybean seedlings were randomly assigned to 4 water treatments of control, with 100 percent (well-watered, W0), 60 percent (mild stressed, W1), 40 percent (intermediate stressed, W2) and 20 percent (severe water stressed, W3) of soil AW. Plants were re-watered to 100 percent AW upon reaching to the assigned water level. During the R4 and R5 reproductive stages leaf samples were collected to measure leaf catalase and peroxidase enzymes and MDA and leaf total nitrogen and seed weight per plant using spectrophotometer and autoanalyzer. Data were analyzed using SAS software and mean comparisons were done using Tukey post-hoc test. Findings of this research showed that water stress significantly (p ≤ 0.05) affected leaf enzymatic activities of catalase and peroxidase, and influenced malondialdehyde (MDA) and total nitrogen contents and seed weight per plant. The catalase enzyme activity increased up to 60% AW, Then declined by further increase in soil available water. A highly significant quadratic relation (R2 = 0.89; P ≤ 0.01) was found between this enzyme activity and soil available water. The trend of variations in peroxidase activity by reduction in soil AW was highly positive and significant (P ≤ 0.01). The amount of this enzyme in water stressed treatments relative to control increased by 18.7, 29.9 and 34.4 percent, respectively. A highly significant, but negative correlation (r = - 0.86; P≥0.01) was found between MDA values and soil available water. The variation trend of leaf soluble proteins relative to soil available water was decreasing and significant differences were found between control and stressed treatments of W2 and W. The leaf total nitrogen contents increased by reduction in soil available water (r = 0.84; P ≤ 0.01). Also, seed weight per plant as an index of plant performance up to 60% of available water was ascending and in the moisture content of 80% available water, due to the increasing trend of all antioxidants to the highest. It could be concluded from these findings that increase of antioxidant enzymes, due to mild water stress, could diminish destructive effects of reactive oxygen species (ROS) in soybean. Antioxidant activities to a certain level of water stress, in this case up to 60% of AW, had an effective protective effect on photosynthesis process, therefore on soybean seed yield. It seems catalase activity was more effective on severe water stresses compared with peroxidase enzyme.