Study of effect of different water shortage levels on biochemical traits and yield of Mung Bean (Vigna radiate L.)

Introduction Water shortage is one of the most important abiotic stresses which impose deleterious effect of growth and yield of crops including mung bean. Part of this effect comes from over production of reactive oxygen species (ROS) including OH·, O2·-, and H2O2. Crops try to scavenge these ROS using antioxidant enzymes like superoxide dismutase, guaiacol proxidase and catalase. This experiment was aimed to study the sensitivity of some traits of Mung Bean to water shortage and to see whether the activity of catalase, superoxide dismutase and guaiacol proxidase antioxidant enzymes is the same under different water shortage intensities or not.   Materials and methods An experiment was conducted using pots with 5 kg capacity in which 5 seeds of mung bean were planted. The pots were put in an open filed to increase the possibility of generalizing the results to field results. At 4-leaf stage, 2 seedlings were removed, and left 3 ones. Treatment levels were control, weak water shortage [irrigation at 65% field capacity (FC)], moderate water shortage (irrigation at 50% FC), and severe water shortage (irrigation at 35% FC) which arranged as on completely randomized block design with 3 replications. At maturity stage, some attributes including biological yield, plant height, number of seed per pod, number of pod per plant, grain yield, and length of pod were measured. The sensitivity index of these attributes to water shortage was calculated using appropriate functions to test whether these attributed differ in terms of value of response to drought or not. For measuring the activity of 3 antioxidant enzymes, the plant samples were taken at flowering stage. Then the activity of catalase, superoxide dismutase, and guaiacol proxidase enzymes were measured as it has been presented in report of Havir and McHale (1987), Van Rossun et al. (1997), and Cavalcanti et al. (2004), respectively.   Results and discussion Results indicated that all measured attributes were significantly affected by water shortage. It was found a low-sloped decreasing trend with increasing water shortage intensity in pod length. The value of plant height for plants experienced weak water shortage was statistically similar to those grown in no water shortage conditions (control). But it decreased sharply for plants treated with medium and severe water shortages. The sensitivity threshold of number of seed per pod was relatively high; because over the field capacities equal to 50% and greater than that, its quantity was similar to control. The sensitivity index of this attribute and number of pod per plant was 0.050182 and 0.038788, respectively. Considering the standard errors of these indices, the difference between number of seed per pod and number of pod per plant for quantitative response to water shortage is statistically negligible. The quantity of both biological yield (straw + grain) and grain yield appeared to be not changed in weak water shortage as compared to no water shortage conditions. Then after, they were negatively affected. Under severe water shortage condition, the percent of decrease in grain yield was higher than in biological yield. The sensitivity index of them was 0.120727 and 0.031512, respectively. The grain weight accumulation is dependent on current photosynthesis and non-structural carbohydrates stored in vegetative organs like stem before flowering. The activity of hydrolytic enzymes including alpha amylase is crucial. The higher decrease in grain yield than in biological yield may imply that the activity of these enzymes has also been negatively affected. Under weak and medium water shortage conditions it was found no change in activity of catalase as compared to control. But under severe water shortage condition, its activity was considerably (3 times) increased. The activity of superoxide dismutase was constant over control and weak water shortage situations; but it doubled under medium water shortage conditions. The guaiacol peroxidase activity did not respond to weak water shortage; subsequently, it showed an upward trend.   Conclusion Grain yield and plant height were the most sensitive traits. Number of seed per pod, number of pod per plant and biological yield with statistically similar values of sensitivity were in 2nd order. The length of pod was the most tolerant trait as it tended to have the lowest sensitivity index. The increase in activity of superoxide dismutase was being witnessed in moderate water shortage. For catalase, it was true only for severe water shortage. The activity of guaiacol porxidase was statistically the same under control and weak water shortage conditions. But its activity increased proportionally with increasing water shortage intensity.