ABSTRACT
Despite the general decreasing trend of seed weight under drought, the seed weight may not reduce uniformly as a function of drought intensity. Therefore, seed weight distribution has become another parameter employed to evaluate the effect of drought on seed weight, through the assessment of weight of seeds of different sizes. Several studies have proposed that roots having large xylem number, diameters, lateral root systems with more root hairs are indicators of drought tolerance. Drought-tolerant soybean cultivar exhibited a greater leaf area rather than less-tolerant cultivar under hydric stress condition.
Under flooding conditions, secondary aerenchyma consisting of white and spongy tissues develops within a few weeks in stems, roots and root nodules of soybean. Aerenchyma formations initiated by ethylene, Ca2+, and ROS signalling through a programmed cell death process are involved in aerenchyma development, a potential role of drought-induced increase in xylem sap ABA in affecting pod growth was suggested. It was proposed that a low pod water potential, which might have led to disruptions in metabolic activities in the pods, is important in determining pod abortion.
It was reported that each 1°C increase in the average growing season temperature leads to a 17% decrease in soybean yield. Photosynthesis was the main physiological process that was affected by HT, often before other cell functions were impaired. There is limited anatomical evidence (ultrastructural changes in the chloroplast, thylakoid membranes, mitochondria, and plasma membrane) to explain the mechanisms of decreased activity of PS-II photochemistry (Fv/Fm, the ratio of variable fluorescence to maximum fluorescence) and decreased photosynthetic rate under HT stress. Anatomical changes under HT stress showed smaller cell size, closure of stomata, and greater stomatal density. Night temperature for example is one of the major important factors for the growth and development of soybeans (Glycine max (L.) Merr.). It is well known that differences between day and night temperatures enhances the seed filling rate and finally increases seed size. Actually, increase in night temperature from 20°C to 25°C after flowering decreased seed size and seed yield.
Soybean is a sensitive plant to salinity stress and both environment and genotype are responsible for the variation in yield and yield components of this crop. Apart from osmotic stress salt is responsible for the generation of ROS in cells that leads to oxidative stress. The generation of these ROSs is due to imbalance between the production and scavenging machinery of ROS. The unquenched ROS react spontaneously with the organic molecules and cause membrane lipid peroxidation, protein oxidation, enzyme inhibition and DNA and RNA damage.
