Physiology of soybean as affected by PEG-induced drought stress

To evaluate the influence of drought stress on soybean physiology, a controlled-environment experiment was conducted at the Institute of Plant Sciences, University of Debrecen, Hungary. Two soybean cultivars, ES Mentor and Pedro, were subjected to four different levels of water deficiency elaborated by different polyethylene glycol (PEG) concentrations; 2.5, 5, 7.5 and 10 % starting from the post-germination phase. The measurements were made at four different stages; second node (V2), fourth node (V4), full bloom (R2) and full pod (R4). ES Mentor plants could not survive under 10 % PEG concentration at V4 stage, and under both 7.5 and 5 % PEG concentrations at R2 stage, and similar reaction was observed under 10 % PEG concentration at V4 stage, and 7.5 % PEG concentration at R2 stage for Pedro. For cultivar ES Mentor, increasing PEG concentration was accompanied by decreasing SPAD values at all stages, and Pedro followed a very similar trend except for a slight, insignificant increase in 2.5 % PEG treatment at V2 stage as compared to control. However, differences were more measurable at later stages. Concerning chlorophyll content, Chla, Chlb and Chlx+c decreased as PEG concentration increased at all stages of ES Mentor; the reduction was insignificant at vegetative stages (V2 and V4 stages) and significant at reproductive stages (R2 and R4), whereas for Pedro 2.5 % PEG treatment had the best Chla and Chlx+c contents at V2 stage. However at the following stages, control treatment could maintain the best values, and the increase in PEG concentration was accompanied by a decrease in both contents. Chlb, on the other hand, was significantly higher for 2.5 % PEG treatment than control at both vegetative stages, whereas in the reproductive stages it insignificantly decreased with increasing PEG concentration. Maximum photochemical efficiency of PSII (Fv/Fm) of both cultivars followed one trend throughout the studied stages; it decreased with increasing PEG concentration. Moreover, increasing PEG concentration was accompanied by a non-significant decrease in the actual photochemical efficiency of PSII (ΦPSII) of ES Mentor in all stages, whereas for Pedro 2.5 % PEG treatment resulted in better ΦPSII compared to control treatment at both vegetative stages, however, control was the highest at later stages and ΦPSII decreased with increasing PEG concentration. Significant differences were recorded for both cultivars in response of stomatal conductance to PEG application; increasing PEG concentration resulted in lower stomatal conductance in all stages (except for a slight increase in 5 % PEG treatment compared to 2.5 % PEG treatment at V2 stage in Pedro plants). It could be concluded that drought stress had different effects on the physiology of the two cultivars; however, the negative effects were more obvious at the late stages of the plant's life cycle of both cultivars, which will presumably reflect on the yield component traits, and consequently, the expected yield.