Genotypic variation for drought stress response traits in soybean. II. Inter-relations between epidermal conductance, osmotic potential, relative water content, and plant survival

As part of a project exploring the potential for using leaf physiological traits to improve drought tolerance in soybean, studies were conducted to explore whether epidermal conductance (g(e)), osmotic potential (pi), and relative water content(RWC) influenced turgor maintenance and ultimately the survival of droughted plants. In a glasshouse study, plants of 8 soybean genotypes that showed different expression of the traits were grown in well watered soil-filled beds for 21 days and then exposed to terminal water deficit stress. The trends in each trait were then monitored periodically until plant death. Genotypic differences were observed in the rate of decline in RWC as the soil dried, in the temporal patterns of change in ge and p, in the duration of survival after watering ceased, and in the critical relative water content (RWCC) at which plants died. In general, ge became smaller and p became more negative as RWC declined and plants acclimated to the increasing stress. Genotypic differences in ge remained broadly consistent as RWC declined. In contrast, the genotypic rankings for p in stressed plants were poorly correlated with those for well watered plants, indicating differential genotypic capacity for osmotic adjustment (OA) in response to stress. Survival times among genotypes after stress commenced ranged from 27 to 41 days, while RWCC ranged from 49% down to 41%. The differences in survival time among the genotypes were able to be explained by genotypic differences in the rate of decline in RWC and in the RWCC, using a multiple linear regression relationship (R-2 = 0.94**). In turn, genotypic differences in the rate of decline in RWC were positively correlated (r = 0.75*) with ge at 70% RWC, and with OA over the drying period (r = 0.98**). In a second study in a controlled environment facility, leaf area retention at 90% soil water extraction was greatest in the one genotype that combined low ge, high OA, and low RWCC. Overall, the responses from the two studies were consistent with the hypothesis that turgor maintenance and ultimately leaf and plant survival of different genotypes during advanced stages of drought stress are enhanced by low ge, high OA capacity, and low RWCC.