Improving drought tolerance in soybean by classical breeding leads to physiological adjustments of photosynthesis and stomata functioning

Abstract

Soybean (Glycine max) is one of the main crops cultivated worldwide and drought is the main abiotic stress affecting its production. However, breeding towards drought tolerance is often set aside due to the difficulties in phenotyping. The aim of this study was to design an effective and simple screening technique that allows to discriminate genotypes according to their drought response and to identify physiological traits responsible for a superior tolerance. This rapid screening evaluated morphological (shoot and root length and weight) and developmental traits (days to germination and plant establishment) of 604 genotypes under control and drought conditions. Two genotypes with contrasting responses (Sensitive Genotype and Tolerant Genotype, SG and TG, respectively) were selected according to their performance in the screening, and were subjected to further testing in order to physiologically characterize the drought response of the chosen cultivars. Wilting, normalized transpiration ratio, leaf gas exchange parameters, water use efficiency (WUE) and stomatal characterizations were performed. TG plants exhibited slow wilting and higher WUE when subjected to drought treatments, product of a reduced water loss due to an anticipated stomatal closure which allowed to preserve soil moisture. Besides, under control conditions, the TG genotype presented a higher stomatal aperture index and a bigger density of epidermal cells and stomata than the drought-sensitive genotypes analyzed. In summary, the analysis of easily quantifiable traits, as the growth of the plant aerial organs and root development, together with the plant development timing, allowed us to select two soybean cultivars with contrasting drought responses. In our opinion, this selection method might be efficiently applied for the selection of drought tolerant cultivars in plant species of agronomic interest.