Using Carbon Isotope Discrimination to Estimate WUE in Soybean and Its Relation with Physiological Traits.

Drought is consistently the most important abiotic stress factor limiting soybean production in the United States. Improved drought tolerance can be achieved through various strategies, however, enhancing WUE to improve soybean response to drought has been understudied. The use of carbon isotope discrimination (CID) as a surrogate measure of WUE has been demonstrated successful in various crops. The objective of this work was to study if CID discrimination was a good surrogate measure of soybean WUE for plants grown under conditions differing in water availability. In addition, several leaf and plant level physiological and anatomical traits that may be responsible for differences in soybean WUE were examined. Four soybean genotypes differing in CID (2 low and 2 high CID genotypes) were grown in pots in the field under well-watered (100-80% SWC), mild drought (50-30% SWC), and severe drought (<30% SWC). As expected, low CID genotypes showed higher WUE, while high CID genotypes showed lower WUE; and this trend was maintained at the different developmental stages and under different water treatments. Low CID genotypes (high WUE) exhibited lower net photosynthetic rates than high CID genotypes, and low net photosynthetic rates were associated with lower stomatal conductance and transpiration. Interestingly, despite of the lower net photosynthetic rates, low CID genotypes accumulated the same amount or more aboveground biomass than high CID genotypes. Additional research is needed to elucidate the impact of differences among genotypes in leaf anatomical characteristics in relation to WUE, and the role of canopy-level dynamics on biomass accumulation and WUE.