Crops Yield Increase Under Water-Limited Conditions. Review of Recent Physiological Advances for Soybean Genetic Improvement

Due to future requirements for more crop production there will be greater needs to increase yields for crops subjected to water deficits. In recent years, substantial progress has been made with soybean (Glycine max (L.) Merr.) in understanding the water deficit limitation on yield using model assessments, physiological investigations, and plant breeding. This knowledge has been applied in developing higher yielding genotypes. This review examines physiological options and genetic advances made with soybean as possible guides for studies with other crops. Three approaches exist for minimizing the negative impact of water deficit on crop production: (1) conserve soil water, (2) access more water, and (3) overcome special water deficit sensitivities. Water conservation in soybean has been achieved by exploiting a genotype that has limited hydraulic conductance in its leaves. A consequence of this trait is that transpiration rate is limited at times of high vapor pressure deficit resulting in soil water conservation for use later in the season. Acquisition of more water is most likely to be achieved by greater depth of rooting or greater root length density deep in the soil. Although promising genetic variability has been identified, breeding efforts for these rooting traits are still required. A special sensitivity in soybean that results in a major limitation in yield is a decrease in symbiotic nitrogen fixation rate with only modest soil drying. Germplasm has now been released that results in increased yields due to a capacity for sustained nitrogen fixation with drying soil. This review highlights that soybean investigations combining physiological investigations, simulations studies, and field-based phenotyping of traits have resulted in the identification of genotypes with increased yield potential in water deficit environments.