Water Use Efficiency in Soybean – Variation, Physiological Basis, and Surrogate Traits.

At the single plant or whole crop level, water use efficiency (WUE) is defined as the ratio of dry matter production to water use, integrated over long time periods (usually, weeks).  This trait may be an important determinant of crop yield under conditions where soil water availability is limiting.  Substantial variation for WUE has been observed in most crop species examined to date, including soybean (Glycine max L. Merr.), but it is very difficult to measure in large screening studies, and nearly impossible to measure in the field.  At the single leaf level, WUE_l is defined as the instantaneous rate of photosynthetic CO₂ fixation to transpiration; according to well-established theory, WUE_l also determines the specific carbon isotope composition of the final crop biomass, and so carbon isotope composition has been used in field studies as a surrogate for WUE_l integrated over time and, by extension, for WUE.  We have reported previously on an alternate trait, minimum epidermal conductance (MEC), which is also very well correlated with WUE in soybean and other species, and is simple to measure.  Here, we report on further studies designed to i) quantify the degree of variation for MEC amongst soybean cultivars adapted to the northern corn belt, as compared to the known range for this trait amongst germplasm examined previously, ii) verify the correlation between MEC and WUE among these cultivars, under both water replete and drought stress conditions, iii) determine the physiological basis of the association between MEC and WUE, and iv) evaluate the correlation between MEC measured in the field and in greenhouse screening studies.