Stomatal and non-stomatal restrictions to carbon assimilation in soybean (Glycine max) lines differing in water use efficiency

Genetic variability for water use efficiency (WUE, the quantity of crop dry matter produced per unit water transpired) has been demonstrated in a wide range of crop species. In agreement with established theory, genotypes with higher WUE are often found to maintain lower leaf internal CO2 concentration (c(i)), as estimated by carbon isotope discrimination. However, lower c(i) may result from reduced stomatal conductance, increased mesophyll (non-stomatal) conductance, or a combination of both. When genotypic variation for WUE is found, it may be important for plant breeding purposes to define whether such variation arises from differences in stomatal or non-stomatal restrictions to CO2 uptake. The soybean cultivar Young was previously shown to have higher WUE than the soybean plant introduction PI416937 when both were grown under cyclic drought stress, but the relative importance of stomatal and non-stomatal factors was not known. In the present work, the difference in WUE between these soybean lines was found to be smaller than previously reported, and was also demonstrated to be constitutive in nature; that is, it occurred under both cyclic drought stress and water-replete conditions. Leaf gas exchange measurements revealed that cv. Young maintained lower c(i) and higher leaf-level WUE than PI416937, as expected. Sensitivity analysis indicated that the observed differences in c(i) under steady-state gas exchange conditions could be attributed entirely to differences in stomatal limitations to photosynthesis. Genotype differences in stomatal conductance and c(i) were also examined under fluctuating photosynthetically active radiation, and in some cases were found to be even more pronounced than under steady-state conditions. (C) 2002 Elsevier Science B.V. All rights reserved.