Variation among Soybean Cultivars in Mesophyll Conductance and Leaf Water Use Efficiency

Improving water use efficiency (WUE) may prove a useful way to adapt crop species to drought. Since the recognition of the importance of mesophyll conductance to CO₂ movement from inside stomatal pores to the sites of photosynthetic carboxylation, there has been interest in how much intraspecific variation in mesophyll conductance (g_m) exists, and how such variation may impact leaf WUE within C₃ species. In this study, the g_m and leaf WUE of fifteen cultivars of soybeans grown under controlled conditions were measured under standardized environmental conditions. Leaf WUE varied by a factor of 2.6 among the cultivars, and g_m varied by a factor of 8.6. However, there was no significant correlation (r = -0.047) between g_m and leaf WUE. Leaf WUE was linearly related to the sub-stomatal CO₂ concentration. The value of g_m affected the ratio of maximum Rubisco carboxylation capacity calculated from the sub-stomatal CO₂ concentration to that calculated from the CO₂ concentration at the site of carboxylation. That is, variation in g_m affected the efficiency of Rubisco carboxylation, but not leaf WUE. Nevertheless, there is considerable scope for genetically improving soybean leaf water use efficiency.