Variable mesophyll conductance among soybean cultivars sets a tradeoff between photosynthesis and water-use-efficiency

Photosynthetic efficiency is a critical determinant of crop yield potential, though it remains below the theoretical optimum in modern crop varieties. Enhancing mesophyll conductance, i.e. the rate of carbon dioxide diffusion from substomatal cavities to the sites of carboxylation, may increase photosynthetic and water use efficiencies. To improve water-use-efficiency mesophyll conductance should be increased without concomitantly increasing stomatal conductance. Here we partition variance in mesophyll conductance to within and among cultivar components across soybeans grown under both controlled and field conditions, and examine the covariation of mesophyll conductance with photosynthetic rate, stomatal conductance, water-use-efficiency and leaf mass per area. We demonstrate that mesophyll conductance varies more than 2-fold and that 38% of this variation is due to cultivar identity. As expected mesophyll conductance is positively correlated with photosynthetic rates. However, a strong...