Have We Selected for Higher Mesophyll Conductance in Domesticating Soybean?

Soybean (Glycine max) is the single most important global source of vegetable protein. Yield improvements per unit land area are needed to avoid further expansion onto natural systems. Mesophyll conductance (g) quantifies the ease with which CO can diffuse from the sub-stomatal cavity to Rubisco. Increasing g is attractive since it increases photosynthesis without increasing water use. Most measurements of g have been made during steady-state light saturated photosynthesis. In field crop canopies, light fluctuations are frequent and the speed with which g can increase following shade to sun transitions affects crop carbon gain. Is there variability in g within soybean germplasm? If so, indirect selection may have indirectly increased g during domestication and subsequent breeding for sustainability and yield. A modern elite cultivar (LD11) was compared with four ancestor accessions of Glycine soja from the assumed area of domestication by concurrent measurements of gas exchange and carbon isotope discrimination (∆C). g was a significant limitation to soybean photosynthesis both at steady state and through light induction but was twice the value of the ancestors in LD11. This corresponded to a substantial increase in leaf photosynthetic CO uptake and water use efficiency.