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_m) quantifies the ease with which CO₂ can diffuse from the sub-stomatal cavity to Rubisco. Increasing g_m is attractive since it increases photosynthesis without increasing water use. Most measurements of g_m have been made during steady-state light saturated photosynthesis. In field crop canopies, light fluctuations are frequent and the speed with which g_m can increase following shade to sun transitions affects crop carbon gain. Is there variability in g_m within soybean germplasm? If so, indirect selection may have indirectly increased g_m 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 (∆¹³C). g_m 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 CO₂ uptake and water use efficiency.