Have We Selected for Higher Mesophyll Conductance in Domesticating Soybean?

ABSTRACT Soybean (<italic>Glycine max</italic>) 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 (<italic>g</italic><italic>m</italic>) quantifies the ease with which CO2 can diffuse from the sub‐stomatal cavity to Rubisco. Increasing <italic>g</italic><italic>m</italic> is attractive since it increases photosynthesis without increasing water use. Most measurements of <italic>g</italic><italic>m</italic> have been made during steady‐state light saturated photosynthesis. In field crop canopies, light fluctuations are frequent and the speed with which <italic>g</italic><italic>m</italic> can increase following shade to sun transitions affects crop carbon gain. Is there variability in <italic>g</italic><italic>m</italic> within soybean germplasm? If so, indirect selection may have indirectly increased <italic>g</italic><italic>m</italic> during domestication and subsequent breeding for sustainability and yield. A modern elite cultivar (LD11) was compared with four ancestor accessions of <italic>Glycine soja</italic> from the assumed area of domestication by concurrent measurements of gas exchange and carbon isotope discrimination (∆13C). <italic>g</italic><italic>m</italic> 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. [ABSTRACT FROM AUTHOR]