Stomatal Conductance Is Essential for Higher Yield Potential of C3Crops

The present review of gas-exchange studies conducted in rainfed or irrigated field conditions clearly indicates that stomatal conductance (gs) of major crops has been inadvertently increased in the last 50 to 80 years as higher yields were being reached through conventional plant breeding. These findings suggest that high gs rates are critical to optimum growth and yield of modern crops, in particular to crop growth rate (CGR) during seed formation and filling. Several hypotheses are presented that may account for this increase. This review also includes studies in which increases in gs were documented for genetically-engineered plants. However, field evaluations of their gas-exchange performance remain sparse. The main finding of this review is that higher rates of gs should be sought, as we forego the usual consideration of crop plants as single organisms losing water, but rather acknowledge that water does exit a crop canopy along a Soil Plant Atmosphere Continuum (SPAC). In non-limiting or moderately-limiting soil water conditions, high yields are generated by an active SPAC that requires sustained levels of gs. In conditions of high evaporative demand, as in summer afternoons, sustained gs allows for cooler canopies with no mid-day depression in photosynthesis as CO2 remains able to diffuse in the leaf through stomata.