Reducing stomatal conductance in C4 plants: A trade off between water and CO2

The CO2 response curve for C4 plants is biphasic, with a steep increase followed by saturation of photosynthetic CO2 uptake. C4 plants which evolved in prehistoric low CO2 concentrations have stomata evolved such that their operating point lies in the transition zone of this biphasic behavior. In the past couple of centuries, global atmospheric CO2 concentrations have more than doubled post industrialization. However, C4 plants have not had the time to evolve and optimize their stomatal behavior for this new changed environment. Under this evolutionary constraint, a further increase in CO2 concentration does not increase photosynthetic carbon uptake. This supra-optimal stomatal behavior results in the operating point moving into the saturation region with no increase in CO2 uptake and higher transpiration with decreased water use efficiency. We hypothesize that by decreasing stomatal conductance and optimizing stomatal behavior, the operating point of C4 leaves can be moved closer to the transition zone resulting in a higher water use efficiency. Here we used a steady-state leaf model of corn leaves to quantify effect of reduced stomatal conductance on the leaf photosynthesis, transpiration and water use efficiency. Simulations performed under full sun conditions show a 22% improvement in water use efficiency (WUE) of a leaf with no drop in CO2 uptake for a 20% drop in stomatal conductance. However, the effects may differ at the crop level where a majority of the leaves are shaded and have altered operating points. To characterize the effect of reduced stomatal conductance at the whole canopy level, we coupled the steady-state leaf model with canopy light attenuation model. Our model results show that for a mature corn crop where a significant portion of canopy leaves are shaded, 40% reduction in stomatal conductance results in a <0.5% drop in CO2 uptake with a 59% increase in WUE. Our model results indicate the potential of substantial water savings in corn plants with minimal to virtually no effect on plant yield. Implementation of this technique in other C4 plants can reduce the dependency of crops on external irrigation, leading to more resilient and sustaining cultivation.