Stomatal responses to light and humidity in sugarcane: prediction of daily time courses and identification of potential selection criteria

Stomatal sensitivities to light and VPD have potential as quantitative selection criteria in programs designed to enhance water‐use efficiency of sugarcane and other crops. These responses were characterized using gas exchange techniques and then simulated by a mathematical relationship describing conductance as a function of photon fluence rates and VPD values. The same form of relationship simulated stomatal responses of well‐watered greenhouse‐ and field‐grown plants. A comparison between simulated and measured conductance values showed a close correlation, indicating that light and VPD responses of stomata are dominant input signals modulating stomatal conductance in sugarcane. Observed conductance of Hawaiian sugarcane in a commerical production area appeared larger than required to support prevailing rates of carbon assimilation, since predicted intercellular CO 2 was greater than required to saturate its C 4 photosynthesis. Manipulation of the relationship describing stomatal conductance allowed us to simulate the responses of plants with hypothetically altered stomatal sensitivities to VPD or to light, using micrometeorological data collected in the field. Further simulation indicated that selection for clones with altered stomatal sensitivity to either light or VPD could improve the water‐use efficiency of sugarcane without inhibiting current high levels of productivity.