The Humidity Factor in Stomatal Cokttrol and Its Effect on Crop Productivity

Abstract

Stomata of various woody and herbaceous plant species respond directly to changes in leaf-to-air vapor pressure difference (VPD). Closure of stomata upon exposure to dry air occurs in many species without changes in bulk leaf water status, suggesting an underlying mechanism different from the well-known closure through reduction in bulk leaf water potential. Recent studies in our laboratory on the response of cassava to water stress demonstrated that plants grown in pots or in the field, with and without soil water stress, were very sensitive to changes in atmospheric humidity. Both CO2 uptake rate and H2O loss decreased greatly as VPD increased. This decrease in gas exchange rate was associated with a reduction in leaf conductance in the absence of changes in leaf water potential. The strong stomatal response to changes in VPD may be of particular importance to perennial crops, such as cassava, that may have to endure a long period of drought. Under these conditions, and in the absence of stomatal response to humidity, both photosynthesis and transpiration will continue at relatively high rates until available soil water is depleted and leaf water potential drops to the level required to induce stomatal closure, at which time both photosynthesis and transpiration will approach zero. In such case, most of the transpirational loss will occur during periods of high VPD and low photosynthesis/transpiration ratio, resulting in a low dry matter accumulation per unit water transpired. On the other hand, with a direct stomatal response to changes in air humidity, available soil water will be depleted slowly, as most of the transpirational loss will occur during periods of the day when VPD is low and water use efficiency is highest. With a prolonged period of limited soil water, the greater water use efficiency will lead to a greater total accumulation of photosynthate over the stress period. Thus, the direct stomatal mechanism is beneficial for those crops that experience long period of drought. However, with nonlimiting soil water conditions or only short periods of soil water stress, optimizing water use efficiency would not be as important as maximizing photosynthesis and consequently crop productivity.