Subject index

Abstract

Parallel time courses of net radiation, canopy conductance and evapotranspiration in a sugarcane canopy demonstrate the interaction of environmental and physiological factors in regulating water loss. The Bowen ratio-energy balance technique was used to evaluate the stomatal response to humidity in the absence of potential artifacts associated with porometer and leaf chamber measurements. Canopy conductance (gc) responded to leaf-air vapor pressure difference (V), supporting the validity of available physiological data showing clear stomatal responses to V in porometers and leaf chambers. Several factors were found to obscure the stomatal response to V under field conditions. The stomatal closing stimulus, V, and the opening stimulus, photon flux density (I), were positively correlated. This caused offsetting stomatal responses to V and I, which reduced the net stomatal response when V changed with radiation-driven changes in leaf temperature. Normalization of gc by I revealed the expected hyperbolic decline of gc with increasing V. Low boundary layer conductance attenuated V imposed at the leaf surface relative to V measured with reference to humidity in the bulk atmosphere. This reduced the stimulus for stomatal response to V, below that indicated by agrometeorological measurements. Use of air saturation deficit (D) rather than V to express evaporative demand overestimates V, minimizing apparent stomatal sensitivity to evaporative demand, when leaves are cooler than the air. These factors may result in small stomatal responses to V that escape detection by indirect measures of stomatal movement, such as those based on the temperature of exposed leaves.