Regulation of transpiration in field-grown sugarcane: Evaluation of the stomatal response to humidity with the Bowen ratio technique

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 ( g c) 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 g c by I revealed the expected hyperbolic decline of g c 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.