Use of a minimally invasive method of measuring leaf stomatal conductance to examine stomatal responses to water vapor pressure difference under field conditions

Most methods of measuring the diffusive conductance to water vapor of individual plant leaves potentially change the leaf environment by enclosing part of the leaf in order to measure the rate of water vapor exchange and the driving force for that exchange. There have been questions about whether leaf to air water vapor pressure difference varies sufficiently in extensive crop canopies to be a significant source of variation in stomatal conductance. Here I combined measurements of temperatures of leaves and metal replicas of leaves with the same size and orientation in an energy balance approach to estimate stomatal and boundary layer conductances under field conditions. The method provides a new way to simultaneously determine the in situ environmental conditions and conductances of leaves. Stomatal conductances of upper canopy leaves of three crop species, Glycine max, Phaseolus vulgaris and Zea mays measured with this method were highly correlated with measurements obtained with a steady-state portable photosynthesis system. However, the photosynthesis system often reduced the leaf to air water vapor pressure difference during the measurements sufficiently to increase stomatal conductance. Measurements made with the minimally invasive method indicated that even in extensive maize and soybean canopies, day-to-day variation in the leaf to air water vapor pressure difference at the surface of upper canopy leaves was sufficiently large to cause about a two-fold variation in mean stomatal conductance on different mid-summer afternoons in both species. (c) 2006 Elsevier B.V. All rights reserved.