The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field

TLDR

Field attempts to link stomatal conductance and leaf water potential to environmental variables are limited by simultaneous influences and diurnal hysteresis, and physiological knowledge does not provide a mechanistic model. The authors propose a simple model that relates leaf water potential to the flow of water through the plant. Model parameters are estimated from field data using non‑linear least squares, allowing prediction and description of seasonal and plant‑to‑plant variation. The model reveals that scatter plots against individual variables persist, yet the estimated parameters enable predictive use and capture inter‑seasonal and inter‑plant differences.

Abstract

Attempts to correlate values of stomatal conductance and leaf water potential with particular environmental variables in the field are generally of only limited success because they are simultaneously affected by a number of environmental variables. For example, correlations between leaf water potential and either flux of radiant energy or vapour pressure deficit show a diurnal hysteresis which leads to a scatter diagram if many values are plotted. However, a simple model may be adequate to relate leaf water potential to the flow of water through the plant. The stomatal conductance of illuminated leaves is a function of current levels of temperature, vapour pressure deficit, leaf water potential (really turgor pressure) and ambient CO 2 concentration. Consequently, when plotted against any one of these variables a scatter diagram results. Physiological knowledge of stomatal functioning is not adequate to provide a mechanistic model linking stomatal conductance to all these variables. None the less, the parameters describing the relationships with the variables can be conveniently estimated from field data by a technique of non-linear least squares, for predictive purposes and to describe variations in response from season to season and plant to plant.