Two methods for studying the role of stomata in controlling leaf water status are compared, using data for field grown spring wheat (Triticum aestivum L.). The first is based on comparisons of leaf water potential and stomatal conductance and their responses to different soil water treatments as obtained by irrigation or by placing plastic rainwater gutters between rows to remove a proportion of the incident rainfall. The second method exploits the varying potential evaporation and the consequent changing leaf water potential and stomatal conductance during the course of a day. Good stomatal control is indicated by large values of the ratio of the amount of variation in stomatal conductance to that of leaf water potential. A particular advantage of the latter method is that it does not require a range of soil water treatments, and it can therefore be more readily used to compare the degree of stomatal control in a range of genotypes. There was generally good agreement between the methods, with the second method showing greater sensitivity.
Stomatal conductances were measured with a continuous flow diffusion porometer which had a thermostatically controlled leaf chamber. This feature had the advantage that conductances could be obtained directly from the output of the humidity sensor without the necessity for routine measurement of, and correction for, leaf temperature.