Dependence of Stomatal Conductance on leaf Water Potential, Turgor Potential, and Relative Water Content in Field-Grown Soybean and Maize

Soybean [Glycine max. (L.) Merr.] and maize (Zea mays L.) were grown in field plots on an Arredondo fine sand (loamy, siliceous hyperthermic Grossarenic Paleudult) and subjected to either five or four water management treatments in 1985 and 1986, respectively. Treatments included daily irrigation, irrigation at either 4- or 8-day intervals, irrigation to maintain a slight reduction in stomatal conductance, and no irrigation. The objective of the study was to investigate the hypothesis that stomatal conductance (gₛ) is more closely related to leaf relative water content (RWC) than to either bulk leaf water (ψL) and xylem potential (ψₓ) or leaf turgor potential (ψₚ), of which have been frequently used to describe leaf water status. An important feature of this study was an attempt to characterize the waterelations and stomatal conductance of field-grown crops subjected only to mild water deficits. A comparison of the coefficients of variation among the parameters used to evaluate leaf water status showed the greatest value for ψₚ, followed by ψL, with RWC having the smallest. When mild plant water deficits were imposed, gₛ was significantly decreased without decreases in either ψL, ψₚ, or RWC. There were no significant relationships between gₛ and any of the measures of leaf water status when data from only the daily irrigated and mildly stressed treatments were pooled. However, linear regressions of gₛ vs. ψL, ψₚ, ψₓ, and RWC were often statistically significant when data from the nonirrigated, severely stressed treatment were included in the analyses. We conclude that bulk leaf measurements of ψL, ψₚ, ψₓ, and RWC cannot be used to describe accurately the response of gₛ to soil dehydrations that result in only mild plant water stress in field-grown soybean and maize.