Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil water potential (Ψs) is better correlated to heliotropic leaf orientation, photosaturated photosynthetic CO2 assimilation and stomatal conductance during periods of limited water availability than is bulk leaf water potential (Ψ1) was examined in greenhouse-grown soybean (Glycine max) plants, submitted to a progressive drought. Paired plants were exposed to either 1000 or 100 μmol m−2 s−1 photon flux densities (PFD) for 45–60 mins. The higher irradiance induced short-term decreases in Ψ1, due to increased transpiration, while Ψl in the plant exposed to low PFD did not decrease. These changes in Ψ1 occurred independently of changes in soil water status. Concurrent to the light treatments, a single attached leaf from each of the two plants was isolated from the rest of the plant by shading, and the pulvinus of its terminal leaflet was exposed to a perpendicular PFD of 500 μmol m−2 S−1. Leaf movement of this leaflet was recorded in response to this light, until a stable leaflet angle was achieved. Values of Ψs and Ψl (before and after light treatment), and photosaturated rates of photosynthesis and stomatal conductance, were then measured on these leaves. Leaflet angle and gas exchange were better correlated with Ψs (r2 = 0.50, 0.50 and 0.57 for angle, photosynthesis and conductance, respectively) than with Ψl especially when Ψl was the result of short-term, high-light induced changes in leaf water status (r2 = 0.36, 0.32 and 0.49, for the same parameters). Leaflet angle was also correlated with stomatal conductance (r2 = 0.61) and photosynthetic rate (r2 = 0.60), suggesting a close association between leaf orientation, leaf metabolism and soil water availability.