STOMATAL AND PHOTOSYNTHETIC RESPONSES TO SHADE IN SORGHUM, SOYBEAN AND EASTERN GAMAGRASS

We studied photosynthetic and stomatal responses of grain sorghum (Sor ghum bicolor [L] Moench cv. Pioneer 8500), soybean (Glycine max L. cv. Flyer) and eastern gamagrass (Tripsacum dactyloides L.) during experi mental sun and shade periods simulating summer cloud cover. Leaf gas e xchange measurements of field plants showed that short-term (5 min) sh ading of leaves to 300-400 mu mol m(-2) s(-1) photosynthetic photon fl ux density reduced photosynthesis, leaf temperature, stomatal conducta nce, transpiration and water use efficiency and increased intercellula r CO2 partial pressure. In all species, photosynthetic recovery was de layed when leaves were reilluminated, apparently by stomatal closure. The strongest stomatal response was in soybean. Photosynthetic recover y was studied further with soybeans grown indoors (maximum photosynthe tic photon flux density 1200 mu mol m(-2) s(-1)). Plants grown indoors had responses to shade similar to those of field plants, except for b rief nonstomatal limitation immediately after reillumination. These re sponses indicated the importance of the light environment during leaf development on assimilation responses to variable light, and suggested different limitations on carbon assimilation in different parts of th e soybean canopy. Photosynthetic oxygen evolution recovered immediatel y upon reillumination, indicating that the light reactions did not lim it soybean photosynthetic recovery. While shade periods caused stomata l closure and reduced carbon gain and water loss in all species, the c onsequences for carbon gain/water loss were greatest in soybean. The o ccurrence of stomatal closure in all three species may arise from thei r shared phenologies and herbaceous growth forms.