Effects of Polyethylene-Glycol-Induced Osmotic Stress on Transpiration and Photosynthesis in Pinto Bean Leaf Discs

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Abstract
A new leaf disc chamber allows measurements of chlorophyll fluorescence and CO2 and H2O vapor exchanges during infusion of solution into the cut edge of the disc. Polyethylene glycol (MW, 6000) was used to apply a mild external osmotic stress to the leaf disc within this chamber. This stress rapidly caused a temporary increase in transpiration. This increase was reversed (5-6min later) and after 20-25 min, the stomates nearly completely closed. Internal CO2 (calculated) and leaf temperature followed the transpiration measurements. Chlorophyll fluorescence (small rise) followed internal CO2 (small rise). This complete sequence of events resembles those caused by exposure of leaves to certain air pollutants which can cause such a transient increase followed by a decrease in stomatal closure.