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

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 (molecular weight, 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 (5-6 minutes later) and after 20 to 25 min, the stomates nearly completely closed. Internal CO₂ (calculated) and leaf temperature followed the transpiration measurements. However, 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 have been seen to cause such a transient increase followed by a decrease in stomatal closure.