Effects of polyethylene-glycol-induced osmotic stress on transpiration and photosynthesis in pinto bean leaf discs.

A new leaf disc chamber allows measurements of chlorophyll fluorescence and CO(2) and H(2)O 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 CO(2) (calculated) and leaf temperature followed the transpiration measurements. However, chlorophyll fluorescence (small rise) followed internal CO(2) (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.