Humidity Responses of Stomata and the Potassium Content of Guard Cells

Humidity responses of stomata and changes in the potassium content of their guard cells were investigated in intact plants and detached epidermal strips of Valerianella locusta (L.) Betcke. Potassium content was determined by Macallum‘s stain. It was found that changes in stomatal aperture caused by decreasing or increasing humidity were followed only after a delay by changes in the potassium content of the guard cells. By comparison, if stomatal movements occurred in response to changes in illumination the relative potassium content of the guard cells correlated continuously with the changes in stomatal aperture. Since the potassium content of the guard cells changed only after most of the stomatal movements in response to changes in humidity were completed changes in potassium content and humidity responses of stomata can be described as following a hysteresis curve.