Regulation of Transpiration in Avena. Responses to Red and Blue Light Steps

The transpiration responses of primary Arena leaves to red and blue light steps were investigated. The response to a red light step was a so‐called slow response (with a rise time of about 8 min). The response to a blue light step consisted of both a slow, and a rapid response (with a rise time of about 2 min). CO 2 ‐free air outside the leaf eliminated only the slow response, i.e. in CO 2 ‐free ait the plant responded to blue light steps but not to red ones. A short exposure of red light prior to a blue light step enhanced the rapid response. The same enhancement of the rapid response could be achieved by means of a temporary pretreatment with CO 2 ‐free air. The magnitude of the rapid response was dependent on the blue light irradiance and no threshold effects could be detected. — The experimental results are discussed by means of a model, based on stomatal regulation by, ion transport between the subsidiary cells and the guard cells. It is suggested that the slow transpiration response is due to CO 2 ‐regulation of the stomatal aperture and that the rapid response reflect a CO 2 ‐independent blue light sensitive process, which acts directly on the ion transport through the subsidiary and guard cell membranes.