Regulation of Transpiration in Avena. Responses to White Light Steps

The transient responses in transpiration rate of the primary leaf of 8-days-old oat plants were studied when step changes in the white light irradiance were administered to the plants. A light or dark step was achieved by an instant increase or decrease, respectively, of the irradiance on the leaf surface. Usually the light step responses consisted of one rapid response (rise time 1–2 min) and one slow response (rise time 5–10 min). The dark step response had a fall time of about 10 min. At a high irradiance level and in CO₂-free air the slow response disappeared. This could mean that the slow response is CO₂-controlled. The rapid response was still present under these conditions and is therefore probably caused by other processes than the slow response. In long time experiments several cycles of a circadian rhythm (period 26–28 h) could be followed in the light step responses. Thus the rapid response was small at that time of the day when the slow response was most pronounced and vice versa. The endogenous nature of the rhythm was established in phase-shift experiments. It is concluded that experiments on stomatal dynamics must take circadian variations into account.