| Abstract: | Meidner, H. 1987. The humidity response of stomata and its measurement.J.exp. Bot. 38:877882. Using artificial membranes of constant conductance with vapourpressures below the membranes of 2?33 kPa (saturated) and 2?26kPa (97% saturated) it was found that an automated water vapourdiffusion porometer registered a change in conductance. In otherwords, it underestimated the conductance when the vapour pressurebelow the membranes was only 97% saturated. This is becausethe programmed computation of the conductance assumes that thespace below the membrane is at all times saturated. The implicationsfor measuring leaf conductance with these instruments is discussedin connection with the humidity response of stomata to a stepwiseincrease in the vapour pressure difference between the leafinterior and the atmosphere. This response appears to involvea lowering of the vapour pressure in the sub-stomatal spaceand, therefore, a transient opening movement which can be detectedby sensitive mass flow porometry. It is concluded that in cases in which a transient opening movementprecedes partial stomatal closure the humidity response to anincrease in vapour pressure difference cannot be due to vapourloss direct from the guard cells. Key words: Humidity, transients, stomata |
