Non-uniform stomatal closure and the apparent convexity of the photosynthetic photon flux density response curve


*Correspondence and present address: Jerry W. Leverenz, Department of Ecology and Environmental Sciences, Swedish University of Agricultural Scienc[/es, Box 7072, S–750 07 Uppsala, Sweden.


The photosynthetic response of leaves to photosynthetic photon flux density (PPFD) may be described by parameters for the rate of dark respiration (R), the initial slope (Ф), the PPFD-saturated rate of net photosynthesis (AmQ) and the apparent convexity (θa). We tested the hypothesis that non-uniformity in stomatal aperture across a leaf results in a clearly lower θa because PPFD saturation will occur at different irradiances in different regions of the leaf. A computer model was constructed to simulate the effects of bell-shaped and other distributions of stomatal conductance for CO2 (gs) across a model leaf. In the model, Ф and AmQ decreased by up to 50% whereas θa decreased by at most 8%, essentially negating the hypothesis. The relationship between photosynthetic rate and gs determined the size of the responses of θa and Ф to non-uniform gs. In support of the model, experiments on sunflower leaves showed that Ф and AmQ decreased by 32 and 52%, respectively, while no measurable change in θa occurred, when abscisic acid was used to induce patchiness in gs. Although not all possible patterns of non-uniform stomatal conductance have been tested, it appears that if large variations in θa occur in nature they do not result from non-uniform gs.