Photosynthetic light-response curves

Abstract

Gradients in photosynthetic capacity through the leaf affect the shape of the irradiance-response curve. These gradients in photosynthetic capacity were manipulated by restraining leaves in different orientations. The shape or curvature of the light-response curve can be defined by Θ, where Θ=0 is a rectangular hyperbola and Θ=1 is a Blackman curve. Horizontal leaves had the highest Θ values when their adaxial (top) surface was illuminated and lowest Θ value when their abaxial (bottom) surface was illuminated. Vertical leaves had intermediate Θ values that were similar for illumination from either direction, indicating that both surfaces had similar photosynthetic capacities. The photosynthetic capacity near each surface was probed by measuring the resistance to photoinhibition by 2000 μmol quanta · m −2·s −1 for 2 h followed by 15 min dark relaxation. Resistance to photoinhibition was consistent with the amount of direct sunlight exposure during growth. By measuring three light-response curves for a given leaf, illuminating the leaf from either the adaxial or abaxial surface or with the adaxial and abaxial surfaces illiminated equally, it was possible to infer gradients in the light absorption and photosynthetic capacity of the leaf using a ten-layer model. The gradient in light absorption was not as steep as expected and the photosynthetic capacity declined from the adaxial surface but increased again approaching the abaxial surface, the increase being more pronounced in vertical leaves. The modelled gradients were qualitatively similar for dorsiventral and isolateral leaves. The gradients in light absorption and photosynthetic capacity were not identical and this results (1) in curvilinear relationships between the quantum efficiency of PSII determined by chlorophyll fluorescence and the quantum efficiency of leaf photosynthesis and (2) in light-response curves that slowly reach saturation rather than being abruptly truncated. The Θ value for the photosynthetic light-response curve will remain a parameter that has to be derived empirically, in contrast to the maximum quantum yield and photosynthetic capacity. The curvature factor, Θ, depends on CO2 partial pressure and the interplay between the gradients in light absorption and photosynthetic capacity through the leaf which can change depending on the light environment during growth.