Effects of flexible leaflet orientations on light capture and photosynthesis were investigated for Arisaema heterophyllum Blume, a perennial herb with a single palmately compound leaf, in two contrasting light environments: a riparian forest understorey and an adjacent deforested open site. Leaf orientations are determined by inclination of leaflet midvein and folding of leaflet blade. Leaves were flatter and had smaller angles of inclination at the forest site than at the deforested site. Directions (angular altitude and azimuth) of leaf surfaces of the forest site plants were close to those predicted to maximize diffuse light capture at each microsite, as determined by the analysis of a hemispherical canopy photograph. Mean light capture efficiency (the ratio of actual diffuse light capture by a leaf to maximal receivable light) reached 98%. In contrast, marked leaflet folding occurred only at the deforested site. The degree of folding varied diurnally with the maximum around noon. Computer simulations showed that PPFDs (photosynthetically active photon flux density) over the photosynthetic saturation level of A. heterophyllum can be effectively reduced by increasing the slope of leaflet surfaces. The importance of decreasing excess irradiance to avoid photoinhibition and to maintain high rates of photosynthesis was confirmed by artificially constraining leaves horizontally.
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