Flexible Leaf Orientations ofArisaema heterophyllumMaximize Light Capture in a Forest Understorey and Avoid Excess Irradiance at a Deforested Site

Effects of flexible leaflet orientations on light capture andphotosynthesis were investigated forArisaema heterophyllumBlume,a perennial herb with a single palmately compound leaf, in twocontrasting light environments: a riparian forest understoreyand an adjacent deforested open site. Leaf orientations aredetermined by inclination of leaflet midvein and folding ofleaflet blade. Leaves were flatter and had smaller angles ofinclination at the forest site than at the deforested site.Directions (angular altitude and azimuth) of leaf surfaces ofthe forest site plants were close to those predicted to maximizediffuse light capture at each microsite, as determined by theanalysis of a hemispherical canopy photograph. Mean light captureefficiency (the ratio of actual diffuse light capture by a leafto maximal receivable light) reached 98%. In contrast, markedleaflet folding occurred only at the deforested site. The degreeof folding varied diurnally with the maximum around noon. Computersimulations showed that PPFDs (photosynthetically active photonflux density) over the photosynthetic saturation level ofA.heterophyllumcan be effectively reduced by increasing the slopeof leaflet surfaces. The importance of decreasing excess irradianceto avoid photoinhibition and to maintain high rates of photosynthesiswas confirmed by artificially constraining leaves horizontally.Copyright1998 Annals of Botany Company Arisaema heterophyllumBlume, forest understorey, hemispherical canopy photograph, high light stress, leaf orientation, light capture efficiency, microsite light availability, midday depression, photoinhibition, photosynthesis.