Heterogeneous reflected light influences asymmetry in leaf anatomy and gas exchange.

Soil and vegetative groundcovers reflect light heterogeneously in habitats lacking a continuous overhead canopy, however the effects of reflected light on vegetation in these habitats has received little attention. We test the hypothesis that reflected light flux affects leaf optical properties, anatomy, and photosynthesis of Ipomoea pes-caprae (Convolvulacae), a common sand dune vine with functionally symmetric leaves, by comparing leaves growing over patches of high and low reflected light flux. Patches of high reflected light were found directly over sand and reflected 26.0 ± 0.9% (mean ± 1 SE) of incident photosynthetically active radiation (PAR) while patches of low reflected light were found over vegetation and reflected 6.1 ± 0.7% of incident PAR. Using a novel in situ method to simultaneously illuminate and measure gas exchange of one leaf surface at a time, we show that abaxial surface photosynthetic maxima and palisade parenchyma in sand patches were, respectively, 2.6 times greater and 20% thicker than those found over vegetation patches. Our results suggest that reflected light strongly influences leaf anatomy and gas exchange of I. pes-caprae, demonstrating that reflected light can be an important component of the light environment for vegetation of habitats characterized by high-albedo substrates.

Leaf movement in Calathea lutea (Marantaceae).

Calathea lutea is a broad-leaved, secondary successional plant which shows complex leaf movements involving both elevation and folding of the leaf surface about the pulvinus. In the plants studied, mean leaf elevation increased from approximately 34 degrees in the early morning to 70 degrees at noon while the angle of leaf folding increased from 13 degrees to 50 degrees over the same time period. During the period from early morning to noon, these movements resulted in a significant decrease in the cosine of the angle of incidence, a measure of the direct solar radiation intercepted. The observed changes in elevational angle significantly reduce the cosine of angle of incidence while folding does not significantly reduce the fraction of direct solar radiation intercepted during the period of direct exposure of the leaf surface to the solar beam. Since elevational changes seem to account for the reduction in exposure to direct solar radiation, the role of folding remains unclear.