Leaf pubescence: effects on absorptance and photosynthesis in a desert shrub.

The presence of leaf pubescence (leaf hairs) in Encelia farinosa, a desert species of the Composite family, reduces the absorptance of photosynthetically active radiation (400 to 700 nanometers) by as much as 56 percent more than a closely related but nonpubescent species, E. californica, a native of the relatively moist southern California coast. Pubescence in E. farinosa, which increases through the growing season, modifies the leaf energy balance and dramatically reduces the photosynthetic rate. The reduction in the photosynthetic rate is caused by decreased light absorption rather than decreased carbon dioxide conductance through the boundary layer.