Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant.

Comparisons of reflection, transmittance, and absorptance spectra of heavily glaucous leaves, glaucous leaves from which the glaucescence was experimentally removed, and naturally occurring non-glaucous leaves of a single species (Dudleya brittonii Johansen) reveal that glaucescence, a powdery wax coating on the leaf surface, is responsible for very high reflectance of ultraviolet radiation (UV) and high reflectances of visible and near-infrared radiation. UV reflectance (up to 83% in UV-B) measured for glaucous leaves of D. brittonii, a succulent rosette-plant occurring in fissures in steep volcanic outcrops, is higher than that reported for any other plant species. Non-glaucous leaves of the species reflect about 10% in UV. I hypothesize that the high UV reflectance of glaucous leaves is ecologically significant in reducing damage to dehydrated leaves from visible and UV-B radiation, thus promoting longevity of the leaves important both in conservation of the mineral nutrient capital of these succulent plants and in their acquisition and storage of water. This and other demonstrated and hypothesized functions of glaucescence are discussed in terms of the evolutionary significance of glaucescence in succulent plants.