Leaf Ultraviolet Optical Properties Along a Latitudinal Gradient in the Arctic‐Alpine Life Zone

Leaf epidermal transmittance of terrestrial solar ultraviolet—B radiation (295—320 nm) was examined along a latitudinal gradient of solar UV—B radiation. In high UV—B radiation zones, e.g., equatorial and tropical regions, mean epidermal transmittance for the species examined was less than 2%. At higher latitudes, mean epidermal transmittance exceeded 5%. Although this latitudinal solar UV—B gradient represents more than a seven—fold difference in daily integrated UV—B irradiance, the calculated mean effective UV—B irradiance at the mesophyll of low—latitude species is not substantially different from that of species at higher latitudes. Species in high UV—B radiation environments appear to attenuate this radiation more effectively than those in lower irradiance environments. In most cases, absorption of UV—B in the epidermis is the major parameter effecting low transmittance. Reflectance from glabrous leaves is generally less than 10%. In some species, pubescent or glaucous leaf surfaces can reflect more than 40% of the UV—B radiation incident on a horizontal leaf, although such surface characteristics do not necessarily indicate high UV—B reflectance. Under controlled conditions, epidermal transmittance in Pisum sativum L. decreased in response to UV—B irradiation. The modification of epidermal transmittance, resulting in lower UV—B irradiance at the mesophyll, may represent a mechanism of plant acclimation to UV—B radiation. Such acclimation may have occurred in several wildland species of temperate—latitude origin that have invaded high UV—B irradiance equatorial and tropical regions.