Leaf anatomical changes in Populus trichocarpa, Quercus rubra, Pseudotsugamenziesii and Pinus ponderosa exposed to enhanced ultraviolet-B radiation

Leaf anatomical characteristics are important in determining the degree of injury sustained when plants are exposed to natural and enhanced levels of ultraviolet-B (UV-B) radiation (280-320 nm). The degree to which leaf anato my can adapt to the increasing levels of UV-B radiation reaching the earth' s surface is poorly understood in most tree species. We examined four tree species, representing a wide range of leaf anatomical characteristics. to d etermine responses of leaf area, specific leaf weight, and leaf tissue para meters after exposure to ambient and enhanced levels of UV-B radiation. See dlings were grown in a greenhouse with photosynthetically active radiation of 39 mol m(-2) day(-1) and under one of three daily irradiances of biologi cally effective UV-B radiation (UV-B-BE) supplied for 10 h per day: (1) app roximate ambient level received at Pullman, Washington on June 21 (1 x); tw o times ambient (2 x), or three times ambient (3 x). We hypothesized the re sponse of each species to UV-B radiation would be related to inherent anato mical differences. We found that the conifers responded anatomically to nea rly an equal degree as the broad-leaved trees, but that different tissues w ere involved. Populus trichocarpa, an indeterminate broadleaf species, show ed significantly thicker palisade parenchyma in recently mature leaves at t he 3 x level and in older leaves under the 2 x level. In addition, individu al leaf area was generally greater with increased UV-B irradiance. Quercus rubra, a semi-determinate broadleaf species, exhibited significantly thicke r palisade parenchyma at the 2 x and 3 x levels as compared to controls. Ps uedotsuga menziesii, an evergreen coniferous species with bifacially flatte ned needles, and Pinus ponderosa, an evergreen coniferous species with a co mplete hypodermis, showed no significant change in leaf area or specific le af weight under enhanced UV-B radiation. Epidermal thickness was unchanged in P. menziesii. However, P. ponderosa increased the thickness and number o f hypodermal layers produced, presumably decreasing penetration of UV-B rad iation into the leaf. We concluded that differences in inherent leaf anatom y of the four species examined are important in the responses to enhanced l evels of UV-B radiation.