RELATING UV-B RADIATION SCREENING EFFECTIVENESS OF FOLIAGE TO ABSORBING-COMPOUND CONCENTRATION AND ANATOMICAL CHARACTERISTICS IN A DIVERSE GROUP OF PLANTS

The ultraviolet-B radiation (UV-B, 300 rum) screening effectiveness of foliage of a diverse group of plants was examined by measuring epidermal transmittance and depth of penetration of UV-B with a fiberoptic microprobe. In addition, the concentration of UV-B-absorbing compounds and various anatomical characteristics were measured to assess whether they were useful predictors of UV-B screening. Sun foliage of naturally growing individuals of seven species were sampled in each of six life forms comprising two evergreen groups (gymnosperms and angiosperms) and four deciduous angiosperm groups (trees, shrubs and vines, herbaceous dicotyledons, and grasses). There was significant life-form variation in epidermal transmittance and depth of penetration of UV-B, concentration of UV-B-absorbing compounds (leaf-area basis), epidermal (including cuticle and hypodermis) thickness, and specific leaf area. Values of these parameters tended to be related to leaf longevity, with the most notable differences apparent between evergreen and deciduous species. The mean epidermal transmittance and depth of penetration of UV-B in foliage averaged 4% and 32 mum in evergreens, compared to 28% and 75 mum in deciduous species. These values are conservative estimates since the microprobe was oriented in foliage such that much of the side- and backscattered UV-B was ignored. The strongest predictors of epidermal transmittance and depth of penetration were epidermal thickness and the concentration of absorbing compounds, which averaged 32 mum and 1.50 A cm-2 in evergreens, but only 19 mum and 0.99 A cm-2 in deciduous foliage. However, the variation found in these relationships implies that additional factors warrant consideration in assessing UV-B-screening effectiveness. The relatively ineffective screening of UV-B by foliage of many deciduous plants suggests they may be more responsive to enhanced UV-B than evergreen species.