Diurnal changes in epidermal UV transmittance of plants in naturally high UV environments

Studies were conducted on three herbaceous plant species growing in naturally high solar UV environments in the subalpine of Mauna Kea, Hawaii, USA, to determine if diurnal changes in epidermal UV transmittance (T_UV) occur in these species, and to test whether manipulation of the solar radiation regime could alter these diurnal patterns. Additional field studies were conducted at Logan, Utah, USA, to determine if solar UV was causing diurnal T_UV changes and to evaluate the relationship between diurnal changes in T_UV and UV-absorbing pigments. Under clear skies, T_UV, as measured with a UV-A-pulse amplitude modulation fluorometer for leaves of Verbascum thapsus and Oenothera stricta growing in native soils and Vicia faba growing in pots, was highest at predawn and sunset and lowest at midday. These patterns in T_UV closely tracked diurnal changes in solar radiation and were the result of correlated changes in fluorescence induced by UV-A and blue radiation but not photochemical efficiency (F_v/F_m) or initial fluorescence yield (F_o). The magnitude of the midday reduction in T_UV was greater for young leaves than for older leaves of Verbascum. Imposition of artificial shade eliminated the diurnal changes in T_UV in Verbascum, but reduction in solar UV had no effect on diurnal T_UV changes in Vicia. In Vicia, the diurnal changes in T_UV occurred without detectable changes in the concentration of whole-leaf UV-absorbing compounds. Results suggest that plants actively control diurnal changes in UV shielding, and these changes occur in response to signals other than solar UV; however, the underlying mechanisms responsible for rapid changes in T_UV remain unclear.