Background/Question/Methods   The accumulation of UV-absorbing compounds in epidermal leaf tissue of higher plants reduces the epidermal transmittance of solar UV radiation and therefore, plays a fundamental role in protecting underlying tissue from UV injury. In a previous study on several plant species growing at a high elevation site in Hawaii, we demonstrated that epidermal UV transmittance (TUV) varied over the course of a day, decreasing at midday and then increasing at night. In the present study, we tested the generality of this phenomenon by screening 52 taxa representing 37 species and 18 families of monocots and dicots (25 forbs, 5 grasses, 3 vines and 4 shrubs/trees of cultivated and wild species) grown in four locations spanning a gradient of ambient solar UV and climate (Hawaii, Utah, Idaho and Louisiana). Non-destructive measurements of adaxial TUV were made at predawn and midday under clear summer skies using a UV-A PAM fluorometer, which determines TUV based on the ratio of chlorophyll fluorescence induced by UV and blue-green (BG) radiation.

Results/Conclusions   Across all species, significant (P<0.05) midday decreases in TUV were detected in 23 of the 37 species tested (62%) with species in all life forms exhibiting this phenomenon. The mean magnitude of the diurnal TUV change was 3.1%, however interspecific variation was great, ranging from 0 in corn (Zea mays) to 31% for okra (Abelmoschus esculentus). Plants grown in Louisiana tended to show larger diurnal changes than those in the other three locations, and across all taxa, the magnitude of the diurnal TUV changes increased with increasing predawn TUV values (R2 = 0.74 for linear regression). Results indicate that diurnal changes in TUV 1) are widespread among higher plants at various locations; 2) vary both within and among species; 3) are highest in species with the lowest baseline levels of UV shielding, and 4) tend to be greater in magnitude for plants growing in warm rather than cool environments.