Diurnal changes in leaf UV-absorbing compounds and epidermal UV-transmittance

Background/Question/Methods

Recent studies have shown that epidermal UV-transmittance (T_uv) can change on a diurnal basis in several plant species growing in the high-elevation, high solar UV environment on Mauna Kea, Hawaii, but how common this phenomenon is among species, and what physiological and environmental factors drive these changes are largely unknown. To test for the generality of this response, we measured T_uv at predawn/dawn and midday using a non-invasive chlorophyll fluorescence technique (UVA-PAM) under clear sky summer conditions in 37 species of native and cultivated plants (n=2-15/species) growing in four locations (Hawaii, Idaho, Utah and Louisiana) that differed in daily biologically-effective UV irradiances (mean daily plant effective UV-B = 3.5 – 11.0 kJ m^-2 d^-1) and air temperatures (mean daily T_air = 11.8 – 29.0 C).  Concurrently, samples in three of these species (Abelmoschus esculentus, Phaeseolus vulgaris, and Zea mays) growing in Louisiana were collected and analyzed for whole-leaf UV-absorbing compounds.  Follow-up studies measured T_uv and UV-absorbing compounds in A. esculentus and P. vulgaris plants growing under ambient clear sky conditions over two consecutive days in late spring (n=14-15 for both T_uv and UV-absorbing compounds).

Results/Conclusions

Across all locations, 25 of the 37 species surveyed (67.6%) showed significant (P<0.10) decreases in Tuv from predawn/dawn to midday with the magnitude of these changes ranging from 0.5% in Typha latifolia to 31.1% in A. esculentus.  When averaged over species, there was a significant effect of location (ANOVA; P = 0.05) with plants growing in Louisiana generally showing larger diurnal changes in T_uv than those growing in other locations.  No significant differences (ANOVA; P=0.64) were found among plants of different growth forms (forb, vine, grass and woody plant).  Diurnal changes in T_uv in A. esculentus and P. vulgaris were associated with significant (P<0.05) changes in whole-leaf UV-absorbing compounds.  No diurnal changes (P>0.05) in T_uv or UV-absorbing compounds were detected in Z. mays.  Results indicate that, 1) diurnal changes in T_uv are widespread among plant species, 2) the magnitude of diurnal changes in T_uv is more closely related to location differences in temperature (higher temperature = greater change in T_uv) than solar UV fluxes, and 3) when diurnal changes in T_uv do occur they are associated with detectable changes in UV-absorbing compounds.  The specific nature of these pigment changes however, has yet to be elucidated.