UV screening in higher plants induced by low temperature in the absence of UV-B radiation
Epidermally located UV-B absorbing hydroxycinnamic acid derivatives and flavonoids serve as a screen against potentially damaging UV-B (280–315 nm) radiation in higher plants. We investigated the effect of low temperature on epidermal screening as assessed by a chlorophyll fluorescence technique. The epidermal UV-transmittance of greenhouse-grown Vicia faba plants was strongly dependent on growth temperatures between 21 and 9 °C, with significant differences already between 21 and 18 °C. There was a good correlation between epidermal UV-A and UV-B absorbance and the absorbance of whole leaf extracts at the respective wavelengths. Whereas in Oxyria digyna and Rumex longifolius no temperature dependence of epidermal transmittance could be detected, it was confirmed for seven other crop plant species, including summer and winter varieties, and for Arabidopsis thaliana. Dicotyledoneous plants showed a stronger response than monocotyledoneous ones. In all investigated species, the response in the UV-A spectral region was similar to that in the UV-B, suggesting that flavonoids were the responsible compounds. In V. faba, mature leaves did not respond with a change in epidermal transmittance upon transfer from warm to cool conditions or vice versa, whereas developing leaves did acclimate to the new conditions. We conclude that temperature is an important determinant of the acclimation of epidermal UV transmittance to environmental conditions in many plant species. The potential adaptive value of this response is discussed.