The Response of Bean Plants to UV-B Radiation Under Different Irradiances of Background Visible Light

Abstract

Plants of <it>Phaseolus vulgaris</it> L. (cv. Stella) were grown in controlled conditions under three different irradiances of visible light with or without UV-B (280–320nm) radiation. The biologically effective UV-B radiation (UV-B<inf>BE</inf>) was 6.17 kJ m−2 d−1, and simulated a c. 5% decrease in stratospheric ozone at 55.7�N, 13.4�E. The photon flux densities of the photosynthetically active radiation (PAR, 400–700 nm) were either 700 μmol m−2−1 (HL), 500, μmol m−2 s−1 (ML) or 230 μmol m−2 s−1 PAR (LL). Under high light (HL) conditions plus UV-B radiation, bean plants appeared most resistant to the enhanced levels of UV-B radiation, and responded only by increasing leaf thickness by c. 18%. A small increase in UV screening pigments was also observed. Both the lower irradiances (ML and LL) increased the sensitivity of the plants to UV-B radiation. Changes in leaf structure were also observed. Photosystem II was inhibited under ML and LL together with UV-B radiation, as determined by Chi fluorescence induction and calculation of the fluorescence half-rise times. Leaf reflectivity measurements showed that the amount of <it>PAR</it> able to penetrate leaves of UV-B treated plants was reduced, and that a possible correlation may exist between the reduced <it>PAR</it> levels, loss of Chi and lowered photosynthetic activity, especially for LL +UV-B grown plants, where surface reflection from leaves was highest. Changes in leaf chlorophyll content were mostly confined to plants grown under LL + UV-B, where a decrease of c. 20% was found. With regard to protective pigments (the carotenoids and UV screening pigments) plants subjected to different visible light conditions responded differently. Among the growth parameters measured, there was a substantial decrease in leaf area, particularly under LL + UV-B (c. 47% relative to controls), where leaf dry weight was also reduced by c. 25%.