Photosynthetic function, lipid peroxidation, and alpha-tocopherol content in spinach leaves during exposure to UV-B radiation

Spinach (Spinacia oleracea L. 'Meridian') plants were irradiated for 9 h d(-1) for 12 d with ultra-violet-B (UV-B) radiation [13.5 kJ m(-2) d(-1) of biologically effective UV-B radiation (UV-B-BE) + 350 mu mol m(2) s(-1) of photosynthetically active radiation (PAR)] or PAR (350 mu mol m(2) s(-1)) to determine if incipient UV-B-induced inhibition of photosynthetic function occurred in photosystem 2 (PS2) reaction centers or in the lipid matrix of thylakoid membranes. Chlorophyll concentrations (leaf area basis) in UV-B-irradiated spinach leaves were significantly lower than in PAR plants after 4, 8 and 12 d of exposure. A significant UV-B-induced rise in initial fluorescence (Fo) at days 8 and 10 was accompanied by a decline in the photochemical efficiency (Fv/Fm) ratio, indicating a reduction in the efficiency of PS2 energy harvesting and trapping. When based upon the chlorophyll content of thylakoid membranes, UV-B radiation did not alter lipid peroxidation but did increase cc-tocopherol content compared with the PAR treatment during the first 8 of the 12 exposure days. These data suggest that incipient UV-B damage to the photochemical apparatus did not occur in PS2 reaction center complexes nor in the lipid matrix of the thylakoid membranes. Photosynthetic function was more susceptible to UV-B-induced impairment than thylakoid membrane lipids were to UV-B radiation damage.