INFLUENCE OF UV-B RADIATION ON MEMBRANE LIPID-COMPOSITION AND ETHYLENE EVOLUTION IN DOYENNE-DHIVER PEAR SHOOTS GROWN IN-VITRO UNDER DIFFERENT PHOTOSYNTHETIC PHOTON FLUXES

In vitro shoots of cv. Doyenne d'Hiver pear (Pyrus communis L.) were irradiated for 6 hr per day with 12 kJ m-2 day-1 of biologically effective UV-B radiation (UV-B(BE) and assessed for modification of membrane lipids. There was no detectable UV-B effect on lipid linolenic acid levels in plants grown at 90 mumol m-2 s-1 of photosynthetic photon flux (PPF). However, at this PPF level, UV-B produced an overall increase in malondialdehyde over a 3-day period, indicating lipid peroxidation. UV-B irradiation reduced the monogalactosyldiacylglycerol (MGDG) concentration, did not affect overall digalactosyldiacylglycerol (DGDG) concentration, but did affect it at specific sampling times, and did not affect total phospholipid levels. UV-B had no consistent effect on glutathione content. In comparison to controls, UV-B irradiated shoots grown under 180 or 270 mumol m-2 s-1 PPF showed an increase in apical necrosis but not at 90 mumol m-2 s-1 PPF. Shoot proliferation was stimulated by 270 mumol m-2 s-1 of PPF, but was depressed by UV-B at this PPF level. Ethylene evolution was increased after a single exposure to UV-B under 90 and 180 mumol m-2 s-1 PPF and after 3 days also under 270 mumol m-2 s-1 PPF. UV-B increased ethane concentration in culture jars, indicating membrane damage; high PPF appeared to ameliorate UV-B induced stress by limiting ethane production. Our results support the hypothesis that MGDG, the major lipid component of the chloroplast, is a target of UV-B irradiation.