RESISTANCE TO OXIDATIVE INJURY IN SUBMERGED RICE SEEDLINGS AFTER EXPOSURE TO AIR

Rice (Oryza sativa L.) seedlings were germinated under water in darkne ss for 5 or 6 days (submerged seedlings) and then in air for 1 day. Co ntrol seedlings were germinated in air, in darkness, for 5 or 6 days ( aerobic seedlings). Changes in levels of antioxidants and in the exten t of oxidative damage after exposure of submerged seedlings to air wer e studied. a-Tocopherol, which inhibits lipid peroxidation, was presen t in submerged seedlings at about 3 times the level found in aerobical ly grown controls, and higher levels than in controls were maintained for 24 h after transfer of the seedlings to air. Products of lipid per oxidation were present at a one-third of the levels found in aerobic c ontrols, and their levels increased after transfer to air. However, th ese levels remained lower than those in aerobic controls even after 24 h of contact with air. Carotenoids, which are considered to protect c hlorophyllous compounds against photooxidation, were not found in subm erged seedlings, but their levels increased after exposure of the seed lings to air. Light at an intensity that did not cause photooxidative damage to chlorophyllous compounds in aerobic controls induced photobl eaching of these compounds in submerged seedlings during the early sta ges of adaptation to air. However, the extent of photobleaching dimini shed as adaptation to air proceeded, and photobleaching was no longer detected after 24 h of adaptation to air. Thus, the system for protect ion of cellular membranes from lipid peroxidation in the post-hypoxic phase appeared already to exist in submerged seedlings. However, the s ystem for protection of pigments from photobleaching was poorly develo ped in submerged seedlings and was fully active only after 24 h of ada ptation to air.