Resistance to oxidative injury in submerged rice seedlings after exposure to air

Rice (Oryza sativa L.) seedlings were germinated under water in darkness for 5 or 6 days (submerged seedlings) and then in air for 1 day. Control seedlings were germinated in air, in darkness, for 5 or 6 days (aerobic seedlings). Changes in levels of antioxidants and in the extent of oxidative damage after exposure of submerged seedlings to air were studied. α-Tocopherol, which inhibits lipid peroxidation, was present in submerged seedlings at about 3 times the level found in aerobically grown controls, and higher levels than in controls were maintained for 24 h after transfer of the seedlings to air. Products of lipid peroxidation were present at a one-third of the levels found in aerobic controls, and their levels increased after transfer to air. However, these levels remained lower than those in aerobic controls even after 24 h of contact with air. Carotenoids, which are considered to protect chlorophyllous compounds against photooxidation, were not found in submerged seedlings, but their levels increased after exposure of the seedlings to air. Light at an intensity that did not cause photooxidative damage to chlorophyllous compounds in aerobic controls induced photobleaching of these compounds in submerged seedlings during the early stages of adaptation to air. However, the extent of photobleaching diminished as adaptation to air proceeded, and photobleaching was no longer detected after 24 h of adaptation to air. Thus, the system for protection of cellular membranes from lipid peroxidation in the post-hypoxic phase appeared already to exist in submerged seedlings. However, the system for protection of pigments from photobleaching was poorly developed in submerged seedlings and was fully active only after 24 h of adaptation to air.