Alcoholic Fermentation and Malate Metabolism in Rice Germinating at Low Oxygen Concentrations

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Abstract
Germinating rice was exposed, in the dark, to low or zero O2 concentrations for 4-5 days by: (1) submergence under 4-5 cm of stagnant solution (3 ppm O2); (2) exposure to a N2 atmosphere; or (3) submergence under solutions flushed with N2. These treatments completely inhibited root growth. Elongation of coleoptiles was stimulated in the stagnant solutions, but not in the N2 treatments. In most experiments, low O2 concentrations resulted in twofold to eightfold increases of malate concentrations in the shoots. Absence of O2 during exposure to H14CO3-, for 30-60 min, inhibited CO2 dark fixation. This inhibition was considerably smaller when seedlings had been raised in N2 rather than in air. Under aerobic conditions during fixation, excised shoots from seedlings raised in N2 fixed more CO2 than shoots from seedlings raised in air. Malate always contained 70% or more of the total fixed 14C, irrespective of the O2 regime during germination and during 14CO2 fixation. Ethanol in stagnant solutions was shown to be formed by the rice seedlings, rather than by bacteria. Ethanol formation during one single day was 20-30-fold greater than the highest recorded amounts of malate in the seedlings. Alcoholic fermentation also responded quickly to changes in aeration regimes, indicating it was an important adaptive factor. Another likely adaptive feature was the high K+ concentration in shoots, even of seedlings grown in the complete absence of O2. It is suggested that these high K+ concentrations have a function in maintaining turgor required for the rapid extension growth of the coleoptiles under low O2 concentrations.