Alcoholic fermentation and malate metabolism in rice germinating at low oxygen concentrations

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 02); (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 0, during exposure to Hi4C03-, for 30-60 min, inhibited C02 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 C02 than shoots from seedlings raised in air. Malate always contained 70% or more of the total fixed 14C, irrespective of the 0, regime during germination and during I4CO2 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 0,. 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 0, concentrations.