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

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.