Carbon Dioxide Effects on Ethanol Production, Pyruvate Decarboxylase, and Alcohol Dehydrogenase Activities in Anaerobic Sweet Potato Roots

The effect of varied anaerobic atmospheres on the metabolism of sweet potato (Ipomoea batatas [L.] Lam.) roots was studied. The internal gas atmospheres of storage roots changed rapidly when the roots were submerged under water. O(2) and N(2) gases disappeared quickly and were replaced by CO(2). There were no appreciable differences in gas composition among the four cultivars that were studied. Under different anaerobic conditions, ethanol concentration in the roots was highest in a CO(2) environment, followed by submergence and a N(2) environment in all the cultivars except one. A positive relationship was found between ethanol production and pyruvate decarboxylase activity from both 100% CO(2)-treated and 100% N(2)-treated roots. CO(2) atmospheres also resulted in higher pyruvate decarboxylase activity than did N(2) atmospheres. Concentrations of CO(2) were higher within anaerobic roots than those in the ambient anaerobic atmosphere. The level of pyruvate decarboxylase and ethanol in anaerobic roots was proportional to the ambient CO(2) concentration. The measurable activity of pyruvate decarboxylase that was present in the roots was about 100 times less than that of alcohol dehydrogenase. Considering these observations, it is suggested that the rate-limiting enzyme for ethanol biosynthesis in sweet potato storage roots under anoxia is likely to be pyruvate decarboxylase rather than alcohol dehydrogenase.