Anaerobiosis in Echinochloa crus-galli (Barnyard Grass) Seedlings : Intermediary Metabolism and Ethanol Tolerance

Tolerance to ethanol and the ability to metabolize key intermediary substrates under anaerobiosis were studied in Echinochloa crus-galli (L.) Beauv. var oryzicola seeds to further characterize the mechanisms which enable it to germinate and grow without O₂.

Our results indicate that E. crus-galli var oryzicola possesses an inherently high tolerance to ethanol and is able to metabolize low levels of ethanol in the absence of O₂. Concentrations of ethanol 45-fold greater than endogenous levels did not prove toxic to germinating seeds.

Five-day anaerobically grown seedlings of E. crus-galli var oryzicola metabolized added ¹⁴C]sucrose primarily to CO₂ and ethanol. Of the soluble compounds labeled, the phosphorylated intermediates of glycolysis and the oxidative pentose phosphate pathway predominated more under anaerobiosis than in air. In addition, organic acids and lipids were labeled from ¹⁴C]sucrose, the latter indicating that metabolism of carbohydrate via acetyl-CoA occurred in the absence of O₂. Lipids were also labeled when seeds were supplied with ¹⁴C]ethanol or ¹⁴C]acetate. Labeling experiments using the above compounds plus ¹⁴C]NaHCO₃, showed further labeling of organic acids; succinate and citrate being labeled under nitrogen, while fumarate was formed in air.

The above metabolic characteristics would allow for the maintenance of an active alcoholic fermentation system which, along with high alcohol dehydrogenase activity, would continue to recycle NAD and result in continued energy production without O₂. In addition, Echinochloa's ability to metabolize carbohydrate intermediates and to synthesize lipids indicates that mechanisms exist for providing the carbon intermediates for biosynthesis, particularly membrane synthesis for growth, even in the absence of O₂.