Anaerobic Metabolism in Germinating Seeds of Echinochloa crus-galli (Barnyard Grass)

Abstract

Echinochloa crus-galli, a problem weed in rice fields, has the rare ability to germinate and to grow in a totally oxygen-free environment. After 7 days growth in the light or dark under N(2), E. crus-galli var. oryzicola produces a 2- to 3-centimeter nonpigmented shoot.Ethanol, malate, and lactate were measured in seeds germinated under N(2) and air, and compared with changes in the activities of alcohol dehydrogenase, malate dehydrogenase, NADP-malic enzyme, and phosphoenolpyruvate carboxylase. During a 7-day time course, ethanol levels increased 661 micromoles per gram dry weight under N(2), with no increase under air. Alcohol dehydrogenase activity increased 5.5 micromoles per gram dry weight per minute in N(2) compared to 1.0 in air. Corresponding increases for lactate were 7.9 micromoles per gram dry weight under N(2) and 2.7 under air, and for malate, 5.3 micromoles per gram dry weight under N(2) and 0.4 under air.Although 85% of the ethanol produced by the seedlings was found in the external solution under N(2), the seeds still contained 90 times more ethanol under anaerobic conditions than under air. No phosphoenolpyruvate carboxykinase activity was detected and phosphoenolpyruvate carboxylase activity was 10 times less under N(2) than air. Malic enzyme activity increased 5-fold under anaerobic conditions, comparable to the change under aerobic conditions. Proposed adaptive mechanisms of flood-tolerant species to anaerobiosis are discussed.