Energetics of Plant Growth Under Anoxia: Metabolic Adaptations of Oryza sativa and Echinochloa phyllopogon

Abstract

Unlike most plant species, Oryza sativa L. cv. S-201 and Echinochloa phyllopogon (Stev.) Koss germinate and grow under anaerobic conditions. In both species, the radicle or shoot emerged by day 3 when the seeds were germinated in air or N2. Under either condition, shoot and/or root dry weight (d. wt) increased linearly from day 3 to day 7, with a corresponding decrease in seed d. wt. In anaerobically grown O. sativa, d. wt accumulation was reduced to 7% of that in air whereas d. wt lost from the seed was reduced to only 37%. No root growth occurred during anaerobic germination and shoot d. wt accumulation accounted for 10% of the d. wt lost from the seed. In E. phyllopogon, d. wt accumulation during anoxia was 25% of that in air, but loss of d. wt from the seed was 44% of the aerobic rate. In air, 48% of the d. wt lost from the seed was converted to shoot or root d. wt. Like O. sativa, E. phyllopogon does not produce a root under N2, but shoot growth accounted for 27% of the d. wt lost from the seed. Thus, either in air or N2, E. phyllopogon was more efficient at converting seed reserves to shoot/root structural dry matter than O. sativa . Based on changes in metabolite pools, O. sativa appeared to shift exclusively to fermentation during anaerobic growth. In E. phyllopogon, however, fermentation alone cannot satisfy the energy requirement for growth without O2. Rather, fermentation, coupled with limited tricarboxylic acid (TCA) cycle operation could supply sufficient ATP for growth under anaerobic conditions. An active oxidative pentose phosphate pathway and lipid synthesis were discussed as important mechanisms for converting NADH to NAD, a necessary cofactor for fermentation and TCA cycle activity.