Anaerobiosis results in low ATP levels and cytoplasmic acidosis. Pyrophosphate (PPi) may play an important role in anaerobiosis as an energy source replacing ATP, as suggested by the hypoxic induction of PPi:fructose 6-phosphate 1-phosphotransferase and sucrose synthase in preference to phosphofructokinase and invertase. Here we show that vacuolar H$ sp+$-translocating pyrophosphatase (PPase) is also strongly induced by anoxia in rice seedlings. The PPase transcript abundance is increased within the first hours of anoxia, and decreases within 2 days after the return of seedlings to air, similarly to alcohol dehydrogenase-1 (Adh1). However, tissue studies show that the highest transcript induction for PPase is in the root whereas the highest induction of Adh1 is in the shoot. Assays of enzyme specific activity indicate a 75-fold increase in PPase activity over 6 days of anoxia, while the vacuolar ATPase changes only slightly. Return of seedlings to air results in rapid disappearance of enzyme activity. Chilling stress in rice seedlings also gives rise to an increase in immunoreactive PPase enzyme, and a progressive 20-fold increase in enzyme specific activity within 6 days. Upon return to room temperature both enzyme level and specific activity decrease. In corn, hypoxic stress results in a small induction in the PPase transcript, and no increase in PPase specific activity, which, however, is constitutively high in this material. It is suggested that in both species, H$ sp+$-PPase may play an important role in hypoxia and chilling stress, not only in conserving ATP, but also in limiting cytoplasmic acidosis.

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