Hypoxia does not affect rate of ATP synthesis and energy metabolism in rice shoot tips as measured by ³¹P NMR in vivo

The cytoplasmic pH, concentrations of phosphate metabolites, and rate of ATP synthesis were measured in vivo in excised rice shoot tips under normoxic and hypoxic conditions using ³¹P NMR. When supplied with glucose, the shoot tips grew rapidly and were relatively unaffected by hypoxia. The cytoplasmic pH decreased transiently by only 0.2 units during hypoxia, and the concentration of ATP was maintained to at least 90% of the normoxic level. Most importantly, the unidirectional rate constant of ATP synthesis from free phosphate decreased less than 25% during hypoxia. This is in contrast to other actively growing tissues such as the maize root tip. γ-Aminobutyrate was the major nonvolatile fermentation end product after 22 h of hypoxia. Other hypoxia-induced changes included a modest increase in [Ala] and [succinate] as well as a substantial decrease in [malate].