Hypoxic pretreatment is known to induce anoxia tolerance in plant species
sensitive to oxygen deprivation. However, we still do not have detailed information
on changes in cytoplasmic and vacuolar pH (pHcyt and pHvac) in plants
under low-oxygen availability (hypoxia) and under anoxia. To investigate this,
we have studied the influence of hypoxia and anoxia on pHcyt and pHvac,
glucose-6-phosphate (Glc-6-P) and nucleotide triphosphate (NTP) contents in
rice (Oryza sativa L.) root tips in comparison with those of wheat (Triticum
aestivum L.) with in vivo 31P-nuclear magnetic resonance. Both cereals
responded to hypoxia similarly, by rapid cytoplasmic acidification (from pH
7.6–7.7 to 7.1), which was followed by slow partial recovery (0.3 units after
6 h). Anoxia led to a dramatic pHcyt drop in tissues of both species (from pH
7.6–7.7 to less than 7.0) and partial recovery took place in rice only. In wheat,
the acidification continued to pH 6.8 after 6 h of exposure. In both plants, NTP
content followed the dynamics of pHcyt. There was a strong correlation
between NTP content and cytoplasmic H+ activity ([H+]cyt = 10^-pHcyt) for
both hypoxic and anoxic conditions. Glc-6-P content increased in rice under
anoxia and hypoxia. In wheat, Glc-6-P was not detectable under anoxia but
increased under hypoxia. In this study, rice root tips were shown to behave
as anoxia tolerant tissues. Our results suggest that the initial cytoplasmic
acidification and subsequent pHcyt are differently regulated in anoxia tolerant
and intolerant plants and depend on the external oxygen concentration.