Effect of oxygen concentration on intracellular pH, glucose‐6‐phosphate and NTP content in rice (Oryza sativa) and wheat (Triticum aestivum) root tips: in vivo 31P‐NMR study

Abstract

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.