Respiratory Activities of Plant Mitochondria Under Osmotic Stress

Abstract

Summary In thin slices of leaves or potato tubers dehydrated in hypertonic sorbitol solutions, photosynthesis or dark-fixation of CO 2 was almost fully inhibited at an osmotic potential of 50 bar, while respiratory oxygen uptake was less than 50% reduced. At the same osmotic potential, coupled oxidation of succinate and exogenous NADH by isolated leaf or tuber mitochondria suspended in media containing sorbitol or sucrose was between 20% and 60% inhibited. As in situ , oxidation was more sensitive to osmotic stress in tuber than in leaf mitochondrial preparations. Coupled oxidation of glycine or malate by leaf mitochondria was only insignificantly reduced when sorbitol was used as osmoticant, while strong inhibition was observed with sucrose or raffinose. The difference appeared to be due to penetration of sorbitol which caused less osmotic shrinkage of mitochondria thus preventing internal solutes from reaching injurious levels. Inhibition of substrate oxidation during osmotic dehydration of mitochondria decreased respiratory control and, less significantly, ADP/O ratios. In contrast to osmotic inhibition of photosynthesis in isolated chloroplasts (Kaiser et al., 1981 a), osmotic inhibition of respiratory activity was found to be reversible on transfer of mitochondria from a hypertonic to an isotonic medium. The low sensitivity of respiration to water stress in vivo compared to the higher sensitivity of photosynthesis is ascribed to different effects. In contrast to chloroplasts illuminated with high light intensities, mitochondria do not work at full capacity in situ . Inhibition of excess work capacity by water stress does not necessarily find expression in performance which reflects ATP turnover and slippage (basal electron transfer). In addition, entry of compatible neutral compounds such as monosaccharides into mitochondria may prevent excessive shrinkage during dehydration. In vitro , this reduces inhibition of respiratory reactions. Leaf mitochondria are more permeable to monosaccharides than are chloroplasts.