The Effect of Reduced Water Potential on Soybean Mitochondria

The respiration of excised hypocotyls and of isolated hypocotyl mitochondria from soybean [Glycine max (L.) Merr., var. Wayne] was determined in various concentrations of sucrose and potassium chloride. Hypocotyl oxygen uptake declined with increasing solute concentration; no specific effects of either solute were apparent. Mitochondrial state III respiration was strongly inhibited as the solute concentrations were raised and there was in addition a specific inhibitory effect of the salt. State IV respiration, however, was unaffected by the presence of osmoticum. ADP/O ratios were also unaffected, except at high potassium chloride concentrations (470 mm). The primary effect of solutes was thus to limit the rate of substrate oxidation.Hydrostatic pressure did not reverse the decline in net phosphorylation accompanying reduced oxidation. It was inferred therefore that the inhibition was not due to lower water potential per se, but rather to some other effect of water or solute concentration.The effect of solutes on a mitochondrial enzyme, malate dehydrogenase, was also examined. Sucrose inhibited malate oxidation by both the mitochondria and the isolated enzyme in parallel, while potassium chloride was more inhibitory on the isolated enzyme. It was concluded that although the addition of solute lowers the water potential, the primary effects are exerted through specific effects of the solute on enzyme activity.