Effects of low water potentials on respiration and on glucose and acetate uptake, by Chlorella pyrenoidosa

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Abstract
Chlorella pyrenoidosa was subjected to a range of water potentials and the effects of these treatments on endogenous respiration and on the uptake and respiration of glucose and acetate were measured. For a given water potential the reductions were greatest for glucose, less for acetate, and least for endogenous respiration. At intermediate water potentials of about-10 atm, glucose respiration was depressed strongly at first, but this respiration approached control levels after two to three hours at low water potentials. The reduced respiration of substrates was caused by inhibition of glucose and acetate uptake, as demonstrated by 14C uptake experiments over short periods. These effects on uptake are attributed to low water potentials, rather than to any possible competition between the molecules of the osmotica and the substrates. Evidence for this view includes the equal inhibitions of “glucose-induced” respiration by osmotica with such diverse molecular structure as mannitol, KCl, and polyethylene glycol 1540. More conclusively, glucose itself was used as an osmotic agent and its inhibition of “glucose-induced” respiration was very similar to that by mannitol solutions of equal water potentials. Respiratory activity was much less reduced than uptake. This was demonstrated by lowering the water potential of cells which had already absorbed glucose from a control medium. The subsequent respiration was much higher than that for cells continuously exposed to low water potential. The findings are discussed in relation to the reduced transport of ions and sucrose, which is known to occur in vascular plants subjected to a water stress. The results demonstrate the advantages of using a unicellular organism in the study of metabolic effects of water deficits in plants.