being used to denote an ideal solvent), R is the universal gas constant, T the absolute temperature, fj the number of moles of species j, V,, the partial molal volume of the solvent (water), and n,, is the number of moles of water.

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A comparison was made of the xerotolerant yeast Saccharomyces rouxii and its non-tolerant counterpart S. cerevisiae when fully adapted to growth media of high and low water activity (u,), and during the period of adaptation from a high u, to a medium containing NaCl (lo%, w/v). Comparisons of the fully adapted yeasts were confined to levels of activity of phosphofructokinase and sn-glycerol-3-phosphate dehydrogenase. The results confirmed our previous findings obtained with different assay procedures that growth at diminished a, dramatically increased the level of glycerol phosphate dehydrogenase in S. cerevisiue but had little or no effect in that respect on S. rouxii. Simultaneously phosphofructokinase activity was about doubled in S. cerevisiue and about halved in S. rouxii. Differences between the two yeasts were even more conspicuous during the period of response to salt stress. Succhurumyces rouxii apparently adapted fully to the salt broth within 6 h (at 30 "C) inasmuch as viability was unaffected by the transfer and glycerol content reached its maximum by then. On the other hand, S. cerevisiae took about 140 h to adapt by a process that could be resolved into two stages. Stage 1 was marked by a catastrophic drop in apparent viability and widely different counts on three plating media. Stage 2 was identified by similar counts on all three plating media and a progressive increase in viability up to the level of the original inoculum. Glycerol phosphate dehydrogenase activity increased during this stage but glycerol accumulation began in both species immediately after transfer. Throughout the entire adaptation period, S. cerevisiae consumed glucose much more slowly than did S. rouxii. The complexity of the response of S. cerevisiue to salt stress has focused attention on this species as a reference for studying yeast water relations in general.

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