Dynamics of Osmotic Adjustment in Rice1

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Abstract
The dynamics of osmotic adjustment in response to the development of plant water deficits were studied in the leaves of several upland rice cultivars. As deficits developed, the osmotic potential of leaves of plants not previously stressed decreased more rapidly than expected if due only to dehydration, with the result that the loss of turgor was delayed. Under rapid soil drying conditions (4 to 6 days), osmotic potential declined nearly linearly with water potential as water potential declined to about −12 bars, but the rate was not sufficient to totally maintain turgor. Under slow soil drying conditions (20 to 22 days), the reduction of osmotic potential, after an initially more rapid decline which was sufficient to totally maintain turgor, was similar to that found under the rapid drying conditions. The final extent of active reduction of osmotic potential was identical under both rapid and slow drying conditions and resulted in minimum “fully hydrated” osmotic potentials of approximately −18 bars. Plants exposed to more than one cycle of stress and 2 days of recovery showed no active osmotic adjustment beyond this minimum during subsequent stress. Thus there was a limit to the extent of osmotic adjustment which was independent of the rate of stress development and stress duration. The actual extent of adjustment depended on stress history and stress intensity.