Effects of water potentials on starch metabolism in light and sucrose metabolism in darkness were investigated with cotton plants grown in a controlled growth chamber. Four glandless cotton plants were grown in each pot for 32 days. At this time, one set of plants (CONTROL) was kept well watered by a daily supply of 250 ml water. Another set (STRESS) was subjected to a gradual dehydration until leaf water potential reached -2.8 MPa at the end of 7 days. Leaves sampled after 4-hr light exposure showed an initial decrease in starch content, which, however, subsequently accumulated during the later period of increasing water potentials. The decrease in starch content was caused by an increase in alpha-amylase activity and the accumulation of this polymer by an enhanced activity of glucan synthetase. It is well-known fact that starch degraded and translocated as sucrose during night. Leaf samplings therefore were made after 6-hr dark exposure in order to understand sucrose metabolism and its relation to starch content in darkness.

Activities of sucrose synthetase and sucrose phosphatesynthase were determined. Striking differences were observed in the responses of these enzyme activities to water potentials. Sucrose phoshate synthase activity was virtually irresponsive to changes in water potentials, which, however, drastically changed sucrose synthetase activity. The latter enzyme activity decreased (about 13% of control) sharply with a rise in water potentials up to about 2.2 MPa at the end of 5-day dehydration. This minimum enzyme activity, however, was soon recovered to about control level during the subsequent 5-day rehydration period. The role of sucrose synthetase is known to be responsible for sucrose translocation at night. This well known functional relationship seemed to be demonstrated by the facts that changes of this enzyme also were closely related to changes of both sucrose (negatively) and starch (positively) in responses to water stresses. This sucrose and starch content relationship found from the present experiments are consistent with the similar observation reported by Huber (1983) from other plant sources.