Effects of canopy defoliation in the dark on the activity of sucrose phosphate synthase

Short-term experiments were conducted with vegetative soybean plants (Glycine max L. Merrill, cv. 'Ransom') to determine whether source-sink manipulations in the dark affected the apparent rate of starch mobilization and the activities of several enzymes involved in sucrose metabolism. When 'demand' for sucrose from a particular source leaf was increased in the dark by defoliation of all other source leaves, leaf sucrose-content was reduced initially, but the rate of starch mobilization was unaffected relative to intact controls. Defoliation altered the magnitude of the diurnal rhythm in sucrose phosphate synthase (SPS) activity such that the activity of SPS was markedly greater in defoliated plants relative to the controls. In contrast defoliation had no effect on the activities of sucrose synthase, cytosolic fructose bisphosphatase, and UDP-glucose pyrophosphorylase. The rate of sucrose formation in the dark was apparently limited by the rate of starch mobilization, and could not be increased by short-term changes in the demand for sucrose.