Characterization of diurnal changes in activities of enzymes involved in sucrose biosynthesis

Experiments were conducted with vegetative soybean plants (Glycine max [L.] Merr., ;Ransom') to determine whether the activities in leaf extracts of key enzymes in sucrose metabolism changed during the daily light/dark cycle. The activity of sucrose-phosphate synthase (SPS) exhibited a distinct diurnal rhythm, whereas the activities of UDP-glucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, and sucrose synthase did not. The changes in extractable SPS activity were not related directly to photosynthetic rates or light/dark changes. Hence, it was postulated that the oscillations were under the control of an endogenous clock. During the light period, the activity of SPS was similar to the estimated rate of sucrose formation. In the dark, however, SPS activity declined sharply and then increased even though degradation of starch was linear. The activity of SPS always exceeded the estimated maximum rate of sucrose formation in the dark. Transfer of plants into light during the normal dark period (when SPS activity was low) resulted in increased partitioning of photosynthate into starch compared to partitioning observed during the normal light period. These results were consistent with the hypothesis that SPS activity in situ was a factor regulating the rate of sucrose synthesis and partitioning of fixed carbon between starch and sucrose in the light.