Experiments were carried out to estimate the elasticity coefficients and thence the distribution of control of
sucrose synthesis and photosynthate partitioning between cytosolic
fructose-1,6-bisphosphatase and
sucrose-phosphate synthase (SPS), by applying the dualmodulation method of Kacser and
Burns (1979, Biochem. Soc. Trans. 7, 1149-1161). Leaf discs of spinach (Spinacia oleracea L.) were harvested at the beginning and end of the photoperiod and illuminated at five different irradiances to alter (i) the extent of feedback inhibition and (ii) the rate of photosynthesis. The rate of CO2 fixation,
sucrose synthesis and
starch synthesis were measured and compared with the activation of SPS, and the levels of fructose-2,6-bisphosphate (Fru2,6bisP) and metabolites.
Sucrose synthesis increased progressively with increasing irradiance, accompanied by relatively large changes of SPS activity and Fru2,6bisP, and relatively small changes of metabolites. At each irradiance, leaf discs harvested at the end of the photoperiod had (compared with leaf discs harvested at the beginning of the photoperiod) a decreased rate of
sucrose synthesis, increased
starch synthesis, decreased SPS activity, increased Fru2,6bisP, a relatively small (20%) increase of most metabolites, no change of the glycerate-3-
phosphate: triose-
phosphate ratio, a small increase of NADPmalate
dehydrogenase activation, but no inhibition of photosynthesis. The changes of
sucrose and
starch synthesis were largest in low light, while the changes of SPS and Fru2,6bisP were as large, or even larger, in high light. It is discussed how these results provide evidence that the control of
sucrose synthesis is shared between SPS and
fructose-1,6-bisphosphatase, and provide information about the in-vivo response of these
enzymes to changes in the levels of their substrates and effectors. At low fluxes, feedback regulation is very effective at altering partitioning. In high light, changes of SPS activation and Fru2,6bisP can be readily overriden by increasing levels of metabolites.