Interaction between Sucrose Synthesis and CO 2 Fixation III. Response of biphasic induction kinetics and oscillations to manipulation of the relation between electron transport, calvin cycle, and sucrose synthesis

Abstract

This article investigates the influence of sucrose synthesis on the light response curve of photosynthesis, on induction kinetics and on photosynthetic oscillations. The occurrence of oscillations and analysis of chlorophyll fluorescence quenching at saturating light intensities shows there is surplus electron transport capacity which cannot be used due to a limitation in carbon metabolism. This limitation is increased when high fructose 2,6-bisphosphate and low sucrose phosphate synthase activity are restricting the rate of sucrose synthesis. After feeding phosphate, raised levels of metabolites allowed higher rates of sucrose synthesis even though sucrose phosphate synthase deactivated and fructose-2,6-bisphosphate rose. The maximum rate of photosynthesis is increased, the light intensities needed to saturate photosynthesis are increased and the transients and oscillations become less marked. Feeding mannose to inhibit sucrose synthesis has the opposite effect on all these parameters. Chemicals which inhibit electron transport (dichlorophenylmethyl urea), energisation and stromal alkalinisation (dinitrophenol, acetate) and ribulose-1,5-bisphosphate carboxylase (cyanide) abolished the oscillations without reducing the maximum steady state rate of photosynthesis. They also, with the exception of cyanide, decreased the initial slope of the light saturation curve. It is discussed how changes in the induction kinetics, in oscillations and in the response of the light response curve can be combined with metabolite analysis to provide a way of screening for chemicals or conditions which modify sucrose synthesis. As an example, it is shown that lithium inhibits photosynthesis via a selective restriction of sucrose synthesis, probably acting by inhibiting the cytosolic fructose-1,6-bisphosphatase.