Interactions between Sucrose Synthesis and CO 2 Fixation IV. Temperature-dependent adjustment of the relation between sucrose synthesis and CO 2 fixation

Abstract

The influence of temperature on the relation between sucrose synthesis and the CO2 fixation has been investigated. Barley leaves were illuminated in a leaf oxygen electrode at temperatures of 8–35 °C, using conditions of saturating CO2 to accentuate the potential limitation due to low rates of sucrose synthesis. Measurements of the light response curve of O2 evolution, oscillations, chlorophyll fluorescence quenching, and the sensitivity to exogenously supplied phosphate and lithium show that sucrose synthesis becomes progressively less limiting as the temperature is increased. In agreement, there are lower levels of metabolites at high temperatures, including hexose phosphate, triose phosphate, 3-phosphoglycerate and ribulose 1,5-bisphosphate. This temperature-dependent shift in the relation between sucrose synthesis and CO2 assimilation was not due to a direct effect of temperature on the concentration of fructose 2,6bisphosphate or the activation of sucrose phosphate synthase. These change in parallel with OZ evolution as the light is increased at 15 °C or at 30 °C. Instead, studies with partially purified enzymes show that sucrose phosphate synthase has a relatively high Q10, while the cytosolic fructose- 1,6-bisphosphatase becomes less sensitive to inhibition by fructose 2,6-bisphosphate and adenosine-5′-monophosphate when the temperature is increased. Simulation of the response of the cytosolic fructose- 1,6-bisphosphatase, and measurements of the metabolite levels maintained at the CO2 compensation point show that the «threshold» for activating sucrose synthesis will be decreased as the temperature increases. This high «threshold» will favour Calvin cycle turnover at low rates of photosynthesis by allowing higher substrate concentrations to be maintained. However, it leads to a lower rate of photosynthesis in high light and CO2 because sucrose synthesis is too slow.