Interactions between Sucrose Synthesis and CO 2 Fixation I. Secondary Kinetics during Photosynthetic Induction are Related to a Delayed Activation of Sucrose Synthesis

Abstract

Summary The regulation of sucrose synthesis has been studied during the induction of photosynthesis in barley leaves in saturating light and CO2. Following illumination, fructose-2,6-bisphosphate increases initially, and then decreases twentyfold over the next ten minutes. Sucrose phosphate synthase activity increases gradually over ten minutes. In agreement, the rate of sucrose synthesis is very low during the initial part of the induction period. The significance of this slow activation of sucrose synthesis was investigated, by measuring chlorophyll fluorescence quenching, metabolite levels, the activation of calvin cycle enzymes, and O2 evolution. There is a delay until rapid O2 evolution is attained, reflecting the activation of the calvin cycle. The low rate of sucrose synthesis aids this activation by ensuring that almost all the newly fixed carbon is retained for autocatalytic build up of the calvin cycle intermediates. However, as phosphorylated intermediates increase, CO2 fixation becomes dependent upon sucrose synthesis. Since the latter has only been partially activated, there is a dramatic reinhibition of photosynthesis. The recovery from this inhibition requires about five minutes, and resembles the adjustment of Fru2,6P2 and SPS to their new value. The changes of photochemical quenching, non photochemical quenching, metabolite levels, and ribulose 1,5-bisphosphate carboxylase activation reveal the cytosol, stroma and thylakoids are interacting to determine the rate of photosynthesis during this second phase of the induction period.