Limitation of Photosynthesis by Sucrose Synthesis

Abstract

During photosynthesis, triose P are exported from the chloroplast and converted to sucrose in the cytosol, releasing P_i which returns to the chloroplast to support further photosynthesis. Thus, sucrose synthesis and CO₂ fixation are mutually interdependent processes which will have to be coordinated if high rates of photosynthesis are to be maintained (1). If sucrose synthesis were to be too slow, metabolites would accumulate and CO₂ fixation would be inhibited by a depletion of P_i. On the other hand, if sucrose synthesis were to be too fast, stromal metabolites would be depleted and the regeneration of Ru1,5P₂ would be inhibited. This problem can be seen in isolated chloroplasts, where the supply of P_i outside the chloroplast has to be carefully adjusted to attain the maximum rate of photosynthesis in any given conditions. How is an analogous control achieved in vivo, and is the optimal answer always found?