The Regulation of Sucrose Synthesis in Leaves

Abstract

Photosynthesis uses the energy of light to produce triosephosphates (trioseP) from carbon dioxide, water and phosphate (Fig. l). In a typical leaf, e.g. from spinach, this trioseP is mainly utilized to synthesize sucrose for transport to other parts of the plant and for storage in the vacuole, and to lesser extent, to synthesize starch for storage in the chloroplasts. As CO2 fixation is a cyclic process, 5/6 of the trioseP formed during CO2 fixation have to remain in the cycle for the regeneration of the CO2 acceptor ribulose 1.5 bisphosphate and only the reminder of 1/6 is available for utilization (Fig. 2). Any withdrawal of trioseP beyond this limit would result in a collapse of the CO2 fixation cycle. On the other hand, CO2 fixation is depending on the recycling of Pi released by sucrose and starch synthesis. To ensure a high rate of photosynthesis it seems therefore of vital importance that sucrose synthesis is adjusted in such a way that the withdrawal of trioseP does not exceed the limit set by the CO2 fixation cycle, and that from this surplus as much as possible is utilized. The mechanism of such a source control of sucrose synthesis will be shown in the following. Furthermore, it will be shown how the synthesis of sucrose might be adjusted to the sink to prevent osmotic damage by excessive accumulation of sucrose in a leaf cell.