The aim of this work was to investigate whether sucrose synthesis in the cytosol of leaf cells is regulated in response to the supply of energy and organic carbon from the chloroplast. Fluxes into sucrose and metabolite levels in wheat (Triticum aestivum var Timmo) leaf protoplasts were compared in a range of light intensities and CO(2) concentrations, showing that sucrose-phosphate synthase and the cytosolic fructose-1,6-bisphosphatase are inhibited in situ when the supply of trioseP from the chloroplasts decreases. Such a regulation might aid CO(2) fixation in limiting conditions by permitting stromal metabolites to be maintained at higher levels than would otherwise be possible.