Water stress stimulates sucrose synthesis and inhibits starch and cell-wall synthesis in tissue slices of growing potato (Solanum tuberosum L. cv. Desiree) tubers. Based on the analysis of fluxes and metabolites, Geigenberger et al. (1997, Planta 201: 502-518) proposed that water deficits up to -0.72 MPa stimulate sucrose synthesis, leading to decreased starch synthesis as a result of the resulting decline of phosphorylated metabolite levels, whereas more-severe water deficits directly inhibit the use of ADP-glucose. Potato plants with decreased expression of adenosine 5'-diphosphoglucose pyrophosphorylase (AGPase) have been used to test the prediction that the contribution of AGPase to the control of starch synthesis should decrease in severely water-stressed tuber material. Freshly cut slices from wild-type and antisense tubers were incubated at a range of mannitol concentrations (20, 300 and 500 mM) and the metabolism of [(14)C]glucose was analysed. A 86-97% reduction of AGPase activity led to a major but non-stoichiometric inhibition of starch accumulation in intact growing tubers attached to the plant (40-85%), and an inhibition of starch synthesis in non-stressed tuber slices incubated in 20 mM mannitol (60-80%). The inhibition of starch synthesis was accompanied by a 2- to 8-fold increase in the levels of sugars in intact tubers and a 2- to 3-fold stimulation of sucrose synthesis in tuber slices, whereas respiration and cell-wall synthesis were not significantly affected. The strong impact of AGPase on carbon partitioning in non-stressed tubers and tuber slices was retained in slices subjected to moderate water deficit (300 mM mannitol, corresponding to -0.72 MPa). In discs incubated in 500 mM mannitol (corresponding to -1.2 MPa) this response was modified. A 80-97% reduction of AGPase resulted in only a 0-40% inhibition of starch synthesis. Further, the water stress-induced stimulation of sucrose synthesis was abolished in the transformants. The results provide direct evidence that the contribution of AGPase to the control of starch synthesis can be modified by environmental factors, leading to a lower degree of control during severe water deficits. There was also a dramatic decrease in the labelling of cell-wall components in wild-type tuber slices incubated with 300 or 500 mM mannitol. The water stress-induced inhibition of cell-wall synthesis occurred independently of AGPase expression and the accompanying changes in starch and sucrose metabolism, indicating a direct inhibition of cell-wall synthesis in response to water stress.