The effect of sulphite on the regulation of photosynthetic sucrose synthesis in poplar leaves

Sulphite at concentrations from 0.05 to 5.0 mM was supplied to illuminated, detached poplar (Populus deltoides Bartr. ex Marsh) leaves via the transpiration stream, The rate of CO2 fixation and partitioning of newly fixed carbon between sucrose and starch were measured and compared with the contents of selected phosphorylated intermediates, the contents of fructose-2,6-bisphosphate (Fru2,6BP) and the activation of sucrose-phosphate synthase (SPS), Supplying leaves with less than or equal to 0.5 mM sulphite led to an increase in the sucrose/starch partitioning ratio without altering the rate of (CO2)-C-14 fixation, The increase in sucrose synthesis compared to starch synthesis was accompanied by relatively small changes of 3-phosphoglyceric acid (PGA), fructose-1,6-bisphosphate (Fru1,6BP), hexose phosphates (hexose-P), uridine 5'-diphosphoglucose (UDPGlc), an accumulation of triose phosphates (triose-P), an activation of SPS, and decreased Fru2,6BP contents, Supplying leaves with greater than or equal to 1.0 mM sulphite decreased (CO2)-C-14 assimilation and increased partitioning of fixed carbon into starch, A selective inhibition of sucrose synthesis was accompanied by an accumulation of triose-P, Fru1,6BP, hexose-P, and a decrease of PGA contents, There was also a large increase of Fru2,6BP contents and a decline in the activation of SPS. It could be argued that sulphite affects the allocation of photosynthetic carbon to sucrose and that sulphite can inhibit photosynthesis via a selective inhibition of sucrose synthesis.