Sulfite at concentrations from 0.5 to 5.0 mM was supplied to illuminated, detached poplar (Populus deltoides Bartr. ex Marsh) leaves via the transpiration stream. Concentrations of chlorophyll a and b, carotenoids, glucose, fructose, sucrose and starch, and extractable specific activities of sucrose synthase (SuSy), acid invertase (AI) and neutral invertase (NI) were measured. In sink and source leaves chlorophylls and carotenoids appeared to be increased by sulfite at concentrations ranging from 0.5 to 5.0 mM. In source leaves the application of 5.0 mM and ≥ 2.5 mM sulfite led to a decrease in total nonstructural soluble sugars (sTNC) and starch, respectively. The specific activity of SuSy was enhanced by ≥ 0.5 mM sulfite. There was only a slight decline in the activity of Al and N1.In sink leaves sTNC decreased and starch increased under the influence of sulfite at concentrations ≤ 1.O mM and ≥ 0.5 mM, respectively. The effect of sulfite on the activity of At, NI and SuSy was negligible. However, in both development groups of leaves alterations in sucrose, glucose and fructose contents were observed. On the basis cf the obtained results we postulate that sulfite changes the sucrolytic activities primarily in source leaves and only marginally in sink leaves

Sulphite at concentrations from 0.5 to 5.0 mM was supplied to illuminated, detached poplar (Populus deltoides Bartr. ex Marsh) leaves via the transpiration stream. Chlorophyll a fluorescence parameters, the contents of fructose-2,6-bisphosphate (Fru2,6BP) and starch, and extractable specific activity of sucrose-phosphate synthase (SPS), sucrose synthase (SuSy), acid invertase (AI), neutral invertase (NI), ATP-dependent fructose-6-phosphate 1-phosphotransferase (PFK) and pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase (PFP) were measured. Chlorophyll fluorescence parameters appeared to be unaffected by sulphite. Application of > 1.0 mM sulphite led to an increase in the content of Fru2,6BP and starch. There was also a decline in the activity of SPS, NI and PFK. On the other hand, the influence of sulphite on the activity of AI and PFP was negligible. Specific activity of SuSy was inhibited by 1.0 and 2.5 mM but activated by 5.0 mM of sulphite. On the basis of the results obtained in the present study, we postulate that sulphite at concentrations > 1.0 mM inhibits primarily sucrose synthesis, favours starch accumulation and has an indirect effect on the sucrolytic activities in poplar leaves.List of abbreviations: AI, acid invertase; Fv/Fm, m a x i m u m photochemical efficiency of PS II; Fv/Fo, efficiency of the photochemistry of PS II; F r u l , 6 B P a s e , f r u c t o s e -1 , 6 -b i s p h o s p h a t a s e ; Fru2,6BP, fructose-2,6-bisphosphate; LPI, leaf plastochron index; NI, neutral invertase; PEG, polyethylene glycol; PFK, ATP-dependent fructose-6-phosphate 1-phosphotransferase; PFP, pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase; PVP, polyvinylpyrrolidone; SPS, sucrose-phosphate synthase; SuSy, sucrose synthase.