Inhibition of phloem loading by sulfite affects sugar distribution in pea leaves

Abstract

Under the influence of sulfite, changes in the rate of ¹⁴CO₂ fixation, ¹⁴C-sucrose uptake and sugar content of the apoplastic and intracellular spaces of pea leaves varied according to the time of treatment and the anion concentration used. The greatest decrease of the rate of ¹⁴CO₂ and ¹⁴C-sucrose uptake in the presence of sulfite at concentrations from 0.1 to 5.0 mM was detected immediately (i.e. 5 min.) after supplying sulfite to the medium. Two hours later, a significant inhibition of photosynthesis and sucrose uptake was observed only with 5.0 and 2.5 and 5.0 mM concentrations of sulfite, respectively. The effect of sulfite (0.1-5.0 mM) on the apoplastic and intracellular sugar content for the first 30 min of treatment was negligible. However, after longer periods under sulfite treatment at concentrations ≥ 1.0 mM, significant increases in the apoplastic sucrose content were observed. An increase in the accumulation of starch was also noted. In light of our observations it suggested that the inhibition of phloem loading by sulfite induces disturbances in sugar distribution.