Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on the transmembrane potential difference

Abstract

Maurousset, L. and Bonoemain, J. L. 1990. Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on the transmembrane potential difference. -Physiol. Plant. 80: 233-237.Sodium sulfite (Na2SO3) decreased uptake from 1 mAf sucrose by the parenchyma and hy the veins of leaves of broadbean {Viciafaba L. cv. Aguadulce). The decrease depended on the concentration of the pollutant and the duration of pretreatment. The inhibition was non-competitive. Sulfite affected the transmembrane potential difference (PD) of the leaf tissues. The short-term response obeyed an 'all or nothing' law. At 0.1 mM and above, sulfite led to a quick depolarization of one-third of the initial potential after a lag phase of about 5 min; for concentrations lower than 0.1 mM, sulfite did not affect the potential. By contrast, the long-term effect of NajSOj on the transmembrane PD strongly depended on its concentration. After 2-12 h of pretreatmemt there was no effect at 10 ixJW, a weak effect at 0.1 mM, and then increasing depolarization as the pollutant concentration increased. The inhibitory effect of Na2SO, on sucrose uptake is thus, at least partly, due to its effect on a component ofthe proton-motive force, AV'. However, the lack of correlation noticed with 0.1 mM Na,SO3 between the effect on sucrose uptake and the long-term effect on transmembrane PD suggests numerous sites of sulfite action.

Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on respiration, photosynthesis and energy charge in the leaf tissues

“…In Vicia faba, inhibition of photosynthesis was greater than that of sucrose uptake by leaf discs (Maurousset and Bonnemain 1990, Fig. 1).…”

“…Then Oj evolution was measured under light (14 W m~'), i.e. in the same conditions as for uptake experiments (Maurousset and Bonnemain 1990).…”

“…Tbe pollutant depolarizes the trans-It appears of interest to study how sulfite can affect membrane potential difference of leaf tissues of Vicia the traesmembrane potential difference. Three effects faba (Maurousset and Bonnemain 1990). After a 6-h are possible: (1) direct inhibition of the plasma menipretreatment (long term effect), the effect was non-brane H+-ATPase, (2) increase of the plasma membrane significant in the presence of 0.1 mM NajSOj, but it permeability to H"^ and/or (3) inhibition of the processes became more and more noticeable as the pollutant con-supplying ATP.…”

Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on respiration, photosynthesis and energy charge in the leaf tissues

“…loading occurs from the apoplast (symport H*-sucrose; Introduction Delrot 1981, Delrot andBonnemain 1981, Bourquin et In broad-bean (Maurousset and Bonnemain 1990) as in al. 1990), it is obvious that the dissipation of the transother species (Noyes 1980, Teh andSwanson 1982, Lo-membrane potential difference due to sulfite (dissiparenc-Plucinska andZiegler 1987a,b, 1988), SO2 and its tion dependent on sulfite concentration), is an imporaqueous derivatives, HSOj and SO3", inhibit the trans-tant parameter that could at least partly explain the location of photosynthetie products, particularly inhibition of phloem loading.…”