Effects of air pollutants on proton and sucrose transport at the plasma membrane of Ricinus communis

The effects of the air pollutants O₃, SO₂ and NO₂ on aspects of sucrose/proton cotransport across the plasma membrane of Ricinus communis plants have been investigated. The H⁺-ATPase hydrolytic activity in cotyledon plasma membrane vesicles purified by phase partitioning showed small stimulations by Na₂SO₃ or NaNO₃ added separately or together to the assay medium. ATPase activity from plants pretreated by fumigation with SO₂ or O₃ also showed an increase, the effect of O₃ being quite marked. Plasma membrane H⁺-pumping in KI-treated microsomal fractions and medium acidification by intact cotyledons both showed small decreases in the presence of Na₂SO₃ or NaNO₂. Both Na₂SO₃ and NaNO₂ at high concentrations (2 mol m^–3) had significant effects on sucrose uptake by intact cotyledons, although sucrose efflux was unaffected. No significant effects on sucrose uptake or efflux by intact cotyledons were observed in plants pretreated by fumigation with SO₂ or O₃. Proton-coupled sucrose transport in isolated plasma membrane vesicles was inhibited in the presence of Na₂SO₃ or NaNO₂. However, both pollutants also significantly inhibited the uptake of acetate by the vesicles, indicating a dissipation of the pH gradient across the membrane. It was concluded that no specific aspect of the sucrose/proton cotransport mechanism was damaged by these air pollutants, and that the effects of these pollutants on carbohydrate partitioning are more likely to be due to general effects on membrane integrity or on other aspects such as leaf carbohydrate metabolism.