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.