LONG-TERM EFFECTS OF SO2 ON PLANTS, SO2 METABOLISM AND REGULATION OF INTRACELLULAR PH

The impact of SO2 on the ionic balance of plants and its implications for intracellular pH regulation was studied to find explanations for long-term effects of SO2. When sulphur, taken up as SO2 by the shoots of plants, is not assimilated in organic compounds, but stored as sulphate, an equivalent amount of H+ is produced. These H+ ions are not buffered chemically, but removed by metabolic processes. On the basis of knowledge on metabolic buffering mechanisms a conceptual model is proposed for the removal of shoot-generated H+ by (i) OH- ions, produced in the leaves when sulphate and nitrate are assimilated in organic compounds and/or by (ii) OH- ions produced by decarboxylation of organic anions (a biochemical pH stat mechanism). The form in which nitrogen is supplied largely determines the potential of the plant to neutralize H+ in the leaves during SO2 uptake by the proposed mechanisms. In field experiments with N2 fixing Vicia faba L. crops, the increase of sulphate in the shoots of SO2-exposed plants was equivalent in charge to the decrease of organic anion content, calculated as the difference between inorganic cation content (C) and inorganic anion content (A), indicating that H+ ions produced in the leaves following SO2 uptake were partly removed by OH- from sulphate reduction and partly by decarboxylation of organic anions. The appearance of chronic SO2 injury (leaf damage) in the field experiment at the end of the growing period is discussed in relation to the impact of SO2 on the processes involved in regulation of intracellular pH. It is proposed that the metabolic buffering capacity of leaf cells is related to the rates of sulphate and nitrate reduction and the import rate of organic anions, rather than to the organic anion content in the vacuoles of the leaf cells.