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The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H + or OH-into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion N + or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations ( H 4 + C a 2~ + Mg 2+ + K+ + Na +) and anions (NO( + CI-+ SOl-+ H2PO~) and charge balance is maintained by excretion of an amount of H + or OH which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H + and in particular OH-ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.The contemporary view is that primary active extrusion of H +, catalyzed by a membrane-located ATPase, is the major driving force for secondary transport of cations and anions across the plasma membrane. However, the fact that net OH-extrusion often occurs (since excess anion absorption commonly takes place) implies there is a yebto-be characterized OH ion efflux mechanism at the plasma membrane that is associated with anion uptake. There is, therefore, a need for future studies of the uptake mechanisms and stoichiometry of anion uptake; particularly that of NO~ which is often the predominant anion absorbed. Another related phenomenon which requires detailed study in terms of cation-anion balance is localized rhizosphere acidification which can occur in response to deficiencies of Fe and P.

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