ANTAGONISM BETWEEN ALUMINUM AND CALCIUM FOR SORPTION BY CALCIUM PECTATE

Aluminium (Al) binding in the apoplasm of the cells of plant roots has been implicated in Al toxicity, but little is known of the reactions between Al and components of the apoplasm. Because of its role in determining root cation-exchange capacity, calcium (Ca) pectate in the root cell wall has been considered especially important in binding Al. Synthetic Ca pectate was prepared at pH 5.4 and 4.0 (separate experiments), and reacted with solutions containing Ca (0.05 to 15 mM) and Al (0 to 200 mu M). A sorption reaction was proposed to explain the preferential binding of Al over Ca by Ca pectate at both pH 5.4 and 4.0. An increase in Al bound by Ca pectate reduced Ca sorption and vice versa, the Ca present in the supernatant increasing by 1.65 +/- 0.15 nmol for each 1 nmol Al sorbed. Further, the volume of solution entrained by the Ca pectate gel decreased with increased Ca and Al concentrations in solution. The reduced pore size suggested by this finding may have important implications for nutrient uptake and cell elongation.