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 dete rmining root cation-exchange capacity, calcium (Ca) pectate in the roo t cell wall has been considered especially important in binding Al. Sy nthetic Ca pectate was prepared at pH 5.4 and 4.0 (separate experiment s), 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 prefere ntial binding of Al over Ca by Ca pectate at both pH 5.4 and 4.0. An i ncrease 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 t he Ca pectate gel decreased with increased Ca and Al concentrations in solution. The reduced pore size suggested by this finding may have im portant implications for nutrient uptake and cell elongation.