LIGAND EFFECTS ON ALUMINUM SORPTION BY CALCIUM PECTATE

Although soluble aluminium (Al) has long been recognised as an importa nt limitation to plant growth on acid soils, the biochemical basis of Al toxicity has not been elucidated. Aluminium accumulation in the cel l wall may be important, especially the reaction of Al with calcium (C a) pectate. A study was conducted to investigate the effects of six li gands, citrate, malate, galacturonate, fluoride, sulfate and chloride, on the sorption of Al by Ca pectate prepared from two sources of pect in that differed in degree of methyl esterification (DE). The sorption of Al by Ca pectate increased linearly with increase in Al added from 25 to 100 mu M (or 50 to 200 mu M in the case of Al-2(SO4)(3)) There was a significant reduction in Al sorption in the presence of those li gands that form strong complexes with Al, especially citrate and, to a lesser extent, malate and fluoride. There was little difference in Al sorption by Ca pectate prepared from pectin of differing DE. Calcium in the supernatant solution increased Linearly by 1.5 nmol for each 1 nmol increase in Al sorbed. The results support the hypothesis that st rong complexes of Al with organic and inorganic ligands reduce Al sorp tion by Ca pectate in the cell wall.