Calcium stimulation of phosphate absorption by annual legumes

Abstract

At calcium and phosphate concentrations within ranges commonly encountered in soil solutions, increasing calcium concentration in flowing culture solutions markedly increased phosphate absorption by several annual legumes. Calcium increased phosphate uptake relatively more at low than at high phosphate concentration. The effect of calcium was greater for two Medicago species than for two cultivars of Trifolium subterraneum and is suggested as at least a partial explanation for the greater sensitivity of Medicago to soil acidity and its greater tolerance to soil alkalinity than Trifolium subterraneum. Pretreatment at different calcium levels had no effect on phosphate absorption, while transfer to solutions of different calcium levels caused an immediate response in phosphate uptake. These results indicate that calcium had a direct effect on phosphate absorption, rather than an indirect effect through root morphology or anatomy. It is suggested that calcium increased phosphate absorption by screening electronegative charges on the roots, thus increasing the accessibility of absorption sites to phosphate.