Calcium Activation of Orthophosphate Absorption by Barley Roots

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Abstract
The mode of Caactiva-tion of phosphate absorption by excised barley roots was considered by kinetic analysis of measurements of steady-state absorption. Roots were obtained from etiolated 5-day-old plants which were grown in a dilute calcium sulfate solution. Phosphate absorption appeared to be mediated by a least 2 concentration-dependent rate-limiting steps. Ca acted as an activator of phosphate absorption mediated through a reaction which had the larger Michaelis constant. The activation was expressed primarily as an increase in the maximum absorption rate of the reaction, with a lesser influence on the Michaelis constant. It was suggested that the increase in phosphate absorption induced by Ca was associated with an increased turnover number rather than a change in the concentration of a rate-limiting intermediate.