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A Kinetic Analysis of Phosphate Absorption by Excised Roots of Millet, Barley, and Alfalfa¹

ABSTRACT

A kinetic analysis was made of P absorption at steady state by excised roots of millet, barley, and alfalfa. Short-time absorption studies indicated that the rate of P absorption is greater for millet than barley and greater for barley than alfalfa. The specific reaction rate constants for the metabolic turnover of the two carriers of P are of the same order of magnitude for the three plant species. They are approximately 1 x 10^-2 moles of P per mole of combined carrier per second. The apparent dissociation constant for carrier "a" is approximately 9 x 10^-4 for all three plant species, but the value for carrier "b" varies from 3 x 10^-6 for millet and alfalfa to 8 x 10^-6 for barley. The major difference between species is in concentration of carrier. These values are 2 x 10^-7, 3 x 10^-8, and 1 x 10^-8 moles per g. of millet, barley, and alfalfa roots, respectively.

¹ Contribution from the Eastern Soil and Water Management Research Branch, SWCRD, ARS, USDA, Beltsville, Md. Presented before Div. II, Soil Science Society of America, Aug. 5, 1958, at Lafayette, Ind.

² Soil Scientists.

Received for publication July 30, 1959. Accepted for publication August 11, 1959.