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

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 × 10 ‐2 moles of P per mole of combined carrier per second. The apparent dissociation constant for carrier “a” is approximately 9 × 10 ‐4 for all three plant species, but the value for carrier “b” varies from 3 × 10 ‐6 for millet and alfalfa to 8 × 10 ‐6 for barley. The major difference between species is in concentration of carrier. These values are 2 × 10 ‐7 , 3 × 10 ‐8 , and 1 × 10 ‐8 moles per g. of millet, barley, and alfalfa roots, respectively.