A kinetic study of phosphate absorption by excised roots of Stylosanthes humilis, Phaseolus lathyroides, Desmodium uncinatum, Medicago sativa and Hordeum vulgare

Abstract

A kinetic analysis was made of phosphorus absorption by excised roots of Stylosanthes humilis, Phaseolus lathyroides, Desmodium uncinatum, Medicago sativa (lucerne), and Hordeum vulgare (barley). The mechanism of phosphorus uptake by the first four species was similar to that already described for barley by Hagen and Hopkins, in that two first order simultaneous reactions (designated a and b) would account for total phosphorus uptake. The phosphorus absorption rate of S. humilis was much greater than that of the other four species at all phosphorus substrate concentrations used. At low phosphate substrate concentrations (1 x 10^-6M KH₂PO₄), absorption rates were in the order: barley > P. lathyroides > D. uncinatum > lucerne; at high concentrations (2 x 10^-4M KH₂PO₄) they were nearly equal. Furthermore, the relative proportion of the total uptake contributed by the b reaction, i.e. associated with HPO₄^2-, was greater for S. humilis at all substrate concentrations used than for the other species. These differences can be explained in part by certain biological constants. The outstanding difference was that S. humilis had a much higher rate constant for both reactions concerned with phosphate uptake, i.e. k_3a and k_3b, than the other species. In addition the b carrier concentration, i.e. Σ Rb, was higher than for the other species, with the exception of barley, and furthermore the ratio of Σ R_a to Σ R_b was very narrow for S. humilis, being 1.61 for S. humilis and 3.97, 7.31, 8.04, and 6.04 for the other species respectively. Similarly the apparent dissociation constant for the b reaction in S. humilis, K_mb, was high, and the ratio of K_ma to K_mb was narrow, relative to the values for the other species. Physical data on the roots, such as damp to dry weight ratios, average length, and number per unit weight, were determined and used in conjunction with the biological constants in an attempt to interpret the phosphate uptake results.