THE INFLUENCE OF PHOSPHATE CONCENTRATION ON THE KINETICS OF UPTAKE BY MAIZE ROOTS

In an experiment with native maize roots depending on different P concentration in the external solution (0.001-50 mM P), the multiphasic character of the kinetics of phosphate uptake has been stated. The single phases are characterized by the different values of Km and Vmax. In the wide range of concentrations the isotherm of phosphate uptake has 5 evident phases. The character of kinetics for the uptake of phosphate is analogus to the kinetics of the enzymatic reactions described by the Michaelis-Menten equation. On the other hand the linear dependence for the inactivated root was determined, i.e., the uptake of phosphate vs. different P concentration in the external solution. The graphic representation of the logarithmic values for the P taken up vs. the different P concentration in the external solution gives the biphasic course including concentration < 1.0 mM P and > 1.0 mM P. Within the framework of the concentration range the following values of Vmax, Km and .phi.in [influx of ions] were calculated under the conditions if the concentration of P is < 1.0 mM P: Vmax = 1.705 .mu.mol P .times. g-1 h-1, Km = 0.057 mM P and .phi.in = 0.83, i.e., if the concentration of P is > 1.0 mM P: Vmax = 40 .mu.mol P .times. g-1 h-1, Km = 16.66 mM and .phi.in = 20. According to these results, the phosphate concentration in the external solution influences the activity of the transport mechanisms concerning their conformative changes which discretely change their working regime of membrane transport. This is also demonstrated in the change of values Vmax, Km and .phi.in.