Kinetics of N-15-labelled nitrate uptake by maize (Zea mays L.) root segments

Isotherms and kinetic constants of nitrate uptake by excised root segments from the apical root zone of 6-d-old maize seedlings pretreated with nitrate were investigated using N-15-labelled nitrate. The isotherms were resolved into two systems namely a multiphasic saturable system at substrate concentrations lower than 2 mol m(-3) and a linear system at higher concentrations, The detailed analysis of the multiphasic saturable system suggested the existence of at least three phases, which followed the Michaelis-Menten kinetics, The I-max and K-m of each phase increase from the lower phase to the upper phase. The distance from the root tip and the presence of stele affected considerably the linear system but only slightly the saturable system.