Radicular uptake kinetics of 15NO3-, CO(15NH2)2, and 15NH4+ in whole rice plants

The mechanism of uptake of nitrate, urea, and ammonium by roots of intact plants (Oryza sativa L.) was studied using enriched nitrogen (15) N as a tracer in order to establish an absorption kinetic model and the influence of external concentration on uptake. Rice plants at 30 and 60 days after germination were submitted to different contact times with two concentrations of an external solution containing these nitrogenous sources. In the treatment with nitrate, the relative uptake rate decreased with increasing plant age, for both concentrations tested. For urea, this rate was higher in dry matter of old plants at low external concentration (1.0 mu mol g(-1)) and for ammonium a higher relative rate of uptake was found in young plants at low external concentration and at high concentration, old plants had higher relative rate of uptake, as had been seen with nitrate and urea uptake. In the experiment related to the influence of external concentration on the radicular uptake, plants were used at 30 days after germination and have shown the occurrence of a multiphasic system (4 isotherms) for the three treatments. Each isotherm is associated with the Michaelis-Menten kinetics and the transition between phases is carried out by jumps. The V-max (maximum uptake rate) and K-m (affinity of carrier to ion) values, calculated by Lineweaver-Burk and Hofstee transformations, increased with the increasing external concentration.