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

The mechanism of uptake of nitrate, urea, and ammonium by roots of intact p lants (Oryza sativa L.) was studied using enriched nitrogen (15) N as a tra cer 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 t hese nitrogenous sources. In the treatment with nitrate, the relative uptak e 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 o f uptake was found in young plants at low external concentration and at hig h 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 s hown the occurrence of a multiphasic system (4 isotherms) for the three tre atments. 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 Lin eweaver-Burk and Hofstee transformations, increased with the increasing ext ernal concentration.