UPTAKE OF NITRATE BY MUTANTS OF ARABIDOPSIS-THALIANA, DISTURBED IN UPTAKE OR REDUCTION OF NITRATE .2. KINETICS

The uptake of nitrate by wildtype plants and chlorate‐resistant mutants of Arabidopsis thaliana (L.) Heynh. was studied by intermittent or continuous measurement of the nitrate concentration of the ambient solution. The uptake rate in the wildtype and the nitrate reductase‐less mutant B 25 showed a dual‐phase relation ship with concentration. Each phase showed Michaelis‐Menten kinetics although multiphasic patterns within each phase could not be excluded. A dual‐phase relationship was also found in the uptake mutant B I. Here, however, phase II did not follow Michaelis‐Menten kinetics and uptake rate of nitrate in the phase II concentration range was considerably lower in the B 1 mutant than in the wildtype. It is concluded that the mutation in B I has disturbed phase II of the nitrate uptake, without affecting phase I, which leads to the suggestion that uptake of nitrate in Arabidopsis is mediated by at least two independent uptake mechanisms. The nitrate uptake rate showed an optimum at pH 8, and it was not stimulated by the presence of calcium. Ammonium had different effects on nitrate uptake: a direct effect, when it was present during the uptake of nitrate, resulting in a release of nitrate and a reduced rate of uptake, and an indirect inhibitory effect, possibly caused by assimilation products of ammonium, which is most pronounced after growth on ammonium as the sole nitrogen source or in long‐lasting uptake experiments in the presence of ammonium. Chlorate also showed a multiple effect, an inhibiting one which proved to be competitive and, at very low concentrations of chlorate, a stimulating one. Evidence was obtained that chlorate and nitrate arc taken up by the same carrier. Copyright © 1979, Wiley Blackwell. All rights reserved