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.

1990. Study on nutrient stress tolerance of nine Taraxacum microspeeies with a contrasting mineral ecology by cultivation in a low nutrient flowing solution. -Physiol. Plant. 79: 389-399.Growth and mineral status of 9 Taraxacum miicrospecies were studied under mineral stress conditions, using a flowing solution of low nutrient concentration. Relative growth rate of (whole) plant dry weight, leaf area, and (whole) plant tissue water were used to describe growth. For 4 microspeeies, specific uptake rates of NOj, H2PO4", K+, Mg-* and Ca'* were investigated. The applied nutrient condition clearly discriminated between the studied Taraxacum mierospecies. With respect to relative growth rate, 3 groups of microspeeies could be distinguished: T. nordsledtii > T. lancidens, T. adamii, T. kollandicum, T. taeniatum > T. sellandii, T. eudontum, T. ekmanii, T. ancistrolobum. These categories coincided well with the mineral ecology of the microspeeies, going from infertile tO' fertile sites. T. nordstedtii, a microspeeies of infertile sites, was most efficient in absorbing NO^, HjPOi^and K*. T. sellandii and T. eudontum, both occurring in fertile grasslands, showed poor uptake performances for all studied ions. In all Taraxacum microspeeies studied, except T. eudontum, internal N conceBtration appeared to limit growth. Efficiencies in N use, at sub-optimal internal N concentrations, varied with the mineral habitat of the microspeeies studied. T. nordstedtii, from infertile sites, and 7". sellandii, from fertile sites, were established as high and low extremes, respectively.