Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system in<i>Pisum</i>and<i>Lemna</i>in relation to nitrogen availability and nitrogen demand

Abstract

Abstract Net nitrate uptake rates were measured and the kinetics calculated in non‐nodulated Pisum sativum L. cv. Marma and Lemna gibba L. adapted to constant relative rates of nitrate‐N additions (R A ), ranging from 0.03 to 0.27 d −1 for Pisum and from 0.05 to 0.40 d −1 for Lemna , V max of net nitrate uptake (measured in the range 10 to 100 mmol m −3 nitrate, i.e. ‘system I’) increased with R A in the growth limiting range but decreased when R A exceeded the relative growth rate (RGR), K m was not significantly related to changes in R A . On the basis of previous 13 N‐flux experiments, it is concluded that the differences in V max at growth limiting R A are attributable to differences in influx rates. Linear relationships between V max and tissue nitrogen concentrations were obtained in the growth limiting range for both species, and extrapolated intercepts relate well with the previously defined minimal nitrogen concentrations for plant growth (Oscarson, Ingemarsson &amp; Larsson, 1989). Analysis of V max for net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower R A values in both species, which is largely explained by the increased relative root size at low R A . A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (R N ), predicted by R A , was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m −3 (R A 0.03 d −1 ) to 44 mmol m −3 (R A 0.21 d −1 ) for Pisum and from 0.2 mmol m −3 (R A 0.05 d −1 ) to 5.4 mmol m −3 (R A 0.03 d −1 ) for Lemna. It is suggested that this value can be used as a unifying measure of the affinity for nitrate, integrating the performance of the nitrate uptake system with nitrate flux and long term growth and demand for nitrogen.