Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system inPisumandLemnain relation to nitrogen availability and nitrogen demand

Abstract

AbstractNet nitrate uptake rates were measured and the kinetics calculated in non‐nodulatedPisum sativumL. cv. Marma andLemna gibbaL. adapted to constant relative rates of nitrate‐N additions (RA), ranging from 0.03 to 0.27 d−1forPisumand from 0.05 to 0.40 d−1forLemna, Vmaxof net nitrate uptake (measured in the range 10 to 100 mmol m−3nitrate, i.e. ‘system I’) increased with RAin the growth limiting range but decreased when RAexceeded the relative growth rate (RGR), Kmwas not significantly related to changes in RA. On the basis of previous13N‐flux experiments, it is concluded that the differences in Vmaxat growth limiting RAare attributable to differences in influx rates. Linear relationships between Vmaxand 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 & Larsson, 1989). Analysis of Vmaxfor net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower RAvalues in both species, which is largely explained by the increased relative root size at low RA. A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (RN), predicted by RA, was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m−3(RA0.03 d−1) to 44 mmol m−3(RA0.21 d−1) forPisumand from 0.2 mmol m−3(RA0.05 d−1) to 5.4 mmol m−3(RA0.03 d−1) forLemna.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.