Efficiency of nitrate uptake in spinach: impact of external nitrate concentration and relative growth rate on nitrate influx and efflux

Regulation of nitrate influx and efflux in spinach (Spinacia oleracea L., c v. Subito), was studied in short-term label experiments with N-13- and N-15 -nitrate. Nitrate fluxes were examined in relation to the N demand for grow th, defined as relative growth rate (RGR) times plant N concentration. Plan ts were grown at different nitrate concentrations (0.8 and 4 mM), with mine ral composition of growth and uptake solutions identical. Nitrate influx, efflux and net nitrate uptake rate (NNUR) were independent of the external nitrate concentration, despite differences in internal nitr ate concentration. At both N regimes, NNUR was adequate to meet the N deman d for growth. RGR-related signals predominantly determined the nitrate flux es. At high RGR (0.25 g g(-1) day(-1)), nitrate influx was 20 to 40% lower and nitrate efflux was 50 to 70% lower than at lower RGR (0.17 g g(-1) day( -1)); efflux:influx ratio (E:I) declined from 0.5 at low RGR to 0.2 at high er RGR. Thus, the efficiency of NNUR substantially increased with increasin g RGR. Differences in nitrate translocation between morning and afternoon c oincided with differences in nitrate efflux, which is in accordance with th e suggested regulation of nitrate efflux by the root cytoplasmic nitrate co ncentration.