Unidirectional fluxes of nitrate into and out of maize roots: measurement and regulation by prior nitrate nutrition

Abstract

A method for rapid assay of concurrent fluxes of nitrate into and out of intact plant roots has been developed. Maize ( Zea mays L.) seedlings were grown on ‘normal’ 14N-nitrate prior to exposure for 5 min to 99 at.% 15N-nitrate. Influx and efflux rates were calculated from the depletion of nitrate and its 15N enrichment in the external solution. Once developed, the assay was used to examine the regulation of nitrate fluxes by prior nitrate nutrition. Maize seedlings were pretreated for 24 h with 0, 20 or 200 μM 14N-nitrate, providing root nitrate concentrations of 5.0, 8.2 and 30.7 μmol/g fw, respectively. Nitrate influx rates increased 55% when the root nitrate concentration rose from 5.0 to 8.2 μmol/g fw, but remained constant when root nitrate increased to 30.7 μmol/g fw. By contrast, efflux of nitrate continued to accelerate as root nitrate concentrations increased. As a consequence of these differential responses, net nitrate uptake was 26–30% higher at a root nitrate concentration of 8.2 μmol/g fw than at the lower and higher concentrations.