Exogenous 15NO3− Influx and Endogenous 14NO3− Efflux by Two Maize (Zea mays L.) Inbreds during Nitrogen Deprivation

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Abstract
The influence of nitrogen stress on net nitrate uptake resulting from concomitant 15NO3- influx and 14NO3- efflux was examined in two 12-day-old inbred lines of maize. Plants grown on 14NO3- were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate contain 98.7 atom % 15N. The nitrate concentration of the roots declined from approximately 100 to 5 micromolar per gram fresh weight during deprivation, and 14NO3- efflux was linearly related to root nitrate concentration. Influx of 15NO3- was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in 15NO3- influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied.