Studies of the Regulation of Nitrate Influx by Barley Seedlings Using 13NO3−

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Abstract
Using 13NO3−, effects of various NO3− pretreatments upon NO3− influx were studied in intact roots of barley (Hordeum vulgare L. cv Klondike). Prior exposure of roots to NO3− increased NO3− influx and net NO3− uptake. This `induction' of NO3− uptake was dependent both on time and external NO3− concentration ([NO3−]). During induction influx was positively correlated with root [NO3−]. In the postinduction period, however, NO3− influx declined as root [NO3−] increased. It is suggested that induction and negative feedback regulation are independent processes: Induction appears to depend upon some critical cytoplasmic [NO3−]; removal of external NO3− caused a reduction of 13NO3− influx even though mean root [NO3−] remained high. It is proposed that cytoplasmic [NO3−] is depleted rapidly under these conditions resulting in `deinduction' of the NO3− transport system. Beyond 50 micromoles per gram [NO3−], 13NO3− influx was negatively correlated with root [NO3−]. However, it is unclear whether root [NO3−] per se or some product(s) of NO3− assimilation are responsible for the negative feedback effects.