Abstract. Net NO3 uptake by NO−3 deficient Chara cells was used to calculate [NO−3]c assuming that the cytoplasm occupies 10% total volume and that nitrate reduction and storage are negligible (i.e. maximum [NO−3]c was calculated). A linear relationship was found between NO−3 efflux and [NO−3]c. There was an initial burst of NO−3 efflux when NH+4 was added, followed by a slower efflux rate which matched influx rate such that net NO−3 uptake was zero. Over 50% of NO−3 that had been taken up in 2 h was lost within the first 5 min of NH+4 addition. The Nernst equation was used to predict the direction of the electrochemical driving force for NO−3 entry. Under the experimental conditions used NO−3 efflux is actively transported. The differential involvement of both NO−3 influx and NO−3 efflux in the regulation of NO−3 uptake is discussed and a model is proposed to account for these results which envisages discrete NO−3 influx and NO−3 efflux carriers.
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