Mechanism of nitrate uptake into Chara corallina cells: lack of evidence for obligatory coupling to proton pump and a new NO3−/NO3− exchange model
Abstract. Nitrate uptake into Chara corallina cells at different external pH (pHo) after different NO3− pretreatment conditions has been investigated. Following NO3− pretreatment (0.2 mol m−3 NO3−) there was little effect of pHo on subsequent net NO3− uptake into Chara cells. After N deprivation (2 mmol m−3 NO3−) there was a pronounced effect of pHo on nitrate uptake, the maximum rate occurring at pHo 4.7. There was no consistent relationship between OH− efflux and NO3− uptake in short term experiments (< 1 h). NO3− efflux was also sensitive to pHo, the maximum rate occurring at ∼ pHo 5.0. An inhibitor of the proton pump, DES, immediately stimulated NO3− uptake into cells pretreated with NO3− and prevented the time-dependent decrease in NO3−, uptake into Chara cells that had been previously treated with low N (2 mmol m−3 NO3−). NO3− efflux was found to be very sensitive to DES with Ki= 0.7 mmol m−3. At the optimum pHo for NO3− uptake the effect of DES on membrane potential (ψm) were slight, and only apparent after 30 min. The results are interpreted in context of current models relating NO3− uptake and H+ pump activity. A new model for NO3− uptake is proposed which involves NO3−/NO3− exchange at steady state.