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 (pH_o) after different NO₃^? pretreatment conditions has been investigated. Following NO₃^? pretreatment (0.2 mol m^?3 NO₃^?) there was little effect of pH_o on subsequent net NO₃^? uptake into Chara cells. After N deprivation (2 mmol m^?3 NO₃^?) there was a pronounced effect of pH_o on nitrate uptake, the maximum rate occurring at pH_o 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 pH_o, the maximum rate occurring at ～ pH_o 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 K_i= 0.7 mmol m^?3. At the optimum pH_o for NO₃^? uptake the effect of DES on membrane potential (ψ_m) 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.