Nitrate Inhibition of Chloride Influx in Barley: Implications for a Proposed Chloride Homeostat

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Abstract
Net accumulation of Cl− by intact barley plants was virtually eliminated in roots and reduced by 40% in shoots when external media (0.5 mol m−3 CaSO4 plus 0–5 mol m−3 KCI) were supplemented with 0.25 mol m− Ca(NO3)2. Plasmalemma 36Cl− influx (Φoc) was shown to be insensitive to external NO3- in plants which had previously been grown in solutions lacking −3, but Φoc became sensitive to NO3-after a lag period of 3–6 h. Kinetic analyses revealed that the inhibition of 36C1− influx by external NO3- was complex. At 0.25 mol m−3 NO3- the Vmax for Cl− influx was reduced by greater than 50%, with insignificant effects upon Km. At 0.5 mol m−3 NO3- there was no further effect upon Vmax but Km for influx increased from 38±5 mmol m−3 to 116±26 mmol m−3. By contrast, Cl− efflux was found to be insensitive to external NO3-. A model for the regulation of Cl− influx is proposed which involves both negative feedback effects from vacuolar NO3- +Cl−) concentration and (external) NO3- inhibition of Cl− influx at the plasmalemma. These combined effects serve to discriminate against Cl− accumulation, favouring NO3- accumulation, when the latter ion is available. Such observations are inconsistent with recent proposals for the existence of bona fide homeostats for chloride accumulation in higher plants.