A comparison of regulatory effects of chloride on nitrate uptake, and of nitrate on chloride uptake into Pisum sativum seedlings

Abstract

Net nitrate uptake, 36ClO−3/NO−3 influx and 36Cl− influx into Pisum sativum L. cv. Feltham First seedlings have been examined following growth in culture medium containing different combinations of chloride and nitrate. When young (6 days old) seedlings, that had been grown in the absence of N were used, nitrate accumulation stimulated net nitrate uptake and 36ClO−3/NO−3 influx (r2= 0.99) while chloride accumulation inhibited nitrate uptake and 36ClO−3/NO−3 influx (r2= 0.65). When nitrate was provided during growth there was no effect of chloride pretreatment on net nitrate uptake and there was little effect of total [NO−3+ Cl−]i on 36ClO−3/NO−3 influx (r2= 0.26). A direct effect of Cl− on 36ClO−3/NO−3 influx was only found when seedlings had been starved of N for more prolonged periods (14 days). When moderate chloride was supplied during growth, 36Cl− influx was insensitive to nitrate or chloride accumulated, but significantly correlated with loge [NO−3+ Cl−]i (r2= 0.75). When trace amounts of Cl− were supplied during growth 36Cl− influx was inhibited by (a) NO−3 in the external medium and (b) Cl− pretreatment, but was insensitive to NO−3 pretreatment. The sensitivity of 36Cl− influx to external nitrate was not found following Cl− pretreatment in the absence of nitrate. The possibility that there are two populations of chloride carriers which differ in their sensitivity to external nitrate is discussed. Tentative schematic models to account for the regulation of nitrate and chloride uptake are proposed in the context of current hypotheses for regulation of ion transport and control systems theory.