Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ³⁶ClO₃⁻

Abstract

The influence of NH4', in the external medium, on fluxes of NO3-and K were investigated using barley (Hordeum vulgare cv Betzes) plants.NWV was without effect on NO3-(3'10C3-) influx whereas inhibition of net uptake appeared to be a function of previous N03-provision. Plants grown at 10 micromolar NO3-were sensitive to external NHW when uptake was measured in 100 micromolar NO3-. By contrast, NO3-uptake (from 100 micromolar NO3-) by plants previously grown at this concentration was not reduced by NI-14 treatment. Plants pretreated for 2 days with 5 millimolar NO3-showed net efflux of NO3-when roots were transferred to 100 micromolar NO3-. This efflux was stimulated in the presence of NH4'. NH4' also stimulated NO3-efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO3-occur via effects upon efflux.By contrast to the situation for NO3-, net K' uptake and influx of 'Rb+-labeled K' was inhibited by NH4' regardless of the nutrient history of the plants. Inhibition of net K' uptake reached its maximum value within 2 minutes of NH4' addition. It is concluded that the latter ion exerts a direct effect upon K' influx.In barley, Lewis et al. (15), for example, showed that NH4' inhibited NO3-accumulation in the root and shoot and NO3-translocation to shoots although NO3-uptake was not measured directly. Recently, MacKown et al. (17,18) have shown that NH14' inhibits NO3-uptake and NO3-reduction in corn plants.Their results suggest that almost all the effect of NH4' on uptake of N03-is via its effect on the reduction step. There have also been reports of stimulated NO3-uptake arising as a consequence of NH4' pretreatment as in the case of seedlings of wheat (21) and corn (18).The almost universal choice of relatively long experimental periods (usually 2 to 24 h) for the investigation ofeffects of NH4+ upon nitrate uptake has made it extremely difficult to distinguish between effects on nitrate uptake per se and subsequent effects upon the properties (selectivity, turnover, etc.) of the nitrate transporter or effects on nitrate reduction and translocation (e.g. 17, 18). Both inhibition (9, 23, 24, 26) and stimulation (21) of in vitro nitrate reductase activity, following NH4' pretreatment, have been documented. Other workers using corn have shown that NH4+ inhibits NO3-reduction in vivo (18). However, the possible direct or indirect effects of NH4+ on the turnover of the nitrate transport proteins (presuming they are in fact proteins) will remain an open question until such proteins can be biochemically characterized.The potential importance of NH4, in the regulation of N03-metabolism has been widely recognized and explored in numerous studies of nitrate assimilation in micro-organisms and higher plants (e.g. 2-4, 12, 14). Considering the intermediary role of NH4+ in nitrate utilization, the former ion must represent a likely candidate for feedback effects on uptake and/or reduction of nitrate. Both short term (direct) effects of NH4' on...

Measurement of Net Fluxes of Ammonium and Nitrate at the Surface of Barley Roots Using Ion-Selective Microelectrodes

“…Net flux of NO3-remained relatively unchanged when Prato barley seedlings were supplied both NO3-and NH4+ at 200 ,uM. This is in contrast to the reports in the literature of significant reductions in NO3-net flux in the presence of NH4+ (5,7,13).…”

“…9A) Many reports have appeared in the literature concerning the influence of ammonium on nitrate uptake in barley (3,5,7,13), maize (14,16,23), and tomato (24). Deane-Drummond and Glass (5) have suggested that the influence of ammonium on nitrate net uptake depends on the amount of nitrate previously available. Using 36C103-as an analog for NO3-, nitrate influx from a solution of 100 Mm NO3-was not affected by the presence of 100 to 500 Mm NH4+ ifthe seedlings had been previously grown at that nitrate concentration.…”

Ammonium Uptake by Rice Roots (I. Fluxes and Subcellular Distribution of 13NH4+)

“…N efflux (either NH4+ or NO3-) has been reported to be quite significant, particularly at elevated concentrations of N (Morgan et al, 1973;Breteler and Nissen, 1982). Indeed, Deane-Drummond and Glass (1983aGlass ( , 1983b suggested that nitrate efflux might regulate net uptake by means of a type of pump and leak mechanism. By contrast, Lee and colleagues have emphasized the importance of influx in the regulation of net uptake of nitrate, although nitrate efflux was equivalent to almost 40% of nitrate influx in barley roots (Lee and Clarkson, 1986;Lee and Drew, 1986).…”

Studies of the Regulation of Nitrate Influx by Barley Seedlings Using 13NO3−

“…According to Deane-Drummond and Glass (6,7) and Glass (10), who used 36C103-and '3NO3-, respectively, to examine NO3-influx after various NO3-pretreatments, influx was independent of prior exposure to NO3-. By contrast, Lee and Drew (17) showed that withholding NO3-for 3 d increased Vma for NO3-influx.…”