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Articles

Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ³⁶ClO₃^–1

II. Regulation of NO₃^– Efflux by NH₄⁺

Botany Department, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B1

The influence of NH₄⁺, in the external medium, on fluxes of NO₃^– and K⁺ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH₄⁺ was without effect on NO₃^– (³⁶ClO₃^–) influx whereas inhibition of net uptake appeared to be a function of previous NO₃^– provision. Plants grown at 10 micromolar NO₃^– were sensitive to external NH₄⁺ 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 NH₄⁺ 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 ³⁶Rb⁺-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.

¹ Financial support from the Natural Sciences and Engineering Research Council of Canada is acknowledged.

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