Studies of the Uptake of Nitrate in Barley : IV. Electrophysiology

Transmembrane electrical potential differences (Δψ) of epidermal and cortical cells were measured in intact roots of barley (Hordeum vulgare L. cv Klondike). The effects of exogenous NO₃^? on Δψ (in the concentration range from 100 micromolar to 20 millimolar) were investigated to probe the mechanisms of nitrate uptake by the high-affinity (HATS) and low-affinity (LATS) transport systems for NO₃^? uptake. Both transport systems caused depolarization of Δψ, demonstrating that the LATS (like the HATS) for NO₃^? uptake is probably mediated by an electrogenic cation (H⁺?) cotransport system. Membrane depolarization by the HATS was “inducible” by NO₃^?, and saturable with respect to exogenous NO₃^?]. By contrast, depolarization by the LATS was constitutive, and first-order in response to external NO₃^?]. H⁺ fluxes, measured in 200 micromolar and in 5 millimolar Ca(NO₃)₂ solutions, failed to alkalinize external media as anticipated for a 2 H⁺:1 NO₃^? symport. However, switching from K₂SO₄ solutions (which were strongly acidifying) to KNO₃ solutions at the same K⁺ concentration caused marked reductions in H⁺ efflux. These observations are consistent with NO₃^? uptake by the HATS and the LATS via 2 H⁺:1 NO₃^? symports. These observations establish that the HATS for nitrate uptake by barley roots is essentially similar to those reported for Lemna and Zea mays by earlier workers. There are, nevertheless, distinct differences between barley and corn in their quantitative responses to external NO₃^?.