Estimation of cytoplasmic nitrate and its electrochemical potential in barley roots using 13 NO 3 − and compartmental analysis

Summary•  13 NO 3 − was used to determine the intracellular compartmentation of NO 3 − in barley roots ( Hordeum vulgare cv. Klondike), followed by a thermodynamic analysis of nitrate transport. •  Plants were grown in one‐tenth Johnson's medium with 1 mol m −3 NO 3 − (NO 3 − ‐grown plants) or 1 mol m −3 NH 4 NO 3 (NH 4 NO 3 ‐grown plants). •  The cytoplasmic concentrations of NO 3 − in roots were only approx. 3–6 mol m −3 (half‐time for exchange approx. 21 s) in both NO 3 − and NH 4 NO 3 plants. These pool sizes are consistent with published nitrate microelectrode data, but not with previous compartmental analyses. •  The electrochemical potential gradient for nitrate across the plasmalemma was +26 ± 1 kJ mol −1 in both NO 3 − ‐ and NH 4 NO 3 ‐grown plants, indicating active uptake of nitrate. At an external pH of 6, the plasmalemma electrochemical potential for protons would be approx. −29 ± 4 kJ mol −1 . If the cytoplasmic pH was 7.3 ± 0.2, then 2H + /1NO 3 − cotransport, or a primary ATP‐driven pump (2NO 3 − /1ATP), are both thermodynamically possible. NO 3 − is also actively transported across the tonoplast (approx. +6 to 7 kJ mol −1 ).