Estimation of cytoplasmic nitrate and its electrochemical potential in barley roots using 13NO3 and compartmental analysis.

13NO3 was used to determine the intracellular compartmentation of NO3 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 m3 NO3 (NO3-grown plants) or 1 mol m3 NH4NO3 (NH4NO3-grown plants). The cytoplasmic concentrations of NO3 in roots were only approx. 3-6 mol m3 (half-time for exchange approx. 21 s) in both NO3 and NH4NO3 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 mol1 in both NO3- and NH4NO3-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 mol1. If the cytoplasmic pH was 7.3 +/- 0.2, then 2H+/1NO3 cotransport, or a primary ATP-driven pump (2NO3/1ATP), are both thermodynamically possible. NO3 is also actively transported across the tonoplast (approx. +6 to 7 kJ mol1).