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

Abstract

• 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 m-3 NO3- (NO3--grown plants) or 1 mol m-3 NH4NO3 (NH 4NO3-grown plants). • The cytoplasmic concentrations of NO3- in roots were only approx. 3-6 mol m-3 (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 mol-1 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 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). © The Authors (2006).