Estimation of cytoplasmic nitrate and its electrochemical potential in barley roots using 13NO3− and compartmental analysis
Author for correspondence: Raymond J. Ritchie Tel: +61 2 9351 4475 Fax: +61 2 9351 4119 Email: email@example.com
- • 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 (NH4NO3-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+/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 mol−1).