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

Authors


Author for correspondence: Raymond J. Ritchie Tel: +61 2 9351 4475 Fax: +61 2 9351 4119 Email: rrit3143@usyd.edu.au

Summary

  • • 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).

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