, Volume 196, Issue 4, pp 674-682

Compartmentation and flux characteristics of nitrate in spruce

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The radiotracer13N was used to undertake compartmental analyses for NO 3 in intact non-mycorrhizal roots ofPicea glauca (Moench) Voss. seedlings. Three compartments were defined, with half-lives of exchange of 2.5 s, 20 s, and 7 min. These were identified as representing surface adsorption, apparent free space, and cytoplasm, respectively. Influx, efflux, and net flux as well as cytoplasmic and apparent-free-space nitrate concentrations were estimated for three different concentration regimes of external nitrate. After exposure to external NO 3 for 3 d, influx was calculated to be 0.09 μmol·g−1·h−1 (at 10 μM [NO 3 ]o), 0.5μmol·g−1·h−1 (at 100 μM [NO inf3 sup− ]o), and 1.2 μmol · g−1· h−1 (at 1.5 mM [NO 3 ]o). Efflux increased with increasing [NO 3 ]o, constituting 4% of influx at 10 μM, 6% at 100 μM, and 21% at 1.5 mM. Cytoplasmic [NO 3 ] was estimated to be 0.3 mM at 10 uM [NO 3 ]o, 2mM at 100 μM [NO 3 ]o, and 4mM at 1.5 mM [NO 3 ]o, while free-space [NO 3 ] was 16 μM, 173 μM, and 2.2 mM, respectively. A series of experiments was carried out to confirm the identity of the compartments resolved by efflux analysis. Pretreatment at high temperature or application of 2-chloro-ethanol, sodium dodecyl sulphate or hydrogen peroxide made it possible to distinguish the metabolic (cytoplasmic) phase from the remaining two (physical) phases. Likewise, varying [Pi] of the medium altered efflux and thereby [NO 3 ]cyt, but did not affect [NO 3 ]free space.

The research was supported by a Natural Sciences and Engineering Research Council, Canada, grant to Dr. A.D.M. Glass and by a University of British Columbia Graduate Fellowship to Herbert J. Kronzucker. Our thanks go to Dr. M. Adam and Mr. P. Culbert at the particle accelerator facility TRIUMF on the University of British Columbia Campus for providing13NO 3 , Drs. R.D. Guy and S. Silim for providing plant material, and Dr. M.Y. Wang, Mr. J. Mehroke and Mr. P. Poon for assistance in experiments and for helpful discussions.