Compartmentation and flux characteristics of ammonium in spruce

Abstract

Using ¹³NH ⁺₄ as a tracer, compartmental analyses for NH ⁺₄ were performed in non-mycorrhizal roots of intact Picea glauca (Moench) Voss. seedlings at four different concentration regimes of external NH ⁺₄ ([NH ⁺₄ ]_o), i.e. 0, 10, 100, and 1500 μM. Three kinetically distinct compartments were identified, with half-lives of exchange of approximately 2 s, 30 s, and 14 min, assumed to represent surface adsorption, Donnan free space, and cytoplasm, respectively. No significant differences were found in half-lives of exchange with changes in [NH ⁺₄ ]_o. Influx was calculated to be 0.96 μmol·g⁻¹·h⁻¹ in N-deprived plants (measured at 10 μM [NH ⁺₄ ]_o), while under steady-state conditions it was 0.21 μmol·g⁻¹h⁻¹ at 10 μM [NH ⁺₄ ]_o, 1.96 μmol·g⁻¹h·⁻¹ at 100 μM [NH ⁺₄ ]_o, and 6.45 μmol·g⁻¹·h⁻¹ at 1.5 mM [NH ⁺₄ ]_o. Efflux measured over the same range constituted approximately 9% of influx in N-deprived plants, 10% at 10 μM, 28% at 100 μM, and 35% at 1.5 mM [NH ⁺₄ ]_o. Cytoplasmic [NH ⁺₄ ] was estimated at 6 m M in N-deprived plants, 2 mM at 10 μM [NH ⁺₄ ]_o, 14 mM at 100 μM, and 33 mM at 1.5 mM. Free-space [NH ⁺₄ ] was 84 μM, 50 μM, 700 μM, and 8 mM, respectively. In comparison with previously published data on fluxes and compartmentation of NO ⁻₃ in white-spruce seedlings, results of this study identify a pronounced physiological preference of this species for NH ⁺₄ over NO ⁻₃ as an inorganic N source in terms of uptake and intracellular accumulation. The significant ecological importance of this N-source preference is discussed.