COMPARTMENTATION AND FLUX CHARACTERISTICS OF AMMONIUM IN SPRUCE

Using (NH4+)-N-13 as a tracer, compartmental analyses for NH4+ were pe rformed in non-mycorrhizal roots of intact Picea glauca (Moench) Voss. seedlings at four different concentration regimes of external NH4+ ([ NH4+](o)), i.e. 0, 10, 100, and 1500 mu M. Three kinetically distinct compartments were identified, with half-lives of exchange of approxima tely 2s, 30s, and 14 min, assumed to represent surface adsorption, Don nan free space, and cytoplasm, respectively. No significant difference s were found in half-lives of exchange with changes in [NH4+](o). Infl ux was calculated to be 0.96 mu mol . g(-1). h(-1) in N-deprived plant s (measured at 10 mu M [NH4+](o)), while under steady-state conditions it was 0.21 mu mol . g(-1). h(-1) at 10 mu M [NH4+](o), 1.96 mu mol . g(-1). h(-1) at 100 mu M [NH4+](o), and 6.45 mu mol . g(-1). h(-1) at 1.5 mM [NH4+](o). Efflux measured over the same range constituted app roximately 9% of influx in the N-deprived plants, 10% at 10 mu M 28% a t 100 mu M, and 355 at 1.5 mM [NH4+](o). Cytoplasmic [NH4+] was estima ted at 6mM in N-deprived plants, 2mM at 10 mu M [NH4+](o), 14mM at 100 mu M, and 33mM at 1.5mM. Free-space [NH4+] was 84 mu M, 50 mu M, 700 mu M, and 8mM, respectively. In comparison with previously published d ata on fluxes and compartmentation of NO3- in white-spruce seedlings, results of this study identify a pronounced physiological preference o f this species for NH4+ over NO3- as an inorganic N source in terms of uptake and intracellular accumulation. The significant ecological imp ortance of this N-source preference is discussed.