ANALYSIS OF (NH4+)-N-13 EFFLUX IN SPRUCE ROOTS - A TEST-CASE FOR PHASE IDENTIFICATION IN COMPARTMENTAL ANALYSIS

(NH4+)-N-13-efflux analyses were conducted with roots of intact Picea glauca (Moench) Voss. seedlings at external NH4+ concentrations of 100 mu M and 1.5 mM. Three kinetically distinct phases were identified with half-lives of exchange of approximately 2 s, 30 s, and 14 min. The presumed identities of the subcellular compartments corresponding to these phases were confirmed by several techniques, including pretreatment of roots (a) at 75 degrees C or with SDS, (b) with alpha-keto-glutarate or L-methionine-DL-sulfoximine, (c) at elevated levels of Ca2+, and (d) at low pH or with Al3+ at low pH. Treatments a and b selectively influenced phase III without affecting phases I and II. Similarly, treatment c selectively perturbed phase II, and treatment d affected phases II and III. Based on these findings and the assumption of an in-series arrangement of root cell compartments, it was concluded that phase III corresponded to the cytoplasm, phase II corresponded to the Donnan free space, and phase I corresponded to a film of solution adhering to the root surface.