NITRATE INDUCTION IN SPRUCE - AN APPROACH USING COMPARTMENTAL ANALYSIS

Using (NO3-)-N-13 -efflux analysis, the induction of nitrate uptake by externally supplied nitrate was monitored in roots of intact Picea glauca (Moench) Voss. seedlings over a 5-d period. In agreement with our earlier studies, efflux analysis revealed three compartments, which have been identified as surface adsorption, apparent free space, and cytoplasm. While induction of nitrate uptake was pronounced, NO3- fluxes in induced plants were decidedly lower and the induction response was slower than in other species. Influx rose from 0.1 mu mol . g(-1). h(-1) (measured at 100 mu M [NO3-](o)) in uninduced plants to a maximum of 0.5 mu mol . g(-1). h(-1) after 3d of exposure to 100 mu M [NO3-]o and declined to 0.3-0.4 mu mol . g(-1). h(-1) at the end of the 5-d period. Efflux remained relatively constant around 0.02-0.04 mu mol . g(-1). h(-1), but its percentage with respect to influx declined from initially high values (around 30%) to steady-state values of 4-7%. Cytoplasmic [NO3-] ranged from the low micromolar in uninduced plants to a maximum of 2 mM in plants fully induced at 100 mu M [NO3-](o). In-vivo root nitrate reductase activity (NRA) was measured over the same time period, and was found to follow a similar pattern of induction as influx. The maximum response in NRA slightly preceded that of influx. It increased from 25 nmol . g(-1). h(-1) without prior exposure to NO3- to peak values around 150 nmol . g(-1). h(-1) after 2 d of exposure to 100 mu M [NO3-](o). Subsequently, NRA declined by about 50%. The dynamics of flux partitioning to reduction, to the vacuole, the xylem, and to efflux during the induction process are discussed.