KINETICS OF NO3- INFLUX IN SPRUCE

Influxes of (NO3-)-N-13 across the root plasmalemma were measured in i ntact seedlings of Picea glauca (Moench) Voss. Three kinetically disti nct uptake systems for NO3- were identified. In seedlings not previous ly exposed to external NO,-, a single Michaelis-Menten-type constituti ve high-affinity transport system (CHATS) operated in a 2.5 to 500 mu M range of external NO3- [NO3-](o). The V-max of this system was 0.1 m u mol g(-1) h(-1), and the K-m was approximately 15 mu M. Following ex posure to NO3- for 3 d, this CHATS activity was increased approximatel y 3-fold, with no change of K-m. In addition, a NO3--inducible high-af finity system became apparent with a K-m of approximately 100 mu M. Th e combined V-max for these discrete saturable components was 0.7 mu mo l g(-1) h(-1). In both uninduced and induced plants a linear low-affin ity system, additive to CHATS and an NO3--inducible high-affinity syst em, operated at [NO3-](o) greater than or equal to 1 mM. The time take n to achieve maximal rates of uptake (full induction)was 2 d from 1.5 mM [NO3-](o) and 3 d from 200 mu M [NO3-](o).