Kinetics of NO₃^- influx in spruce

Influxes of ¹³NO₃^- across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Three kinetically distinct uptake systems for NO₃^- were identified. In seedlings not previously exposed to external NO₃^-, a single Michaelis-Menten-type constitutive high-affinity transport system (CHATS) operated in a 2.5 to 500 μM range of external NO₃^- [NO₃^-]_o. The V_max of this system was 0.1 μmol g^-1 h^-1, and the K_m was approximately 15 μM. Following exposure to NO₃^- for 3 d, this CHATS activity was increased approximately 3-fold, with no change of K_m. In addition, a NO₃^--inducible high-affinity system became apparent with a K_m of approximately 100 μM. The combined V_max for these discrete saturable components was 0.7 μmol g^-1 h^-1. In both uninduced and induced plants a linear low-affinity system, additive to CHATS and an NO₃^--inducible high-affinity system, operated at [NO₃^-]_o ≥ 1 mM. The time taken to achieve maximal rates of uptake (full induction) was 2 d from 1.5 mM [NO₃^-]_o and 3 d from 200 μM [NO₃^-]_o.