KINETICS OF NH4+ INFLUX IN SPRUCE

influxes of (NH4+)-N-13 across the root plasmalemma were measured in i ntact seedlings of Picea glauca (Moench) Voss. Two kinetically distinc t uptake systems for NH4+ were identified. In N-deprived plants, a Mic haelis-Menten-type high-affinity transport system (HATS) operated in a 2.5 to 350 mu M range of external NH4+ concentration ([NH4+](o)). The V-max of this HATS was 1.9 to 2.4 mu mol g(-1) h(-1), and the K-m was 20 to 40 mu M. At [NH4+](o) from 500 mu M to 50 mM, a linear low-affi nity system (LATS) was apparent. Both HATS and LATS were constitutive. A time-dependence study of NH4+ influx in previously N-deprived seedl ings revealed a small transient increase of NH4+ influx after 24 h of exposure to 100 mu M [NH4+](o). This was followed by a decline of infl ux to a steady-state value after 4 d. In seedlings exposed to 100 mu M external NO3- concentration for 3 d, the V-max for NH4+ uptake by HAT S was increased approximately 30% compared to that found in N-deprived seedlings, whereas LATS was down-regulated. The present study defines the much higher uptake capacity for NH4+ than for NO3- in seedlings o f this species.