AMMONIUM FLUXES INTO PLANT-ROOTS - ENERGETICS, KINETICS AND REGULATION

Ammonium uptake across the plasma membranes of seedling roots of intac t rice plants is thermodynamically active at low external concentratio ns, and consequently, electrogenic uniport is an unlikely mechanism fo r influx. At higher NH4+ concentrations uptake is passive and electrog enic uniport is a possibility. While passive permeation of NH3- is als o possible at high external [NH4+], influx measurements at 10 mM NH4demonstrated a pH dependence which was inconsistent with significant N H3 permeation. Kinetic studies using (NH4+)-N-13 established that infl ux at low external [H-4(+)] occurred via high affinity transport syste ms (HATS) in rice and spruce, while at higher [NH4+], influx was media ted by low affinity transport systems (LATS), that showed linear conce ntration dependence. Ammonium influx via the HATS was shown to be up-r egulated or down-regulated in response to changes of N status, whereas influx in the LATS was insensitive to N status. The identity or ident ities of the regulatory signals responsible for controlling influx are discussed.