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PLANT PHYSIOLOGY , Vol 104, Issue 3 899-906, Copyright © 1994 by American Society of Plant Biologists
Ammonium Uptake by Rice Roots (III. Electrophysiology)
M. Y. Wang, ADM. Glass, J. E. Shaff and L. V. Kochian
Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (M.Y.W., A.D.M.G.)
The transmembrane electrical potential differences ([delta][psi]) were
measured in epidermal and cortical cells of intact roots of 3-week-old rice
(Oryza sativa L. cv M202) seedlings grown in 2 or 100 [mu]M NH4+ (G2 or
G100 plants, respectively). In modified Johnson's nutrient solution
containing no nitrogen, [delta][psi] was in the range of -120 to -140 mV.
Introducing NH4+ to the bathing medium caused a rapid depolarization. At
the steady state, average [delta][psi] of G2 and G100 plants were -116 and
-89 mV, respectively. This depolarization exhibited a biphasic response to
external NH4+ concentration similar to that reported for 13NH4+ influx
isotherms (M.Y. Wang, M.Y. Siddiqi, T.J. Ruth, A.D.M. Glass [1993] Plant
Physiol 103: 1259-1267). Plots of membrane depolarization versus 13NH4+
influx were also biphasic, indicating distinct coupling processes for the
two transport systems, with a breakpoint between two concentration ranges
around 1 mM NH4+. The extent of depolarization was also influenced by
nitrogen status, which was larger for G2 plants than for G100 plants.
Depolarization of [delta][psi] due to NH4+ uptake was eliminated by a
protonophore (carboxylcyanide-m-chlorophenylhydrazone), inhibitors of ATP
synthesis (sodium cyanide plus salicylhydroxamic acid), or an ATPase
inhibitor (diethylstilbestrol). The results of these observations are
discussed in the context of the mechanisms of NH4+ uptake by high- and
low-affinity transport systems operating across the plasma membranes of
root cells.
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