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Plant Physiol, January 2000, Vol. 122, pp. 243-254
The Delivery of Salts to the Xylem. Three Types of Anion
Conductance in the Plasmalemma of the Xylem Parenchyma of Roots of
Barley1
Barbara
Köhler2 and
Klaus
Raschke*
Albrecht-von-Haller-Institut für Pflanzenwissenschaften,
Universität Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany.
To
explore possible pathways for anions to enter the xylem in the root
during the transport of salts to the shoot, we used the patch-clamp
method on protoplasts prepared from the xylem parenchyma of barley
(Hordeum vulgare L.) plants. K+ currents
were suppressed by tetraethylammonium or
N-methylglucamine in the solutions in the pipette and
the bath, and the permeating anions were Cl or
NO3. We recorded the activities of three
distinct anion conductances: (a) an inwardly rectifying anion channel
(X-IRAC), characterized by activation at hyperpolarization and open
times of up to several seconds; (b) a quickly activating anion
conductance (X-QUAC), important for anion efflux at voltages between
 50 mV and the equilibrium potential of the prevailing anion; and (c)
a slowly activating anion conductance (X-SLAC), activating above 100
mV. Both X-IRAC and X-QUAC were permeable for Cl and
NO3; X-QUAC was also permeable for malate.
The occurrence of X-IRAC became more frequent with an increase in
cytoplasmic Ca2+, while the occurrence of X-QUAC decreased.
Anion currents through X-SLAC, and particularly through X-QUAC, were
estimated to be large enough to account for reported rates of xylem
loading, which is in accordance with the notion that xylem loading is a
passive process.
1
This work was supported by grants from the
Deutsche Forschungsgemeinschaft to K.R.
2
Present address: Laboratory of Plant
Physiology and Biophysics, University of London, Wye College, Wye, Kent
TN25 5AH, UK; e-mail sbs99bk{at}wye.ac.uk.
*
Corresponding author; e-mail kraschk{at}gwdg.de; fax 49-551-397823.
© 2000 American Society of Plant Physiologists
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