ISSN:
1432-1424
Keywords:
stomatal guard cell
;
(difference-) current-voltage relation
;
conductance-voltage relation
;
H+ pump
;
kinetic carrier model
;
K+ transport
;
Vicia
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary The steady-state, current-voltage (I–V) characteristics of stomatal guard cells fromVicia faba L. were explored by voltage clamp using conventional electrophysiological techniques, but with double-barrelled microelectrodes containing 50mm K+-acetate. Attention was focused, primarily, on guard cell response to metabolic blockade. Exposures to 0.3–1.0mm NaCN and 0.4mm salicylhydroxamic acid (SHAM) lead consistently to depolarizing (positive-going) shifts in guard cell potentials (V m ), as large as +103 mV, which were generally complete within 60–90 sec (mean response half-time, 10.3±1.7 sec); values forV m in NaCN plus SHAM were close or positive to −100 mV and well removed from the K+ equilibrium potential. Guard cell ATP content, which was followed in parallel experiments, showed a mean half-time for decay of 10.8±1.9 ([ATP] t=0, 1.32±0.28mm; [ATP] t=60−180sec, 0.29±0.40mm). In respiring cells, theI–V relations were commonly sigmoid aboutV m or gently concave to the voltage axis positive toV m . Inward- and outward-rectifying currents were also observed, especially near the voltage extremes (nominally −350 and +50 mV). Short-circuit currents (atV=0 mV) were typically about 200–500 mA m−2. The principal effect of cyanide early on was to linearize theI–V characteristic while shifting it to the right along the voltage axis, to decrease the membrane conductance, and to reduce the short-circuit current by approx. 50–75%. The resulting difference-current-voltage (dI–V) curves (±cyanide) showed a marked sensitivity to voltages negative from −100 mV and, when clamp scans had been extended sufficiently, they revealed a distinct minimum near −300 mV before rising at still more negative potentials. The difference currents, along with changes in guard cell potential, conductance and ATP content are interpreted in context of a primary, ATP-consuming ion pump. FittingdI–V curves to reaction kinetic model for the pump [Hansen, U.-P., et al. (1981)J. Membrane Biol. 63:165; Blatt, M.R. (1986)J. Membrane Biol. 92:91] implicates a stoichiometry of one (+) charge transported outward for each ATP hydrolyzed, with pump currents as high as 200 mA m−2 at the free-running potential. The analysis indicates that the pump can comprise more than half of the total membrane conductance and argues against modulations of pump activity alone, as an effective means to controlling K+ transport for stomatal movements.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01871188
Permalink
