Perturbation of Chara Plasmalemma Transport Function by 2[4(2',4'-Dichlorophenoxy)phenoxy]propionic Acid

Abstract

Electrophysiological measurements on internodal cells of Chara corallina Klein ex Willd., em. R.D.W. revealed that in the presence of (2-[4-(2',4'-dichlorophenoxy)phenoxy]propionic acid) (diclofop) the membrane potential was very sensitive to the pH of the bathing medium. At pH 5.7, 100 micromolar diclofop caused a slow reduction in the electrogenic component of the membrane potential to the value of -123 +/- 5 millivolts. Membrane resistance initially decreased, recovered transiently, then stabilized at approximately 65% of the control value. At pH 7.0, the potential appeared to plateau around -200 millivolts before rapidly declining to -140 +/- 4 millivolts; removal of diclofop resulted in recovery of the electrogenic component. Diclofop reduced cytoplasmic ATP levels by 96.4% and 36.6% at pH 5.7 and 7.0, respectively. At pH 8.2, diclofop did not change the ATP concentration significantly, but induced a hyperpolarization of the membrane potential to near -250 millivolts, and also reduced or inhibited the dark-induced hyperpolarization; the light-induced depolarization was reduced to a lesser extent. DCMU applied in the light elicited the same response at the plasmalemma as placing cells in the dark. When K(+) channels were opened and cells depolarized with 10 millimolar K(+), diclofop induced a further depolarization of approximately 30 millivolts. Cells decoupled with HPO(4) (-2) were still sensitive to diclofop. Currents associated with OH(-) efflux and HCO(3) (-) influx, as measured with a vibrating probe technique, became spatially destabilized and reduced in magnitude in the presence of diclofop. After 60 minutes, most of the cell surface was engaged in a low level of OH(-) efflux activity. The results indicate that diclofop may be a proton ionophore at pH 7.0 and 5.7. At pH 8.2, diclofop may inhibit the operation of the H(+)-ATPase and OH(-) efflux systems associated with HCO(3) (-) transport by perturbing the control processes that integrate the two, without a reduction in ATP concentration.