Plant root model cell systems have provided insight into the biophysical mechanism by which extremely low frequency electric fields (EF; f ≤ 100 Hz) affect nonexcitable eukaryotic cells. The evidence indicates that the plasma membrane is the site of interaction with applied extremely low frequency EF, and that cells respond to field exposure via a sensing mechanism involving the induction of extremely low frequency membrane potentials (<tex-math>$V_{{\rm m}}^{{\rm i}}$</tex-math>). We suggest a mechanism by which <tex-math>$V_{{\rm m}}^{{\rm i}}$</tex-math> may be transduced into EF-induced root growth inhibition. Suspensions of excised Zea root tips were used to test the hypothesis that growth-inhibiting extremely low frequency EF exposures inhibit net H+ excretion from protoplasts, a process mediated by a plasma membrane H+- ATPase which is intimately involved in cellular extension. Rates of acidification of root tip suspensions were measured as an analog for net H+ efflux. The experimental results support this hypothesis. At the apparent threshold for inhibition of H+ excretion, the associated 60-Hz EF strength was about <tex-math>$220\ V\cdot {\rm m}^{-1}$</tex-math> (root mean square). Estimates of <tex-math>$V_{{\rm m}}^{{\rm i}}$</tex-math> associated with inhibition of net H+ excretion are in agreement with those known to affect Na+/ K+ transport in human erythrocytes.

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