Effect of Mentha x piperita essential oil and monoterpenes on cucumber root membrane potential

Peppermint (Mentha piperita L.) essential oil and its main components were assessed for their ability to interfere with plant plasma membrane potentia ls. Tests were conducted on root segments isolated from etiolated seedlings of cucumber (Cucumis sativus L.). Increasing the concentration of peppermi nt essential oil from 5 to 50 ppm caused a decrease in membrane potential ( V) hyperpolarization of 10 3 mV, whereas concentrations from 100 up to 900 ppm caused an increasing depolarization of Vm (from 5 to 110 mV). When test ed at 300 ppm. ( +)-menthyl acetate, (-)-limonene and 1,8-cineole did not e xert any significant effect on V-m. whereas (+)-menthofuran (73 mV), (+)-pu legone (85 mV), (+)-neomenthol (96 mV), (-)-menthol (105 mV) and (-)-mentho ne (111 mV) showed increased ability to depolarize V-m. A plot of log of oc tanol-water partition coefficient (K-ow) against their depolarizing effect showed a significant negative correlation, suggesting that among all monote rpenoids increased membrane depolarization depends on lower K-ow. However, among monoterpene ketones, alcohols and furans, increased membrane depolari zation is associated with a decline in water solubility. The possible effec t of monoterpenoids on membrane ion fluxes is also discussed, since changes in the bioelectric potential of cells imply changes in tile flux of ions a cross the plasma membrane. (C) 2001 Published by Elsevier Science Ltd.