Effect of (+)-pulegone and other oil components of Mentha x piperita on cucumber respiration

Peppermint (Mentha x piperita L.) essential oil and main components were assessed for their ability to interfere with plant respiratory functions. Tests were conducted on both root segments and mitochondria isolated by etiolated seedlings of cucumber (Cucumis sativus L.). Total essential oil inhibited 50% of root and mitochondrial respiration (IC50) when used at 324 and 593 ppm, respectively. (+)-Pulegone was the most toxic compound, with a 0.08 and 0.12 mM IC50 for root and mitochondrial respiration, respectively. (-)-Menthone, followed (+)-pulegone in its inhibitory action (IC50 values of 1.11 and 2.30 mM for root and mitochondrial respiration respectively), whereas (-)-menthol was the less inhibitory compound (IC50 values of 1.85 and 3.80 mM respectively). A positive correlation was found for (+)-pulegone, (-)-menthone and (-)-menthol between water solubility and respiratory inhibition. The uncoupling agent, carbonyl-cyanide-m-chlorophenyl-hydrazone (CCCP), lowered (-)-menthol and (-)menthone inhibition and annulled (+)-pulegone inhibition of mitochondrial respiration, whereas salicyl-hydroxamic acid (SHAM) 2-hydroxybenzohydroxamic acid, the alternative oxidase (AO) inhibitor, increased (-)-menthone inhibition and annulled both (+)-pulegone and (-)-menthol inhibitory activity. The possible interaction of (+)-pulegone and (-)-menthol with AO and the mechanism of action of ( +)-pulegone, (-)-menthone and (-)-menthol on mitochondrial respiration are discussed.