Electrical Impedance Analysis of Tissue Properties Associated with Ethylene Induction by Electric Currents in Cucumber (Cucumis sativus L.) Fruit

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Abstract
A study based on electrical impedance analysis of tissue properties was conducted in order to understand the relationship between impedance components and ethylene biosynthesis induced by direct current in cucumber (Cucumis sativus L.) fruit. Impedances were measured at a range of alternating current frequencies from 500 Hz to 1 MHz. We calculated capacitances representing the plasma membranes, C1, and organelle membranes, C2, and resistances representing extracellular space, R1, cytoplasm, R2, and organelle interior, R3. Direct current of 1 to 3 mA induced ethylene synthesis with a sharp peak at 1 h. The rate of production was greater with a stronger current. This abrupt induction of ethylene synthesis was accompanied by an equally abrupt activation of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase within 1 h, but not that of ACC oxidase, which was activated only at the later stages of the treatment at a time when ethylene production and ACC synthase activity were declining. Using direct current of 0 to 3 mA, C2, R1, and R2 increased abruptly, and C1 increased gradually after 3 h. The rates of increases were greater with currents of larger magnitude, R3 was not affected during passage of the current. Diazocyclopentadiene, an inhibitor of ethylene action, eliminated the direct current induction of R1 but had no effect on the increases in C2 and R2. Diazocyclopentadiene counteracted the stimulative effects of exogenously applied ethylene with respect to respiration and activities of ACC oxidase and phenylalanine ammonia-lyase. These results indicate that an externally applied current may generate signal(s) by altering the functions of organelle membranes and/or cytoplasmic pH to induce ACC synthase.