Induction of ELF transmembrane potentials in relation to power-frequency electric field bioeffects in a plant root model system. I. Relationship between applied field strength and cucurbitaceous root growth rates.

Seminal roots of Cucumis sativus and Cucurbita maxima were exposed to 60 Hz electric fields of 100-500 V X m-1 in a conducting aqueous inorganic growth medium. Root growth rates were measured to produce a dose-response relationship for each species. The species were selected for study because of their familial relationship, reported sensitivity to 60 Hz, 360 V X m-1 electric fields, and differing average root cell sizes. The latter characteristic influences the magnitude of ELF membrane potentials induced by constant-strength applied electric fields, but does not affect the magnitude of the electric field strength tangent to the cell surface. The difference in average root cell size between C. sativus (smaller cells) and C. maxima (larger cells) was used to evaluate two alternate hypotheses that the observed effect on root growth is stimulated by the electric field tangent to the cell surface, or a field-induced perturbation in the normal transmembrane potential of the cells. The results of the dose-response relationship studies are qualitatively consistent with the hypothesis that the effect is elicited by induced transmembrane potentials. The smaller-celled roots showed a substantially higher response threshold [C. sativus; E0TH approximately 330 V X m-1] than did the larger-celled species [C. maxima; E0TH approximately 200 V X m-1]. At field strengths above the response thresholds in both species, the growth rate of C. sativus roots was less affected than that of C. maxima roots exposed to the same field strength.