Electric Control of Polar Growth in Roots of Allium cepa.

Abstract

Numerous experiments have been performed to determine the effects of continuous electric currents passed through growing roots immersed in various aqueous media. In practically all, the electric field was oriented at right angles to the long axis of the root. Under such conditions a minimum of two diverse stimuli, viz., gravity and applied E.M.F., operate at right angles to one another, thus causing a complicated type of variable growth response. 1 , 2

To provide simpler experimental conditions, an electric current having parallel isopotential lines and appropriate current density must be passed up or down the polar axis of the root and must coincide with the orientation of the general axis of the inherent electric field of the root as well as that of gravity.

An apparatus was designed to comply with this requirement and to maintain the roots under normal conditions of growth except for the applied current. Three sets of roots were used in each experiment: (1) control roots, (2) roots through which an electric current was passed upward, and (3) roots in which the same current was passed downward. Since individual roots vary slightly with respect to the threshold of current density for reversible inhibition of growth and also in their rates of elongation, each experiment involved 3 or more duplicate sets of roots. Preliminary experiments led to a choice of a current giving an IR drop of 40 mV/mm, with the root immersed in tap water as the growing medium and the current passed upward through 25 mm of the apical end of the growing root. This gives complete inhibition of growth. The same current passed downward gives a very slight inhibition of growth, Fig. 1, or none at all. At 37.8 mV/mm, complete but reversible growth inhibition is obtained with acdending current.