Electrical and Curvature Responses of the Avena Coleoptile to Unilateral Illumination.

Introduction One fact that is agreed upon in the literature regarding the mechanism of phototropism is that the growth curvature results from an unequal distribution of auxin. There are at least three hypotheses as to how unilateral illumination causes the unequal distribution: 1. Photoinactivation of auxin on the lighted side of the plant. 2. A differential change in the rate of synthesis of auxin on the lighted and shaded sides. 3. Unilateral displacement of auxin toward one of the sides. It is the last of these mechanisms with which this paper will be primarily concerned. If lateral transport of auxin serves as a mechanism for phototropism, it requires a transverse polarity which is induced by the tropic stimulation. This is essentially a statement of the familiar Cholodny-Went theory, which applies to other tropisms as well as phototropism. One logical possibility is that such a transverse polarity is electrical in nature. Because of this possibility, the transverse electrical changes which occur in the Avena coleoptile as a result of most of the tropic stimuli have been established. The results of these studies have been reviewed recently by SCHRANK (10). Apparently only two investigators, (OPPENOORTH, 6 SCHRANK 8), have attempted to measure the transverse electrical changes resulting from stimulation of the Avena coleoptile by unilateral illumination. OPPENOORTH'S findings (6) indicate that the lighted side of the Avena coleoptile becomes electropositive to the shaded side when the plant is unilaterally illuminated with a dosage of light which causes positive phototropic curvature (500 mCs). SCHRANK (8), on the other hand, measured the changes in the transverse electrical polarity in the apex of the Avena coleoptile resulting from continuous unilateral illumination and found that the lighted side became electronegative to the shaded side. It is the purpose of this investigation to make additional mieasurements of the electrical changes of the Avena coleoptile when it is stimulated by a dosage of light which causes positive phototropic bending and to observe the electrical changes resulting from illumination which causes negative bending. If the transverse electrical polarity indeed serves as a polar force which directs the auxin to the side of the coleoptile which becomes convex, the orientation of the light induced polarities in these two instances should be opposite.