Bimodal Polarotropism of Vaucheria to Polarized Blue Light: Parallel Polarotropism at High Fluence Rate Corresponds to Negative Phototropism

Vaucheria

(Xanthophyceae) exhibited cruciform polarotropism when they were grown under polarized white or blue light for several days. The coexistence of two groups of branches growing perpendicular and parallel to the electric vector (E-vector) resulted in cruciform polarotropic orientation. Such polarotropic bending was, however, not detected within 24 hr. As the fluence rate of polarized white or blue light increased, parallel orientation to the E-vector became dominant. Polarized red light produced exclusively perpendicular polarotropism. This shift in pattern was much obvious in V. terrestris sensu Götz than V. sessllis and V. dichotoma. Since the photoperception is restricted to the tip of the apical dome and since this region receives maximum photons when the E-vector is at a right angle to the cell axis, Vaucheria becomes oriented normal to the E-vector as far as the fluence rate is optimum. The direction of growth is expected to change into parallel to the E-vector when the fluence rate is supraoptimum. The perpendicular (normal) and parallel polarotropism of Vaucheria, thus, correspond to positive and negative phototropism, respectively. Orientation of photoreceptor molecules is suggested to be predominantly parallel to the surface of the apical dome.