Bimodal polarotropism of Vaucheria to polarized blue light: Parallel polarotropism at high fluence rate corresponds to negative phototropism

Species of 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 polar-otropic orienta tion. Such polarotropic bending was, however, not detected within 24 hr. As the fluence rate of polarized white or blue light increased, parallel orie ntation to the E-vector became dominant. Polarized red light produced exclu sively perpendicular polarotropism. This shift in pattern was much obvious in V. terrestris sensu Gotz than V. sessilis and V. dichotoma. Since the ph otoperception is restricted to the tip of the apical dome and since this re gion 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 perpen dicular (normal) and parallel polarotropism of Vaucheria, thus, correspond to positive and negative phototropism, respectively. Orientation of photore ceptor molecules is suggested to be predominantly parallel to the surface o f the apical dome.