Responses of moss protonemata to red and far-red polarized light: evidence for disc-shaped phytochrome photoreceptors.

Protonemata of Physcomitrium were grown in a liquid medium of sucrose and mineral nutrients. Under these conditions, the filaments respond phototropically most readily to red and far-red irradiation by "growing out" on the apical flank nearest the light source. This response has been examined as a function of the angle of polarization for red (662 nm) and far-red (732 nm) light given singly and simulataneously. Results indicate that both red and far-red light, striking the growing cell surface perpendicularly, are absorbed independently of their angle of polarization. However, radiation striking the flank tangentially is absorbed if the plane of polarization (vibrational plane of the electric vector) is parallel to the cell surface but not if it is vertical to the cell surface. It is concluded that the active photoreceptors are dichroic and coin- or disc-shaped. Synergistic interaction between red and far-red irradiation supports the hypothesis that the photoreceptor is phytochrome and that the light growth response is proportional to the total quanta absorbed by the pigment and/or the consequent cycling of the phytochrome between the Pr and the Pjr states. No evidence was found for a difference in receptor orientation between the red and far-red absorbing states.