XVI International Botanical Congess

Light oriented movement of the Mougeotia chloroplast from profile to face position is controlled by phytochrome and at short wavelengths independently by UV-BL-photoreceptor(s). Polarized BL is most effective if the E-vector vibrates parallel to the cell axis, but UV-A with the E-Vector perpendicular. Microbeam experiments confirm the inverse action dichroism for UV-A and BL. The data indicate flavin photoreceptors in the ectoplasm with orientation of the transition moments for BL preferentially parallel and for UV-A normal to the cell surface. Chloroplast movement can also be induced by UV-B, showing a similar action dichroism as in UV-A. At high fluence rates of UV-B, chloroplasts reorient from face position to profile position (high intensity movement) in order to avoid photodamage. The responsiveness to red light decreases after extended exposure to UV-B.