Is there a third photoreceptor involved in the control of chloroplast movements in mougeotia?

The photometric method was used to test a possibility proposed recently that a new photoreceptor with maximum activity at 620 nm is involved in mediating chloroplast rotation in Mougeotia (Z Lechowski, J Bialczyk [1988] Plant Physiol 88: 189-193). The hypothesis was tested under conditions of continuous dichromatic unilateral or mutually perpendicular irradiation with red light of wavelengths 620 or 660 (680) nanometers and far-red. When the red light was polarized parallel to the long cell axis, chloroplast response could be monitored by changing the direction of far-red irradiation. The level of the response obtained with red and far-red applied from the same direction depended on far-red intensity: at higher fluence rates the maximum response was shifted to longer wavelengths of red light. A high fluence rate of far-red inhibited the response. The absorption coefficients of Mougeotia chloroplasts were measured for the studied wave-lengths using the microphotometric method. Possible impact of absorption by the chloroplast on photoreception has been discussed. Current and previous results can be interpreted in terms of phytochrome action and do not support the involvement of the hypothetical 620 nanometer photoreceptor.