The action spectrum of far‐red light in interaction with orange light on orientation of chloroplasts in Mougeotia

summary Chloroplast rotation from profile to face position in Mougeotia can be controlled by an interaction of far‐red and orange light during dichromatic irradiation. In experiments to determine the action spectrum in the far‐red light range during dichromatic irradiation, light conditions prevented the formation of a gradient of the far‐red‐absorbing form of phytochrome in the cell. Linearly polarized monochromatic light of 620 nm vibrating parallel to the cell axis or unpolarized far‐red light (700 run to 900 nm) when given alone had no effect on chloroplast reorientation to the face position. Unpolarized far‐red light in the range from 700 nm to 775 nm was active in an interaction with 620 nm monochromatic light and had its maximum effect on face chloroplast orientation at 710 nm. Wavelengths above 775 nm were ineffective. In this kind of chloroplast photoorientation in Mougeotia , high irradiance of both kinds of light together with prolonged exposure were necessary to obtain the full response. The time‐course response showed three phases: lag, movement and steady‐state. At the saturating dose of both kinds of light there was a 10–15 min lag period between the first detectable change in chloroplast position. Then the response was linear (movement phase) until after approximately 30 min when all the chloroplasts had become reorientated. Simultaneous irradiation of Mougeotia cells with green and far‐red light did not change the maximum activity in far‐red light. The effect was similar when treatment with one kind of light e.g. orange or far‐red light, was followed by the other. The photo responses of Mougeotia chloroplasts are typical of the high irradiance reactions that control many photomorphogenetic processes.