INTERPRETATION OF THE ‘DICHROIC ORIENTATION’ OF PHYTOCHROME

The dichroic orientation of phytochrome observed both in the phytochrome‐mediated phototropism in Adiantum protonemata and in the phytochrome‐mediated chloroplast movement in Mougeotia were analyzed in terms of the orientation of the transition moment associated with the long‐wavelength absorption band, assuming that phytochrome, associated with the plasma membrane, rotates around the normal to the membrane. The orientation of the long‐wavelength transition moment of the phytochrome chromophore was calculated using the zero‐differential overlap approximation of the molecular orbital theory for ir‐electrons. The results indicate that the orientation of the long‐wavelength transition moment mainly changes later than 2 ms after red light excitation of P_r, and that the different dichroic orientations of P_r and P_fr can be attributed to the change in the angle of the long‐wavelength transition moment of phytochrome with the plasma membrane from 18^o to 72^o during phototransformation.