Phytochrome Conversion by Ultraviolet Light

Light absorbed primarily by the protein of phytochrome is active in transforming both the red and far‐red absorbing forms. Pr and Pfr. The ratio of quantum yields for the conversions of Pr and Pfr by u.v. radiation (φr/φfr)u.v.= 1.5 and does not differ significantly from the ratio obtained with red and far‐red light absorbed directly by the chromophores (φr/φfr)vvis. Thus, the efficiency of energy transfer from protein to chromophore is essentially the same for both forms of the chromoprotein. The ratio of the relative quantum yields for u.v. and visible light (φr)u.v./(φr)vis was 0.32 indicating that 30–35 per cent of the light energy absorbed by the protein was transferred to the chromophore.