Phytochrome is the bluish green pigment which is responsible for flowering. It has a light absorbing portion called chromophore attached to small proteins. Phytochrome occurs in two forms i.e., PR and Pra (i.e., red light and far red light absorbing forms) and these two forms are interconvertible. P Red Kgto x p xfar red light During day time PFR form accumulates in plants which is inhibitory to flowering in SDP and stimulatory to flowering in LDP. During night time PR form accumulates in plants whose effect is vice versa.