Following observations indicate that phytochrome is involved in the initiation of stimulus in plants:
1) Long dark periods of short day plants, if interrupted mid way by a brief exposure of red light (660 mu), the flowering is inhibited.
2) If the exposure of red light is immediately followed by far-red light (730 mu), the flowering is promoted.
Short-day plants: At the end of light period most of the phytochrome is converted to Pfr form, which inhibits flowering in short day plant. During long dark period Pfr form is converted to PR form which promotes the formation of flowering stimulus.
Long-day plants: Stimulus in these plants is promoted by Pfr form of phytochrome.Long light period favours conversion of Pr form to Pfr form.