What is the site of perception of photoperiod necessary for...

Photoperiodism is a phenomenon that describes the requirement of light to stimulate the flowering on a vegetative shoot.
The concept of Photoperiodism was discovered by W. W. Garner and H. A. Allard in 1920. In the tobacco mutant variety 'Maryland mammoth' flowering is different at different places. This was found to be a function of length of the day.
Photoperiodism is precisely the response to the duration of light and timing of light and dark phases. All plants can perceive the induction to photoperiodic effect in their leaves.