The photoperiod in plants is perceived at

1. meristem

2. flower

3. floral buds

4. leaves

(d) Fully developed leaves receive the stimulus of light photoperiodism is the effect of light and its duration on the growth and development especially flowering in plants.
Even a single leaf as its part is capable of inducing photoperidism but very young leaves are insensitive to light. Other parts meristem flower and floral buds denote perceive the stimulus.
Page 252, XI NCERT