what is photoperiod and what is photoperiodism ?

Photoperiod is the duration of light which is required by a plant for flowering. It can also be called as day length. It varies with different zones on earth and also according to seasons.

Photoperiodism is the biological response of an organism whether plants or animals towards the  change in intensity of light during day and night periodically. The response usually is observed by the changing behavior or growth of the organism.