Differentiate between.
Phototropism and photoperiodism

It is a light-dependent orientation of a plant which is periodic in nature. It may either be positive phototropism (towards the source of light) or negative phototropism (away from the source of light).

Photoperiodism: 

It is the dependency of a plant to the lengths of day or night to be able to flower properly. Any interruption in that will prevent the plant to flower.