Explanation:

It has been discovered that duration of light also plays a major role in inducing flowering in plants. This flowering response in plants to the relative length of light and dark periods within a 24 hour cycle is called
'Photoperiodism'. Every plant has a specific critical period of light duration needed for flowering. In order to flower they must be exposed to either more or less light hour than that of critical period. Based on flowering response to light duration, plants have been classified into Long day plants, short day plants and day neutral plants.

Long day plants (LDP): They are plants which produce flower only when they are exposed to light duration more than the critical period. They produce flower in summer. E.g.-Oat and Rye plants.
Short day plants (SDP): They are plants which produce flower only when they are exposed to light duration shorter than the critical period. They produce flower in spring or winter. E.g.- Chrysanthemum, Tobacco and Coffee.
Day neutral plants: They produce flowers throughout the year irrespective of length of light hours to which they are exposed. E.g.- Tomato.