Photoperiodism is the influence of the relative lengths of day and night on the activities of an organism. Although the best known example of photoperiodism can be found in flowering plants and many other responses in both animals and plants are regulated by day-length, i.e. the duration of photoperiod.
The influence of the photoperiod on flowering can be demonstrated by exposing certain plants to a brief flash of light in the middle of the night. If this treatment is continued for a sufficient number of days, flowering will be delayed. This is made use of by horticulturalists in guaranteeing a supply of plants like chrysanthemume and poinsettias at Christmas. Such plants can be made to flower early by giving them extra darkness. In some cases one long night is all that is necessary.
With certain other plants, petunias for example, the reverse is true: light treatment induces early flowering whereas dark treatment delays it. It should be noted that adjustment of the photoperiod, early and late varieties of plants can be made to flower simultaneously, thereby enabling plant breeders to cross them.
On the basis of their referring responses to light and dark, flowering plants can be divided into three groups:
1. Those that require long days and short nights, long-day plants, for instance, petunias, spinach, radishes and lettuce. Long-day plants flower only when the light period exceeds a certain critical length in each 24-hour cycle. This varies, but on average is about 10 hours.
2. Those that require short days and long nights, short-day plants, for instance, chrysanthemums, poinsettias, cockleblur. Short-day plants flower only when the light period is shorter than critical length in each 24-hour cycle. For cocklebur this is 141/2 hours.
3. Those that are indifferent to day-length, day-neutral plants, for instance, tomato and cotton.
There is no hard and fast dividing line between long and short-day plants: all gradations between the two exist. Not surprisingly, long-day plants tend to flower in the summer, or to inherit temperature regions where days are long and nights are short. Short-day plants, on the other hand, tend to flower in the winter and spring, or to live nearer the equator. For both long- and short-day plants, the critical factor influencing flowering is not the length of the light period but the length of the dark period. Thus long-day plants can be induced to flower by nights that are shorter than a critical length, and short-day plants can be induced to flower by nights that are longer than a certain critical length.