What is Photoperiodism - Types of Photoperiodism.

  It is defined as " the response of plants to relative lengths of day and night ". The relative length of day and night affects the activities of organisms e.g. flowering in plants. The illuminated period is called photo period. Photo period affects the flowering in plants. The plants are divided into three types on the basis of production of flowers.

1. Long Day Plants:  These plants produce flowering in long photo period i.e. they require long days and short nights. The long day plants produce flowering when length of photo period is more then critical length in each 24 hours cycle. In these plants critical varies from 11 hours to 14 hours. E.g. Petunias, spinach, radishes and lettuce.
2. Short Day Plants: These plants which produce flowering in short photo period i.e. they require short and long night. These plants produce flowering only when the photo period is shorter the critical length in each 24 hours. e.g. Chrysanthemums, poinsettias, cockle bur. The critical length for cockle bur is 14/2 hours.
3. Day Neutral Plants:  The plants which are in different to day length, are called day neutral plant. They produce flowering in both long and short photo period. They require only five hours continuously. e.g. Tomato, Cotton.

"Transmission of message of Photoperiodism"

   The message for flowering is produced in leaves and is transmitted to the buds where flower formation is initiated. There is a hormone florigen. Which is responsible for flowering. This hormone can be transmitted from one plant to another plant when two plants are grafted together.

  Take two short day plants, one plant is kept short day condition and other plant is kept long day condition.

   Short day plant, which is kept in long day condition. Now both plants are joined together by grafting. The florigen will be transmitted from short day plant to the other plants short plant, which is kept in long day condition. Now both plants will produce flowering. The hormone florigen has not been isolated so far. It is called Transmission of message of photoperiodism.

"Mechanism Involving light quality and biological Clock":

   The plants have metabolic clock, which measures the day light time. Plants have light detecting system to set the clock. All organisms have biological clock, which can measure the time even without environmental clues, like light can reset the clock. It is called mechanism involving light quality and biological clock.