Question

What is critical photoperiod? On the basis of this, plants are categorized in how many types?

Answer 1

Plants require a certain day length to flower. This relative length of day and night needed for flowering is called as photoperiod. The length of the day and night above and below which a plant would not flower is called critical photoperiod. Depending upon the duration of photoperiod plants are divided into following categories:

1. Short Day Plants (SDP)

Short day plants generally require light period of less than 12 hours i.e. 8-10 hour and continuous dark period of about 14-16 hour for subsequent flowering. Most of the winter flowering plants belong to this category e.g. Xanthium, Dahlia, sugarcane, rice, potato.

2. Long Day Plants (LDP)

Long day plants require light period of 14-16 hour for subsequent flowering. The distinctive feature of these plants is that long night period entirely prevent flowering. But long nights interrupted by light, even briefly, cause the inhibitive effect of the night to be lost and the plants flower. These plants are sometimes also called as short night plants. E.g. Henbane, Spinach, Wheat, Oat.

3. Day Neutral Plants

These plants flower in all photoperiods and can blossom throughout the year. Examples are tomato, cotton, sunflower, cucumber.