Photoperiodism in plants - Practice Questions & MCQ

• The plants in order to flower require a certain day length i.e., the relative length of day and night which is called as photoperiod. 
• The response of plants to the photoperiod expressed in the form of flowering is called photoperiodism.

Depending upon the duration of the photoperiod, they classified plants into three categories.

1. Long Day Plants:

• These plants require a longer day light period (usually 14-16 hours) in a 24 hours cycle for subsequent flowering.
• Some examples of these plants which are also called as short night plants are Hyoscyamus niger (Henbane) Spinacia (spinach) Beta vulgaris (Sugar beet).
• In long day plants the light period is critical.

2. Short Day Plants:

• These plants require a relatively short day light period (usually 8-10 hours) and a continu­ous dark period of about 14-16 hours for subsequent flowering.
• Some examples of these plants which are also known as long-night-plants are Maryland Mammoth variety of to­bacco (Nicotiana tabacum) Biloxi variety of Soybeans (Glycine max), Cocklebur (Xanthium pennsylvanicum).
• In short day plants the dark period is critical and must be continuous. If this dark period is interrupted even with a brief exposure of red light, the short day plant will not flower.
• Maximum inhibition of flowering with red light occurs at about the middle of critical dark period. 
• However, the inhibitory effect of red light can be overcome by a subsequent exposure to far-red light (730-735 mu wavelengths).

3. Day Neutral Plants:

• These plants flower in all photoperiods ranging from 5 hours to 24 hours continuous ex­posure. 
• Some of the examples of these plants are tomato, cotton, sunflower, cucumber and certain varieties of peas and tobacco.

TIP: Critical photoperiod is that continuous duration of light which must not be exceeded in a short day plant and must always be exceeded in a long day plant. In order to make them flower.