Question

One set of a plant was grown at 12 hours a day and 12 hours a night period cycles and it flowered. While in the other set, the night phase was interrupted by a flash of light and it did not flower. Under which one of the following categories will you place this plant?
A. Day neutral
B. Short day
C. Long day
D. Darkness neutral

Answer 1

Plants react to light in different ways and this is called photoperiodism. On the basis of response of plants to light, they are divided in five categories. Flowering in some plants also depends on light. Photoperiodism was developed by W.W. Garner and H.A. Allard. They performed specific experiments to study the effects of photoperiodism.

Complete answer:
Response of plants to duration and timing of light and dark is defined as photoperiodism. Most flowering plants require a particular day length or light period. It was discovered by W.W. Garner and H.A. Allard. They observed this physiological response of plants in a tobacco mutant, ‘Maryland mammoth’, which flowered at different times at different places. After controlling the other factors like nutrition, temperature, etc., they reported that it was the length of the day that affected the plant’s growth. On the basis of response of plants to light, they are divided into five categories.

-Short day plants- these plants flower when the length of the day is short. If kept under light for longer than a critical point, they do not flower. These plants are also called long night plants as they require long uninterrupted darkness to bloom. Example: Cannabis sativum, Glycine max, etc.
-Long day plants- these plants flower the length of the day. They do not flower if kept in dark for longer periods. These plants are also called short night plants as they require less darkness to flower. Example: Beta vulgaris, Plantago, etc.
-Short-long day plants- these plants when in their early life require short day conditions but as they grow old, they become long day plants. Example Canterbury, Triticum vulgare, etc.
-Long short-day plants- these are actually short-day plants but require long day conditions when young. Example- Bryophyllum, Kalanchoe, etc.
-Day neutral plants- these plants grow in any photoperiod. They remain unaffected by the length of the day. Example: Tomato, Phaseolus vulgaris, etc.

Hence, the correct answer is option (B).

Note: Critical day length is an essential factor of photoperiodism. Critical day length is the length of photoperiod above or below which flowering occurs in a plant. For example, in Hyoscyamus niger, it is a long day plant whose critical day length is eleven hours, that is, above eleven hours flowering will take place.