Question

Which of the following is a long day plant
(a) Poppy
(b) Xanthium
(c) Paddy
(d) Soybean

Answer 1

Hint: Photoperiodism is the physiological reaction of organisms to the length of the night or a dark period. It occurs in plants and animals.
‘Poppy' is a long day plant.

Complete answer:
Photoperiodism in plants is classified under three groups according to the photoperiods: short-day plants, long-day plants, and day-neutral plants.
Cryptochromes and Phytochromes are the type of photoreceptor that is important in photoperiodism. Cryptochromes absorb blue light and UV-A. 
1. Long day plants- They begin flowering when the days are longer than their critical day length. They will flower when they are exposed to a short period of darkness and a long period of light. Example- Poppy, potato, lettuce, spinach.
2. Short day plants- They begin flowering when the days are shorter than their critical day length. They require a long period of darkness and a short period of light to flower. Example- rice, tobacco, cotton, Xanthium, soybean, etc.
3. Day-neutral plants- They do not depend upon photoperiodism for flowering. They rather require a period of low elements. Example- cucumber, rose, tomato, etc.

So, the correct answer is ‘Poppy'.

Note:
1. Phytochrome usually occurs in two forms: ${ P }_{ r }$ and ${ P }_{ fr }$. During the day, red light converts phytochrome to its active form, ${ P }_{ fr }$. This then triggers the plant to grow. In turn, far-red light converts phytochrome from ${ P }_{ fr }$ to ${ P }_{ r }$ in shade or in dark. ${ P }_{ r }$ is the inactive form of phytochrome and will not allow for plant growth. 
2. The threshold or the minimum amount of daylight required by the plant is known as the critical day length.