Question

Which ones are long day plants?
A.Wheat, Poppy, Soyabean
B.Wheat, poppy, Beat
C.Wheat, Oat, Soyabean
D.Wheat, Xanthium, Paddy

Answer 1

In order to classify plants as long day plants or short day plants, we need to understand the concept of photoperiodism.

Complete answer:
Angiosperms use a photoreceptor protein, phytochrome, to sense seasonal changes in night length, or photoperiod, which is their signal for flowering. Furthermore, obligate photoperiodic plants necessarily require a long or short enough night before flowering, whereas facultative photoperiodic plants can produce flowers under one condition.
Red light (present during daytime) converts phytochrome to its active form ($Pfr$). This triggers the plant to produce flowers. Far-red light is present in the shade or in the dark. It converts phytochrome from $Pfr$ to $\Pr$. $\Pr$ is the inactive form of phytochrome. So, flowering will be inhibited. This system of conversion of $Pfr$ to $\Pr$ helps the plant to sense nighttime and daytime. $Pfr$ can be converted back to $\Pr$ by a process called dark reversion, where long periods of darkness initiate this conversion. This is important for plant flowering. A short-day plant can not flower if light is provided for a few minutes in the middle of the night and a long-day plant can flower if given more red-light in the middle of the night.
It is not the day length that controls flowering, but the length of the night is the controlling factor. Photoperiodic flowering plants are classified as –
Long-day plants
Short-day plants
Though night is the controlling factor, still the classification is done like this because of the initial misunderstanding that day length controls flowering. There are some plants that are called long-short-day plants (LSDP) or short-long-day plants (SLDP). LSDPs produce flowers after a series of long days followed by short days whereas SLDPs produce flowers after a series of short days followed by long days. Photoperiod length or more precisely, night length is different for different plants.
Long-day plants produce flowers when the night length falls below their critical photoperiod. They flower during late spring or early summer when days are getting longer.
Some long-day plants are: Carnation (Dianthus), Henbane (Hyoscyamus), Oat (Avena), Pea (Pisum sativum), Barley (Hordeum vulgare), Lettuce (Lactuca sativa), Wheat (Triticum aestivum)
Short-day plants produce flowers when the night lengths exceed their critical photoperiod. They cannot produce flowers under short nights or if a night time is interrupted for a few minutes by giving artificial light. This is called scotoperiod (the minimum requirement of night time).
Photoperiodism affects flowering by inducing the shoot to produce floral buds in place of leaves and lateral buds.
Some short-day facultative plants are: Kenaf ( Hibiscus cannabinus), Marijuana (Cannabis), Cotton (Gossypium), Rice (Oryza), Jowar (Sorghum bicolor), Green Gram (Mung bean, Vigna radiata), Soyabeans (Glycine max).

Hence option B is correct.

Note:
Day-neutral plants such as cucumbers, roses, tomatoes, etc. do not initiate flowering based on photoperiodism. We need to keep in mind that it is the night time or the scotoperiod that is of more importance than the day length.