Question

Importance of day length in flowering plants/photoperiodism was first observed in
A. Cotton
B. Tobacco
C. Lemna
D. Petunia

Answer 1

Hint: Physiological response of plants and animals to the length of day and night period is defined as photoperiodism. It can also be defined as the developmental responses of plants to the relative day and night period. An example is when a plant doesn’t bloom during the night time in winters.

Complete answer:
Photoperiodism is an organisms' ability to adjust their physiology and behaviour to seasonal changes in the environment according to the length of a day. The photoperiodic plants are classified into three types:- short-day, long-day, and day-neutral. The hormone responsible for photoperiodism is Auxin. Auxin also helps in apical dominance. Auxin moves to the darker aspect of the plant, affecting the cells there to grow larger in size than the corresponding cells on the lighter aspect of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism. The first plant where photoperiodism was discovered is Tobacco.

Hence, the right answer is B( Tobacco).

Additional info: In 1920, two employees of the US Department of Agriculture, W. W. Garner and H.A.Hallard discovered a mutation in Tobacco plant. This mutated variety was called Maryland and Mammoth. These varieties prevented them from flowering in summer which they usually do and didn’t bloom till December. Experimenting with artificial lighting in winter and artificial darkening in summer, the results were that Maryland Mammoth was affected by photoperiod.
Some examples of short-day plants are Chrysanthemum, Rice, Morning Glory, Onion, Soyabean. Some examples of long-day plants are Spinach, Hibiscus, Lettuce, Radish, Potato. Some Day-neutral Plants are rice, corn, cucumber, Tomato, sunflower, and pea.

Note:
Because of photoperiodism, flowering and other responses within an ecotype population of plants are synchronized in time. This is certainly a bonus if the plants need cross-pollination; it is essential that each one bloom at the same time with others. Photoperiodism also affects plant growth as it’s regulated by a PGR (auxin).