Question

Which wavelengths are the most effective in photoperiodism?
A. Green and yellow
B. Blue and red
C. Blue and violet
D. Red and far-red

Answer 1

Photoperiodism is made up of two words, Photo meaning light and Periodism meaning duration. Therefore, it is the effect of the duration of the light over plants. The lights of 660-730 nm cause photoperiodism. In addition, the wavelength of red light is usually taken around 660-700 nm.

Complete step by step answer: The leaves of a plant receive the photoperiodic effect. This is because the leaves contain a pigment known as the phytochrome. The phytochrome is a bluish-green pigment. Now the phytochromes are present in two alternate forms. These two forms are Phytochrome red $\left( {{P_r}} \right)$, and Phytochrome far-red $\left( {P{f_r}} \right)$. The phytochrome red receives the light of the red range. This range varies from 660-700 nm. The phytochrome far-red receives the light of a far-red range. This range varies from 700-730 nm. The phytochrome red is the inactive form and the phytochrome far-red is the active form. Moreover, both these forms are interchangeable in each other. The phytochrome red promotes flowering in short-day plants and it inhibits seed germination. On the other hand, the phytochrome far-red promotes flowering in long-day plants and it promotes seed germination.
Thus, based on the above information, we can conclude that the red and far-red wavelengths are the most effective in photoperiodism.
Hence, the correct answer is option (D).

Note: Photoperiodism is the effect of the duration of the light or the photoperiod on the flowering of the plants. This phenomenon was discovered by Garner and Allard in 1920, in a tobacco plant. On the basis of the photoperiod, the plants are classified into three groups as Short-day plants, Long-day plants, and Day-neutral plants.