Question

What is the effect on the flowering of a plant if a flash of red light is followed by a flash of far-red light?
A.Flowering is increased
B.Flowering is decreased
C.Flowering is stopped
D.Effect of red flash is reversed

Answer 1

Phytochromes are a class of light sensing protein-chromophore complex called photoreceptors. There are two types of phytochromes: ones that absorb red ($P_RED$; 600-700nm) and ones that absorb infrared or far red light ($P_iRED$;700-800). Light absorption by $P_RED$ converts it into $P_iRED$ and vice versa. First flash of red light would increase the total amount of $P_iRED$ and the following infra-red light will increase $P_RED$. Hence, the ratio of $P_RED$ and $P_iRED$ will remain unchanged.

Complete answer:
Flowering in plants is affected by many environmental factors and the duration of light is one such important factor. This developmental response of plants to light is called photoperiodism.
Light sensing is carried out by protein-chromophore complexes called photoreceptors. Phytochromes are one such photoreceptor absorbing at the red (600-700 nm) and far/infra-red (700-800) region of the light spectrum.

Based on time of flowering plants can be : short cycle plants ( flowering when daylight time is shorter than night), long cycle plants ( flowering when duration of day is longer than night) and neutral cycle plants (flowering when lengths of day and night are similar).
Different ratios of $P_RED$ and $P_iRED$ trigger this flowering time in different types of plants. Currently accepted idea is that higher $P_RED$ induce flowering in short cycle plants and higher $P_iRED$ induce flowering in long cycle plants.

Hence, the correct answer is option (D)

Note: Phytochromes are essential photoreceptors in different plants and fungi. Phytochromes play essential roles in germination, length of seedlings and stems, gene expression, and flowering. Phytochromes work in red and infrared regions of the light spectrum. One type $P_RED$ red light and gets converted to $P_iRED$ and $P_iRED$ absorbs infrared light and gets converted to $P_RED$ . Different plants have evolved to switch on genes for flowering at different rations of $P_RED$ and $P_iRED$.