SSC
BIOLOGY
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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Pfr turns into Pr in the light, causing short-day plants to flower
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Pr turns into Pfr in the light, causing long-day plants to flower.
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Pfr turns into Pr in the dark, causing long-day plants to flower.
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Pr turns into Pfr in the dark, causing short-day plants to flower.
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Detailed explanation-1: -A daylength extension with incandescent light is more effective in promoting flowering of long-day plants like Hyoscyamus niger than fluorescent light. A low phytochrome photoequilibrium (Pfr/Ptot), attained by a far-red irradiation at the close of long days under fluorescent light, also promotes flowering.
Detailed explanation-2: -Absorption of red light by Pr converts the protein to the far-red-absorbing conformation, Pfr. Absorption of far-red light by Pfr converts the conformation back to Pr (Figure 1). Hence, to a first approximation, phytochromes can be thought of as reversible red/far-red light-activated molecular switches.
Detailed explanation-3: -In these plants, when the day is long and the night is short, fewer Pfr molecules change into Pr during the night, leading to Pfr-dependent repression of flowering; by contrast, when the day is short and the night is long, more Pfr molecules change into Pr during the night, diminishing this repression.