Question

Which one is correct with respect to long day plants?
A. $Pfr/\Pr > 1$
B. $\Pr /Pfr > 1$
C. Red light inhibit flowering
D. Far red light promote flowering

Answer 1

If a plant flowers after being exposed to longer light periods, which is more than certain critical length, then it is called a long-day plant. Lettuce, spinach and some wheat varieties are examples of long day plants. Florigen is responsible for floral transition in meristem. It leads to flowering of plants. In long day plants, Florigen is given by longer time of sunlight.

Complete answer:
Option A $Pfr/\Pr > 1$: Red light exposure converts chromoprotein to functionally active form ($P{f_r}$), whereas darkness/far-red light exposure makes chromoprotein inactive (${P_r}$). $P{f_r}$ activate florigen production in long day plants. When $P{f_r}$ is greater than ${P_r}$, more daylight is needed. It is true in the case of long day plants.
Option A is correct.
Option B $\Pr /Pfr > 1$: When ${P_r}$ is greater than $P{f_r}$, there is lesser daylight, which is not true in the case of long day plants.
Hence, Option B is not correct.
Option C Red light inhibits flowering: Red light helps in accumulating florigen in long day plants. So, it is not possible to inhibit flowering.
So, Option C is not correct.
Option D Far red light promotes flowering: Increase in far red light decreases the accumulation of florigen, which in turn decreases flowering.
Option D is not correct.

Hence, Option A is the correct answer.

Note:
Rather than the length of daylight, darkness plays a big role in the flowering of plants. Short nights requiring plants are long day plants. More than $12$ hours of light are needed for blooming of long day plants. Also, they need just $8$ hours of darkness for initiation of flowering. Thus, during summers, they bloom well. By shortening the light hours, scientists can postpone the flowering season.