Question

Assertion: Chrysanthemum is flashed with 660 nm of light, the flower does not bloom even if the light period is well below the critical photoperiod. Reason: Chrysanthemum is a long-day flowering plant.

• A. Both assertion and reason are correct statements and reason is the correct explanation to the assertion
• B. Both assertion and reason are correct statements and reason is an incorrect explanation to the assertion
• C. Only assertion is correct
• D. Both assertion and reason are incorrect

Answer 1

The correct option is C Only assertion is correct

Photoperiodism is the response of plants to the relative length of day and night. The part of the day that would experience light is called the light period or photoperiod. This periodic exposure of light leads to flowering in some plants. Certain plants require a definite photoperiod to induce flowering called critical photoperiod. Based on this plants are divided into:

i. Long-day plants: These are those plants that require a longer period of light than a critical photoperiod.

ii. Short-day plants: These are those plants that require a shorter period of light than a critical photoperiod.

iii. Day-neutral plants: The flowering in these plants is not affected by the exposure to light duration.

If a flash of red light, which is light having a wavelength of 660 nm, is given to Chrysanthemum, it is seen that the plant does not bloom. This condition persists even if the light period is below the critical photoperiod. This is known as the light break reaction. Hence, the assertion is correct.

However, Chrysanthemum is a short-day plant and not a long-day plant. Hence the reason is incorrect.

This kind of photoperiod response in Chrysanthemum can be reversed if the plant is exposed to far-red light, which is light with a wavelength of approximately 730 nm. This photoperiodic response is mediated by certain molecules like phytochrome which can detect red and far-red light. The phytochrome has two forms - Pr (red-absorbing form of phytochrome) and Pfr (far-red-absorbing form of phytochrome). During the day, Pr absorbs red light at 660 nm and gets converted into the Pfr form. This leads to an accumulation of Pfr during the day. Pfr on the other hand absorbs far-red light at 730 nm and gets converted into the Pr form. This conversion happens slowly in the dark. In this manner, by alternating between the two forms, Pr and Pfr, phytochrome regulates the photoperiodic response. Hence, this is the reason that correctly explains the given assertion.