Question

What type of plants require a period of uninterrupted darkness to induce flowering?

• A. Short-day plants
• B. Long-day plants
• C. Day-neutral plants
• D. Facultative Long-day plants

Hint:

To prevent short-day plants (like chrysanthemums) from flowering out of season in a greenhouse, growers use artificial lights to introduce brief "night breaks" during the middle of the dark period.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Photoperiodism is the physiological reaction of organisms to the relative lengths of daylight and darkness. In plants, photoperiodic pathways regulate the transition from vegetative to reproductive growth (flowering).

Step 2: Detailed Explanation:
- Short-Day Plants (SDPs) are photoperiodic plants that flower only when the daylength is shorter than a critical threshold.
- Early physiological experiments revealed that the term "short-day" is actually a misnomer; these plants do not measure the length of the light period, but rather the length of the continuous, uninterrupted dark period (night).
- For a short-day plant to flower, the continuous dark period must exceed a specific critical duration.
- If this dark period is interrupted even briefly by a flash of light (a "night break"), the plant's internal molecular clock is reset, and it will fail to flower. This night-break effect is highly sensitive to red light ($660 \text{ nm}$) and can be reversed by a subsequent exposure to far-red light ($730 \text{ nm}$), demonstrating the involvement of the photoreceptor phytochrome.
- Phytochrome exists in two interconvertible forms: the inactive $\text{P}_r$ form and the active $\text{P}_{fr}$ form. During the day, sunlight converts $\text{P}_r$ to $\text{P}_{fr}$. During the long night, $\text{P}_{fr}$ slowly reverts to $\text{P}_r$ (dark reversion).
- In short-day plants, a high concentration of $\text{P}_{fr}$ inhibits flowering. Therefore, a long, uninterrupted dark period is required to allow sufficient conversion of $\text{P}_{fr}$ back to the inactive $\text{P}_r$ form, releasing the molecular block on the flowering locus genes.

Step 3: Final Answer:
Short-day plants require a continuous, uninterrupted period of darkness to induce flowering.