If a long-day plant has a critical night length of 9 hours, which 24-hour cycle would prevent flowering?
a. 16 hours light/8 hours dark
b. 14 hours light/10 hours dark
c. 4 hours light/8 hours dark/4 hours light/8 hours dark
d. 8 hours light/8 hours dark/light flash/8 hours dark

Verified step by step guidance

1. Understand the concept of critical night length: Long-day plants require a night shorter than their critical night length to initiate flowering. In this case, the critical night length is 9 hours.
2. Evaluate option a (16 hours light/8 hours dark): The dark period is 8 hours, which is shorter than the critical night length of 9 hours, allowing flowering.
3. Evaluate option b (14 hours light/10 hours dark): The dark period is 10 hours, which is longer than the critical night length of 9 hours, preventing flowering.
4. Evaluate option c (4 hours light/8 hours dark/4 hours light/8 hours dark): The longest continuous dark period is 8 hours, which is shorter than the critical night length, allowing flowering.
5. Evaluate option d (8 hours light/8 hours dark/light flash/8 hours dark): The light flash interrupts the dark period, making the longest continuous dark period less than 9 hours, allowing flowering.

Key Concepts

Photoperiodism

Photoperiodism is the physiological reaction of organisms to the length of day or night. In plants, it affects flowering and other developmental processes. Long-day plants require a light period longer than a critical length to flower, meaning they need shorter nights. Understanding photoperiodism is crucial for determining how different light/dark cycles influence plant behavior.

Critical Night Length

Critical night length is the minimum duration of uninterrupted darkness required for a plant to either initiate or inhibit flowering. For long-day plants, flowering is triggered when the night is shorter than this critical length. In the given question, a night longer than 9 hours would prevent flowering, as it exceeds the plant's critical night length.

Interruption of Dark Period

Interruption of the dark period can affect a plant's perception of night length. A brief light exposure during the night can reset the plant's internal clock, making it perceive the night as shorter. This is particularly relevant for long-day plants, where a light flash during the night can prevent the plant from recognizing a long night, thus promoting flowering.