USA Biology Olympiad (USABO) Practice Exam

Which statement regarding long day plants is correct?

1. They need continuous light to flower.

2. They will flower if they receive a short flash of light during the night.

3. They will not flower in any darkness.

4. They require darkness longer than 12 hours to initiate flowering.

The correct answer is: They will flower if they receive a short flash of light during the night.

Long day plants are those that require longer periods of light (or shorter nights) to initiate flowering. The key feature of these plants is their sensitivity to light duration, specifically the length of the dark period they experience. The correct statement regarding long day plants is that they will flower if they receive a short flash of light during the night. This phenomenon is known as photoperiodism, where even a brief exposure to light during a long period of darkness can interrupt the dark night cycle, signaling the plant to start flowering. This allows long day plants to take advantage of the increasing daylight of late spring and summer. Other statements about long day plants do not accurately reflect their physiological responses. For example, while they do not need continuous light to flower, enough light in the daytime and a shorter night are crucial for flowering. Additionally, long day plants do flower in conditions with dark periods shorter than 12 hours, which is contrary to the idea that they require darkness longer than this duration to initiate flowering.