Photoperiodism is a plant response to the relative lengths of day and night. This cheat sheet explains how plants use light signals to time flowering, dormancy, and seasonal growth. It is especially useful for understanding why some plants flower only in spring, summer, or fall.
Students need these ideas to connect environmental cues with hormone signaling and gene expression.
The key concept is that many flowering responses depend more on uninterrupted darkness than on the total hours of daylight. Phytochrome is the light-sensing pigment system that helps plants measure night length by switching between Pr and Pfr forms. Short-day plants flower when the night is longer than a critical length, while long-day plants flower when the night is shorter than a critical length.
Florigen is the mobile flowering signal produced in leaves and transported to shoot apical meristems.
Key Facts
- Photoperiodism is the physiological response of a plant to the relative lengths of light and dark periods in a 24-hour cycle.
- Short-day plants flower when night length is greater than the plant's critical night length.
- Long-day plants flower when night length is less than the plant's critical night length.
- Day-neutral plants flower based on factors such as maturity, temperature, or hormones rather than photoperiod.
- Phytochrome exists in two interconvertible forms: Pr absorbs red light and converts to Pfr, while Pfr absorbs far-red light and converts to Pr.
- In most flowering responses, the length of continuous darkness is the key signal, so a brief light flash during the night can interrupt flowering in short-day plants.
- Florigen is a mobile flowering signal made in leaves that travels through the phloem to the shoot apical meristem.
- Vernalization is the promotion of flowering after a prolonged cold period, and it can interact with photoperiod in some species.
Vocabulary
- Photoperiodism
- A plant's biological response to the relative lengths of day and night.
- Critical night length
- The threshold length of uninterrupted darkness that determines whether a photoperiod-sensitive plant will flower.
- Short-day plant
- A plant that flowers when the night is longer than its critical night length.
- Long-day plant
- A plant that flowers when the night is shorter than its critical night length.
- Phytochrome
- A plant pigment system that detects red and far-red light and helps regulate flowering and other light responses.
- Florigen
- A mobile chemical signal produced in leaves that triggers flowering at the shoot apical meristem.
Common Mistakes to Avoid
- Calling short-day plants daylight-measuring plants is wrong because their flowering usually depends on a long, uninterrupted night rather than a short day by itself.
- Ignoring critical night length is wrong because a plant does not flower simply because a day is labeled long or short, it flowers only when its threshold is met.
- Assuming all plants respond to photoperiod is wrong because day-neutral plants flower mainly according to age, development, or environmental conditions other than day length.
- Forgetting the effect of a night interruption is wrong because a brief red light flash during darkness can reset phytochrome signals and prevent short-day plants from flowering.
- Confusing phytochrome forms is wrong because Pr absorbs red light and becomes Pfr, while Pfr absorbs far-red light and becomes Pr.
Practice Questions
- 1 A short-day plant has a critical night length of 12 hours. If it receives 10 hours of light and 14 hours of uninterrupted darkness, will it flower?
- 2 A long-day plant has a critical night length of 9 hours. If it receives 16 hours of light and 8 hours of dark, will it flower?
- 3 A plant flowers only when it experiences nights shorter than 10 hours. Classify it as short-day, long-day, or day-neutral, and explain your choice.
- 4 Explain why a brief flash of light in the middle of the night can change flowering in a photoperiod-sensitive plant even if the total day length is unchanged.