Many plants sense day length (the length of light exposure) and regulate when they flower. This mechanism is called photoperiodism. The molecular mechanism of photoperiodism is central to plants’ adaptation to the seasons.
Basic Knowledge
Classification of photoperiodism: ①Long-day plants (LDP): Form flower buds when the day length exceeds the critical day length (also called short-night plants). Examples: rapeseed, wheat (winter wheat), spinach, and iris. ②Short-day plants (SDP): Form flower buds when the day length becomes shorter than the critical day length (also called long-night plants). Examples: morning glory, chrysanthemum, cosmos, and tobacco (Nicotiana tabacum). ③Day-neutral plants: Form flower buds regardless of day length. Examples: corn, tomato, and cucumber.
Plants sense “the length of the night”: The length of the dark period (night) is important. Flowering is determined by whether it is longer (short-day plants) or shorter (long-day plants) than the critical dark period. If light is given during the dark period (night interruption), short-day plants do not flower, whereas long-day plants flower.
Photoreceptor: phytochrome: The photoreceptor for photoperiodism. The Pr form absorbs red light (660 nm), and the Pfr form absorbs far-red light (730 nm); the two forms convert into each other. During the day, the Pfr form accumulates, while at night it slowly converts to Pr in darkness. The Pfr/Pr ratio near the end of the dark period acts as the signal for flower-bud formation.
Flowering hormone (florigen): A flowering-promoting signal produced in leaves after day length is sensed there. In Arabidopsis thaliana, the FT (FLOWERING LOCUS T) protein is transported through the phloem to the shoot apical meristem, where it induces flower-bud formation.
Vernalization: A phenomenon in which treatment with low temperature for a certain period promotes flowering. It is an essential condition for winter wheat to flower in spring.
Explain why night interruption suppresses flower-bud formation in short-day plants, based on changes in the state of phytochrome.
Answer: Short-day plants require a continuous dark period longer than the critical dark period to form flower buds. When red light is given during the dark period, Pr is converted to Pfr. When Pfr accumulates, the plant determines that the dark period has not been continuous, and the flower-bud formation signal is blocked. Therefore, the plant “interprets” the dark period as having become shorter, and the short-day plant does not form flower buds.
- Long-day plants = flower when day length > critical day length (short-night plants)
- Short-day plants = flower when day length < critical day length (long-night plants)
- What plants sense is “the length of the night (dark period)”
- Night interruption = light exposure during the dark period suppresses flowering in short-day plants
- Phytochrome = Pr (absorbs red light) ⇔ Pfr (absorbs far-red light)
- Florigen = FT protein transported through the phloem to the shoot apex
- Vernalization = low-temperature treatment promotes flowering (winter wheat)
Points to Note
① It is correct to say that “long-day plants flower on long days,” but it is more accurate to understand that they flower during a “short night (a dark period shorter than the critical dark period).” ② Phytochrome consists of interconvertible forms that absorb red and far-red light, but cryptochrome (a blue-light receptor) also participates in circadian rhythms and phototropism. ③ The identity of florigen was unknown for many years, but it was identified as the FT protein in 2007.
Practice
- Give one example each of a long-day plant and a short-day plant, and state the day-length conditions under which each forms flower buds.
- What wavelengths of light do the Pr and Pfr forms of phytochrome absorb, respectively? Also state how they change at night.
- What is vernalization? Explain it using winter wheat as an example.