1. How Night Lighting Affects Chrysanthemums in Aichi Prefecture
Aichi Prefecture is the top producer of chrysanthemums in Japan, accounting for about one-third of the national total. The main area for this is Tahara City [7]. At night, farmers use a method called "electric light cultivation." They turn on lights in the field before flower buds form. This makes the plant feel like the day is long, which delays flowering. By combining this with covering the fields with cloth to block light, they can ship flowers all year round [7].
For chrysanthemums, if you interrupt the night with lights for a few hours, the formation of flower buds is suppressed. If you suppress them for about 60 days and then stop the lights, the flowers bloom. This allows farmers to sell chrysanthemums even in winter [6]. Recently, more farmers are switching from old bulbs to LED lights [7]. But why does making the night just a little bright stop the flowers? The clue to this mystery was found over 100 years ago, in the observation of a single type of tobacco plant.
2. Tobacco Plants That Bloomed Only in a Winter Greenhouse
In 1920, American researchers Garner and Allard reportedly noticed a tobacco variety called "Maryland Mammoth." When grown in the south, this variety bloomed. However, north of Washington D.C., it did not bloom during the growing season and died from the autumn frost. But when grown in a greenhouse during winter, it bloomed [2].
Furthermore, when they used artificial light to make the day longer in the winter greenhouse, the plant stopped blooming. The two researchers thought that the length of the day controlled when flowers appeared. Their paper was published in March 1920 in the Journal of Agricultural Research [2]. The property where living things react to changes in the length of day and night is called photoperiodism [1].
Plants that flower when days get shorter are called "short-day plants." Many of these bloom from summer to autumn, such as morning glories, chrysanthemums, cosmos, rice, and soybeans [3]. In contrast, "long-day plants" flower when days get longer. Examples include spinach and thale cress [4]. But here is a question: which clue do plants use to know the season, the length of the day or the length of the night?
3. Why Plants Respond to the Length of the Night, Not the Day
Experiments showed that plants react to the length of the night. For example, in the plant called beggar-ticks, flowers begin to form if there is a dark period of more than 8.5 hours. However, this 8.5-hour limit is specific to beggar-ticks; the number changes depending on the plant species [3].
Even if the dark period is long enough, if you shine light for just a very short time in the middle of the night, the formation of flower buds in short-day plants is interrupted. What is needed is not just that the total night is long, but that the night is "uninterrupted and long" [4]. The lights in chrysanthemum fields use this property. They cut the night in the middle to make the plant think the night is short. It looks like the plant is measuring the night with a stopwatch, but it does not actually have a clock.
So, does the color of the light used to interrupt the night matter? If only the total length of darkness mattered, interrupting the night would not work.
4. How Red Light Stops Flowering and Far-Red Light Cancels the Effect
Red light, with a wavelength around 660 nanometers, is particularly effective at interrupting the night. However, if you shine far-red light, with a wavelength around 730 nanometers, right after the red light, the effect of the red light is canceled. The plant returns to a state where it can flower [4].
A similar switching effect of light was found in lettuce seeds. In the early 1950s, Borthwick and others found that if you alternate red and far-red light several times, whether the seed sprouts depends on the "last light" applied. If the last light was red, it did not sprout; if it was far-red, it did [5].
Because of this, Hendricks thought that a molecule receiving light switches back and forth between two forms. It is like a light switch where red means "on" and far-red means "off." But it is not really a switch; it is a phenomenon where the form of the molecule changes [5]. Was this molecule ever actually found?
5. Phytochrome: The Molecule in Plants That Senses Red Light
The molecule that receives light is called phytochrome. It is a pigment-protein that absorbs light energy. It has two forms: one that absorbs red light (Pr) and one that absorbs far-red light (Pfr). When it receives light, they change into each other [5].
In 1959, Butler and others succeeded in extracting phytochrome from etiolated corn seedlings. These are sprouts grown without light, similar to bean sprouts [5].
Phytochrome helps regulate how plants grow. It can either encourage or suppress the formation of flower buds. It is thought that red light works to interrupt the night, and far-red light cancels it, because this molecule changes between two forms [5]. But sensing light is not enough to make a flower bloom. How does the information travel from where it is sensed to where the flower forms?
6. How Florigen Carries the Flowering Signal From Leaves to the Stem Tip
Light is sensed in the leaves, but flowers form at the tip of the stem. Connecting these two separate places is a signal called "florigen," which tells the plant to flower. When leaves sense the length of the day, florigen is made in the leaves. It travels through a pathway called sieve tubes to the stem tip, where it causes flower buds to form [8].
The identity of this signal is thought to be a small protein called FT (FLOWERING LOCUS T), or a relative of it [8].
The idea of florigen was proposed about 70 years ago, but its true identity remained unknown for a long time. In 2007, research by Tamaki and others showed that in rice, a protein called Hd3a, which is similar to FT, moves from the leaf to the stem tip and promotes flowering. It is thought to be the florigen of rice [9]. From the light-sensing molecule that measures the night to the protein that makes flowers, plants exchange information between distant parts of themselves.
7. How to Investigate Chrysanthemums From Aichi at Shops Near You
Today, you can look for chrysanthemums at flower shops or supermarkets. Check if the place of origin is written on them. If you find chrysanthemums from Aichi Prefecture, think about why they are sold even in winter, using what you learned in this article.
If you travel by train or car at night, look for bright spots in fields or greenhouses. If you see them, tell a family member. It might be electric light cultivation. Do not go out alone at night. For safety, always go out at night with a parent or guardian.
The Aichi Prefectural Agricultural Experiment Station has a page about flowers that includes information on chrysanthemum electric light cultivation and the region's production. You can look up terms like "photoperiodism" and "phytochrome" in online dictionaries like Kotobank if you are curious.