1. Morning Glories Are Already Open by the Time You See Them in the Morning
Many people imagine morning glories opening as they bathe in the morning sunlight, because of their name. However, an explanation from the Faculty of Science at Niigata University states that the blooming time shifts with the seasons. In July, they open just before dawn. As the season progresses, the time gets earlier, and by October, they open in the middle of the night [1].
In every season, they are open before dawn. Therefore, the explanation describes them not as flowers that bloom in the morning, but as flowers that are already open in the morning. The reason they are fully open at 6:00 a.m. is not because they reacted to the morning sun, but because they had already opened several hours before 6:00 a.m. [1].
If the opening time moves with the seasons, the signal is probably not the sunrise itself. So, what is it?
2. Morning Glories Open About 10 Hours After Sunset, Based on the Length of the Dark Time
Experiments at Niigata University showed that when the length of the day (the light period) was longer than 11 hours, morning glories opened about 10 hours after it became dark [1]. In Japan, the seasons when morning glories grow have days longer than 11 hours. This means they open about 10 hours after the previous day's sunset [1]. A research introduction from the University of Tsukuba states that they open in the morning, 10 to 12 hours after it gets dark [3].
In an example provided by Weathernews, if sunset in July is at 18:58, they bloom around 5:00 a.m. the next day, about 10 hours later. If sunset in September is at 17:50, they bloom before 4:00 a.m., about 10 hours later [2]. As sunset gets earlier, the blooming time tends to get earlier too. However, the time from sunset to blooming changes depending on the season and temperature. These examples are specific to certain regions, so the times are not the same across the country.
Darkness triggers the opening. If placed in constant light, morning glories do not open and the buds wither [1]. It has been reported that if you grow them with artificial light to reverse day and night, they open in the evening. By changing the time it gets dark, you can make them open at a desired time [1].
But a question remains: If they count from when it gets dark, what happens if they are kept in a room that is always dark?
3. Morning Glories Open at the Same Time Even in Total Darkness
An experiment described in the Niigata University explanation kept potted plants with many buds in total darkness for three days. The flowers still opened. On the first day, they began to open around 3:00 to 4:00 a.m. and were fully open by 8:00 a.m. On the second and third days, the times were almost the same [1].
Since they open at a set time even without changes in brightness, it is thought that there is a biological clock inside the flower. The same result occurred when buds were cut off and put in water, so it is the bud itself that judges light and dark [1].
The clock is linked to gene activity. Research at the National Institute for Basic Biology measured the amount of all genes working in morning glory petals every three hours, from three days before blooming until half a day after opening, and created a database. Among them, genes working on a cycle of about 24 hours were found [4]. The University of Tsukuba introduction also states that the blooming time is influenced by an internal body clock (circadian rhythm) with a cycle of about 24 hours [3].
One person who studied this experiment for a long time is Junko Kaihara. According to the Niigata University explanation, much of the research on morning glory blooming is by Kaihara, and her research, which started when she was a middle school student, was recognized by Professor Atsushi Takimoto of Kyoto University [1].
Is the body clock accurate enough to bloom at the exact same time every day? Or is it also influenced by the surrounding temperature?
4. A Hot Night Makes Morning Glories Open Later
There was an experiment where the night temperature was changed. At 23°C or below, they always bloomed about 10 hours after it became dark. However, at 25°C or above, the higher the temperature, the later the blooming was [1].
Even in the same season, they open earlier on days when the morning temperature is low, and later on days when it is high [1]. The body clock may not be just a clock, but something that progresses while watching both light and temperature.
Since darkness triggers blooming, places exposed to light at night may be affected. Weathernews, supervised by a researcher from the Japan Flower Association, reports that artificial light from streetlights can cause poor flowering [2].
So, at that set time, how do the petals open all at once?
5. How Growing Petals Curve Outward to Open the Flower
The flower opens because the petals grow rapidly. Within a single petal, the inner side grows faster than the outer side, so the petal curves outward [1]. It is said that if you cut the petal the night before blooming, leaving only a thick vein called "yo" in the center, it still curved outward by the next morning [1]. The bud is twisted in the opposite direction to the way the vine curls, and it opens as if that twist is unraveling [1].
According to research on other flowers like the Satsuki azalea, the reason petal cells swell is thought to be that starch inside the cells breaks down into sugar, increasing osmotic pressure and drawing in water. This is a guess based on research on other flowers, not confirmed for morning glories themselves [1].
Some varieties change color when they open. In the morning glory variety Heavenly Blue, it is reported that the pH inside the cells containing surface pigment rises from 6.6 to 7.7 during blooming, changing the color from red to blue [5]. It is not clear from this material if the same happens in other varieties.
6. How the EPH1 Gene Controls When Morning Glories Wilt
Research by the National Agriculture and Food Research Organization and others found a gene called EPHEMERAL1 (EPH1) that regulates the lifespan of morning glory flowers. When the action of this gene is suppressed, the flower lives about twice as long [6]. This research used a variety called "Murasaki" that is less affected by ethylene to study how petals age over time [6].
This means that morning glory flowers are designed to age not just because of outside conditions, but because of the passage of time. Both the mechanism that responds to the darkness of night to open, and the mechanism that ages after opening, are involved in the flower's passage of time.
In the short life that ends by noon, mechanisms that measure time are involved in both opening and wilting.
7. Try Recording Sunset Times and Flower Opening Times Yourself
If you are growing morning glories, try writing down the time of sunset and the time the flowers were open. You can find sunset times on weather forecast websites. The next morning, wake up early with your family, check if the flowers are open, and write down the time if you can. Do not go outside at night; only look in the morning.
If you write this down for several days, you can see if the opening time gets earlier as sunset gets earlier. It is also good to compare if there is a difference in how flowers bloom near streetlights versus places without them, even in the same garden [2].
If you want to read more, opening the explanation from the Faculty of Science at Niigata University [1] or the research introduction from the University of Tsukuba [3] will let you read the experimental conditions and diagrams in detail.