Every day the sunflowers wake up and move towards the sun following their route from east to west, like the hands of a clock. The phenomenon is called “heliotropism”, a set of movements thanks to which plants direct their leaves and flowers towards the sunlight. Until now, scientists attributed this ability to a phenomenon called phototropism, but a new study published Tuesday in PLoSBiology found that several types of photo responses are required for dancing in sunflower. Researchers at the University of California examined the behavior of these plants and overturned previous assumptions about their dependence on light.
Plant biologist Stacey Harmer and her team discovered in 2016 that young sunflower plants follow sunlight in different patterns during stem growth. In the experiment, they observed that plants grown in the lab grew symmetrically and most of their development took place at night. In contrast, flowers exposed to natural light grew during the day on the eastern side of the stem, so they began the day leaning in that position and bent toward the west as the hours passed. When night came, the east side stopped growing and they grew on the west side of the tree, which ended up facing east again before dawn.
The same scientists are now wondering how these plants perceive sunlight and which molecular pathways are responsible for their growth patterns. Their “surprising” results, Harmer said, suggest that the sunflower dance is more complex in the real world than studied in the laboratory. For the research, they observed which genes were activated in sunflowers grown indoors and which in naturally grown ones. Inside they grow directly to artificial light, activating the genes associated with phototropin. Those grown outdoors, to the surprise of the researchers, did not show significant differences in response to the movement of the sun. But in these plants they identified other light-receiving systems, including one to avoid red light—generated in the shade—that was activated in the western part of the sunflower stem early in the day, when the sun is in heaven. .
Due to these characteristics, the researchers concluded that there are many pathways that respond to different wavelengths of light to achieve the same goal, but they were unable to identify the genes involved in heliotropism. “Understanding the molecular pathways involved in solar tracking will provide tools to help breeders produce plants that can retain this capacity,” explained the scientist.
Among the things that most surprised the authors of this new study was how quickly these plants learned. When the lab-grown sunflowers were moved outside, they “started following the sun from day one,” suggesting, Harmer said, that the lab plants underwent a kind of “rewiring,” and that the allowed to follow the sun will grow better than those. which is not.
Why do old sunflowers lose that ability?
The ability of sunflowers to follow the sun is exclusive to the youngest. When they reach adulthood, their dancing stops. They never return again in their lifetime and remain facing west until they die. To understand this behavior, in a previous study, the plant biologist and his colleagues restrained the stems of young plants to prevent them from moving towards the sun. Others were placed in pots facing east, so that at the next sunrise they found themselves with their backs to the star. Others are exposed to intense blue light, which alters day/night cycles, changing to 30 hours instead of 24 in the grow room.
Because of this, they find that following the sun is good for their development. Manipulated sunflowers lost up to 10% of their biomass and their leaf size was markedly reduced compared to non-manipulated flowers. They also found that those with the blue LED all the time continued to move from east to west for days. Those who were led to believe that they were on 30-hour days, their repetitions eventually turned out to be bad. This is due to a circadian clock, the internal clock of sunflowers, which dictates when and how it should rotate. Thus, they take advantage of sunlight, essential for their photosynthesis, while promoting growth hormone production.
However, getting older has its advantages too. When they stop their growth they stop dancing in search of the sun, but they start to give more heat, which makes them more attractive to pollination insects and that pollination, in turn, allows the old sunflower to reproduction.