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 sunlight. Until now, scientists had attributed this ability to a phenomenon called phototropism, but A new study published this Tuesday in PLoSBiology has found that multiple types of photoresponses are needed for sunflower dancing. Researchers at the University of California delve into the behavior of these plants and overturn previous assumptions about their dependence on light.
Plant biologist Stacey Harmer and her team discovered in 2016 that young sunflower plants follow sunlight thanks to different patterns during stem growth. In the experiment, they observed that the lab-grown plants grew symmetrically and most of their development occurred at night. Instead, 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. As night came, the east side stopped growing and they increased on the west side of the stem, ending up facing east again just before dawn.
The same scientists have now considered how these plants perceive sunlight and what molecular pathways are responsible for their growth patterns. Their “surprising” results, Harmer says, suggest that the sunflower dance is more complex in the real world than what was studied in the laboratory. For the research, they observed which genes were activated in sunflowers grown indoors and which in those grown naturally. Indoors they grew directly toward artificial light, activating 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 reception systems, including one to avoid red light—which is generated in the shade—that was activated on the west side of the sunflower stem early in the day, when the sun is in the sky. this.
Given these behaviors, the researchers conclude that there are multiple pathways that respond to different wavelengths of light to achieve the same goal, but they have not been able to identify the genes involved in heliotropism. “Understanding the molecular pathways involved in solar tracking will provide tools to help breeders generate plants that retain this capacity,” explains the scientist.
Among the things that most surprised the authors of this new study is how quickly these plants learn. When the lab-grown sunflowers were moved outside, they “started following the sun from day one,” suggesting, Harmer said, that the lab plants underwent some sort of “rewiring,” and that those allowed to follow the sun grow better than those that are not.
Why do old sunflowers lose that ability?
The ability of sunflowers to follow the sun is exclusive to the youngest ones. Once they reach maturity, their dance stops. They don’t spin again for the rest of their lives and They stare west indefinitely until they die. To understand this behavior, in a previous study, the plant biologist and her colleagues immobilized the stems of young plants to prevent them from moving following the sun. Others were placed in pots facing east, so that the next sunrise they found themselves with their backs to the star. Some more were exposed to intense blue light, altering the day/night cycles, changing to 30 hours instead of 24 in the growth chamber.
As a result, they found that following the sun is good for their development. Manipulated sunflowers lost up to 10% of their biomass and the size of its leaves was visibly reduced compared to that of unmanipulated flowers. They also found that those that had the blue LED on them all the time continued moving from east to west for several days. Those who were made to believe that they were on 30-hour days, their turnaround ended up being erratic. This is due to a circadian clock, the internal clock of sunflowers, that marks when and how much it should rotate. Thus, they make the most of sunlight, vital for their photosynthesis, while promoting the production of growth hormone.
However, being older also has its advantages. When they stop their growth they stop dancing in search of the sun, but they begin to give off additional heat, which makes them more attractive to pollinating insects and that pollination, in turn, will allow the old sunflower to reproduce.
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