With their bright yellow manes and sturdy stems, sunflowers might seem like a simple summer delight. But researchers say the plants are surprisingly enigmatic after discovering they don’t use conventional processes to track the sun across the sky.
Over the course of a day, sunflowers follow the path of the sun overhead – a process known as heliotropism – with their heads tilting progressively westwards as a result of cells elongating on the east side of the stem.
At night, cells elongate on the opposite side of the stems resulting in the heads reorienting themselves towards the east.
However researchers say the processes behind the solar tracking are an unexpected mystery.
“So many of our results were different than we anticipated,” said Prof Stacey Harmer, an author of the research at University of California, Davis.
Many plants grow towards a light source – this is because when blue light receptors, called phototropins, are activated on the lighted side of the stem, they cause hormones called auxins to become concentrated on the other “dark” side.
These auxins stimulate an elongation of cells, resulting in the stem bending towards the light.
While new research suggests this mechanism explains how sunflowers bend towards artificial light indoors, it appears it does not account for the way sunflowers naturally track the path of the sun.
Writing in the journal Plos Biology, Harmer and colleagues reported that they first analysed which genes were switched on within the stems of sunflowers grown indoors, when artificial blue light was shone from one direction.
The results suggest genes relating to phototropins were primarily “switched on” within the lighted side of the stem, with those related to and auxins and cell growth switched on within the shady side, suggesting sunflowers grown indoors turn towards blue light as a result of conventional processes.
But when the team turned to the stems of outdoor sunflowers, they found something different. The east side of the stem, the side in sunlight, and the west side, the side in shade, showed little difference in terms of which of these genes were switched on.
The researchers then tried blocking blue and ultraviolet or red and far-red light on their own, found neither attempt alone affected heliotropism in the outdoor plants, suggesting multiple light-signalling pathways are likely at play at the same time.
In another twist, the team found when sunflowers grown indoors were moved outdoors, the plants began tracking the sun immediately.
However, a different pattern of genes was switched on across their stems on their first day outside compared with when they were indoors or on subsequent days outdoors – suggesting some sort of adjustment mechanism was at play.
Harmer said the work suggests findings obtained in controlled environments do not necessarily reflect what is happening in the wild.
“Our results suggest that both solar tracking and phototropism in real world conditions is much more complicated than we had expected,” she said.
