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Sunflowers use multiple light response systems to track the sun

Sunflowers have long fascinated observers with their ability to follow the sun from east to west throughout the day, a behavior known as heliotropism. But the science behind this intricate dance has remained somewhat of a mystery. Now, a study led by Stacey Harmer and her team at the University of California Davis sheds light on the complexities of this behavior. The study has revealed that sunflowers rely on multiple types of light responses to execute this movement.

Phototropic (light) response

In the past, it was believed that the sunflower’s ability to move towards light sources was primarily driven by a light-dependent response pathway known as the phototropic response. This system operates when blue light, falling unevenly on a plant, gets detected by proteins known as phototropins. 

These proteins then cause a redistribution of a specific plant hormone, resulting in the plant’s growing tip bending towards the light. It’s nature’s own way of helping plants, which are anchored to one spot, to reach out for the light they need for photosynthesis.

However, Harmer and her colleagues’ research indicates that the sunflower’s heliotropism isn’t solely based on this phototropic response. 

Focus of the study

In their study, they contrasted the gene activity patterns in sunflowers bending towards blue light in a lab setting to those of sunflowers in a natural environment, following the sun’s movement. 

The researchers discovered that only a handful of genes responsible for the phototropic bending in the lab exhibited significant changes in activity in response to the sun’s movement.

Shade avoidance system

Interestingly, their findings also highlighted the activation of other light-response systems. One such system, the shade avoidance system, detects far-red light typically found in shaded areas. 

The shade avoidance system was activated on the west side of the sunflower stem during the early hours of the day when the sun was still in the east. This revelation paints a complex picture of the mechanisms behind sunflower movement. 

Implications of this light response system

Even more intriguing is the fact that even when either the red and far-red or blue light was removed, the sunflowers still managed to track the sun. This suggests that multiple systems collaborate to ensure the heliotropic response, making it possible even if one or more light triggers are absent.

“We’ve been continually surprised by what we’ve found as we study how sunflowers follow the sun each day,” said Harmer. “In this paper, we report that they use different molecular pathways to initiate and maintain tracking movements, and that the photoreceptors best known for causing plant bending seem to play a minor role in this remarkable process.”

The research not only provides a deeper understanding of a well-observed phenomenon but also challenges previous assumptions about plant behavior and adaptation. 

The study is published in the journal in PLOS Biology.

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