The enigma that is, the sunflower

Research shows that sunflower solar tracking and phototropism is more complicated than expected.

Sunflowers in a sunny field
Photo by todd kent on Unsplash

Plants have an extraordinary ability to alter their growth in response to environmental cues. In a dense herbaceous border, plants will sense their neighbours and alter their growth to outgrow competitors (shade avoidance) to optimise light conditions.

Sunflowers are famous for their charming ability to track the sun through the process of heliotropism. They tilt their heads westwards by the cells in their stems elongating more on the east side of the stem during the day, and on the west side at night. Heliotropism is believed to be a specialised version of phototropism, which is the ability of plants to re-orient shoot growth towards a light source to optimise photosynthesis.

To date, plant scientists assumed that sunflowers’ heliotropism, would be based on the same process, governed by phototropin and responding to light at the blue end of the spectrum. New research conducted by Stacey Harmer and colleagues at the University of California Davis in the US however, revealed that sunflowers use a different, new mechanism from what was previously thought.

Researchers compared gene activity patterns of sunflowers in a laboratory to sunflowers growing in a field, in sunlight. The research showed that indoors, the sunflowers grew straight towards the light, activating genes associated with phototropin. The plants grown outdoors though, showed a completely different gene expression pattern. There was no apparent difference in phototropin between one side of the stem and the other. Hence upending previous assumptions, as scientists have therefore not (yet) identified the genes involved in the remarkable process of heliotropism.

As stated in the research report, “The results suggest that the transcriptional regulation of heliotropism is distinct from phototropin-mediated phototropism and likely involves inputs from multiple light signalling pathways.”