Sunflower Heliotropism

The new cover story of last week’s science magazine (August 5, DOI: 10.1126/science.aaf9793) was very riveting, although I do not have a background in biology, I was tempted to read the article, and it turned out to be a fairly straight forward read.

The big question was, how and why do young sunflowers gear themselves towards the sun? It turns out this solar tracking phenomenon is called heliotropism, where the aerial portion of the sunflowers turn towards the sun’s motion throughout the course of a day. Also at night, the plant will shift back to face the east and prepare for the sun to come out again.

To better understand how this works and its impact, researchers at UC Davis, UC Berkeley and UVA tried to disrupt the event by rotating the plant manually by 180 degrees at night so that they would face west when the sun comes up, and by creating resistance for the plant to self-orient. It turned out after multiple trials, those plants resulted in 10% less in dry biomass and leaf area. It demonstrates that the sun tracking promotes growth.

Researchers also did experiments to understand why do the flowers direct itself at night. Is it due to the internal circadian rhythm or hourglass timing? flowers placed in an environment where there is constant light with the normal hour but no directional change, experienced dampened circadian rhythm.

Next question, how do the plant actuate itself to move? There is something called a “pulvinus” that exist in some plants that act as motors. (Just by looking up pulvinus, I encounter the paper below:

Click to access Bio-Intelligent-Materials.pdf

discussing bio-inspired actuation mechanism. It looks very interesting with a brief skim, I may go back and read it in full details and write something about it later if I have time.) However, sunflowers do not have pulvinis. Researchers hypothesized that it is due to stem growth. They then monitored the stem elongation rate and the sun tracking rhythm of sunflowers and discovered a linear correlation which supports their hypothesis. Furthermore, researchers found out that there is a differential elongation rate on opposite sides of the stem to support daily rhythms.

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