Why do sunflowers behave this way?
Anyone who undertakes a train journey in late summer from Helsinki to Beijing (like I did quite some years ago), will see extensive fields of sunflowers before the train leaves Europe and enters Asian territory. What is remarkable about the sunflowers is that they all face one direction and observing that, I thought the sunflowers probably orient themselves towards the sun. But according to a recent study by Horvath and co-workers, this is not so, because following a short period of full bloom during which the flowers do orient towards the sun, they then gradually bend downward and become ‘locked’ in a direction to face the East. (Now the beautiful and patriotic song “The East is Red” 东方红, never mind the words, enters my head and reminds me of the time I heard it daily while spending 3 weeks in China by invitation of the Chinese Academy: you can guess how long ago that must’ve been!). However, sunflowers, which like many fruit and vegetable plants originated in the ‘New World’, have other reasons to turn eastward.
In the past, several reasons had been advanced to explain why after the flowering period sunflower inflorescences no longer track the sun, but remain east-oriented. Yet none have been tested reliably and the new explanation for the first time takes into consideration the place of origin of the sunflower plant, the astronomical data of the sun’s position, the meteorological data of diurnal cloudiness, the time-dependent elevation angle of mature sunflower heads and the absorption spectra of the inflorescence’s front and back. An earlier suggestion was that a non-skyward orientation of mature sunflower heads would make it more difficult for birds to peck at the seeds. While true, it does not explain why the flowers should face east. Another attempt to explain the eastward orientation was that it would reduce the heat load at noon, but west-facing flowers would have the same advantage, so why ‘east’? It has also been assumed that east-facing allows greater light reception in the morning and speeds up drying of morning dew, thereby reducing fungal attack. That an easterly orientation promotes attractiveness to pollinators has been suggested, but by the time the sunflower heads get ‘locked’ in the easterly position, pollination has long been finished and the idea that an easterly orientation and the lower head temperature could be advantageous for seed maturation was not supported experimentally.
What appears to be crucial is that there is a 10-50% surplus energy absorbed by an east-facing sunflower inflorescence compared with other directional orientations. This could indeed accelerate the evaporation of morning dew, but what is the easterly orientation due to? It has seemingly something to do with the region the sunflower plant evolved, namely Boone County in North America, which regularly encounters cloudy afternoons. If afternoons are cloudier than the mornings, then east-facing inflorescences have an energy advantage of around 10% over west-facing flowers and an up to 50% radiation excess over south-facing flowers, taking into consideration absorption spectra of the inflorescence and the back of the heads. Maximum radiation absorption should be advantageous for seed production and maturation. The easterly orientation seen even in domesticated European sunflowers is likely to be a genetic trait that evolved in response to the meteorologic conditions of cloudy afternoons in the region that sunflowers evolved in North America.
Given that solar panels are usually directed south and direct sunlight is most intense at noon, are sunflowers ill-adapted, or do sunflowers perhaps ‘know more’ than solar panel engineers? Sunflower heads are tilted, looking downward and under such conditions the lower angle of the sun in westerly and easterly position is crucially important. But adding afternoon cloudiness into the calculation is what then causes the East to turn into ‘the winning formula’ for Helianthus annuus (the sunflower).
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021.
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