What is it about a sunflower that makes it a sunflower? There seems to have been a bit of a flurry of papers on phototropism in sunflowers recently. The recent study in Annals by Kutschera and Briggs has picked up a few citations. For example Science published a paper by Atamian et al. on how sunflowers use circadian rhythms to track the sun.
This internal clock explains the how, but why does the final flower face east?
Experiments with heating flowers showed that warm flowers got more visits from pollinators. This would be consistent with an explanation for the heliotropism of Arctic Poppies, which are said to track the sun to warm the flower (and provide a comfortable hotspot for visiting pollinators)
However, a recent article by van der Kooi in Current Biology recently argues that heat is not the only possibility for orientation. He suggests that a few other factors should be tested for.
If sunflowers face east, then their heads are illuminated by the morning sun. The sunlight on the gold of the flowers simply makes them for striking to insect eyes, attracting pollinators to the newly opened flowers of a plant.
It’s not just pollen in the flowerheads, later there will be seeds and with a food source like that, there could also be unwanted visitors. Facing the east means the flower will dry out faster from any dew it gathers overnight, reducing the danger from fungal infection.
This might seem odd to suggest that the flower could be warmed and cooled, but the sun moves during the day. Afternoons tend to be hotter than mornings and during this higher temperature period an east-facing flower is a facing away from the sun. This means the bulk of the flowerhead is between the sun and the pollen, preventing the sun’s rays from striking the pollen when the heat is high.
If you can get access to Current Biology, then van der Kooi’s article is definitely worth browsing. Aside from putting the Atamian et al. paper in context, it also highlights plenty of other opportunities for research projects. I’m particularly interested in the idea that through protecting their pollen sunflowers could also be anti-sunflowers.
Experiments certainly support the warming hypothesis, but it is possible that some or all of the other reasons could be true too. If you’re doing an experiment on this (or have done and I’ve missed it) then we’d love to hear from you at AoB Blog.
Ulrich Kutschera, Winslow R. Briggs, 2015, 'Phototropic solar tracking in sunflower plants: an integrative perspective', Annals of Botany, vol. 117, no. 1, pp. 1-8 http://dx.doi.org/10.1093/aob/mcv141
H. S. Atamian, N. M. Creux, E. A. Brown, A. G. Garner, B. K. Blackman, S. L. Harmer, 2016, 'Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits', Science, vol. 353, no. 6299, pp. 587-590 http://dx.doi.org/10.1126/science.aaf9793
W. R. Briggs, 2016, 'How do sunflowers follow the Sun–and to what end?', Science, vol. 353, no. 6299, pp. 541-542 http://dx.doi.org/10.1126/science.aah4439
Casper J. van der Kooi, 2016, 'Plant Biology: Flower Orientation, Temperature Regulation and Pollinator Attraction', Current Biology, vol. 26, no. 21, pp. R1143-R1145 http://dx.doi.org/10.1016/j.cub.2016.08.071