How and why do sunflowers follow the sun?

Sunflowers are known worldwide for their movements following the sun. These beautiful yellow flowers are the visible proof that plants are not static organisms, and their movements have been inspiring poets, musicians – and also scientists – throughout decades. In the morning, sunflowers face the east, awaiting for the rising sun and then follow its light through the day. But how are these movements possible?

Most living organisms, from bacteria to complex multicellular groups, display daily rhythms, usually called circadian rhythms, that are mainly regulated by light, temperature and the endogenous circadian clock. In plants, circadian rhythm can be observed, for instance, in differential growth rates during the day and at night. This process is regulated by complex physiological interactions between the amount of growing-related hormones, light-signaling components and different cells’ sensitivities, both to light and to hormones, during distinct periods of a 24h natural cycle.

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Fig. 1. Sunflower growth, represented by dry mass and leaf area of control individuals (C) and plants that were rotated to face the opposite side at nightfall (R).

Plants need light for photosynthesis and, as a strategy to increase photosynthesis rates, they tend to grow their photosynthetic organs, mainly leaves, to the direction of incoming light. This type of growth is called phototropism and it was recognized long time ago. A more dynamic type of phototropism – the heliotropism – is observed in sunflowers and allows aerial portions of the plant, such as flowers, to follow sun’s movement during the day. Moreover, sunflowers also move during the night, in a manner that the flower faces east even before sunrise.

Heliotropism is possible because of deferential cellular growth of sunflower’s stem sides. During the day, more intense cellular growth occurs at the east side of the stem, driving the plant to face west. At night, stem’s west side grow faster, and the flower “looks” again to the east, awaiting the rising sun (watch here). Researchers from the University of California performed a series of experiments to better understand how this process happens and why it is important.

The scientists have performed experiments, in which they rotated potted plants every evening (so sunflowers faced east at nightfall) or limited stems’ movements. They found that when sunflowers’ regular movements were either manipulated or restricted, their growth were significant impaired, indicating that heliotropism is essential for normal growth of young individuals. Also, they discovered that sunflower’s movements are led by genes that are differentially activated by light and hormones in each size of the stem at different periods of a 24h cycle.

Next question that scientists wanted to answer was: what if there were no sunlight directional cues? Would a sunflower continue its day/night cycle? Yes! They found that when sunflowers were moved to constant, fixed overhead lightning, they maintain directional growth for several days. Flowers tended to face east during the “subjective night” and west during “subjective day”. This result is extremely curious, since many animals also display an endogenous physiological rhythm in the absence of environmental circadian cues.

In nature, it’s easily observed that movements following sunlight are exhibited only by young sunflowers. By performing another series of observations, scientists discovered that apices’ growth ceases gradually at the final stage of floral development. But is there any ecological reason for that? – it was another question the group has asked.

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Fig. 2. Temperatures of east-facing (E) and west-facing (W) sunflowers at hourly intervals

When a flower faces the sun, it increases its temperature, what attracts more pollinators. So, researchers thought that an eastward orientation may promote sunflower attractiveness to pollinators. To experimentally test this hypothesis, they rotate some individuals orientation in the field, and monitored their temperature and pollinators’ visits throughout the day. As expected, they found that non-manipulated plants were warmer and were visited by pollinators 5 times more than the rotated ones. Interestingly, when rotated plants were artificially warmed, they recovered a great number of visitors. Thus, temperature really determines differential attractiveness of flowers to pollinators, and heliotropic movements are essential for that! After pollination, there’s no need for heliotropic movements to continue, so it makes sense that the movement ceases in older individuals.

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Fig. 3. Pollinators visits to east-facing and west-facing sunflowers (A); Floral temperature (B) and pollinators visits (C) to sunflowers with east or west (with or without supplemental heat) orientations.

The direction and the amount of solar irradiation undergo predictable daily changes and all living organisms might coordinate their physiological process with these variations in the environment. Watch the video below to better understand why, in sunflowers, such coordination generates heliotropic movements of young individuals, enhancing plant growth and playing an essential role for reproduction.

Bruna de Oliveira Cassettari

Reference:

Atamian, H. S.; Creux, N. M.; Brown, E. A.; Garner, A. G. Blackman, B. K. Harmer, S. L. (2016) Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science 353 (6299), 587-590. [doi: 10.1126/science.aaf9793]

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