Go for the sun. It’s a prime directive for most plants on Earth. Gather enough light to drive carbon dioxide, an atmospheric gas, into sugars to fuel growth. The rest of the needed ingredients to build a plant come from underground and the roots. To build more roots, however, photosynthesis has to happen.
Go for the sun. It’s a prime directive
for most plants on Earth.
There are biological processes involved to achieve a goal. As with humans achieving great things, process matters . Plants tempered by selective pressures reach ever upward. Height has been something plants have striven for since colonizing land, and is of course relative to other plants competing for the light. There are early plants like Cooksonia and even giant mosses that were the giants of their day relative to other plants (up to 6″ and 24″, respectively). Fairly quickly, larger, modern tree-size scale plants appeared, like gigantic horsetails. The goal is to collect enough light to grow and reproduce, to make it to the next generation.
The sun is the ultimate source of energy on Earth– even the fossil fuels we use today. For at least 3.7 billion years , photosynthesis has taken sunlight, fixed carbon, evolved oxygen, and formed the base of most food webs.
Today our planet teems with interconnected life in both the sea and on land. The land is dominated by plants. Most of them are reaching beyond their neighbors, going for the sun. Going for the sun has been one driver of plant height. It’s energy intensive to be a tree, but height has its advantages. When an opening in a forest opens up, dormant seeds germinate and grow up as fast as they can to achieve height and shade or crowd out their neighbors. Some plants live on trees, or climb up them.
There are some exceptions to plants reaching for the sun, of course. Some plants are parasitic. Some are able to make it under the shade of other plants, living on the shafts of light that make it through a forest canopy. Some ferns have done just that, dominating many forest floors. In the desert, sunlight is not a limiting factor. Water is.
Growing to a Beat
Plants, indeed most life, have a rhythm to schedule resource management, rally defenses during times most likely for attack by diseases or herbivores, and optimizing growth while the sun shine(Greenham and McClung, 2015).
Just how do plants temper their growth and timing of events?
In 1729, French natural philosopher Jean-Jaques de Mairan did an experiment on what was probably Mimosa pudica, more famously known as the sensitive plant. An experiment only possible because of the relatively new global plant trade/exchange as Mimosa pudica is native to The Americas.
Left to themselves, Mimosa leaves fold down at night and re-open again during the early morning to catch as much sun as possible during the day. He noticed that this folding and unfolding happened even in the absence of the light, the first evidence of a biological clock within any organism. Internal clocks have now been found in most living organisms.
Anticipating the dawn, and ‘gating’ other responses– making them more or less intense at different times of day are two things an internal clock does. There is evidence that this includes defense responses to be raised at the time of day when pathogen attack is most likely to occur. More classically, flowering time is, in part, under control of the clock as well. Circadian rhythms are adaptive. They can be entrained by the environment, but will continue to run absent external cues as well, as de Mairan showed (Greenham and McCLung, 2015). And given the variable environment and parts of a plant not having uniform entrainment, there are, in fact, many clocks running at once (and similar things have been found in animals too- many organs have their own circadian clocks, synced with the brain’s master clock).
Heliotropism, or suntracking
Recently, heliotropic sunflowers have their sun-tracking movement dictated by their circadian clock (Atamian et al., 2016). It’s another example of a plant’s physiology being an output of the clock. Sunflowers (Helianthus annus) have been well known to track the sun. However, the precise mechanism is unknown. In Mimosa, leaf movements are driven by changes in water pressure at the leaf blade nodes. Sunflowers don’t track the sun via this mechanism. They require active stem growth to reorient each day.
Showing the the heliotropism of sunflowers is adaptive, the UC Davis, UC Berkeley, and University of Virginia based scientists show that early morning east-facing sunflowers are healthier plants (they rotated experimental plants to face west at dawn). They also observed that heliotropism slows over time and stops once the flower is produced. The Eastward reorientation in growth is more powerful than the Westward one, meaning that the flowers end up facing East most of the time. This means the final position of the floral head when growth ceases is to the rising sun, heating them up early in the day, attracting more pollinators. Sunflowers that had been tricked into having west-facing floral heads by rotating them 180º that had measurably fewer pollinators could have as many pollinators as their eastward facing counterparts through artificial heating.
The re-orientation throughout the 24 hour day is driven by the hormone auxin (Atamian et al. 2016; Briggs, 2016). Whichever side of the stem has a greater amount of auxin elongates more than the other side of the stem and it bends. Another interesting aspect of Atamian et al. is that gibberellins, another plant hormone are required for stem elongation. They showed a sunflower unable to make gibberellin couldn’t track the sun, showing that stem elongation is a key part of the heliotropic mechanism. This is one example of how hormones can interact to generate a 10 foot tall, sun-tracking, sunflower.
Sunflowers in Context
Sunflowers belong to one of the most successful groups of flowering plants (~10% of all flowering plants– only orchids have a similar number of species), the Asteraceae, or composite family (daisies are part of this family too). These plants are notable for their “composite” flower with many florets (including the ‘ray florets’ that look like petals) in their flower head able to be pollinated and produce seeds. What we see as the ‘sun flower’ is actually many tiny florets, most able to be pollinated and produce a seed.
Sunflowers are important economically and culturally (and native to North America, though they have been spread worldwide). We eat their seeds and use them as an oil producing crop (Russia’s Peter the Great started the trend of sunflower in the 1850s) and were popular with the Russian Orthodox church as sunflower oil was a cooking oil not banned during lent. Famously, Van Gogh painted sunflowers and modern geneticists have likely figured out the mutation causing the appearance of some of the sunflowers he decided to paint. And last, The Hopi sunflower, a semi-domesticated sunflower land race was used by Native Americans as a dye (Thanks to University of Georgia scientist Rishi R. Masalia for these sunflower facts).
What We Have in Common With Sunflowers
We humans have tracked solar cycles for a long time and recently started to reach for our host star. We also have circadian clocks operating in our bodies. Getting closer than any plant likely has with our far flung satellites and astronauts on the ISS. Those people exploring are literally fueled by plants reaching for the sun generation after generation. Star Trek is 50 this year. Something both humans and plants can celebrate is aiming for the stars. If humans do boldly go where no one has gone before, they’ll encounter alien plants– or plant-like primary producers on strange other worlds too, boldly growing towards their planet’s star(s).
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 353:587-590. doi: 10.1126/science.aaf9793
Briggs, W.R. (2016). How do sunflowers follow the sun– and to what end?. Science 353:541 doi: 10.1126/science.aah4439
Greenham, K., McClung, C.R., (2015). Integrating circadian dynamics with physiological processes in plants. Nature Rev. Genetics 16: 598-610 doi: 10.1038/nrg3976