Education, Outreach & Diversity

The Beauty and Versatility of Sunflowers

By Andrew F. Galloway

Sunflowers 2


For millennia, sunflowers (Helianthus) have symbolised happiness, being synonymous with the golden rays of the sun, as described by their botanical name Heli [relating to the sun] anthus [flower]These plants are cherished by many, particularly with school children who aim to grow the world’s tallest; although they would have to grow over 9.2 metres if they wish to surpass the Guinness World Record holder in Germany from 2014 [1].

Sunflowers belong to the wider daisy family (Asteraceae; named after the Aster), which has an estimated 70 species varying in a plethora of colour and size. Sunflowers use their petals to advertise that their nectar is available to pollinating insects such as bees. Unlike people, insects use a different type of light (ultra-violet or UV) in order to see. To a passing person, sunflowers have vibrant yellow petals tethered to a dark centre. However, to a bee, that sunflower displays a bullseye that strongly stands out to them compared to the background, indicating where they can collect nectar [2]. Each flower head is formed of thousands of miniature flowers, called florets that can produce up to 2,000 seeds [3]. The arrangement of these seeds, commonly 34 seeds clockwise and 55 seeds anti-clockwise follows to the Fibonacci sequence. This means that all sunflowers have their seeds arranged using the golden angle (137.5º) to form a mesmerising double spiral pattern [3-4].

Sunflowers were domesticated about 5,000 years ago in the eastern part of the United States by Native Americans, who consumed their seeds and extracted their oil for use in bread [3]. The Native Americans also used sunflowers for medicinal purposes, their fibres, and as dye for clothing [3]. In the 1500s, Spanish explorers brought these plants to Europe where they were popularised as an ornamental curiosity for the rich [3]. Many years after their introduction, they were commercialised as an agricultural crop in Russia [5]. As of 2020, Ukraine is the world’s largest producer of sunflowers at 16.5 million metric tonnes, where it is unsurprisingly the country’s national flower [3&6]. Broadly speaking there are two types of sunflower seeds which are harvested, black and stripped. The black seeds contain more oil that is extracted for margarine and standalone cooking oil. Whereas, the stripped seeds are used for consumption as they are rich in protein and unsaturated fats, and a good source of fibre and vitamin E [7].

Ever since they were grown, it was believed that sunflower leaves and flowers could track the sun. However, this is not entirely true. A young sunflower’s leaves can indeed orientate themselves towards the moving sun, a response called phototropism [8]. When sunflowers mature, they remain in a fixed easterly position to capture the morning’s sun. In a study where sunflowers were grown inside within an isolated growth chamber that had a static light from above, the young sunflowers kept moving from east to west [9]. An inbuilt molecular clock known as the circadian clock, which also wakes us in the morning and makes us feel tired at night, enables sunflowers to discriminate between east to west. Sunflowers orientate themselves through their growth by elongating one side of their stem compared to the other, and thus causing their stem to bend in one particular direction [10]. Having an easterly fixed position benefits the plant by increasing the evaporation of morning dew, which can reduce the probability of fungal infection. It can also reduce excess heat when the sun peaks at noon, therefore protecting pollen and developing seeds from scorching [11].

As well as being used as an agriculture crop, sunflowers can be used to decontaminate soils from heavy metals, which are highly toxic elements (such as arsenic, lead and mercury) to people [12]. Industrial sites, mining, landfill and sewage are major sources of these pollutants [12-13]. Traditional methods of decontaminating soils are expensive and arduous, requiring large amounts of chemicals and equipment to remove them. An alternative method that is estimated to reduce costs by half cost compared to the traditional method is phytoremediation, or plainly put, the use of plants to remove these hazardous elements [14]. Some varieties of sunflowers are able to accumulate these elements from deep within the soil into their tissues, including leaves and roots, without a determinantal effect on their health [14]. After accumulating these toxic elements, a grower can simply remove the plants and place within a secured area for processing. Processing involves burning the contaminated plants at high temperatures to remove organics, collecting the heavy metals, and compacting them into glass-like material for stable storage [13].

In addition to heavy metals, sunflowers can accumulate radioactive elements such as uranium and cesium without too greater impact on their health. The Chernobyl nuclear disaster was one of the worse in history, contaminating vast areas of fertile land around the power plant. Common annual sunflowers were used to reduce levels of radioactive cesium and strontium pollution from the nearby containment land [15-16]. Following the success of using sunflowers to reduce radioactive elements around Chernobyl, volunteers planted millions of sunflowers near the Fukushima Daiiachi nuclear plant (within 100 km) to reduce radiation within the soil [17]. However, this turned out not to be so successful as they unknowingly planted the wrong variety of sunflower, which was unable to accumulate as much of these radioactive elements. It remains unclear how some varieties can accumulate toxic pollution, whereas other cannot. 

Throughout the millennia, many have used the sunflower as a symbol for happiness, as well as being associated with positivity, the sunflower has been demonstrated to be a highly versatile crop for the modern world. Under UV, they appear as bright bullseyes guiding in pollinating insects like landing strips. Once pollenated each flower head can produce up to 2,000 seeds, which can be eaten or turned into oil. The humble sunflower has also become a natural source of decontamination from leftover toxic heavy metals and radioactive elements.

Sources

[1] Guinness World Records (2014). Tallest sunflower. https://www.guinnessworldrecords.com/world-records/tallest-sunflower#:~:text=The%20tallest%20sunflower%20measures%209.17,held%20this%20record%20twice%20previously

[2] Moyers, BT. Owens, GL. Baute, GJ. Rieseberg, LH. (2017). The genetic architecture of UV floral patterning in sunflower. Annals of Botany. 120: 39–50. https://doi.org/10.1093/aob/mcx038

[3] Helianthus annuus L. (Asteraceae). Oxford Plants 400: https://herbaria.plants.ox.ac.uk/bol/plants400/profiles/GH/Helianthusannuus

[4] Kuhlemeier, C. (2007). Phyllotaxis. Trends in Plant Science. 12: 143-150. DOI: https://doi.org/10.1016/j.tplants.2007.03.004 

[5] Putt, ED. (1997). Early History of Sunflower. In Sunflower Technology and Production, A.A. Schneiter (Ed.). https://doi.org/10.2134/agronmonogr35.c1

[6] Food and Agriculture Organisation of the United Nations. (2020). Food Outlook, Biannual Report on Global Food Markets. http://www.fao.org/3/ca9509en/ca9509en.pdf

[7] Adeleke, BS. Babalola, OO. (2020). Oilseed crop sunflower (Helianthus annuus) as a source of food: Nutritional and health benefits. Food Science & Nutrition. 8: 4666-4684. https://doi.org/10.1002/fsn3.1783

[8] Wyk, AS. Prinsloo, G. (2019). Challenging current interpretation of sunflower movements. Journal of Experimental Botany. 70: 6049 6056 https://doi.org/10.1093/jxb/erz381

[9] Atamian, HS. Creux, NM. Brown, EA. Garner, AG. Blackman, BK. Harmer, SL. (2019). Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science. 353: 587-590. https://doi.org/10.1126/science.aaf9793

[10] Moulia, B. Bastien, R. Chauvet-Thiry, H. Leblanc-Fournier, N. (2019). Posture control in land plants: growth, position sensing, proprioception, balance, and elasticity, Journal of Experimental Botany. 70: 3467–3494. https://doi.org/10.1093/jxb/erz278

[11] Vandenbrink, JP. Brown, EA. Harmer, SL. Blackman, BK. (2014). Turning heads: The biology of solar tracking in sunflower. Plant Science. 224: 20-26. https://doi.org/10.1016/j.plantsci.2014.04.006

[12] Jun, L. Wei, H. Aili, M. Juan, N. Hongyan, H. Yunhua, Z. Cuiying, P. (2020). Effect of lychee biochar on the remediation of heavy metal-contaminated soil using sunflower: A field experiment. Environmental Research. 188:109886. https://doi.org/10.1016/j.envres.2020.109886

[13] Greipsson, S. (2011) Phytoremediation. Nature Education Knowledge 3:7. https://www.nature.com/scitable/knowledge/library/phytoremediation-17359669/#:~:text=reductase%20(merA).-,Phytoextraction,of%20contaminants%20in%20the%20soil

[14] Zehra, A. Sahito, ZA. Tong, W. Tang, L. Hamid, Y. Khan, MB. Ali, Z. Naqvi, B. Yang, X. (2020). Assessment of sunflower germplasm for phytoremediation of lead-polluted soil and production of seed oil and seed meal for human and animal consumption. Journal of Environmental Science. 87: 24-38. https://www.doi.org/10.1016/j.jes.2019.05.031

[15] Revkin, AC. (2001). New Pollution Tool: Toxic Avengers With Leaves. The New York Times. https://www.nytimes.com/2001/03/06/science/new-pollution-tool-toxic-avengers-with-leaves.html?mtrref=www.google.com&gwh=E07A1758D21602B0C107873E02A6735B&gwt=pay

[16] Soudek, P. Valenová, S. Vavříková, Z. Vaněk, T. (2006). 137Cs and 90Sr uptake by sunflower cultivated under hydroponic conditions. Journal of Environmental Radioactivity. 88: 236-250. https://doi.org/10.1016/j.jenvrad.2006.02.005

[17] Slodkowski, A. Nakao, Y. 2011. Sunflowers melt Fukushima's nuclear "snow". Reuters. https://www.reuters.com/article/us-japan-disaster-sunflowers-idUSTRE77I0PG20110819

Andrew Galloway

Andrew Fife Galloway

Research Coordinator, Wellcome Centre for Integrative Neuroimaging, 
University of Oxford

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