Sunflowers are turning heads

Sunflowers in Fargo, North Dakota, USA. Image released into the public domain by the United States Department of Agriculture.

Sunflowers in Fargo, North Dakota, USA. Image released into the public domain by the United States Department of Agriculture.

It is August and the sun is shining here in Norwich*, so what better plant to be the Organism of the Month than the sunflower?

Sunflowers are the subjects of some of Vincent Van Gogh’s most famous paintings, but they are cultivated for more than just their beauty. Sunflower oil, made from compressing sunflower seeds, is commonly used in cooking and to make biodiesel. It is the 4th most highly consumed oil in the World (behind palm, soybean and oil seed rape) (1). The seeds are also edible and are promoted as good sources of some vitamins, minerals and cholesterol-lowering compounds.

Sunflowers come from from the Americas and have been cultivated there for thousands of years (2). Native American tribes found a variety of uses for sunflowers. The seeds were ground up to make flour and the fleshy flower head used as a vegetable in cooking. Red, blue, purple and black pigments were extracted from the seeds to make dyes. The fibrous leaves and stems were used for the weaving of fabric and baskets. Sunflowers even had a role in worship of the Aztec’s Sun God. Although sunflowers first arrived in Spain in the early 16th century, it was Russia who first started producing sunflower oil commercially in the late 18th century (2). Russia remained the World’s largest producer of sunflower oil until the end of the 20th century, when it was overtaken by Argentina.

Image by Tim Bartel  via Wikimedia Commons. Licensed under CC BY-SA 2.0.

Image by Tim Bartel via Wikimedia Commons. Licensed under CC BY-SA 2.0.

Sunflowers (latin name Helianthus annuus) belong to the Daisy (Compositae/Asteraceae) family  of plants. The members of this family have what are known as compound flowers, where the flower heads are actually made up of many individual tiny flowers. The centre of a sunflower flower head is full of black florets that produce the sunflower seeds. Around the outside of the flower-head is a set of ray florets, which each have a single large yellow petal. The ray florets are infertile and their main function is to attract insect pollinators.

If you observe sunflowers over the course of the day, you will notice that the flower heads move to track the sun in the sky overhead. In the morning they face East for sunrise and then slowly change position so that by sunset they face West. Overnight, the flower heads reposition to face East again. How this process, known as heliotropism, works in sunflowers is a bit of a mystery. Since the flower heads stop tracking the sun when they mature, it has been suggested that the turning of the flower head might be due to localized growth of cells on one side of the plant stem near the flower head.

How is heliotropism in sunflowers regulated? It is assumed that light-sensing plays a role, but it isn’t the only factor because flower heads still track East to West on a cloudy day when the sun isn’t visible. Also, if you rotate sunflowers by 180 degrees overnight so that the flower heads face West at dawn instead of East, the flower heads continue to move the way they would have done in the old position. It takes a few days for them to change their movement to track the sun again. This indicates that the day-night cycle of flower head movement in sunflowers is mostly regulated by the plants own internal (circadian) clock, with light-sensing making adjustments when needed.

Along with not understanding HOW heliotropism in sunflowers works, we don’t understand WHY it is beneficial. One possibility is that the increase in light falling on the flower head leads to increased photosynthesis (plant food production using light energy) in the flower head. Another possibility is that by tracking the sun, the flower heads heat up more over the course of the day, which could help them attract insect pollinators, and/or increase the speed of seed formation.

* Or at least it was when the author wrote this. Sunshine is not guaranteed, especially if you live in the UK!

References:

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12 thoughts on “Sunflowers are turning heads

  1. Thank you Sarah – love sunflowers. Question – might the tracking be not so much to do with light as to do with heat? If the sun shines on one side of the plant (even through cloud) that side will receive more (infra-red?) heat rays – now if that affects the size or shape of the cells on that side of the plant (by say making them contract or expand or stretch) might not that result in the plant twisting (think of shrinking the fibres in one half of a skein of wool)?

    Just an idea.

    Best wishes

    Huw

    • I guess thats possible Huw, although plant cells are bound by a cell wall so shape changes are quite restrictive. In some other species, heliotropism works by cells on one side of the stem taking in more water so they become turgid (more rigid) causing the stem to bend/straighten. However, sunflowers dont appear to have this set up.

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  3. Another great post, Sarah! The part about the circadian test of heliotropism in sunflowers was really interesting, although I guess it makes sense if you think about it or the flowers wouldn’t return to their starting positions overnight!

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