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Heliotropism

Sunflowers turn their heads to follow the sun. One of the most striking sights in the countryside is a field of yellow sunflowers whose heads slowly turn in unison as the sun moves across the sky.


I had never really thought much about sunflower behaviour other than having picked up this fun fact at some point. One August evening in Wisconsin, however, we were driving past a field of sunflowers. The sun was low in the sky and I suddenly realised the sunflowers were facing in the opposite direction from the sun. Why? I had to investigate further!


Heliotropism - the movement of plant parts to slowly track the motion of the sun across the sky during the day, and then they drift back to face the East during nighttime.

Sunflowers are among the most strikingly recognisable and easy to grow of all flowers. Although some varieties are short, many are 2-3 metres tall, and some selected varieties can reach 5 metres or even taller. They are natives of North America and early travellers carried them back and forth across the Atlantic, humans and plants journeying together across seas and continents, finding new places to become established, sometimes returning home years later. Their name in many languages reflects their solar tracking behaviour and their sunny appearance: tournesol, girasol, girasole, helianthus, suncokret, slunečnice, solsikke, zonnebloem.


The shoots and buds of sunflowers track the sun from morning until evening, the young plants absorbing as much light energy as they can by always turning toward it. Each new dawn finds them facing the east again, ready to catch the first light of the morning. As the plants grow taller and the flower heads begin to open, the daily cycle continues. Entire fields, row upon row, grow tall together and slowly turn in perfect unison.


Since the early morning is the busiest time for bees, sunflowers produce their pollen early to profit from the activity. The bees adore warm flowers, so they prefer to visit those which catch the sun in the morning as it first appears. The yellow sunflower heads with black centres act like little warming stations for pollinators, attracting them to visit when their pollen load is at its most abundant and by warming the bees they help to ensure efficient pollination. In experiments, the plants whose flowers face east in the morning grow better and produce more seeds than those which are turned to face away from the dawn, so beneficial is their sun-tracking behaviour. The tracking behaviour continues until the flowers are in full bloom.


The movements of flower heads to follow the sun is initiated by the plant itself in accordance with its internal daily clock, although the exact control mechanisms are not completely understood. The flowers move even when the weather is cloudy and the cyclical movements continue for a few days after the plant is rotated to face a different direction, showing that there is indeed an intrinsic control mechanism and biological clock directing the behaviour. The movement of the flowers to face east again during the night is also likely to be related to the internal circadian clock.


The sun is a very important influence on the plants’ movements in several ways. The shadier side of the sunflower stem grows faster due to more auxins being produced in reduced light. Auxins are plant hormones which promote growth, so the shady side of the stem grows more than the sunny side, gently bending the stem towards the sunlight. By changing the acidity of plant tissues through a cell signalling system, sunlight also increases the level of some cell wall proteins which allow the plant cells to be more extensible or stretchy and therefore allows the stems to make bigger movements. Buds and young flowers bend and stretch in response to the glorious sunlight in a daily dance from east to west followed by a nightly return to their starting position.


As the season advances, the seeds swell and weigh increasingly on the stems. Under the extra load, cell walls thicken and become less flexible; as the stems become older and stronger, they become more rigid. The sunflowers’ range of movement diminishes until eventually they stop tracking the sun altogether. From around the time of full bloom, their faces remain fixed most usually towards the east..



Once the seeds are mature, the plants enter a phase called senescence. The nutrients are withdrawn from the leaves and stems so that the once splendid plant withers to the ground where roots and scattered seeds quietly wait for the soil to warm again in the spring.

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Love these sunflowers. We had a wild one that grew over 6 ft. Our bees loved it too.

Thanks for sharing its history.

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