Sunflower plants exhibit a wide variety of behaviours designed to encourage successful growth, including heliotropism, phototropism and gravitropism. Some sunflower plant behavioural processes, such as circadian rhythms, are innate and naturally occurring. Other behaviours, such as flower heads that grow facing down, are developed in hybrid plants for particular benefits.
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Sunflower plants exhibit heliotropism, or solar tracking, as soon as the sprouts emerge from the soil. As the sun rises, seedlings, buds and leaves face the east, absorbing warmth and energy. As the day progresses, the plants track the movement of the sun across the sky, turning to the west at dusk. During the night, the plants return to an east-facing position, ready for the morning sunrise. About one day before sunflower buds blossom into fully developed flowers, the heliotropic behaviour ends and the flower heads remain in an eastward facing position.
Sunflower plants also display phototropism, meaning that the plants will display a positive reaction to light and grow toward a light source. This is due to hormones in the plant called auxins. The auxins retreat from the light source and gather in the dark side of the seedlings and stems. Once gathered, these hormones encourage growth on one side of the plant, causing the darker side to elongate and curve the plant toward the light source.
Gravitropism causes sunflower plants to naturally grow upward, responding to the gravitational pull. If a sunflower seedling is grown in a vertical container, then placed on its side, the plant will bend itself and continue to grow skyward. Like phototropism, auxin concentration is responsible for this behaviour. Auxins will gather on the underside of a sunflower plant placed on its side. This encourages growth and elongation on the underside, bending the plant to an upright position.
The innate circadian rhythms of the sunflower plant function as an internal clock and calendar, controlling the plant’s responses to the sunlight available at different times of the day or during different seasons. This allows the sunflower plant to maximise exposure to sunlight even on dark, overcast days. Even when placed in a dark closet, the sunflower plant continues its rotation from east to west, finally reorienting itself to the east, as if waiting for sunrise.
Sunflower seeds are a valuable North American crop used for oil, meal, food products and birdseed. Unfortunately, sunflower seeds are also favoured by wild birds, which can feed relentlessly on crops leaving little for the harvest. To protect commercial sunflower seed crops, bird-resistant hybrids have been developed that display new sunflower plant behaviours. After reaching full height and full bloom maturity, the flower heads of these commercial cultivars begin growing downward, hiding the ripening seeds from hungry birds.
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- Indiana University Department of Biology: Sunflower Plants
- Claude E. Phillips Herbarium: Gravitropism
- Claude E. Phillips Herbarium: Phototropism
- Indiana Public Media: Keeping Sunflowers in the Dark
- University of Tennessee Institute of Agriculture: Sunflower: An Alternative Crop for Tennessee Producers