Phototropism in Sunflowers

Defining Phototropism

• Phototropism is the attraction of plants to a light source. In nature, this light source is the sun, but artificial alternatives like indoor lighting can also stimulate phototropism. Plants will grow in the direction of the strongest light source. This is an evolutionary adaptation developed by plants to maximize the amount of sunlight received by the plant. This allows the plant to better undergo photosynthesis and produce glucose for metabolic needs and storage.

How Phototropism Works

• Receptors made of chemical complexes known as phytochromes absorb visible wavelengths of light. When they absorb this light, they emit a chemical signal that produces a hormone known as auxin. Auxin promotes the growth of cells located in the stem of the plant, specifically the darker side of the sunflower. The growth of cells on the light-receiving side of the plant is inhibited. As the shaded side of the plant extends and the lighted side remains the same in length, the new growth of the plant will eventually be "turned" towards the light source.

How Sunflowers Exhibit Phototropism

• Sunflowers exhibit a rapid form of phototropism. They are sometimes considered heliotropic, as before the buds have fully blossomed they will follow the sun from east to west. However, they are not truly heliotropic, as sunflowers do not exhibit this behavior after the flower has fully opened. This is because the stem of the sunflower becomes much stiffer after the bud has fully blossomed. Sunflowers typically do not grow in light-restricted conditions and do not require heliotropic behavior to grow optimally.

Heliotropism is Not Phototropism

• Often, heliotropism will be confused with phototropism. Heliotropism is a plant behavior in which the flower of a plant will follow the sun throughout the course of the day. This is done to maximize the light received during the day and is common among plants that have native environments that are low in light or have short days. Heliotropism is a much more accelerated process than phototropism, which will typically happen over the course of a few days rather than a few hours.