Plant Movement: Heliotropism in Sunflowers
This year at Reiman Gardens our theme is ‘Movement’ and the Children’s Garden is exploring the movement of plants through heliotropism. Heliotropism is the motion of plant parts in response to the sun. Heliotropic plants slowly track the motion of the sun across the sky during the day, and then drift back during the nighttime. Sunflowers display heliotropic movement even on cloudy days, suggesting that there is a learned diurnal rhythm that is followed in the absence of a strong directional light.
Leaves and floral parts both exhibit heliotropism. Floral heliotropism is controlled by motor cells located just below the flower head. These cells move potassium ions between adjacent cells – drawing potassium in and expanding while in shade, and pushing potassium out and retracting in the sun. Sunflower (Helianthus annuus) exhibits heliotropism in apical buds and developing flowers but stops once the flower fully opens, leaving the head facing east. Not all sunflowers exhibit heliotropism – varieties that have more than one flower exhibit little to no heliotropism, possibly due to self-shading of the motor cells.
There are two cultivars of sunflower planted in the Children’s Garden ‘Teddy Bear’ and ‘Vincent’s Choice Deep Orange’. Come out to see them and learn about other kinds of plant movement such as Seismonastic and Photonastic movement and Geotropism.
To learn more about heliotropism in sunflowers, click here for a scholarly article from Plant Science.
Prepared by Lindsey Smith, Collections Curator