Researchers have found that sunflowers orient themselves according to circadian rhythms. This allows for better growth and the attraction of pollinator animals. The study was just published in the journal Science.
Growing sunflowers face east in the morning, turn to the west as the day progresses, and then return to facing east at night. As plants mature, however, they slow down and stop moving during the day. The researchers wanted to investigate the actual mechanisms involved as well as the benefits of this movement.
When the sunflowers were moved into an indoor growth chamber with an overhead light, the flowers continued to move as if they were still outside. This showed that they were following a natural rhythm and not simply detecting sunlight. If the researchers used overhead lighting to mimic an artificial day-night cycle, the sunflowers adapted to this new cycle and changed their movements. The sunflowers appeared to be using a circadian clock, the same mechanism that helps control sleep cycles in humans.
The movement of the sunflowers was controlled by uneven gene expression. The genes caused one side of the stem to grow faster than the other side, allowing the flower to sway in the proper direction. This not only explains the flowers’ movement but also provides an explanation for why mature flowers (which are no longer growing) stop moving.
There are a few benefits to tracking the sun. Sunflowers that had been immobilized with stakes showed reduced growth and leaf biomass. Moving to face the sun seemed to be important for healthy growth. The researchers also noticed another benefit. By facing the morning sun, the flowers warmed up quickly in the morning. Pollinators, such as bees and butterflies, prefer warmer plants. By facing east and absorbing extra sunlight, the sunflowers could attract more pollinating insects.
This study showed the first known case of a circadian clock in a plant. The findings also provide an explanation for how flowers orient themselves throughout the day-night cycle.
Hagop S. Atamian et al. Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science (2016).