Science

Why young sunflowers follow the sun

Every day, young sunflowers follow the sun like spectators at an incredibly slow tennis match. But scientists have never known why, or why the tracking stops when they become adults. Now, a new study suggests that this daily sun worship is guided by circadian rhythms during development. In a series of experiments, scientists tied down young plants to prevent them from moving or rotated them so they were facing the wrong way when the sun rose. When the plants’ normal movements were thwarted, they grew far more slowly than regular sunflowers, with about a 10% decrease in both biomass and leaf area. Researchers say the rhythmic tracking helps the plants grow bigger, allowing them to add cells on whichever side is doing the “stretching”: the east side during the day and the west side at night. When the sunflowers finally settle down and stop this daily tracking, their position also gives them an advantage, researchers report this week in Science. A second set of experiments showed that mature plants’ permanent eastward orientation makes for warmer flower faces, which may be responsible for a large increase in pollinator visits—five times more than in mature plants that face west. Looks like following the sun is more than just a spectator sport!