Young sunflowers turn and swing every day. New findings add to evidence that the plants are animal-like.
Harmer, a professor in the University of California at Davis, Department of Plant Biology, carried out a series of experiments on sunflowers in the field, in pots outdoors and in indoor growth chambers.
By staking plants so that they could not move, Harmer showed that he could destroy their ability to track the sun. He also noticed that sunflowers prevented from moving were not as tough and leafy as those that were free to move. When plants were moved indoor with a settled overhead light, they continued to swing back and forth for a few days.
The indoor plants did start tracking the “sun” again when the apparent source of lighting was moved across the room. The plants could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours.
When sunflowers track the sun, the east sides of their stems grew more rapidly than the west sides. At night, the west sides grew faster as the stem swung the other way. The team identified a number of genes that were expressed at higher levels on the sunward side of the plant during the day or on the other side at night. A plant growth-regulating hormone (激素), called auxin, appears to be a key driver.
The “dance” to the sun cycle obviously slows when the sunflower matures (成熟) and its flowers open up. At that point, the plants stop moving during the day and settle down facing the sun in the east.
“Bees like warm flowers.” Harmer said, adding that the bees are cold-blooded, so landing on a warm flower saves them energy and perhaps feels really good.
“The morning warmth changes the flowers in a way to make them more appealing to insects, perhaps causing them to release more attractive scents earlier in the day.” he said “We're currently testing this idea.”
