How Sunflowers Worship the Sun
By Bob Engel
Marlboro College Professor Emeritus of Biology and Environmental Science
One of the icons of the fall season is the abundance of blooming members of the Asteraceae. Things like asters, goldenrods, black-eyed susans, and ragweeds are everywhere. And so are the sunflowers you planted last spring. Plate-sized flowers (it’s more complicated than this) full of seeds (also more complicated) standing tall and proud. The spring is owned by violets, the fall by asters and allies. Why this is so is a bit obscure, but that didn’t stop me from asking that very question of numerous graduating seniors in biology (most of the plants use the fall winds for dispersal; increasingly uncommon birds need not apply).
We have something hot off the presses (Science, v. 353, 5 August) about sunflowers, and because they’re blooming now, I decided to pass on a bit the story.
Like a lot of plants, the young, flowering stems of sunflowers track the sun — they are heliotropic.
The young stems start out the day facing east, then track the sun across the sky where they end up facing west. And then, by dawn, they’re peering east again. Nice trick. I have friends whose solar arrays do the same things. It's a win for both: the panels and the plants collect more sunlight by tracking, with the moving plants growing faster than experimentally tethered controls that can’t move. Fine, but how do plants move at all?
There are only two possible ways. The first is mediated by a hormone (Auxin) that causes the plant cells on the shady side of the stem to elongate, thus pushing the plant toward the sun. So, early in the day, the west side of the stem causes the movement, but later, the east side is in the shade and keeps the process going. A second possible mechanism for movement is driven by pumping ions, creating an osmotic gradient that water eventually follows. Here, it’s a hydraulic pumping mechanism that effects the movement. Both are pretty neat (the slamming shut of a Venus fly trap is hydraulic), but it’s the hormone-mediated response that is involved in sunflower heliotropism.
So does the stem get back east? It happens at night and is driven by a typical biological rhythm that provides the cue for the auxin instead of the sun.. At night, the auxin is slowly relocated to the west side of the stem, slowly turning the stem, and, BINGO, it’s all ready to go again the next morning. We know it’s a rhythm, because like almost all other examples, the cycle persists for a while in constant low light and because it’s screwed up if the plants are placed in an artificial 30-hour day (a kind of jet-lag). The scientists also noticed that the amount of auxin in the stems fluctuates on a rhythmic 24-hour cycle.
The final question has to do with why the mature flowers all point east (check it out). Too heavy to move? Maybe, but they also heat up quickly that way and get more pollinator visitation than flowers experimentally oriented toward the west. And, it doesn’t hurt that the sensitivity of the whole system is more east-directed than west-directed.
By the way, the two complications mentioned at the start of these ramblings have to do with the aster family. It used to be called the Compositae because most of its members have two types of flowers that compose what we call the flower. In the case of the sunflower, the yellow “petals” are actually sterile flowers that help attract pollinators, with the small, almost black fertile flowers doing the reproducing in the center. The other complication? They’re fruits, not seeds, because the real seed that we and a lot of other animals eat resides in that dry husk. Chickadees have to peck through this — at or near your feeder — to get at that very energy-dense seed. The fruit is called an achene. So think maybe 200 flowers, some that look like petals, the rest that do the deed, all in a dense head. Don’t believe me? Check out the “petals” on a aster. Sometimes you can see a small, sterile filament, the remnant of the female style in a typical flower.