ANN ARBOR—A brush with low doses of the herbicide dicamba spurs oxalis, a weed commonly known as wood sorrel, to produce more flowers, University of Michigan researchers found.
The finding suggests that farmers and homeowners are inadvertently making weeds growing around the perimeter of their fields or lawns stronger, the researchers say. Their results are published in the journal New Phytologist .
"The general public uses herbicides in their lawn to kill weeds. They're not using herbicides to make the weeds do better. They probably don't even know this is a potential side effect or something that can happen," said senior study author Regina Baucom , professor of ecology and evolutionary biology. "We wanted to study it at a fine level to figure out what it means in terms of population persistence and evolution."
During the application process, dicamba has a tendency to become whisked aloft on air currents and settle in low doses on plants adjacent to the target area of the herbicide. These plants often don't receive toxic doses of the herbicide, but can be affected by it. After conducting other research that involved dicamba, the scientists noticed that plants growing where the herbicide drifted seemed to produce more flowers.
Scientists first noted this phenomenon in the early 1900s, when botanists found that stress from low concentrations of toxic substances such as copper and zinc could boost growth in plants, algae and fungus. German scientists first called this a "stimulation effect," and later it became known as "hormesis."
To study this trend in oxalis, the researchers, led by recent U-M graduate Anah Botran, planted a field with oxalis and exposed half of the field to low levels of dicamba. The other half of the field acted as a control and was not treated with the herbicide. Botran, a graduate student, along with undergraduate students Olive Tatara, Elise Kanefsky and Betania Cornejo, measured the plants' growth, flower number, seed number and other measures of plant health.
The first year, the plants produced more flowers in the treated half of the plot compared to the control. The researchers also saw some evidence that the plants hit by dicamba produced more seeds in the first year. They repeated the experiment for a second year, but did not see the same increase in the number of seeds.
The researchers also examined whether the oxalis that overproduced flowers passed that trait down to its descendents—meaning that the ability to produce more flowers could have evolved in the population.
"We found no evidence of genetic variation—so what we've captured is basically just a single-generation response of the plants getting exposed to low doses of dicamba," Baucom said. "And it may translate to increased fitness in some years, but there's no evidence this trait would increase in the population, or evolve."
The team also wanted to understand if the change in oxalis blooms also changed the way pollinators interacted with the plants. Botran and the team of undergraduates observed pollinator visits to the plants, and found that plants exposed to dicamba—those with more flowers—actually had lower pollinator visitor rates. The researchers think that's because the pollinators actually had more flowers to visit, driving down the rate at which each flower was visited individually. The researchers also showed that flowers of plants exposed to dicamba were smaller.
"We didn't find a genetic effect of this increased production, and we didn't find that it consistently translated to fitness," Baucom said. "But we did find that the overproduction of flowers led to a different pattern of plant pollinator visitations."
Baucom says the next steps for this type of research will be looking at whether pollinators get enough sustenance from these smaller flowers. And while the team found no evidence that the response could evolve in this oxalis population, Baucom says other weed species or populations could respond differently, and their approach gives researchers a way to test that.
"What I think is scientifically interesting about this is that folks in both weed science and ecotoxicology have been interested in hormesis, and have been interested in whether this over production is heritable," she said. "We put this question in a framework where you could examine the potential that this trait will evolve."
New Phytologist