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July 20, 2009

Weed killer may make maize more nutritious

by Emily Sohn

Herbicides get a bad rap for their hazardous effects on the environment, but there may be a silver lining.

A new study found that a common weed-killer can actually make sweet corn more nutritious. It's the first study to show health-boosting properties of any herbicide on any crop. While the same surely won't be true for all chemically treated produce, the researchers involved in the work are inspired enough by their results to start looking for more examples.

"We think this is a new area of crop science," said Dean Kopsell, a vegetable crop physiologist at The University of Tennessee in Knoxville. "We've been applying herbicides to these crops, but no on has ever looked at their nutritional value."

For their first inroads into the subject, Kopsell and colleagues investigated a common herbicide called mesotrione, which farmers have been using for about a decade to keep weeds away from field corn, sweet corn and popcorn.

Mesotrione works by interfering with a weed's ability to produce carotinoids -- pigments that simultaneously help the plants absorb light, protect them from the sun's harmful rays, and make them appear orange when green chlorophyll pigments die every autumn.

Carotinoids (which include beta-carotene in carrots, lutein in spinach, and lycopene in tomatoes) also act as antioxidants, protecting the people who eat them from cancers, heart disease, and other illnesses.

When sprayed with mesotrione, weeds lose carotinoids and their protective effects, turn white and die. "They get sort of fried to death," Kopsell said.

He and colleagues tested the herbicide on three varieties of sweet corn: one that can tolerate the chemical well, one that takes a beating along with the weeds, and one that turns a little white soon after being sprayed but usually recovers and survives. Forty-five days later, the researchers harvested mature corncobs and tested kernels in the lab.

In the intermediate strain, the scientists were surprised to find as many as 15 percent more carotinoids in sprayed corn compared to unsprayed corn. A type of carotinoid called zeaxanthins got the biggest boost.
Zeaxanthins don't show up naturally in many foods, but they play an important role in protecting the eyes from cataracts and macular degeneration. The more of them we can get, the better.

"If I can say one thing," Kopsell said, "This is an example that there are herbicidal chemicals that can increase the nutritional value of our crops."

Stress probably explains the results of the new study, Kopsell suspects. In response to the battering that corn gets from mesotrione, the plant seems to produce extra carotinoids, which in turn produces cobs that are healthier for us.

Stress may be one of the same reasons some organic crops have been shown to have extra nutrients, he added. Without the cushy protection of pesticides and herbicides, organic crops have to fend for themselves more than conventional versions do. His analyses, he added, found no residual mesotrione on the treated kernels.

While more work is needed to know whether the effect will be consistent, the new study shakes up conventional notions of how herbicides work and what kinds of roles they might play, said Vincent Fritz, a horticulturalist at the University of Minnesota in Waseca.

"It really represents a paradigm shift on herbicides and what they do," Fritz said. "This is something we overlooked. We need to rethink the potential use of herbicides for the benefit of plants and human health other than weed control."

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