How weed killers might protect our eyes: It’s corny

Farmers — and the rest of us — often employ modern chemistry to protect our botanical babies from bullying weeds. And while our crops and ornamentals may benefit, weeds have always been the real target of the herbicides that we apply. But that may change.

A new study finds that at least one herbicide, when applied directly onto young sweet corn plants, can boost the pigment content of the kernels that mature a month or more later. Dietary sources of those pigments appear to play a critical role in helping the human eye stay healthy.

The newly observed corn effect is highly selective, says Dean Kopsell of the University of Tennessee, in Knoxville. It’s also far from what this vegetable crop physiologist had expected when his team began its experiments.

The herbicide they’ve been studying — mesotrione — works by mucking up the leafy machinery by which plants produce a family of important chemicals known as carotenoids. They’re best known as the plant pigments responsible for making carrots orange and watermelons red. But they’re produced in almost all plants, even those that are simply green.

Carotenoids can serve as solar collectors, Kopsell notes, harvesting light just as chlorophyll does (but from a range of wavelengths chlorophyll doesn’t handle effectively or at all). Carotenoids also help dissipate heat so that sunlight doesn’t fry tender plants. Bottom line: In broad-leaf weeds, mesotrione will bleach the greenery and eventually kill the plant.

The expectation was that if mesotrione did this to weeds, it couldn’t possibly be good for corn, Kopsell says. At best, his team suspected, crops might find the herbicide mildly harmful. And indeed, hitting young corn plants with mesotrione bleached their leaves and temporarily stunted their growth. Not good things.

But over a period of weeks, the corn recovered. And to the UT scientists’ surprise, they witnessed evidence of that old adage: What doesn’t kill you only makes you stronger. When temporarily inhibited carotenoid production in the corn resumed, it did so with gusto. The result: Concentrations of two of these pigments in corn kernels — lutein and zeaxanthin — were some 15 percent higher than normal. The findings will appear in an upcoming issue of the Journal of Agricultural and Food Chemistry.

A large and growing body of evidence indicates that these carotenoids play an important role in the human eye by absorbing blue light. Without sufficient concentrations of lutein and zeaxanthin in the retina’s macula, this region of the eye may become damaged.

Indeed, a review of the literature by scientists at Tufts University and the Agriculture Department’s Jean Mayer Human Nutrition Research Center on Aging (both in Boston) found “ample evidence” that the lower the concentration of these carotenoids in the macula, the higher an individual’s risk “of age-related macular degeneration, an irreversible process that is the major cause of blindness in the elderly.”

And where does the eye get these compounds? From the diet. Except that “it’s really hard to find zeaxanthin in a vegetable crop,” Kopsell notes. “Sweet corn has some. Tabasco peppers have some. But other crops have low to minor amounts. So I think than any manipulation that we can have in zeaxanthin is very important.”

But how significant is a 15 percent increase? “I’m asked that a lot,” Kopsell acknowledges, and “we think it is important.” However, he adds, “I think we could probably manipulate the response even more. “ And not just in corn. Indeed, he notes, “I have a research project right now with scallion onions to see if we can extend this work to them.”

The initial sweet corn trials included Merit, a cultivar especially sensitive to mesotrione toxicity; Temptation, which is pretty resistant; and Incredible, a cultivar with intermediate vulnerability. Incredible was the only sweet corn to respond to the weed killer by boosting its eventual carotenoid concentrations.

The first carotenoid that corn plants make is the colorless phytoene. An enzyme normally transforms this chemical into its better known, pigmented kin — from beta-carotene to lutein. But the weed killer knocks out the enzyme just downstream of phytoene, short-circuiting production of additional carotenoids.

“What I theorize happened,” Kopsell says, is that as the herbicide temporarily knocked out the enzyme, “phytoene began to accumulate.” Weeks later, when the enzymatic pathway recovered, it ended the phytoene logjam so that the production of downstream carotenoids could resume.

Merit was too sensitive to the concentration of herbicide used for this cultivar to recover successfully and Temptation was probably so tolerant that it never experienced much of a phytoene logjam. But Kopsell says it should be possible to tailor herbicidal concentrations so that most cultivars respond as Incredible did. In fact, when he co-administered both mesotrione and another widely used herbicide (atrazine), this common herbicidal cocktail appeared to boost stress in the corn and further enhance Incredible’s eventual carotenoid content beyond what was seen with mesotrione alone.

So who knows? Such studies might one day transform the reputation of some herbicides — from plant killers into nutrient boosters.