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Victims no one mourns.

It's enough to put a soap opera to shame: Neighbors killing neighbors, poisoning the environment, boosting a favored few at the expense of others.

But the setting isn't some steamy small town; it's a pastoral meadow or a thriving crop field. And the shocking goings-on aren't a scandal; they're just allelopathy.

That's the term scientists use to describe the ability of certain plants to produce natural chemicals that suppress or even kill other plants. The term was coined more than 50 years ago, in 1937, and researchers are still trying to sort out all the killers--and the victims.

One victim that no one will mourn is ducksalad, the most troublesome aquatic weed in water-seeded rice such as that grown in California and parts of the South.

Happily for rice farmers, researchers with USDA's Agricultural Research Service have found a surprising 347 rice accessions with some natural ability to repel ducksalad--a boon not only for farmers' finances, but also for the environment, since herbicide needs could eventually be reduced.

This discovery began accidentally in the early 1980's, according to Robert H. Dilday, a plant geneticist at the ARS Rice Production and Weed Control Research Unit at Stuttgart, Arkansas.

"In 1983 and '84, I was screening accessions from the world rice germplasm collection for herbicide tolerance," Dilday recalls. "We had a natural infestation of ducksalad in our test plots. I noticed the ducksalad would grow right up to some of the rice plants, but other rice plants didn't have any of the weed around them."

His interest piqued, Dilday obtained funds to begin evaluating rice accessions specifically for allelopathic qualities. In 1988 and 1989, Dilday, fellow ARS researcher Roy J. Smith, Jr., and Palo Nastasi of the University of Arkansas evaluated some 10,000 rice accessions.

From those, the researchers pinpointed 347 with allelopathic activity against ducksalad--about 3.5 percent of the accessions checked.

At that rate, says Dilday, the rice germplasm collection as a whole could contain as many as 500 accessions that could fend off ducksalad to some degree.

In 1988, the first year of the allelopathy evaluations, Dilday also identified 132 rice varieties that repelled redstem, another aquatic weed, and six that looked promising for resistance to broadleaf signalgrass.

And it looks like there are two other weeds that allelopathy may be promising against--rice flatsedge and barnyardgrass.

"We have natural infestations of those two in our rice plots, and we're seeing some activity against them in rice lines from India, the Philippines, and Bangladesh," Dilday says.

In some instances, the rice accessions that repel ducksalad are able to maintain a weed-free area with a radius of up to 10 inches around their base. More typical, though, is a distance of 5 to 6 inches, Dilday says.

Now ARS and university scientists are working to identify the chemistry involved in the allelopathy, Dilday says.

"We're looking at water samples, root samples, and aboveground plant parts," he notes.

"If we can identify the chemicals involved, chemical companies might be able to copy them. I'm also testing to see if the allelopathic substances linger in the soil for more than a year."

Another possibility is breeding the allelopathic characteristic into commercial rice varieties, Dilday adds.

"I have been able to breed it into third-generation plants, so we know the allelopathy is passed to off-spring," he says.

Especially heartening is the knowledge that the allelopathic capability exists in a wide variety of rice cultivars--the 347 come from 30 different countries--with a great variety of other characteristics.

"For example, it is found in plants of all three grain types--short, medium, and long; in different plant heights, ranging from semi-dwarf to extremely tall; and in early and late maturing plant types," Dilday points out.

"That diversity tells me it's not genetically linked to specific physical characteristics, and that could make it easier to transfer through breeding into commercial varieties.

"I think allelopathy is just coming into its own as a method of weed control," he continues. "I think down the road we will not have as many herbicides available because of environmental restrictions. So if we don't develop something like this to help control weeds, it could be hard to stay in the rice business."

Weeds That Fight Back

At least a dozen weeds have been identified as having allelopathic abilities of their own.

At the ARS Plant Protection Research Unit in Ithaca, New York, weed scientist Roger D. Hagin found that one of these, quackgrass, leaves behind a substance that kills alfalfa seedlings when the quackgrass itself is killed.

In researching whether the same sort of substances are exuded from living quackgrass, Hagin has identified the substances as 5-hydroxyindoles or their derivatives. These are natural compounds known in only a few other plants, such as Griffonia, a legume from West Africa.

One of these, 5-hydroxyindoleacetic acid (5-HIAA), acts as a plant growth hormone, he notes. At the proper levels, it will stimulate plant growth, but too much can kill a plant. Another compound, 5-hydroxytryptophan (5-HTP), acts as a growth inhibitor in quackgrass and some other plants.

A carboline compound derivative of 5-HTP kills garden or field slugs and does not appear to affect plants.

All three compounds are given off by dead quackgrass plants, and their effects can linger in a field for up to a year, Hagin says.

"Generally, the worst is past after 2 months," he adds. "But it will inhibit a lot of weeds; you can get dandelion kill from it."

Hagin says at least one chemical company is contemplating manufacture of a synthetic version of the carboline as a molluscicide. "The other 2 compounds may be adapted as selective weed killers," he adds.

Quackgrass isn't the only plant determined to take something else with it when it goes. A 3-year pasture rejuvenation project in the late 1970's and early 1980's revealed that killed Kentucky bluegrass leaves a hostile setting for at least one legume, birdsfoot trefoil.

Tests indicate the compounds from the grass were similar to those in quackgrass. I've seen it kill dandelions and broadleaf plants, so it has potential as a herbicide, too, just like quackgrass."

PHOTO : Plant geneticist Robert Dilday checks a rice variety that keeps weeds at bay by releasing a natural chemical. (K-3603-2)

PHOTO : Weed scientist Roger Hagin examines various grass varieties with allelopathic characteristics. (K-4389-1)

Robert H. Dilday is in USDA-ARS Rice Production and Weed Control Research, P.O. Box 287, Stuttgart, AR 72160. Phone (501) 673-2661. Roger D. Hagin is in USDA-ARS Plant Protection Research, Tower Road, Ithaca, NY 14853. Phone (607) 255-1712.
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Title Annotation:weeds suffer a setback from allelopathic crops
Author:Hays, Sandy
Publication:Agricultural Research
Date:Feb 1, 1992
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