Herbal Herbicides
Weed killers manufactured by Mother Nature
Certain plants are picky about the company they keep. Once established, walnuts and some sandy shrubs, for instance, create a virtually barren border of ground around them. Many other plants aren’t quite so antisocial. They permit numerous species into their neighborhoods, while barring a few plant types.
Chemical defenses play a major role in determining which plants flourish in woodlands, meadows, farms—or even in suburban lawns. Although this herbal warfare has been recognized since Biblical times, its study is “still regarded as a relatively young and immature field of science,” notes Yoshiharu Fujii of Japan’s National Institute for Agro-Environmental Sciences in Tsukuba.
Only in the past few decades have scientists focused on the chemical warfare underlying botanical standoffishness. They’ve demonstrated that many plants manufacture compounds that sicken or kill intruders.
The potential payoff from influencing this defense is huge, notes Alan R. Putnam, a retired Michigan State University horticulturist who spent 18 years studying allelopathy, or plants’ chemical defenses against other plants. By inhibiting crop growth, “allelochemicals undoubtedly cost world agriculture billions of dollars annually,” he says. By understanding chemical-defense mechanisms, he argues, “we could put them to work to benefit agriculture.”
Fujii and other agricultural scientists have been working aggressively to identify the defensive chemicals. Some of the researchers look to cultivate plant varieties that naturally keep weeds at bay, while others are scouting for bodyguards that will protect a high-valued crop from nutrient- and light-robbing bullies. A few scientists intend to model new commercial pesticides on the agents that plants naturally produce.
“Public awareness of environmental hazards from synthetic herbicides has opened new doors for scientists working on allelopathy to develop safe, environmentally friendly, and more productive farming methods,” says Fujii, president of the International Allelopathy Society.
No plant is yet marketed for its allelopathic potential, notes Francisco A. Macías of the University of Cadiz in Spain. However, his group and others are identifying and boosting weed-fighting activity in plants ranging from wheat and rice to lawn grasses and mustards. Macías says that such work may eventually slash the economic, labor, and environmental costs associated with society’s current heavy reliance on commercial herbicides.
On guard
The goal of Putnam and others when they began exploring allelopathy for agricultural gain was the cultivation of plants that could defend themselves without the help of commercial pesticides, produce rich yields, and cost little more than conventional varieties.
By these criteria, Leslie A. Weston of Cornell University has already found some winners. When the weed scientist screened a host of fine-fescue grasses, she identified several that appear to be a lawn manager’s dream. They create dense carpets of bright green grass that grow slowly, so they need little mowing. They also resist disease, tolerate shade or full sun, and inhibit at least 20 of the most common urban weeds, thereby needing no help from herbicides.
Weston’s group found that the best performers among those fescues make ample use of allelopathy. Their roots exude copious amounts of m-tyrosine—an unusual variant of a common amino acid. Weeds readily absorb m-tyrosine, mistaking it for the nutritious tyrosine. Their roots soon become deformed and stunted, and death quickly follows.
Of the many fescues that exude the toxic amino acid, a few are stellar performers. One, known as Intrigue, generally keeps a planted field 95 percent weedfree without use of supplemental herbicides, Weston notes. This cultivar has been marketed for years as a low-maintenance turf. But only recently has Weston’s team demonstrated allelopathy’s central role in the fescue’s weed-controlling prowess. Reports of this work are slated to appear in several journals later this year.
Weston’s program will next address whether the fescues will share their turf with other grasses or instead chemically muscle them out.
Besides these ornamental grasses, most cereal grains examined—which include rice, wheat, corn, and barley—exude one or more allelochemicals. Some of the oldest research on allelopathy focused on rice.
Currently, most rice growers around the world experience serious weed problems, often having to put up with yield-robbing interlopers that have become resistant to commercial herbicides. A few rice varieties, however, have shown unusual success at poisoning some of their most pesky competitors, such as a weed called barnyard grass.
In years when weather conditions are especially favorable, certain experimental varieties of rice can be grown without herbicides, notes David Gealy of the U.S. Department of Agriculture’s rice-research center in Stuttgart, Ark. At other times, those plants need assistance from herbicides but require far smaller quantities than conventional rice cultivars do.
However, rice plants must also perform well in the field, the mill, and the kitchen. One of the most weed-suppressive varieties that Gealy’s group has studied has been a disappointment in those regards. Despite amazing weed control, plant stalks of the variety known as PI312777 tend to fall over during storms and the seeds break during milling.
However, Gealy’s team has found that some hybrid rice varieties suppress weeds well under special planting regimens. So, too, do some noncommercial Chinese-derived lines in the U.S. federal germplasm collection. The role of allelopathy in their weed-fighting performance has not, to date, been evaluated, he notes.
Bodyguards
The findings on allelopathy explain, in part, why some crops do better when they aren’t continuously planted in a field, but are instead included in a rotation cycle with sorghum, mustards, or other plants. Scientists have recently found that some of the most effective of these alternative species produce abundant weed-killing chemicals.
USDA’s Natural Products Utilization Research Unit in University, Miss., focuses on a suite of allelochemicals produced by sorghum. The plant releases them as an oily secretion known as sorgoleone.
“Only the root hairs produce it, and they exude it as quickly as it’s made,” explains Stephen O. Duke, a plant scientist on the project. “In fact,” he says, “we think that the last step in [sorgoleone’s] synthesis occurs as it’s leaving those root hairs”—which is fortunate because it’s toxic even to its parent plant.
Duke describes sorgoleone as a controlled-release herbicide, entering the environment gradually and only as needed. USDA has genetically altered sorghum for enhanced sorgoleone production and expects to field-test that crop in a couple of years.
A rotational-cropping cycle that includes sorghum might offer organic farmers, who don’t use synthetic pesticides, an all-natural option for controlling weeds. Alternatively, Duke notes, farmers might sparingly interplant sorghum with wheat to let sorgoleone protect both crops.
Duke’s team is considering altering the genetic machinery of other plants so that they will also make sorgoleone. In some cases, that capability might require only a fine-tuning of a plant’s existing make-up. For instance, Duke notes, rice “has most of the genetics to do this already.”
Stephen Machado of Oregon State University in Pendleton has been screening other crop plants that fight weeds. In the January/February Agronomy Journal, he suggests that organic farmers might plant meadowfoam (Limnanthes alba Hartw.)—a plant grown for the multipurpose oil that it yields—between higher-value plantings. In soil, meadowfoam’s allelochemical—glucolimnanthin—degrades into several compounds.
As another protective measure, farmers might apply to their fields the mealy waste that’s left after meadowfoam’s seeds have been pressed to extract their oil. Machado is exploring whether this meal, which is rich in glucolimnanthin, can act as an all-natural herbicide.
Chemicals produced by the brassica family—which includes cabbages, mustards, and the rapes from which growers harvest canola oil—also fight weeds. For the past 18 years, Matthew J. Morra of the University of Idaho in Moscow has been investigating whether these plants can be rotated with other crops to improve the soil and thereby serve as “green manures.”
Brassicas contain glucosinolates throughout their tissues, Morra reports. Once those tissues are crushed—by a feeding insect or a plow’s blade—an enzyme in the plant converts the glucosinolates into powerful allelochemicals called isothiocyanates. In the soil, an isothiocyanate can degrade into any of half a dozen different compounds, some even more potent against weeds than the chemicals the plant initially released. Moreover, Morra found that some of the brassicas release nitrogen, which fertilizes the soil, at the same time that they whack weeds.
Morra notes that his university has a patent pending for the use of mustards to fight weeds.
The new guard
Beyond identifying plants as a source of potent herbicides that may be appropriate for organic agriculture, some scientists are exploring different strategies.
Macías and his group, for example, are studying natural allelochemicals as models for new synthetic herbicides. To design commercial analogs of allelochemicals, the researchers are starting with chemicals found in soil around the roots of wheat, corn, and other grains. Although DIBOA and DIMBOA—the compounds released by wheat—are allelopathic, their degradation products can be even more so, the researchers reported in the Feb. 22, 2006 Journal of Agricultural and Food Chemistry. The most potent breakdown product identified, which is known as APO, is also unusually long-lived. It can persist in soil for 3 months.
Recently, Macías began tinkering with DIBOA’s structure. One of its natural breakdown products, DDIBOA, at trace levels kills 100 percent of target weeds. Adding a short chain of carbon atoms to the molecule can increase its weed-fighting potency 1,000-fold. Changing a few atoms within a ring-shaped feature of the molecule increases that potency 100-fold more, so that even smaller amounts kill weeds.
The Cadiz researchers are in the process of patenting these new super-DIBOAs. Macías expects to launch field trials of some of the compounds next month. Fortunately, his group has found, the parent plant is virtually immune to these new analogs. It detoxifies them by the same means through which it neutralizes APO and natural chemicals derived from DIBOA.
Three major agrochemical companies are working with the Cadiz team to investigate the development of new herbicides based on APO and DIBOA variants. One goal is to coat crop seeds with these weed killers.
Scientists applying another strategy note that plants tend to make allelochemicals only after receiving signals of an apparent invasion by weeds or other pests. Biological chemist John A. Pickett of Rothamsted Research in Harpenden, England, and his colleagues intend to use those signaling agents to fool a crop into acting as if it were under siege.
Many plants generate cis-jasmone in response to weeds. Pickett’s group found that this signal, in turn, triggers allelochemical production by grains and many other plants. One could “directly treat a crop with cis-jasmone, which is what we would probably do in Europe or the United States,” Pickett says.
For developing countries, Pickett favors a less costly approach. “We’d like to get plants to naturally switch cis-jasmone on earlier than they do now—perhaps at the first sign of attack by pests,” he says. Scientists could create hypersensitive plants of either the crop species or monitor plants whose sole function would be to signal neighboring crop plants to ramp up their allelochemical defenses.
Currently, Pickett notes, “we’re working with the British Wheat Breeders Association to develop plants with a more potent [allelopathic] response to cis-jasmone.”
No bull’s-eye
Scientists haven’t yet hit the target in developing allelopathic weed fighters. Current methods are neither precise nor potent enough, Morra says.
Weed scientist Regina G. Belz of the University of Hohenheim in Stuttgart, Germany, agrees, noting that allelopathy may not defend a plant that’s also besieged by harsh weather, poor nutrition, or bad soil.
In most cases, however, Belz says that allelopathy probably offers the most environmentally friendly approach to tackling weeds.
It also has another major advantage, Macías says: novelty. Among commercial herbicides, “we have not seen one with a new mode of action in the last 30 years,” he notes. Most products work by inhibiting photosynthesis.
Some plants, however, produce allelochemicals that simultaneously poison weeds by three or more mechanisms, all different from those employed by commercial herbicides. Moreover, a single chemical may poison in more than one way, Macías adds.
Because it’s unlikely that weeds will quickly overcome multiple vulnerabilities, allelopathic weed control may keep its potency longer than existing single-action commercial chemicals do, Macías argues.
“With allelopathy, our guiding philosophy is simple,” he says. “Learn from nature.”
