It's a jungle out there, even for the plants.

Scientists have found that many plants conduct chemical warfare against their neighbors, exuding toxins from their roots to prevent the growth of nearby plants.

This appears to be one method by which weeds compete so effectively against cultivated plants.

But, as an international meeting of chemists here was told, agricultural researchers are learning how to exploit these anti-plant toxins to develop natural methods of weed control.

The reports were presented this week to the International Chemical Congress of Pacific Basin Societies, which has drawn more than 4,000 chemists from 45 countries bordering the Pacific Ocean.

"It's been obvious for years that many weed species had to have some method of attacking crop plants. They can take over a field much faster than you would expect from simple competition for water and nutrients," said Alan R. Putnam of Michigan State University. "Now we're beginning to zero in on how they do it."

Quack grass, he said, has been found to secrete a substance from its roots that alters the roots of nearby legume plants, such as soybeans and alfalfa, so that valuable nitrogen-fixing bacteria do not grow on them.

Normally, legumes thrive because their roots play host to colonies of bacteria that can extract nitrogen from the air and turn it into fertilizer. Legumes growing in the vicinity of quack grass have no bacterial colonies and are stunted and yellow.

Even when farmers kill the quack grass with herbicides, Putnam said, damaging residue from the weeds can persist in the soil for a year.

Putnam said another weed called velvet leaf produces a toxin that damages the roots of corn and other vegetables.

C.S. Tang, a biochemist at the University of Hawaii, reported that many plants begin waging chemical warfare as mere seeds. As the seed absorbs water in preparation for sprouting, toxins are diffused into the surrounding soil, preventing any other seeds within a "sphere of influence" from sprouting.

Some long-lived plants can cause so much toxin to accumulate in the soil that they damage themselves. The coffee plant, a long-lived tree, is a prime example, said George Waller of Oklahoma State University in Stillwater. The toxin is caffeine.

"We think this is the cause of what coffee farmers call 'tired soil,' " Waller said. "Coffee farmers everywhere have found that after about 10 to 25 years the tree just isn't as productive."

Waller said soil tests showed heavy concentrations of caffeine which, in the laboratory, can stop cell division in the plant's roots. He suggested that similar toxin buildups could be responsible for "tired soil" problems reported with other long-lived crops such as citrus fruits and grapes.

In some cases, scientists reported, the chemical weapons that plants wield can be turned to the farmer's advantage. Plants that produce toxins against weeds can rid a field of weeds before a cash crop is planted.

A cover crop of rye, said Douglas Worsham of North Carolina State University in Raleigh, can leave enough toxin in the soil to inhibit weed growth while allowing crop plants to grow. He said this works only if the soil is not tilled.

In "no-till" farming, the soil is not plowed. Planting is done with a drill that pushes seeds below the surface. Herbicides can be used to kill weeds. No-till farming has become increasingly popular because it reduces soil erosion and the costs of plowing or, as advocates say, it saves soil, toil and oil.

As long as the dead rye plants are left on top of the soil, Worsham said, they give off toxins that inhibit weed seeds, which are near the surface, from sprouting. Crop seeds, below ground, are unaffected.

"We think this is a method of reducing the amount of herbicides used in agriculture," Worsham said.