Chemical warfare in garden happens naturally

By Gary Raham
Writer and Illustrator

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If you thought you were the arms-bearer when it comes to chemical warfare on behalf of the "good plants" in your garden, think again.

Plants themselves raise havoc with their insect enemies, warn neighboring plants of eminent invasion and adjust the biochemistries of color to attract different pollinators. In fact, scientists are discovering that we might be better off enhancing the natural ability of plants to defend themselves rather than treating them as immobile green wimps incapable of doing anything but feed us, smell nice or make us sneeze.

Corn can be merciless with armyworm caterpillars, for example. Scientists studying a variety of corn that grows on the Caribbean island of Antigua found that it responds within an hour to armyworm munching behavior by producing a protein-slicing enzyme that tears the caterpillars' innards to tatters. Scientists believe that combining this enzyme with other defense mechanisms could make it difficult for insects to adapt in their usual rapid fashion.

Radishes, when attacked by cabbage butterfly caterpillars, produce 10 times the concentration of mustard oil glycosides and 30 percent more spikes than unharmed plants. Not only do these plants set more seed, they somehow create seedlings already primed with enhanced defenses against caterpillar depredations.

Researchers at the University of California in Davis during the late '90s revealed even "spookier" behavior: plants communicating by chemicals from one species to another. Sagebrush leaves damaged in a manner similar to the injury inflicted by the attentions of hungry insects released a chemical called methyl jasmonate from their leaves. Tobacco plants downwind responded within minutes by producing a chemical (mercifully abbreviated to PPO) that made their own leaves less appealing to insects.

Scarlet gilia performs chemistry tricks with respect to flower color. Flower color shifts from a preponderance of red to shades of pink and white during its July to September season. The changes correlate to the relative abundance of hummingbirds (that prefer red flowers) and hawkmoths (that prefer the lighter shades). At elevations where hummingbirds stay all summer, the plants keep their red blooms. At higher elevations where hummingbirds move away in August, the plant shifts to lighter colors in apparent accommodation to hawkmoths.

Some of the more fascinating studies reveal that plants release the chemical equivalent of a scream for help that attracts the predators of those insects feeding on them. A variety of corn under attack by armyworms released a chemical that attracted a species of predatory wasp. The wasp, like the motherly alien in "Alien," lays its eggs in the caterpillars and the resulting larvae eat the caterpillars from the inside out. Similarly, German researchers have shown that tobacco plants injured by hawkmoth larvae released chemicals that attracted hawkmoth predators to the scene.

Agricultural scientists would like to genetically engineer some of these chemical-call-for-help abilities into other crop plants. If successful, it would change the way farmers battle insect pests. They would have to avoid traditional chemicals that kill insects indiscriminately - pests and their predators alike. The advantage would be that pest species, adept at evolving resistance to specific chemicals, would be less likely to develop effective resistance against their normal predators.

And it appears that our own meanderings in the garden don't go unnoticed by our green charges. Studies stretching back to the '70s indicate that plants respond to touch. In the case of Arabidopsis, a member of the mustard family, Stanford University researchers found that plants touched twice daily did not grow as tall as untouched plants. They discovered that certain genes turned on in response to touch and that calcium metabolism was involved in some way.

It would appear that our chemical salvos in the garden only add to much more targeted discharges by plants that have been working on their defensive relationships with insects for eons. Plants also seem to support the notion of a biological "uncertainty principle" not too unlike the one in physics that states that one can never know both the position and momentum of a particle because the act of observing it changes the energy of the system. Our presence in the garden changes plant responses in subtle and amazing ways. But maybe we already knew that all along.