A little thirst can aid plant defenses.

A little thirst can aid plant defenses

Every gardener knows that parched plants become especially vulnerable to insects. The idea that all but well-watered plants are jeopardized "has been the conventional wisdom -- probably for a couple hundred years," notes insect ecologist William J. Mattson of the U.S. Forest Service in East Lansing, Mich. But this conventional wisdom fails to recognize that "plants don't respond linearly to stress," he says. Over the past three years, he and others have accumulated data suggesting a new hypothesis: While a lot of water stress is undeniably bad, a little may prove beneficial. Now, researchers at the University of Virginia in Charlottesville have completed what appears to be the first study offering strong confirmatory evidence.

Last year, the Virginia researchers showed that mild water stress made the leaves of white oak saplings less attractive to the oak lace bug. In a far more detailed series of just-completed experiments, Edward F. Connor and Grant McQuate investigated a similar relationship involving soybeans, a leading U.S. crop. Given a choice, Connor told SCIENCE NEWS, the larvae of Mexican bean beetles -- a major U.S. pest -- "definitely preferred" dining on foliage from well-watered plants as opposed to mildly thirsty ones.

Leaves from the mildly thirsty plants contained 11 percent less water and proved a little tougher -- and therefore harder to penetrate. But the bugs had a more serious reason to prefer thoroughly watered plants. Larvae forced to feed on mildly water-stressed foliage grew more slowly, entered adulthood smaller and were less hardy than those reared on leaves from well-watered plants. Connor says these findings suggest that stress-induced chemical defenses, called toxicants, in the thirsty plants are at least partly responsible.

In one growth-chamber experiment, Connor fed some larvae leaves harvested from chronically thirsty plants. Only on the night before leaf harvesting were these plants well watered. The last-minute dousing brought leafwater and toughness levels to normal, eliminating the potential physical deterrents. Yet compared with larvae raised on leaves from always-well-watered plants, larvae eating leaves from these generally thirsty plants took about 15 percent longer to mature, gained about 10 percent less weight during high-growth stages and suffered 10 to 15 percent greater mortality. Mortality was twice as high among larvae feeding on intact, mildly thirsty plants in the greenhouse and field.

In spite of the last-minute watering, leaves used for the growth-chamber experiments maintained their stress-induced chemistry. With reduced water, Connor explains, plants tend to break down some of their proteins into free amino acids. Among the mildly thirsty soybeans, he found free-amino-acid levels 36 percent higher on average than in well-watered plants, though some individual amino acids increased sixfold.

Such changes may mean that a water-stressed plant diverts some of its resources from normal growth to the development of other compounds, including waxes, resins and oils, some of which can aid in the plant's defense, Mattson says. For example, ongoing experiments in loblolly pines by Peter L. Lorio Jr. of the U.S. Forest Service in Pineville, La., show that as maturing (30-year-old) trees become thirsty -- even mildly thirsty -- they step up resin production. While resins normally serve to seal up tree wounds, at high levels they can also thwart bark beetles -- the pine's archenemy -- from establishing lethal infestations in the tree's outer vascular tissue, or phloem.

In a small study in 1986 and '87, Lorio fostered a bark-beetle infestation on loblollies. He found the trees "greatly resisted a beetle attack" during a slow-growth, dry period but not during a high-growth, well-watered period.

Connor says these observations suggest growers should reconsider watering plants when there's little threat to their growth or yields: "This watering could just increase pest problems."