Brittle plants point to ozone damage.

Brittle plants point to ozone damage

Ozone (O.sub.3.)--the primary photochemical irritant in smogis also a major crop hazard, costing U.S. farmers an estimated $5 billion annually in reduced yields. New research at the University of Colorado in Boulder now suggests that an early warning sign of impeding smog-ozone damage is plant embrittlement.

Eight-week-old ryegrass plants were exposed to 15 parts per million ozone for 7 hours. That level, while high, is not beyond what's encountered in some ozone-polluted regions experiencing periodic high peak-exposure levels, notes Jane Bock, a plant ecologist on the project. Moreover, she points out, it is below the level needed to inflict damage that can be seen under a microscope. However, when her colleague Alan Greenberg, a materials scientist, measured the tensile properties of leaves from these plants, he found they were "predictably, more brittle" than leaves from plants unexposed to ozone, she says.

The degree of embrittlement was small but significant. The "tensile modulus," or stiffness measurement, of the ozone-exposed leaves was 19.3 percent higher (13,000 pounds per square inch) than that of unexposed leaves. Ozone-treated leaves were also 30.4 percent less ductile, required only 73.1 percent as much energy to fracture and had a breaking strength 17.5 percent smaller than unexposed leaves. Similar embrittlement also was discernible in ozone-exposed soybeans, green beans, barley and wheat.

Recent studies by others have shown that ozone-damaged plants tend to attract more insects, Bock says. One explanation that's been offered is that ozone may alter the amino acid and sugar levels in plants, making them more attractive to pests. However, Bock says, "It may also be that insects can more easily bite and tear plants affected by ozone because of their brittleness." She has begun investigating that possibility.