By Janet Raloff
Nanoscale pollutants can enter crop roots, triggering a host of changes to plants’ growth and health, two studies find. These tiny particles can stunt plant growth, boost the plants’ absorption of pollutants, and increase the need for crop fertilizers.
Nanomaterials that get released in the exhaust from diesel-fueled tractors can rain down onto crop fields. Those used in fabrics, sunscreens and other products collect in the solids separated out of sewage and wastewater — nutrient-rich solids that are routinely spread on U.S. fields to improve soils. The new studies offer a glimpse at the toxic effects such nanoparticles may pose to future crops as exposures rise.
The new data now “forewarn of agriculturally associated human and environmental risks from the accelerating use of manufactured nanomaterials,” Patricia Holden of the University of California, Santa Barbara, and her colleagues report in one of the studies, published online August 20 in the Proceedings of the National Academy of Sciences.
Manufacturers have been turning to nanoparticles in recent years because they perform differently than larger-scale versions of the same product, notes Jason White of the Connecticut Agricultural Experiment Station in New Haven. If nanoscale materials behave differently, both chemically and physically, he asks, “Why should we have assumed they’d behave the same biologically?”
To study the impact of such materials on crops, Holden’s team exposed soy plants from germination through bean production to soil treated with either of two widely marketed metal-oxide nanomaterials: the cerium oxide used as a catalyst in diesel fuel and other products, or the zinc oxide particles used in sunscreens and as antibacterial agents.
Compared to untreated plants, those grown in soil spiked with zinc oxide nanoparticles developed fewer leaves at the highest dose used. By contrast, nano-cerium stunted plant growth at all concentrations, “but most dramatically at the lowest level used,” Holden notes. Zinc oxide accumulated in soy leaves and beans grown in treated soil. Cerium oxide’s entry into the plants, however, stopped at the roots’ nodules. In plants receiving the highest cerium-oxide doses, those nodules didn’t contain the bacteria that normally take the nitrogen from the air and convert it into a chemical form (ammonia) that soy and other crops use as a fertilizer.
The ability of soy and other legumes to fix nitrogen “is one of the most important microbial processes in agriculture,” notes White, an environmental toxicologist. So the ability of nano-cerium to shut this process down “was the most significant new finding” — and the most troubling.
This is the first investigation of the effects of nanomaterials on plants exposed via soil. “That’s cool and obviously relevant to how plants would be affected in their native environment,” notes analytical chemist Mark Schoenfisch of the University of North Carolina at Chapel Hill.
In a second study, published online August 2 in Environmental Science & Technology, White’s team exposed the roots of tomato, zucchini and soy plants to fullerenes, widely used nanomaterials manufactured from pure carbon. Because trace residues of toxic pesticides lace soils long after they were last applied, White’s group looked to see if nontoxic quantities of fullerenes in the root zone affected how plants respond to any breakdown residues of DDT.
When fullerenes were present, all three types of plants removed more of the pesticide from the material in which they had been grown — in this case, vermiculite. Since plants were not grown to the fruiting stage, White notes, there’s no way to know if the pollutant would also accumulate in the crop — “but it was certainly in the shoot system.”
