Getting to the Root

branches with pink blossoms
G.M. Greene / Courtesy of Larry A. Hull

From the blossom comes the fruit, but only if insects haven't damaged the roots, leaving the tree vulnerable to diseases in the soil.

The wire worm is an orange-tan insect recognizable by its considerable mouth parts. A small curveless pest, it lays its eggs in the soil; its larvae swim around dirt particles until they bump into a root. Then they attack, gnawing away at the finest fibers, leaving the plant vulnerable to fungi and bacteria.

"In the past gardeners assumed insecticide was the cure for this pest problem, but nobody ever confirmed it," said entomologist Jack Schultz. By placing miniature video cameras in the earth, Christina Wells, who received her Ph.D. in plant physiology at Penn State, was able to do just that. "This work will really enhance our knowledge of root longevity," added David Eissenstat, an associate professor of woody plant physiology who, along with Schultz, advised Wells on the project.

Working with USDA scientists in an experimental orchard in West Virginia, Wells conducted a two-year experiment on a plot of six peach trees. After digging four holes around each tree, she sank a 60-centimeter clear plastic "root observation tube" into each one. Once a month, a USDA technician poured a broad-spectrum insecticide and water around the circumference of half the trees and untreated water around the other half. Every two weeks he slipped a minicam attached to the end of a long pole into Wellsís observation tubes.

At Penn State, Wells digitized each individual frame from the tapes onto a computer to construct a life history database for each individual root. "Overall, I analyzed at least 18,000 frames from a total of 1,400-plus roots," Wells said. "I would trace the root diameter, when each was born and when it died, whether it turned brown, and note any lateral roots it had." Finally, she transferred all the data onto a spreadsheet. Each data point on a graph represents 20 to 25 hours of work, she said.

Her findings were significant. Soil insecticide treatment of roots resulted in a 50 percent increase in survivorship compared to the untreated roots. "This suggests insects may be responsible for a large fraction of fine root death," Wells said.

She also learned something about pigmentation. "Roots start out white and as they age they turn brown," Wells said. "Applications of soil insecticide delayed the onset of this browning by an average of ten days. This information suggests that browning may be stimulated by interactions with soil insects."

Wells also found that different classes of roots lived for different amounts of time. For instance, roots with large diameters lived longer than smaller ones. But the life spans of roots of all diameters were improved equally when treated with insecticide, as were all orders of roots (determined by the number of fine roots branching off a single larger root). Finally, Wells found that younger roots responded more to the pesticide treatment than did older roots.

"The significant aspect of Christinaís work is that she took the time to watch roots be born, grow, and die on a much more frequent schedule than anyone else," said Schultz. "She confirmed it is indeed insects influencing root performance. Nobody has ever done that before."

Christina Wells received her Ph.D. from the Intercollege Graduate Degree Program in Plant Physiology. David Eissenstat, Ph.D., is associate professor, 218 Tyson Bldg., University Park, PA 16802; 814-863-3371; dme9@psu.edu. John C. Schultz, Ph.D., is professor, 1 Pesticide Lab; 863-4438; ujq@psu.edu. Both are in the College of Agricultural Sciences. Wells received a Collaborative Research Fellowship in Root Biology from NSF, USDA, and the Department of Energy.

Last Updated January 01, 2000