Sweetpotatoes Pack Potent Protection

Sweetpotatoes earned U.S. producers about $395 million in 2008—and they’re packed with carotenoids, vitamin C, potassium, and fiber. Now Agricultural Research Service (ARS) scientists have found that they also contain high levels of protective phenolic compounds called “caffeoylquinic acids.” These compounds, which are known for their antioxidant activity, are found in many plants, though the levels vary widely.

ARS agronomist Howard Harrison teamed up with plant pathologist Pat Wechter and plant physiologist Joseph Peterson (now retired) to measure the levels of caffeoylquinic acids in sweetpotatoes. All three scientists work at the U.S. Vegetable Laboratory in Charleston, South Carolina. Other ARS collaborators included ARS chemists Maurice Snook and Trevor Mitchell, who work at the Richard B. Russell Research Center Toxicology and Mycotoxin Research Unit in Athens, Georgia.

The scientists assessed the levels of four types of caffeoylquinic acids in 16 sweetpotato varieties from the United States, Brazil, and Africa. They also studied caffeoylquinic acids in bigroot morningglory, a sweetpotato relative with roots that can weigh more than 60 pounds.

The research team found measurable amounts of all four types of caffeoylquinic acids in the sweetpotatoes they tested. On average, the highest levels of the compounds were found in the layer of tissue just under the skin. Intermediate levels were found in the stele—the interior of the sweetpotato—and the lowest levels were found in the skin. Bigroot morningglory roots also had high levels of all four of the defensive compounds.

The scientists found that three of the compounds they tested provided some protection against Rhizopus soft rot, a fungus that infects sweetpotatoes after harvest by invading through breaks in the skin. All of the compounds inhibited growth of plant-infecting bacteria, and one inhibited another infectious plant fungus, Fusarium solani.

The team noted that the compound levels varied significantly among the sweetpotato varieties they evaluated, despite the study’s small sample size. “If we screened a larger sweetpotato germplasm collection, we’d probably find varieties with even higher compound levels,” Harrison says. He thinks that plant breeders may be able to use the compounds as biochemical markers to develop sweetpotato varieties with enhanced pest resistance and other beneficial traits.—By Ann Perry, Agricultural Research Service Information Staff.

Howard Harrison is in the USDA-ARS U.S. Vegetable Laboratory, 2700 Savannah Hwy., Charleston, SC 29414; phone (843) 402-5300 ext. 5323, fax (843) 573-4715.

"Sweetpotatoes Pack Potent Protection" was published in the October 2009 issue of Agricultural Research magazine.