Natural Variability in Phenolic and Sesquiterpene Constituents Among Burdock (Arctium lappa L. and Arctium minus L.) Leaves for Potential Medicinal Interests
Arctium lappa and Arctium minus, commonly known as burdock, are introduced, feral, biennial species found throughout the United States. Native to Europe, these plants have been valued for centuries for their medicinal properties and/or as part of a healthy diet by both western and eastern cultures. In the United States, burdock is used to treat burns by holistic medical practitioners (i.e. the Amish communities) as it is believed to relieve pain, reduce scarring, prevent infection, speed healing, and minimize the need for narcotics, antibiotics, and skin grafting. Since burdock is cross-pollinated, genetic diversity among burdock individuals is likely to result in substantial levels of phenotypic variability among and within species, as well as among and within accessions within species with respect to many traits, including levels of potentially health-beneficial compounds found in medicinal or edible plant parts. Moreover, because burdock thrives in diverse habitats, its production under conditions that might affect these compounds is probable. Constituent variation may also occur among different sized leaves, between first and second year plants, and between rosette leaves and stalk leaves. Therefore, the objectives of this research were to: a) develop techniques to evaluate biochemical components of burdock leaves; b) characterize sources of variability in leaf composition; c) and evaluate the effects of horticultural management techniques (i.e., irrigation, irradiance, and temperature levels) on the biochemical profile of leaves. The chemical composition of these leaves was analyzed spectrophotometrically for their phenolic content and antioxidant power and then quantified on an HPLC-DAD instrument. The variability in potentially bioactive chemical constituents among burdock individuals within the population was substantial. Arctium lappa and A. minus had high levels of variability for flavonoids and hydroxycinnamic acids among accessions and among plants within accessions. In general, A. minus plants contained more quantifiable chromatographic compounds (peaks) than those of A. lappa. Arctium lappa individuals produced primarily hydroxycinnamic acids, but A. minus individuals also synthesized high levels of specific hydroxycinnamic constituents along with several flavonoid compounds. Larger leaves contained higher amounts of phenolic compounds than did smaller ones, and second year rosettes contained levels of chemical constituents three-fold higher than these found in first year rosettes. Environmental conditions of high light, frequent irrigation, and cooler temperatures increased phenolic levels in burdock leaves. The data acquired from this project justify further efforts in burdock domestication and be used to develop future ideas and proposals on the commercialization and domestication of burdock and its use in the medical field.
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