Anti-Microbial Activity of Phenolic Acids in Pteridium aquilinum

Abstract

All land plants are continually exposed to a wide variety of microorganisms ranging from those which are beneficial to those which are pathogenic. Most plants remain remarkably healthy, and this seems to be especially true of ferns, which reportedly are rarely subject to diseases (Braid, 1940; Hutchinson, 1976; Bandoni, pers. comm.). It is now generally recognized that the resistance of plants to pathogens (and other predators) is mainly due to their containing fungicidal and bactericidal secondary compounds. The few surveys which have been carried out on ferns have confirmed that they do contain such antibiotic substances (Maruzzella, 1961; Khrisagar & Mehta, 1972; Lynch-Brathwaite et al., 1975; Banerjee & Sen, 1980; Cooper-Driver et al., in prep.). One major class of secondary compounds with known antimicrobial activity is the phenolics, which range from simple phenols to complex polymeric tannins (Harbore, 1982, pp. 230-264). Their main role in plants appears to be as protective compounds against fungi, bacteria, and viruses (Friend, 1979; Swain, 1979). For example, the flavonoids present in the farinose exudate on the leaf surfaces of some ferns have antimicrobial properties (Chowdhury et al., 1974; Wollenweber, 1978). Simple phenolics, such as p-hydroxybenzoic, protocatechuic, and vanillic (hydroxybenzoic) acids, and p-coumaric, caffeic, and ferulic (cinnamic) acids (Fig. 1) have been shown to be present in the fronds of ferns (Bohm & Tryon, 1967; Bohm, 1968; Cooper-Driver, 1976) and to play an important role in the ecology of Bracken (Pteridium aquilinum (L.) Kuhn) by inhibiting the germination and growth of neighboring plants (Stewart, 1975; Glass, 1976; Gliessman & Muller, 1978). Such acids are also known to have inhibitory effects on feeding by insects (Jones & Firn, 1979). The concentration of these acids in the leaves is known to vary quantitatively during growth and development (Glass & Bohm, 1969; Cooper-Driver et al., 1977; Jones, 1983), and thus must affect the antimicrobial, allelopathic, and anti-insecticidal properties of the plants differentially during the growing season. Simple phenolic and cinnamic acids must also play a major role in the defense of fern gametophytes against pathogens, particularly since more complex phenols, such as flavonoids, have been shown to be absent during the pre-antheridial stage (Petersen & Fairbrothers, 1980). However, it has been reported that fern prothalli are readily susceptible to several species of off' fungi, like Pythium species (Gregor, 1938, pp. 141-154), and gametophytes grown in pure culture can be readily invaded by Botrytis cinerea, Pythium debaryanum, and Rhizoctonia solani (Hutchinson & Fahim, 1958). Gametophytes grown in pure culture appear to be more susceptible to molds and damping off diseases than to the more specific pathogens of angiosperms, Fusarium solani and Verticillium dahliae (Hutchinson, 1976). In angiosperms, many of these