Frequency of Cyanogenesis in Bracken in Relation to Shading and Winter Severity

Abstract

A number of studies have shown that cyanogenic phenotypes of plants are less palatable to, and less damaged by, many herbivores than are acyanogenic phenotypes of the same species (Jones, 1962; Angseesing & Angseesing, 1973; Cooper-Driver & Swain, 1976; Dritschilo et al., 1979; Schreiner et al., 1984). Yet many plant species are polymorphic for cyanogenesis, and the acyanogenic morphs are common (Fikenscher & Hegenauer, 1977). Studies on this polymorphism in Trifolium repens and Lotus corniculatus have shown that a variety of environmental stresses are more deleterious to cyanogenic plants than they are to acyanogenic ones. In Trifolium, low winter temperatures are correlated with low frequencies of plants containing cyanogenic glucoside (Daday, 1954; De Araujo, 1976). However, the correlation between low winter temperatures and a high incidence of acyanogenic plants is poorer for L. corniculatus (Jones, 1977). Cyanogenic Lotus and Trifolium plants were less tolerant of drought (Abbott, 1977; Foulds & Grimes, 1972). In some situations, salt spray may also select more strongly against cyanogenic Lotus plants (Keymer & Ellis, 1978). Cooper-Driver (1976) found that Bracken, Pteridium aquilinum (L.) Kuhn, was polymorphic for cyanogenesis and that cyanogenic Bracken fronds growing in the shade had higher cyanide levels than those growing in open, sunny habitats (Cooper-Driver et al., 1977). The incidence of cyanogenic and acyanogenic fronds in the two habitats was not measured. In an attempt to follow up on this work, the current study examined the incidence of cyanogenic and acyanogenic fronds in open and shady habitats. A survey was also made to determine if there was any correlation between winter severity and the incidence of cyanogenesis in Bracken stands.