Defensive Ecology of the Cruciferae

Abstract

The glucosinolates (mustard oil glucosides), present in all crucifer species examined, seem to provide a major line of chemical defense against bacteria, fungi, insects, and mammals. Circumstantial evidence suggests that other classes of secondary compounds, each restricted to one or a few genera, represent a second line of chemical defense. Survival of wild crucifers depends partly on escape from adapted enemies in time and space. Discovery of crucifers by several enemy species is aided by behavioral responses to glucosinolates or their breakdown products. Allylglucosinolate (sinigrin) in the leaves of Thlaspi arvense releases allylthiocyanate instead of the more typical allylisothiocyanate, which is used as a host-finding attractant by several insect species. This change in secondary chemistry may reduce the rate of discovery of Thlaspi plants by crucifer-adapted enemies. The defensive ecology of crucifers seems to typify that of herbaceous plants generally: chemical resistance, in the form of small amounts of toxic compounds, combined with low apparency to enemies which are adapted to the chemical defenses. The importance of the Cruciferae and other families of herbaceous plants as sources of food-plants for man may result in large part from their relatively low concentrations of toxins. The mature foliage of trees, shrubs, and grasses, by contrast, remains poor food for man, just as for other plant