Plant Phenolics as Natural Toxicants and Herbivore Deterrents

Plants and herbivores have a long history of coevolution. From a plant perspective, these interactions include both the beneficial activities of animals (e.g., pollination and seed dispersal), and their harmful activities, such as the consumption of plant tissues. Plants produce many chemicals that affect grazers. Alkaloids and phenolics are particularly important in this regard.

Tannins (condensed and hydrolyzable) can reduce the palatability, nutritional quality, and digestibility of forage to grazers, and, in some types of plants, tannins are indu-cible by grazing. The phenolic content of the leaves of northern birch, for example, increases rapidly (<48 h) in response to herbivory. But the phenolics contents of leaves can also be affected by light intensity and nutrient levels, so relationships between herbivores and phenolics can vary with environmental factors, as well as with the type of plant and herbivore.

In poultry, tannins added to the diet at low concentrations (0.5-2%) reduce growth and egg production. Higher concentrations (3-7%) can cause death. Cattle also can die from consuming oak leaves (Quercus incana or Q^ havardii).

Some animals have mechanisms allowing them to consume tannin-rich plant tissues as food. These mechanisms include an alkaline gut pH, a peritrophic membrane (this absorbs tannins, which can then be excreted with the feces), or the production of mucin (a tannin-binding, pro-line-rich protein in the saliva).

Sweet clover (Melilotus spp.) contains high concentrations of the phenolic, coumarin. If sweet clover is allowed to become moldy, the coumarin can be converted to dicoumarol by fungal enzymes. Dicoumarol is a potent anticoagulant. Cattle that are fed moldy sweet clover can become lethargic and anemic, and may suffer severe internal bleeding.

Many members of the Anacardiace (cashew family) contain phenolic compounds that can cause severe dermatitis. Urushiol, for example, is an alkylated catechol that is the main active ingredient of poison ivy, poison oak, and poison sumac, and the main constituent in Japanese lacquer (Figure 2). Urushiol also occurs in mangos, the seeds of ginkos, and the shells of cashew nuts. As little as 50 mg of urushiol is sufficient to cause a complicated delayed allergic reaction with the body's immune system in most humans. Mammals other than humans do not seem to be vulnerable to it.

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