The Chemical Defenses of Plants to Pathogens and Herbivores

Populations of plants are subject to attack by pathogens and herbivores at all stages of their life cycles. Pest pressure may be devastating, as illustrated by the outbreaks of insects apd diseases in cultivated and native plants (121, 173, 178, 198, 329) and in the success of weed control by biotic instruments (6, 141, 338, 340). Epidemics or episodes of intensive herbivory may lead directly to the local extinction of a plant species, or may weaken plants and make them vulnerable to subsequent pressures. Loss of photosynthetic capacity due to disease or partial defoliation lowers resistance to abiotic stresses or to infection by other pests, and reduces competitive ability (198, 240, 270). Defense against pathogens and herbivores may be secured by many different mechanisms; it may depend on the texture and composition of the plant surface (156, 182, 196, 242), the presence of anatomical structures such as thorns or resin ducts (123, 199, 229), the absence of nutrients required by the pest (140), the presence of hormonelike substances that affect the development of insects (335), unsuitable pH or osmotic pressure (18, 199, 229), or the accumulation of secondary products (47, 92, 169, 298). These products are diverse, ranging from alkaloids to terpenes; phenolics; and steroidal, cyanogenic, and mustard oil glycosides. Some (e.g. phenolics) are ubiquitous in seed plants, others (e.g. alkaloids) occur in several thousand species and others (e.g. cyanogenic glycosides) occur in only a few hundred species (61). Secondary products may be sequestered within cells or organelles therein and released only when the cells are broken. They also may be stored in and secreted by epidermal glands which are sessile or on trichomes (19, 69, 156, 182, 187, 269). Typically, glandular exudates contain only one class of compound, but this class varies among species. The glandular exudates usually are terpens, alkaloids, res-