Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

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332 KARBAN & MYERS Table 1 A dichotomous key for induced responses Does stress or injury change plant quality? 1 NO: No response l' YES: INDUCED RESPONSE (proceed to 2) Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only. Does the induced response decrease herbivore preference or performance? 2 NO: No effect or induced susceptibility 2' YES: INDUCED RESISTANCE (proceed to 3) Does reduced herbivore preference/performance increase plant fitness? 3 NO: The plant is not defended by the response 3' YES: INDUCED DEFENSE in plant quality following injury caused by drought (104), nutrient deficiency (70), solar radiation (66), low temperature (46), high temperature (94), air pollution (20), and previous damage caused by herbivory (107). Much of the evidence for changes in resistance associated with induced responses comes from bioassays of induced foliage under laboratory or artificial field conditions (reviewed in 28, 84). While the proportion of cases in which induced responses act as defenses against herbivores may be uncertain, we would like to consider in this review the characteristics of changes that relate to their role as defenses. What are the changes, why and how might they occur, and what might be done to further understand their influence on plant-herbivore interactions? Specifically, which changes are likely to act as effective defenses and how might they work? Which herbivores are likely to be affected? Have these responses evolved as defenses against herbivores? Under what conditions might selection favor facultative induced defenses rather than preformed constitutive defenses? WHAT CHANGES FOLLOW DAMAGE? Secondary Metabolites and Phytoalexins Injuries to plant tissues cause a wide array of plant responses. The nature of the response varies with plant type. One area of progress has been to recognize that the way trees respond is associated with their growth pattern and nutrient status (14). A cataloging of plant responses is beyond the scope of this review, although a few representative examples are provided. Many studies of induced responses have considered changes in tannins and phenols, products of the shikimic acid pathway. Relative activity of the enzyme phenylalanine ammonia lyase (PAL) can determine the production of phenolics, including lignin (19). Induction of the phytohormone ethylene by tissue damage may influence the production of PAL and therefore the concentration
INDUCED RESPONSES 333 Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only. of secondary metabolites (110) and leaf toughness (47). The exact role of ethylene in this process remains controversial (72). Many agents of environmental stress correlated to herbivory can also cause increases in secondary metabolites (71). Patterns often vary depending on the history of the plant. The balance between many primary and secondary metabolites influences the response of plants to stress and also the effects that these plant responses will have on herbivores. Herbivore damage often affects the concentrations of available nitrogen and other important nutrients in foliage (7, 97). A major problem facing workers in this area is determining which of the many secondary plant chemicals and plant nutrients that change following damage or stress are responsible for the overall effects on herbivores. The range of induced changes is so great that it is impossible to investigate all these factors and difficult to determine rationally which are worthy of study. In some instances, herbivores elicit plants to synthesize phytoalexins (1, 68, 100). Phytoalexins are low molecular-weight, antimicrobial compounds (63) usually present in plants at extremely low concentrations prior to infection. These can be synthesized de novo by plants following microbial infection, and effectiveness is determined by the speed and magnitude at which they are produced and accumulated (62). Limited evidence suggests that phytoalexins may be active against insects as well as plant pathogens (90, 95, 74, 32). Physiological and Morphological Changes The response of plants to herbivores can be more extensive than simply modifications of secondary metabolite concentrations. For example, spider mites cause widespread changes in the cytology, histology, and physiology of their host plants, including modifications of photosynthetic and transpirational rates, and they can inject substances that can act as plant growth regulators (reviewed by 59). Herbivores can influence the morphology of their food plants by causing increases in the density of prickles, spines, and hairs (reviewed by 79), by causing the return to juvenile growth form (11), or by affecting the phenology of plant processes such as leaf abscission (106). Many herbivores are "specialists" on plant tissue of a particular physiological age, so that altering the synchrony between plant and insect could act to make the plant appear more resistant. All of these changes could have an influence on herbivores, or on the extent of further herbivory. DYNAMICS OF PLANT CHANGE FOLLOWING HERBIVORE DAMAGE Plants respond to herbivore damage over spatial scales ranging from single leaves to whole trees and over temporal scales ranging from minutes to
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