Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

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340 KARBAN & MYERS WHY INDUCED RESPONSES RATHER THAN CONSTITUTIVE ONES? 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. Some fraction of the induced responses elicited by damage result in greater resistance to herbivores. If these changes increase the resistance of plants to their herbivores, why are they inducible rather than constitutive? The problem becomes more perplexing for those cases in which induced responses are general reactions to many stresses and have activity against many different herbivores and parasites. The problem applies only to active responses since passive deterioration can only be inducible, by definition. We consider four hypotheses. Phytotoxic Responses and Packaging Problems If the products induced by damage are toxic to herbivores and plant diseases, they may also be toxic to the plants themselves, and self-toxicity may increase if the effect is maintained for an extended time. For example, some phytoalexins are toxic to plants at concentrations that inhibit microorganisms (62). Repeated applications of fungus-derived elicitors of these phytoalexins to the foliage of beans caused severe necrosis and stunted growth. This autotoxicity is avoided by a system in which the phytoalexins are only produced when needed. Many plant products that are released following herbivory are locally toxic to the plant. However, precursors may be stored safely in vacuoles so that enzymes and substrates are mixed only after the vacuoles are ruptured by feeding damage (reviewed in 21). Plants Are Induced Much of the Time For some plants, the induced state might be the most common one. For example. tomato plants must be carefully protected in the greenhouse to prevent the induction of high levels of proteinase inhibitors. Plants in the field are likely to be in the induced state most of the time following stimulation from wind (R. M. Broadway. personal cummunication). This argument probably does not apply to those examples of induced resistance in which an effect on herbivores has been demonstrated in the field. This is not really an explanation for why a particular response should be inducible but rather an observation that the distinction between induced and constitutive traits may be largely semantic. in some cases. The Induced Response Creates a Changing Target Most studies measure induced responses by looking at only a restricted group of chemicals or by doing a bioassay. Even so. results often vary considerably
INDUCED RESPONSES 341 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. among parts of plants, and among different plants within a population. The responses to damage of different plants and different parts within an individual plant may be quite idiosyncratic. The plant may not simply be in an "induced state." Rather, induction likely involves differential changes occurring in different types of organs of a plant, and in different organs of the same type (leaves on a tree). Induced responses include many traits that affect herbivores, all of which can change, rather than the turning on of a single "defensive chemical." Each of these traits in each plant part may respond with its own rate of induction and relaxation. A changing, heterogeneous target may allow for a more rapid response and may retard or prevent the adaptation of herbivores or diseases to the plant defense (105). A changing plant phenotype may allow the plant to respond more rapidly to herbivores and other parasites than it could if it relied on constitutive defenses that changed only in evolutionary time. Constitutive defenses have no lag time at all, but also no ability to change when herbivores circumvent them. Induced responses may allow the plant to respond to unpredictable environmental variability (65). Phenotypic plasticity in resistance is expected to be more effective than genetic adaptation in response to selective factors, such as herbivores and plant pathogens, that may vary during the life span of an individual plant. This hypothesis predicts that induced resistance will be most common for plants that experience unpredictable selective pressures from herbivores sporodically in relation to the generation time of the plant. This argument would be strengthened if we knew that phenotypic plasticity in resistance to herbivores was heritable, as plasticity of some other plant traits appears to be (92). Induced Defenses Are Less Costly Much of the recent theory concerning the evolution of plant defenses has centered around the notion that defenses are costly (18, 27, 76, 85, 88). Accordingly, plants should allocate resources to defenses only when and where such allocation will result in increased fitness. This leads to several testable predictions: (a) Herbivory should reduce plant fitness and induced plants should have greater fitness than noninduced plants when herbivores are present. (b) Plants without induced defenses should have higher fitness in environments without herbivores. (c) Plants that are well defended by constitutive defenses against a particular herbivore should not allocate resources to induced defenses against that same herbivore. If constitutive defenses are effective, induced defenses, which are presumably costly, would be redundant. In other words, these two should be negatively correlated. These predictions have not been tested adequately. Cotton plants that were induced at the cotyledon stage supported smaller populations of spider mites
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