Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

More documents

Recommendations

Info

334 KARBAN & MYERS 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. evolutionary time. Most of the studies that point to induced resistance, assayed as a decrease in herbivore performance, have found that the response was systemic at least to other parts of the damaged shoot. However, one study measuring rapid increases in foliage phenols found that this chemical response was not systemic in birch trees (99). The spatial extent of the induced response may determine whether the response acts as a defense. A localized response may encourage herbivores to feed elsewhere on the same plant; damage to the plant will be spread but not reduced. Surprisingly, no study has explicitly mapped the spatial extent of induced resistance in all parts of the entire plant. Despite the exciting suggestion by Rhoades (86) and Baldwin & Schultz (4) that plants may become more resistant in response to cues released by damaged neighbors, subsequent experiments have been few and have not supported the idea (80, 28). Some responses are known to occur within several hours after damage, as in the case of proteinase inhibitors in damaged foliage of solanaceous plants (reviewed in 108) or latex in damaged cucurbits (16). What component of damage signals rapidly induced responses is generally not known. Damage to tissues may release cell wall fragments that are translocated to other parts of the plant where they activate genes that code for enzymes, such as proteinase inhibitors (91). In this case the signal is transported systemically within injured tomato plants but is directed primarily up the stem from older leaves to younger ones (69). The proteinase inhibitors accumulate in vacuoles of uninjured cells of injured plants and are deleterious to some caterpillars 00). Induced resistance need not involve de novo synthesis; damage may bring preformed enzymes and substrates into contact, causing the production of active agents (21). Enzymatic activation of compartmentalized precursors is responsible for many reactions, including the cyanogenic response of plants to herbivores (21, 49). Damage to tissue may release ethylene that stimulates the production of PAL and increases in phenolics (110, see also 72). Phenolics are not transported from damaged to undamaged birch leaves; rather, they are synthesized in undamaged leaves following increases in PAL activity (34). The mechanisms of responses that occur over several years are also poorly understood. Plant tissue that dcvelops in the growing season after marked defoliation often shows increases in phenolics and fiber, declines in nutrient concentrations, regrowth of juvenile tissue, and changes in plant morphology (rcvicwed in 102, 79). MECHANISMS: ACTIVE RESISTANCE OR PASSIVE DETERIORATION? While enzymatic activation of precursors and synthesis of phytoalexins and proteinase inhibitors are clearly active processes, changes in plant chemistry
INDUCED RESPONSES 335 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. following defoliation may result from a passive rearrangement of resources within the plant. The distinction is that active responses involve de novo synthesis or energetically costly enzymatic processes, whereas passive responses involve only the consequences of tissue removal. Passive responses have been described as nutrient stress by Tuomi and coworkers (98, 99), as carbon-nutrient imbalance by Bryant and his associates (13, 14), and as passive deterioration by Myers & Williams (80). According to this hypothesis, a tree growing in an area with abundant soil nutrients (a fast growing tree) loses proportionately more nitrogen and other nutrients and less carbon during defoliation because it had proportionately more nitrogen in its leaves. Subsequently, carbon may be replaced in the leaves at a faster rate than nitrogen, and the surplus allocated to carbon-based allelochemicals (terpenes, resins, tannins, and other phenolics) and fiber. These foliar changes are expected to reduce the preference and performance of herbivores on trees that were previously defoliated. On the other hand, trees growing in nutrient-poor conditions or which store proportionately more carbon in their leaves (evergreens) may respond in the opposite way; defoliation may reduce the concentrations of carbon-based chemicals and increase the palatability of leaves of these slow-growing trees in the next growing season (14, 15, 24, 98). This model leads to several testable predictions (see also 98). (a) Nitrogen fertilization of defoliated trees should negate the nutrient imbalance and cancel the induced response; (b) carbon stress should result in a collapse of carbon-based resistance; (c) if herbivory and plant crowding reduce the same nutrients, then the effects of these two stresses should be qualitatively similar (57). Experimental N fertilization of birch trees increased foliar nitrogen and reduced phenolics, while root damage, which reduced nutrient uptake, reduced foliar nitrogen and increased phenolics (97). Larsson et al (64) found similar patterns between carbon availability (light) and carbon-based phenolics. Shading (reduced C) increased the palatability of willows to snowshoe hares, presumably because of reduced carbon-based defenses (12). Clipped and shaded willows produced regrowth shoots with lower concentrations of carbon-based secondary compounds, that were more preferred than clipped and unshaded trees. The resource rearrangement model does not explain all observations, however. Nitrogen fertilization of artificially defoliated birch trees did not negate the induced resistance as assayed by autumnal moth caterpillars (39). Crowding cotton plants reduced their suitability to spider mites; however, crowding and herbivore damage did not act additively to reduce foliage quality for mites or verticillium fungus (57). On the contrary, induced resistance was only apparent when plants were not crowded, suggesting that resources are required for the induced response to occur. These tests of the passive model are not easy to interpret. For example,
Page 1 and 2: Annu. Rev. Ecol. Syst. 1989. 20:331
Page 3: INDUCED RESPONSES 333 Annu. Rev. Ec
Page 7 and 8: INDUCED RESPONSES 337 Annu. Rev. Ec

Induced Plant Responses to Herbivory - Terrestrial Systems Ecology

Create successful ePaper yourself

Delete template?

Save as template?