Factors Affecting Expression of Defenses

Some plant groups are characterized by particular defenses. For example, ferns and gymnosperms rely primarily on phenolics, terpenoids, and insect hormone analogues, whereas angiosperms more commonly produce alkaloids, phenolics, and many other types of compounds. However, most plants apparently produce compounds representing a variety of chemical classes (Harborne 1994, Newman 1990). Each plant species can be characterized by a unique "chemical fingerprint" conferred by these chemicals. Production of alkaloids and other physiologically active nitrogenous defenses depends on the availability of nitrogen (Harborne

1994). However, at least four species of spruce and seven species of pines are known to produce piperidine alkaloids (Stermitz et al. 1994), despite low N concentrations. Feeding by phytophagous insects can be reduced substantially by the presence of plant defensive compounds, but insects also identify potential hosts by their chemical fingerprint.

Defensive compounds may be energetically expensive to produce, and their production competes with production of other necessary compounds and tissues (e.g., Baldwin 1998, Chapin et al. 1987, Herms and Mattson 1992, Kessler and Baldwin 2002, Strauss and Murch 2004). Some, such as the complex phenolics and terpenoids, are highly resistant to degradation and cannot be catabolized to retrieve constituent energy or nutrients for other needs. Others, such as alkaloids and nonprotein amino acids, can be catabolized and the nitrogen, in particular, can be retrieved for other uses, but such catabolism involves metabolic costs that reduce net gain in energy or nutrient budgets. Few studies have addressed the fitness costs of defense. Baldwin (1998) evaluated seed production by plants treated or not treated with jasmonate, a phytohomone that induces plant defenses. Induction of defense did not significantly increase seed production of plants that came under herbivore attack but significantly reduced seed production of plants that were not attacked.

Given the energy requirements and competition among metabolic pathways for limiting nutrients, production of defensive compounds should be sensitive to risk of herbivory or predation and to environmental conditions (e.g., Chapin et al. 1987, Coley 1986, Coley et al. 1985, Hatcher et al. 2004, Herms and Mattson 1992, M. Hunter and Schultz 1995, Karban and Niiho 1995). Plants that support colonies of predaceous ants may reduce the need for, and cost of, chemical defenses. L. Dyer et al. (2001) reported that several amides produced by Piper cenocladum deter generalist herbivores, including leaf-cutting ants and orthopterans, whereas resident Pheidole bicornis ants deter specialist herbivores that oviposit on the plant. Plants hosting P. bicornis colonies produced lower concentrations of amides, indicating a tradeoff in costs between amides and support of ants. Nevertheless, redundant defenses are necessary to minimize losses to a diversity of herbivores.

Organisms are subjected to a variety of selective factors in the environment. Intense herbivory is only one factor that affects plant fitness and expression of defenses (Bostock et al. 2001). Plant genotype also is selected by climatic and soil conditions, various abiotic disturbances, etc. Factors that select intensively and consistently among generations are most likely to result in directional adaptation. The variety of biochemical defenses against herbivores testifies to the significance of herbivory in the past. Nevertheless, at least some biochemical defenses have multiple functions (e.g., phenolics as UV filters, pigments and structural components, as well as defense), implying that their selection was enhanced by meeting multiple plant needs. Similarly, insect survival is affected by climate, disturbances, condition of host(s), as well as a variety of predators. Short generation time confers a capacity to adapt quickly to strong selective factors, such as consistent and widespread exposure to particular plant defenses.

Plants balance the tradeoff between the expense of defense and the risk of severe herbivory (Coley 1986, Coley et al. 1985). Plants are capable of producing constitutive defenses, which are present in plant tissues at any given time and determine the "chemical fingerprint" of the plant, and inducible defenses, which are produced in response to injury (e.g., Haukioja 1990, Karban and Baldwin 1997, Klepzig et al. 1996, Nebeker et al. 1993, M. Stout and Bostock 1999, Strauss et al. 2004). Constitutive defenses consist primarily of relatively less specific, but generally effective, compounds, whereas inducible defenses are more specific compounds produced in response to particular types of injury (Hatcher et al. 2004). Induced defense is under the control of plant wound hormones, particularly jasmonic acid, salicylic acid, and ethylene (Creelman and Mullet 1997, Farmer and Ryan 1990, Karban and Baldwin 1997, Kessler and Baldwin 2002, Thaler 1999a, Thaler et al. 2001), that are triggered by injury or herbivore regurgitants (McCloud and Baldwin 1997). For example, pitch, consisting of relatively low-molecular weight terpenoids, is a generalized wound repair mechanism of many conifers that seals wounds, infuses the wound with constitutive terpenoids, and physically prevents penetration of the bark by insects (see Fig. 3.2). Successful penetration of this defense by bark beetles induces production of more complex phenolics that cause cell necrosis and lesion formation in the phloem and cambium tissues surrounding the wound and kill the beetles and associated microorganisms (Klepzig et al. 1996, Nebeker et al. 1993). Proteinase inhibitors are commonly induced by wounding and interfere with insect digestive enzymes (Kessler and Baldwin 2002, Thaler et al. 2001).

Studies indicate that plants often respond to injury with a combination of induced defenses that may be targeted against a particular herbivore or pathogen species but that also confer generalized defense against associated or subsequent herbivores or pathogens (Hatcher et al. 2004, Kessler and Baldwin 2002, M. Stout and Bostock 1999). Klepzig et al. (1996) reported that initial penetration of Pinus resinosa bark by bark beetles and associated pathogenic fungi was not affected by plant constitutive defenses but elicited elevated concentrations of phenolics and monoterpenes that significantly inhibited germination of fungal spores or subsequent hyphal development. Continued insect tunneling and fungal development elicited further host reactions that were usually sufficient to repel the invasion in healthy trees. Plant defenses can be induced through multiple pathways that encode for different targets, such as internal specialists versus more mobile generalists, and interaction ("crosstalk") among pathways may enhance or compromise defenses against associated consumers (Kessler and Baldwin 2002,Thaler 1999a,Thaler et al. 2001).Whereas emission of jasmonate from damaged plants can communicate injury and elicit production of induced defenses by neighboring, even unrelated, plants (see Chapter 8), herbivorous insects may not be able to detect, or learn to avoid, jasmonic acid (Daly et al. 2001).

Tissues vary in their concentration of defensive compounds, depending on risk of herbivory and value to the plant (Dirzo 1984, Feeny 1970, McKey 1979, Scriber and Slansky 1981, Strauss et al. 2004). Foliage tissues, which are the source of photosynthates and have a high risk of herbivory, usually have high concentrations of defensive compounds. Similarly, defensive compounds in shoots are concentrated in bark tissues, perhaps reducing risk to subcortical tissues, which have relatively low concentrations of defensive compounds (e.g., Schowalter et al. 1992).

Defensive strategies change as plants or tissues mature (Dirzo 1984, Forkner et al. 2004). A visible example is the reduced production of thorns on foliage and branches of acacia, locust, and other trees when the crown grows above the graz ing height of vertebrate herbivores (Cooper and Owen-Smith 1986, P. White 1988). Seasonal growth patterns also affect plant defense. Concentrations of condensed tannins in oak, Quercus spp., leaves generally increase from low levels at bud break to high levels at leaf maturity (Feeny 1970, Forkner et al. 2004). This results in a concentration of herbivore activity during periods of leaf emergence (Coley and Aide 1991, Feeny 1970, M. Hunter and Schultz 1995, R. Jackson et al. 1999, Lowman 1985,1992, McKey 1979). Lorio (1993) reported that production of resin ducts by loblolly pine, Pinus taeda, is restricted to latewood formed during summer. The rate of earlywood formation in the spring determines the likelihood that southern pine beetles, Dendroctonus frontalis, colonizing trees in spring will sever resin ducts and induce pitch flow. Hence, tree susceptibility to colonization by this insect increases with stem growth rate.

Concentrations of various defensive chemicals also change seasonally and annually as a result of environmental changes (Cronin et al. 2001, Mopper et al. 2004). Cronin et al. (2001) monitored preferences of a stem-galling fly, Eurosta solidaginis, among the same 20 clones of goldenrod, Solidago altissima, over a 12 year period and found that preference for, and performance on, the different clones was uncorrelated between years. These data indicated that genotype x environmental interaction affected the acceptability and suitability of clones for this herbivore.

Healthy plants growing under optimal environmental conditions should be capable of meeting the full array of metabolic needs and may provide greater nutritional value to insects capable of countering plant defenses. However, unhealthy plants or plants growing under adverse environmental conditions (such as water or nutrient limitation) may favor some metabolic pathways over others (e.g., Herms and Mattson 1992, Lorio 1993, Mattson and Haack 1987, Mopper et al. 2004, Tuomi et al. 1984, Wang et al. 2001, R. Waring and Pitman 1983). In particular, maintenance and replacement of photosynthetic (foliage), reproductive, and support (root) tissues represent higher metabolic priorities than does production of defensive compounds, under conditions that threaten survival. Therefore, stressed plants often sacrifice production of defenses so as to maximize allocation of limited resources to maintenance pathways and thereby become relatively more vulnerable to herbivores (Fig. 3.9).

However, N enrichment may permit plants to allocate more C to growth and reduce production of nonnitrogenous defenses, making plants more vulnerable to herbivores, as predicted by the Carbon/nutrient balance hypothesis (Holopainen et al. 1995). Plant fertilization experiments have produced apparently contradictory results (Kyto et al. 1996, G. Waring and Cobb 1992). In some cases, this inconsistency may reflect different insect feeding strategies (Kyto et al. 1996, Schowalter et al. 1999). Kyto et al. (1996) also found that positive responses to N fertilization at the individual insect level were often associated with negative responses at the population level, perhaps indicating indirect effects of fertilization on attraction of predators and parasites.

Spatial and temporal variability in plant defensive capability creates variation in food quality for herbivores (L. Brower et al. 1968). In turn, herbivore employment of plant defenses affects their vulnerability to predators (L. Brower

WOOD PRODUCTION / UNIT LEAF AREA, gm-2 yr-1

| The density of mountain pine beetle attacks necessary to kill lodgepole pine increases with increasing host vigor, measured as growth efficiency. The blackened portion of circles represents the degree of tree mortality. The solid line indicates the attack level predicted to kill trees of a specified growth efficiency (index of radial growth); the dotted line indicates the threshold above which beetle attacks are unlikely to cause mortality. From R. Waring and Pitman (1983) with permission from Blackwell Wissenschafts Verlag GmbH.

WOOD PRODUCTION / UNIT LEAF AREA, gm-2 yr-1

| The density of mountain pine beetle attacks necessary to kill lodgepole pine increases with increasing host vigor, measured as growth efficiency. The blackened portion of circles represents the degree of tree mortality. The solid line indicates the attack level predicted to kill trees of a specified growth efficiency (index of radial growth); the dotted line indicates the threshold above which beetle attacks are unlikely to cause mortality. From R. Waring and Pitman (1983) with permission from Blackwell Wissenschafts Verlag GmbH.

et al. 1968, Malcolm 1992, Stamp et al. 1997,Traugott and Stamp 1996). Herbivore feeding strategies represent a tradeoff between maximizing food quality and minimizing vulnerability to predators (e.g., Schultz 1983, see later in this chapter).

The frequent association of insect outbreaks with stressed plants, including plants stressed by atmospheric pollutants (e.g., V.C. Brown 1995, Heliovaara 1986, Heliovaara and Vaisanen 1986,1993, W. Smith 1981), led T. White (1969, 1976, 1984) to propose the plant stress hypothesis (i.e., that stressed plants are more suitable hosts for herbivores). However, experimental studies have indicated that some herbivore species prefer more vigorous plants (G. Waring and Price 1990), leading Price (1991) to propose the alternative plant vigor hypothe sis. Reviews by Koricheva et al. (1998) and G. Waring and Cobb (1992) revealed that response to plant condition varies widely among herbivore species. Schowalter et al. (1999) manipulated water supply to creosotebushes, Larrea tridentata, in New Mexico and found positive, negative, nonlinear, and nonsignificant responses to moisture availability among the assemblage of herbivore and predator species on this single plant species. These results indicated that both hypotheses can be supported by different insect species on the same plant.

Regardless of the direction of response, water and nutrient subsidy or limitation clearly affect herbivore-plant interactions (Coley et al. 1985, M. Hunter and Schultz 1995, Mattson and Haack 1987). Therefore, resource acquisition is moderated, at least in part, by ecosystem processes that affect the availability of water and nutrients (see Chapter 11).

Some plant species respond to increased atmospheric concentrations of CO2 by allocating more carbon to defenses, such as phenolics or terpenoids, especially if other critical nutrients, such as water or nitrogen, remain limiting (e.g., Arnone et al. 1995, Chapin et al. 1987, Grime et al. 1996, Kinney et al. 1997, Roth and Lindroth 1994). However, plant responses to CO2 enrichment vary considerably among species and as a result of environmental conditions such as light, water, and nutrient availability (Bazzaz 1990, Dudt and Shure 1994, P. Edwards 1989, Niesenbaum 1992), with equally varied responses among herbivore species (e.g., Bezemer and Jones 1998, Salt et al. 1996,Watt et al. 1995). Such complexity of factors interacting with atmospheric CO2 precludes general prediction of effects of increased atmospheric CO2 on insect-plant interactions (Bazzaz 1990, Watt et al. 1995).

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