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- Materials and methods
1. It is claimed that the quality of foliage following defoliation depends on carbon/nutrient balance of the tree. To study the importance of sink/source regulation for the quality of foliage, as well as for its quantity, Pinus sylvestris trees were defoliated and fertilized both in southern Finland and at the tree line in northern Finland.
2. The pattern of defoliation in a shoot was more decisive for quantitative changes in new foliage than its extent: removal of similar amounts of foliage from different branch parts led to different outcomes.
3. Defoliation of 50% spread evenly within a shoot, or applied to the basal part of a shoot only, did not alter production of new foliage, whereas defoliation applied to the apical part of a shoot decreased the mass and length of needles in the new shoot. Defoliation of apically located 1-year-old needles of the branch leader shoot, but not of 2-year-old ones, significantly reduced the mass and length of needles in new shoots.
4. These results are consistent with the explanation that damage alters the ability of shoots and branches to form strong meristematic sinks and that sink strength determines the ability of these meristems to draw resources from the common pool of the tree.
5. Defoliation of the main photosynthate source lowered concentrations of the fructose and glucose, indicating shortage of carbon. However, whole-tree defoliation did not affect the concentrations of individual foliar sugars.
6. Traits describing pine shoot growth correlated negatively with foliar phenolic concentrations but not with concentrations of other secondary compounds. Concentrations of foliage phenolics consistently increased after defoliation, while terpenoids, putatively the main class of defensive compounds in Scots Pine, did not respond to defoliation. Defoliation of a branch or a whole tree had only slight effects on the concentrations of fibre, mono- and sesquiterpenes, resin acids or nitrogen.
7. Likewise, fertilization significantly increased the concentration of some sesquiterpenes only in pine foliage. Whole-branch defoliation and fertilization together had no effect on the concentration of fibre or nitrogen in pine foliage.
8. Altogether, the amount of foliar biomass removed, nutrients or carbon did not explain in any consistent way the qualitative changes in the pine foliage. Instead, results were consistent with simple physiological dependence of foliar traits on sink strength. Production of terpenoids reflected increased sink strength, but the production of phenolics was negatively correlated with sink strength.
9. The difference between shoot growth characteristics and foliage concentrations of phenolics and, on the other hand, terpenoids, may have a biosynthetic, instead of an ecological or evolutionary explanation. Protein synthesis, and thereby possibilities for growth, competes with phenolic synthesis (via phenylalanine) but not with terpenoid synthesis.
10. These results indicate that in Scots Pine the predicted trade-off between growth and production of pooled carbon-rich secondary compounds was found in phenolics but presumably for reasons external to the carbon/nutrient balance and growth/differentiation balance hypotheses. Instead, terpenoids did not behave as predicted by these theories.
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- Materials and methods
The nature and significance of herbivore-induced, potentially defensive responses in plants have been controversial for more than a decade (e.g. Haukioja 1980, 1990; Fowler & Lawton 1985; Tallamy & Raupp 1991; Karban 1993). The earliest models explaining herbivore-induced responses assumed at least implicitly that plant quality for herbivores had been fine tuned by herbivory and emphasized that plant chemical defence varied with exposure to herbivores (e.g. McKey 1979; Rhoades 1979, 1985; Haukioja 1980). Subsequent hypotheses have particularly emphasized the role of the plant's resources in modifying defensive responses: e.g. nutrient vs carbon availability for the plant, amounts of carbon vs nutrients lost in defoliation, as well as the inherent growth rate of plants (Bryant, Chapin & Klein 1983; Coley, Bryant & Chapin 1985; Herms & Mattson 1992). According to the growth/differentiation balance hypothesis (Herms & Mattson 1992), plant growth rates may be linked to their chemical defence owing to physiological constraints between cell growth and differentiation. According to the carbon/nutrient balance hypothesis (Bryant et al. 1983), on the other hand, both the type and amount of chemical defence will vary with the environmental availability of nutrients and on the basis of the carbon/nutrient balance in the plant tissue. The cornerstone for both hypotheses is the strong negative correlation between concentrations of plant nitrogen and phenolics, the trade-off having a physiological basis in phenylalanine, a product of the shikimate acid pathway, which can be used either for protein or for phenolic synthesis (Margna 1977; Margna, Vainjärv & Laanast 1989).
Plants, as modular organisms, do not necessarily respond to environmental challenges such as herbivory as whole individuals. Instead, the behaviour of the individual plant results from responses of individual, competing modules. Furthermore, different modules within the same individual plant may carry foliage of different ages. The age of foliage often determines its value for herbivores. Accordingly, the value of foliage for herbivores may vary even within an individual plant as much as, or more than, the value of foliage of different plant species. Such within-plant variability, and the consequences of responses of individual modules, have received much less attention in plant-herbivore studies than the responses of whole plants (but see e.g. Whitham & Slobodchikoff 1981; Suomela, Kaitaniemi & Nilson 1995). All this indicates that the elements of whole-plant physiology also have to be explicitly incorporated into ecological and evolutionary explanations of herbivore-induced plant responses.
Within-plant control among meristems is exerted via differences in their sink strengths (Clifford 1992) and operates by means of a complex interplay involving at least hormonal cues (Hillman 1984; Cline 1991). Effective control at the whole-plant level assumes that meristems in the most favourable positions (in relation to abiotic factors but also in relation to other meristems) for genet fitness dominate over meristems located in inferior positions (Haukioja 1991; Sachs, Novoplansky & Cohen 1993). Interactions among modules form the basis for the sink/source hypothesis explaining herbivore-induced responses in plants (Haukioja 1990; Honkanen & Haukioja 1994; Honkanen, Haukioja & Suomela 1994; Haukioja & Honkanen 1996). According to this hypothesis, one primary way in which the plant is affected by damage is via damage-induced changes in the abilities of meristems to compete for resources (Haukioja et al. 1990; Honkanen & Haukioja 1994; Ruohomäki et al. 1997). Damage by herbivores may modify sink strengths either by directly disturbing them or via disturbances to source leaves during sink formation (Honkanen et al. 1994). Recognizing that the lost biomass may have temporarily and spatially variable physiological roles, sink vs source, this scenario allows for several alternative outcomes even after damage which removes the same amount of biomass and nutrients. For instance, if the leaves on a branch, at a given time, feed developing local meristems, damage to these leaves will presumably lead to weak new meristems. The insufficient sink strength of weak meristems seems to be an important reason why a branch which has been defoliated at a time when the meristems for the next-year leaves are still developing, in the following growth season will compete poorly with other branches and will not receive its normal share from the total resource pool of the tree. Among-module alterations alone may lead to herbivore-induced responses (Haukioja & Honkanen 1996; Honkanen & Haukioja 1998). This emphasizes the need to understand whole-plant sink/source relationships at the time when the damage occurs and the importance of the timing of the damage in relation to seasonal shifts in resource allocation (Marquis 1992; Haukioja & Honkanen 1996).
There were two aims in this study. First, to test the predictions of the sink/source hypothesis, sink/source relationships were altered by debudding and defoliation experiments were conducted on whole trees and on individual branches and shoots of Scots Pine Pinus sylvestris. Second, to test the role of sink/source regulation on the quality of Scots Pine foliage for herbivores after fertilization and defoliations, the concentrations of the main putative defensive compounds, phenolics, resin acids and terpenoids, which in Scots Pine are crucial defences, were determined (Niemelä, Mannila. & Mäntsälä 1982; Haukioja et al. 1983; Danell, Gref & Yazadani 1990; Björkman, Larsson & Gref 1991; Sjöberg & Linden 1991; Saikkonen, Neuvonen & Kainulainen 1995). In addition, the concentrations of structural carbohydrates (fibre and lignin) which contribute to toughness of foliage as well as some sugars and nitrogen, i.e. important metabolites for insect growth, were determined.