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Abstract

1. Prominent theories of plant defence have predicted that plants growing on nutrient-poor soils
produce more phenolic defence compounds than those on richer soils. Only recently has the Protein
Competition Model (PCM) of phenolic allocation suggested that N and P limitation could
have different effects because the nutrients are involved in different cellular metabolic processes.
2. We extend the prediction of the PCM and hypothesize that N will have a greater influence on
the production of phenolic defensive compounds than P availability, because N limitation
reduces protein production and thus competition for phenylalanine, a precursor of many phenolic
compounds. In contrast, P acts as a recyclable cofactor in these reactions, allowing protein
and hence phenolic production to continue under low P conditions.
3. We test this hypothesis by comparing the foliar concentrations of phenolic compounds in (i)
phenotypes of 21 species growing on P-rich alluvial terraces and P-depleted marine terraces in
southern New Zealand, and (ii) 87 species growing under similar climates on comparatively
P-rich soils in New Zealand vs. P-depleted soils in Tasmania.
4. Foliar P concentrations of plants from the marine terraces were about half those of plants
from alluvial soils, and much lower in Tasmania than in New Zealand. However, foliar concentrations
of N and phenolic compounds were similar across sites in both comparisons, supporting
the hypothesis that N availability is a more important determinant of plant investment in phenolic
defensive compounds than P availability. We found no indication that reduced soil P levels
influenced plant concentrations of phenolic compounds. There was wide variation in the foliar N
and P concentrations among species, and those with low foliar nutrient concentrations produced
more phenolics (including condensed tannins).
5. Our study is the first trait comparison extending beyond standard leaf economics to include
secondary metabolites related to defence in forest plants, and emphasizes that N and P have different
influences on the production of phenolic defence compounds.

Item Type:	Article
Authors/Creators:	Wright, DM and Jordan, GJ and Lee, WG and Duncan, RP and Forsyth, DM and Coomes, DA
Keywords:	carbon-nutrient balance;herbivory;phenolics;plant–herbivore interactions;protein competition model
Journal or Publication Title:	Functional Ecology
ISSN:	0269-8463
DOI / ID Number:	https://doi.org/10.1111/j.1365-2435.2009.01597.x
Additional Information:	The definitive published version is available online at: http://onlinelibrary.wiley.com/
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