Variation of defence-related metabolites in the foliage of adult beech and spruce: A conceptual approach to approximating traded-off carbon

To prevent carbon (C) loss to consumers, trees need to defend their primary production. The tree-internal conflict in resource allocation between growth and defence demands has been the subject of various hypotheses but still requires quantification. A conceptual approach to approximating the C amount dispensable in favour of primary production at the expense of defence is demonstrated which is based on nine defence-related metabolite groups. Quantification is exemplified at the level of sun and shade leaves of adult Fagus sylvatica and Picea abies trees, two species contrasting in foliage type, under oxidative stress as induced by ozone exposure. The difference between maximum and minimum metabolite levels sampled several times throughout four consecutive growing seasons were conceived as dispensable between growth and defence-related metabolism and expressed in proportion of the mean annual gross primary production (GPP) of the foliage. In both species, this proportion amounted to between 2 and 5% of GPP (on a molar C basis). Remaining uncertainties are discussed as concerning functional overlap of substances between growth and defence-related metabolism, estimated classification of metabolite turnover rates and "third-party" trade-offs across C demands. Given the complexity of plant defence, simplification is needed for modelling allocation trade-offs in plants. The presented conceptual approach meets this need in approximating C transfer capacities between competing physiological demands and in stimulating empirical assessments towards mechanistic understanding.