REGULATION OF WOODY PLANT SECONDARY METABOLISM BY RESOURCE AVAILABILITY - HYPOTHESIS-TESTING BY MEANS OF METAANALYSIS

Our aim in this study was to determine how well phenotypic variation i ri foliar concentrations of carbon-based secondary compounds (CBSCs) i n woody plants can be predicted on the basis of two resource-based hyp otheses, i.e. the carbon-nutrient balance (CNB) and growth-differentia tion balance (GDB) hypotheses. We conducted a meta-analysis of literat ure data with respect to responses of CBSCs, carbohydrates and nitroge n to six types of environmental manipulations (fertilization with nitr ogen or phosphorus, shading, CO2 enrichment, drought stress, ozone exp osure): Plant responses to nitrogen fertilization, shading and CO2 enr ichment in terms of pooled CBSCs and carbohydrates were consistent wit h predictions made with the two hypotheses. However, among biosyntheti cally distinct groups of CBSCs only concentrations of phenylpropanoid- derived compounds changed as predicted; hydrolyzable tannins and terpe noids, in particular, were less responsive. Phosphorus fertilization d id not affect concentrations of CBSC or primary metabolites. Plant res ponses to drought and ozone exposure presumably were driven by plant d emands for particular types of compounds (osmolites in the case of dro ught and antioxidants in the case of ozone exposure) rather than by ch anges in resource availability. Based on the relative importance of th e treatment effects, we propose a hierarchical model of carbon allocat ion to CBSCs. The model implies that CBSC production is determined by both resource availability and specific demand-side responses. However , these two mechanisms work at different hierarchical levels. The doma in of the CNB and GDB hypotheses is at the high hierarchical levels, p redicting the total amount of carbon that can be allocated to CBSCs. P redicting altered concentrations of individual CBSCs, i.e. low hierarc hy levels, probably demands biosynthetically detailed models which als o take into account the history of plant interactions with biotic and abiotic factors.