Phenolic and phenolic-related factors as determinants of suitability of mountain birch leaves to an herbivorous insect

We investigated the role of phenolic and phenolic-related traits of the lea ves of mountain birch (Betula pubescens ssp, czerepanovii) as determinants of their suitability for the growth of larvae of the geometrid Epirrita aut umnata. As parameters of leaf suitability, we determined the contents of to tal phenolics, gallotannins, soluble and cell-wall-bound proanthocyanidins (PAS and PAB, respectively), lignin, protein precipitation capacity of tann ins (PPC), and leaf toughness. In addition, we examined concentrations of s oluble carbohydrates and protein-bound amino acids as background variables describing the nutritive value of leaves. The correlation of the leaf trait s of our 40 study trees with the tree-specific relative growth rate (RGR) o f E. autumnata showed that the only significant correlation with RGR was th at of PAS - the largest fraction of total phenolics - and even that explain ed only 15% of the variation in E. autumnata growth. The nonlinear estimati on of the relationship between RGR and PAS by piecewise linear regression d ivided the 40 study trees into two groups: (i) 19 trees with good leaves fo r E. autumnata (RGR ranging from 0.301 to 0.390), and (ii) 21 trees with po or leaves (RGR ranging from 0.196 to 0.296). The suitability of leaves with in these two groups of trees was determined by different phenolic traits. W ithin the good group, the suitability of leaves for larvae was determined b y the PPC of extracts, which strongly correlated with gallotannins, and by the total content of gallotannins. In contrast, the leaves of poor trees ha d significantly higher contents of both PAS and PAB, but leaf toughness cor related only negatively with the RGR of E. autumnata larvae. We also discus s the causes of variation in the phenolic and phenolic-related factors that determine the suitability of leaves for E. autumnata larvae in different g roups of trees. (C) 2001 Elsevier Science Ltd. All rights reserved.