Leaf surface traits: overlooked determinants of birch resistance to herbivores and foliar micro-fungi?

Abstract

Previous studies on the mechanisms of birch resistance to herbivores and foliar micro-fungi (both pathogenic and endophytic) have focused mainly on the role of internal leaf chemistry. In the present study, we examined genetic correlations between leaf surface traits (glandular trichome density and total concentrations of surface flavonoid aglycones) and occurrence of three species of foliar micro-fungi, one pathogenic rust (Melampsoridium betulinum) and two endophytic fungi (Fusicladium sp. and Melanconium sp.), and performance of autumnal moth larvae (Epirrita autumnata) in two birch species, Betula pubescens ssp. czerepanovii and B. pendula. The performance of autumnal moth larvae on B. pubescens ssp. czerepanovii was negatively correlated with density of glandular trichomes (RGR: r=−0.855; pupal mass: r=−0.709). In addition, rust infection was negatively correlated with trichome density in B. pendula (r=−0.675) and with epicuticular flavonoid aglycones in B. pubescens ssp. czerepanovii (r=−0.855). The frequency of the endophytic fungus Fusicladium sp., was related to epicuticular flavonoid aglycones (r=−0.782), while another endophytic fungus, Melanconium sp., showed no associations with any of the studied variables in B. pubescens ssp. czerepanovii. Our results indicate that leaf surface traits may be at least as important determinants of herbivore performance and micro-fungi abundance in birch as leaf internal chemistry.