NITROGEN-INDUCED REDUCTION IN LEAF PHENOLIC LEVEL IS NOT ACCOMPANIED BY INCREASED RUST FREQUENCY IN A COMPATIBLE WILLOW (SALIX MYRSINIFOLIA)-MELAMPSORA RUST INTERACTION

Phenolic compounds are potential antimicrobial agents in willows (Sali x spp.). However, little is known about their role in willow pathogen interactions. Earlier studies have shown that phenolic levels may vary depending on genotype and environmental factors (such as nutrient ava ilability). In the present study, the combined effects of nitrogen fer tilization and infection by pathogenic rust (Melampsora sp.) on phenol ics and growth in willow (Salix myrsinifolia) clones were investigated . Cutting-propagated plants of eight willow clones were subjected to l ow and optimum levels of nitrogen Fertilization and inoculated with ru st urediniospores or mock-inoculated with water. The plants were harve sted 20 days after the inoculations and the dry weight of the leaves, stems and roots determined. Phenolics were analysed from leaf material using HPLC. The willow clones differed significantly in their phenoli c content, rust frequency and phytomass production. Plants under optim um nitrogen availability showed increased shoot phytomass production a nd reduced root production. Rust infection increased the root phytomas s of willow under both fertilization treatments, but had no consistent effect on the shoot growth of the clones. Low nitrogen availability i ncreased the phenolic levels in willow leaves, but the rust frequency did not differ significantly between plants grown at low and optimum l evels of nitrogen fertilization. In several clones, rust infection inc reased the concentrations of individual phenolic compounds, but there were no significant correlations between phenolic concentrations and t he level of rust infection. The results indicate that resistance in co mpatible willow-rust interactions may not be directly governed by phen olic compounds.