Herbivory, induced resistance, and interplant signal transfer in Alnus glutinosa

Field experiments with manually defoliated black alders (Alnus glutinosa) showed that defoliation affected herbivory by the major alder antagonist, the leaf beetle Agelastica alni. Herbivore damage increased with increasing distance to the defoliated tree, suggesting induced resistance not only on the damaged tree, but also on the neighbouring trees. The beetles also avoided leaves from the nearest neighbours for both feeding and oviposition in a laboratory assay, so the alders showed interplant resistance transfer. Natural enemies did not appear to shape this pattern, because the number of entomophagous arthropods and predator-prey ratios even increased with increasing distance to the defoliated tree. The numbers of all specialist, but not the generalist, herbivore species paralleled the increase in the attack of the specialist A. alni, supporting the view that specialists are more affected by plant resistance than generalists. Mechanisms causing this pattern, found in the field, were studied in more detail using biochemical analyses and further bioassays. Responses of alder leaves to herbivory of A. alni were shown to include ethylene emission and the release of a blend of volatiles with mono-, sesqui- and homoterpenes. Changes in leaf chemistry after herbivory included increases in the activity of oxidative enzymes (polyphenoloxidase, PPO, lipoxygenase, LOX, and peroxidase, POD) and proteinase inhibitors (PIs), and an increase in the phenolic contents of the leaves. Quantification of the endogenous jasmonic acid (JA) showed the activation of the octadecanoid pathway following herbivory. The active components in mediating a possible interplant signal transfer via airborne volatiles may have included ethylene, beta-ocimene, 4,8-dimethylnona-1,3,7-triene (DMNT), and 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). The incubation with volatiles resulted in an increase in the activity of catalase (CAT) and PIs (after MeJA application) and in an increase in the content of phenolics and PI activity (after ethylene application). Further evidence that airborne interplant communication may be important in the response of alder trees to beetle attack came from container experiments. In airtight chambers, unattacked leaves significantly increased the activity of proteinase inhibitors when they were associated with leaves previously attacked by beetle larvae. In conclusion, field experiments, bioassays in the laboratory as well as biochemical analyses suggest the existence of interplant resistance transfer in A. glutinosa, with airborne volatiles as a possible mechanism. However, the relative importance of airborne and possible soil-borne signals as well as unknown effects of intensified nutrient absorption of defoliated trees, possibly reducing foliage quality of undamaged neighbours, remains to be shown.