Volatile organic compound emissions from Alnus glutinosa under interacting drought and herbivory stresses

Plant volatile organic compounds (VOCs) elicited in response to herbivory can serve as cues for parasitic and predatory insects, but the modification of VOC elicitation responses under interacting abiotic stresses is poorly known. We studied foliage VOC emissions in the deciduous tree Alnus glutinosa induced by feeding by the larvae of green alder sawfly (Monsoma pulveratum) under well-watered and drought-stressed conditions. Drought strongly curbed photosynthesis rate and stomatal conductance, but there were no effects of insect feeding on photosynthetic characteristics. Feeding induced emissions of volatile products of lipoxygenase pathway and monoterpenes, and emissions of stress marker compounds (E)-β-ocimene and homoterpene DMNT. The emissions were more strongly elicited and reached a maximum value earlier in drought-stressed plants. In addition, methyl salicylate emissions were elicited in herbivory-fed drought-stressed plants. Herbivores were more strongly attracted to well-watered plants and consumed more than a four-fold greater fraction of leaf area than they consumed from drought-treated plants. Overall, this study demonstrates an important priming effect of drought, suggesting that plants under combined drought/herbivory stress are more resistant to herbivores.