Massive release of volatile organic compounds due to leaf midrib wounding in Populus tremula

We investigated the rapid initial response to wounding damage generated by straight cuts to the leaf lamina and midrib transversal cuts in mature aspen (Populus tremula) leaves that can occur upon herbivore feeding. Wound-induced volatile emission time-courses of 24 compounds were continuously monitored by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS). After the mechanical wounding, an emission cascade was rapidly elicited, resulting in emissions of key stress volatiles methanol, acetaldehyde and volatiles of the lipoxygenase pathway, collectively constituting ca. 99% of the total emission. For the same wounding magnitude, midrib cuts lead to six-fold greater emissions of volatiles per mm² of surface cut than lamina cuts during the first emission burst (shorter than seven minutes), and exhibited a particularly high methanol emission compared to the emissions of other volatiles. This evidence suggests that feeding by herbivores capable of consuming the leaf midrib can result in disproportionally greater volatile release than feeding by smaller herbivores incapable of biting through the major veins.