Something in the air: Seedling volatiles and anti-herbivore defence

Background/Question/Methods

Seedling herbivory is an important recruitment filter, influencing species diversity in many different plant communities. Nonetheless, how and why different species are selected or rejected by herbivores is poorly understood. We do know that seedlings of different species vary greatly in the expression of constitutive chemical defences, and that these defences often increase with age. Nonetheless it remains unclear whether or how herbivores detect these defences prior to attack, despite the presumably pressing need seedlings have to signal defensive capability before tissue is damaged or lost. Recently tremendous interest has focussed on the role of Volatile Organic Compounds (VOCs) as herbivore deterrents in established plants, and the most commonly reported response is release of VOCs following attack and subsequent avoidance of the same plant by other herbivores. It would seem likely that VOCs could be used by seedlings to signal defensive capability, but the role of VOCs at the recruitment stage is poorly understood. As part of a new 3-year research project we are using a combination of behavioural, chemical and neurophysiological techniques to elucidate whether seedlings release VOCs as a defensive signal, determine VOC identity, and examine the response of molluscs to seedling VOCs.

Results/Conclusions

Initial experiments using Y-tube olfactometers have shown that selection of macerated tissue derived from 14-d-old seedlings of nine European grassland species by the snail Helix aspersa was strongly related to gustatory acceptability; i.e. seedling selection may be influenced by olfactory selection on the part of the herbivore. A second Y-tube experiment showed that selection of Plantago lanceolata seedlings was strongly negatively correlated with increasing age; pellets derived from 1-wk old seedlings were three times more likely to be selected as those from 8-wk old plants. In elucidating a link between seedling acceptability and olfactory response to macerated seedlings, our initial work suggests that even from an early ontogenetic stage plant selection by snails may be governed by olfactory cues. This finding highlights the need for ecologists to focus more on the role of plant volatiles in seedling-herbivore interactions and our on-going work aims to identify the key VOCs involved in altering herbivores to seedling defenses and to examine how terrestrial mollusks perceive and respond to seedling VOCs.