Background/Question/Methods Volatile terpenes produced by plants serve multiple biological roles including tree resistance against insect herbivores. The increased frequency and severity of drought stress observed in forests across the globe may hinder trees from producing defense-related volatiles in response to biotic stress. To assess the ways in which drought stress physiology alters herbivore-induced volatile emissions, we monitored pre-dawn water potential, gas-exchange, and terpene volatile emissions of ponderosa pine (Pinus ponderosa) saplings for three drought durations and in response to simulated herbivory via methyl jasmonate application. To determine if this limited induction was due to the reliance of de novo herbivore-induced emissions on new photosynthates, we then employed a 13CO2 pulse-chase labeling technique and measured incorporation into terpenes emitted from well-watered trees under simulated herbivory.
Results/Conclusions Although three-, six-, and seven-week drought treatments reduced photosynthetic rates by 20, 89, and 105%, respectively, constitutive volatile emissions were generally resistant. Herbivore-induced emissions, however, exhibited threshold-like behavior; saplings were unable to induce emissions when pre-dawn water potentials were below -2 MPa, the approximate zero-assimilation point for this species. Even under well-watered conditions, less than 8% of herbivore-induced volatiles became labelled, suggesting that de novo induced emissions from ponderosa pine are synthesized predominantly from older carbon sources. These data suggest that drought likely constrains emissions through sophisticated molecular mechanisms that alter carbon metabolism and allocation, as opposed to substrate limitation cause by the cessation of photosynthesis. These results highlight the importance of considering drought severity when assessing herbivore-induced volatiles and the need to identify the specific mechanisms governing induced emission dynamics once critical physiological drought thresholds are surpassed.