Herbivore-induced terpene emissions by ponderosa pine are predominantly synthesized from stored carbon and constrained by sustained drought

Shealyn Malone, Forest and Wildlife Ecology, University of Wisconsin, Madison, WI, Austin Simonpietri, Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, W. Berk Knighton, Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT and Amy Trowbridge, Entomology, University of Wisconsin-Madison, Madison, WI

Background/Question/Methods
Drought and herbivory are predicted to increase in frequency and duration as the climate warms. Individually, drought and herbivory alter chemical defense compounds in conifers, yet despite their common co-occurrence and role in shaping forest ecosystems, the interactive effect of drought and herbivory on tree chemistry remains unclear. Trees simultaneously experiencing drought and herbivory will face trade-offs in terms of carbon allocation to defense compounds, but how these trade-offs manifest and when they occur remains elusive. To address these gaps in our knowledge, we quantified mono- and sesquiterpene composition of whole-canopy volatile emissions, one year-old needles, and stem tissue from ponderosa pine saplings. Trees were exposed to three drought durations (3, 6 and 7 weeks) and subsequently treated with methyl jasmonate (MeJA) to simulate herbivory. To link primary physiology with secondary metabolism, pre-dawn water potential and net assimilation rate were also measured. Finally, to determine the source of carbon substrate (i.e., recently fixed versus stored carbon) for the synthesis of constitutive and induced terpene emissions, we used 13C pulse-chase labeling techniques with well-watered trees.
Results/Conclusions
Results show that drought modulates features of the terpene chemical defense syndrome—volatile emission rates, needle concentrations, and woody tissue concentrations—at different timescales and levels of drought severity. Under mild drought when net assimilation rate was reduced by ~20%, induced emissions were unaffected. However, as net assimilation rate reached zero under prolonged drought, induced emissions were 12x lower than the induced emissions of well-watered trees. Interestingly, terpene concentrations in the needle and woody tissue trended higher as drought progressed, suggesting that the physiochemical mechanisms generally understood to control volatile emissions by terpene-storing species may be incomplete, particularly when describing emissions during drought. Constitutive terpene emissions were predominantly unlabeled, corroborating observations that emissions by ponderosa pine primarily originate from stored pools (i.e., resin ducts). Interestingly, induced emissions were also unlabeled, suggesting that de novo synthesis of induced terpenes are also synthesized from stored carbon, not recent assimilates. These observations suggest that drought-suppression of induced terpene emissions, despite occurring concomitantly with net zero assimilation, is not a direct effect of reduced photosynthetically-derived carbon substrate. Our study highlights the need to better understand the biophysical mechanisms controlling volatile emissions from terpene-storing species during drought. Further, our findings suggest that drought-suppression of herbivore-induced terpene emissions may impact tree resistance against forests pests and pathogens, providing a potential mechanistic link between drought-induced shifts in metabolism and insect driven patterns of mortality.