Effects of elevated growth temperature and enhanced atmospheric vapour pressure deficit on needle and root terpenoid contents of two Douglas fir provenances

Abstract

In the present work, we studied the effects of elevated air temperatures, which were above the optimum for photosynthesis, in combination with enhanced atmospheric VPD on two Douglas fir provenances grown under controlled conditions in a climate chamber. Provenance Monte Creek (MC) from the menziesii-glauca transition zone, Southern British Columbia, Canada, was derived from a dry environment receiving ca. half of the precipitation at its natural site than the interior provenance Pend Oreille (PO) from a mesic site in Northeast Washington State, US. We determined the terpenoid contents in needles and roots of the trees as well as terpene emission from needles and terpenoid synthase activities observing clear provenance-specific patterns. Whereas total terpenoid contents in needles dropped significantly in provenance PO in response to thermal stress, they remained unaffected in MC. The drop in terpenoid content in PO was due to decreased abundance of almost all identified terpenoids with exception of five compounds. Terpene emission was significantly enhanced in thermal-stressed provenance MC but it was unaffected in provenance PO. Oppositely, root terpenoid contents were rather stable in both provenances upon high temperature and enhanced atmospheric VPD. Similarly, we did not observe stress effects on terpenoid synthase activity, which was used as a proxy for the formation of terpenoids. The results indicate that features of the original habitat of the trees determine plant chemotypic properties, for example, thermal stress related responses. The observed decrease of terpenoid levels in needles of PO after long-term exposure to elevated temperature/enhanced atmospheric VPD, might weaken stress-exposed trees. Since terpenoids are essential components of the conifers’ defense arsenal against herbivores, decreased terpenoid levels might increase susceptibility of stressed trees to above- and belowground herbivore challenges.