Extreme Heat Event Alters BVOC Responses to Elevated Ozone: From Physiology to Emission Patterns

Extreme heat and elevated ozone (O₃) significantly affect plant growth and secondary metabolism, including biogenic volatile organic compound (BVOC) emissions. However, the impact of the extreme heat events (EHs) on the O₃-induced BVOC emissions remains unclear. Here, Quercus nuttallii(an isoprene emitter) and Q. acutissima (a monoterpene emitter) were exposed to five O₃ treatments for three months and subjected to a two-week EH. The emission rates of isoprene and monoterpenes, their synthesis capacities, and reactive oxygen species (ROS) levels were determined to investigate how heat stress modulates O₃ effects. Both BVOC types showed a hormetic response to O₃, with low doses of O₃ stimulating emissions and high doses suppressing them. Isoprene (Hedges'g = 2.91) and monoterpene (Hedges'g = 2.52) emission rates were inhibited by short-term EH across the five O₃ treatments. However, isoprene emission rates generally recovered during the post-EH period, except under the highest O₃ exposure (NF80). EH also shifted isoprene's response to O₃ from being synthesis-driven to ROS-driven. Although the monoterpene emission rate response to O₃ was driven by synthesis capacity in the pre-EH period, this response became less explainable by either factor post-EH. These findings emphasize the profound effects of EH on the BVOC response to O₃, providing crucial insights for predicting regional BVOC emissions under future climate scenarios.