Heat stress strongly induces monoterpene emissions in some plants with specialized terpenoid storage structures

Four different plant species, sunflower (Helianthus annuus), western redcedar (Thuja plicata), American sweetgum (Liquidambar styraciflua), and red ironbark (Eucalyptus sideroxylon), were enclosed in glass or Teflon chambers, exposed to different temperatures typically ranging from 30 degrees C to 43 degrees C, and their terpenoid emission rates were quantified using gas chromatography techniques. The results show that sunflower, western redcedar, and American sweetgum were not significant constitutive monoterpene emitters at 30 degrees C. Monoterpene emissions from western redcedar and American sweetgum remained negligibly low up to 35 degrees C-37 degrees C, however, at higher temperatures, the emissions began to increase at a much more rapid rate than predicted by current emission models. The monoterpene emissions from sunflower, western redcedar, and American sweetgum increased by an average factor of 22, 98, and 5900 for a temperature increase (Delta T) of 13 degrees C, 13 degrees C, and 15 degrees C, respectively, thereby transforming them from negligible into high monoterpene emitters. The observed emission increase was substantially higher than the model-predicted increase of 3.7-fold (Delta T = 13 degrees C) and 4.5-fold (Delta T = 15 degrees C). We demonstrate that current emission algorithms cannot accurately model the emission behavior of these plant species at the high temperatures that are characteristic of severe heatwaves. Furthermore, monoterpene emis-sions from western redcedar and American sweetgum remained elevated for 1-7 days after the heat stress. In contrast to the other three species, red ironbark was a significant constitutive monoterpene emitter (at 30 degrees C) and its emission response to temperature was close to model predictions. Each of the four species investigated in this study possesses specialized foliar terpene-storing structures, for example, glandular trichomes and oil glands. The strong monoterpene emission induction observed in some of these species may be attributed to heat-induced damage to these storage structures. However, the wall permeability of these structures may vary by species and could limit the magnitude of the heat-induced emission response.