Elevated CO2 offsets the alteration of foliar chemicals (n-icosane, geranyl acetate, and elixene) induced by elevated O3 in three taxa of O3-tolerant eucalypts

Eucalypts are important forest resources in southwestern China, and may be tolerant to elevated ground-level ozone (O 3 ) concentrations that can negatively affect plant growth. High CO 2 may offset O 3 -induced effects by providing excess carbon to produce secondary metabolites or by inducing stomatal closure. Here, the effects of elevated CO 2 and O 3 on leaf secondary metabolites and other defense chemicals were studied by exposing seedlings of Eucalyptus globulus, E. grandis, and E. camaldulensis × E. deglupta to a factorial combination of two levels of O 3 (< 10 nmol mol −1 and 60 nmol mol −1 ) and CO 2 (ambient: 370 μmol mol −1 and 600 μmol mol −1 ) in open-top field chambers. GC-profiles of leaf extracts illustrated the effect of elevated O 3 and the countering effect of high CO 2 on compounds in leaf epicuticular wax and essential oils, i.e., n -icosane, geranyl acetate and elixene, compounds known as a first-line defense against insect herbivores. n -Icosane may be involved in tolerance mechanisms of E. grandis and the hybrid, while geranyl acetate and elixene in the tolerance of E. globulus . Elevated O 3 and CO 2 , singly or in combination, affected only leaf physiology but not biomass of various organs. Elevated CO 2 impacted several leaf traits, including stomatal conductance, leaf mass per area, carbon, lignin, n -icosane, geranyl acetate and elixene. Limited effects of elevated O 3 on leaf physiology (nitrogen, n -icosane, geranyl acetate, elixene) were commonly offset by elevated CO 2 . We conclude that E. globulus , E. grandis and the hybrid were tolerant to these O 3 and CO 2 treatments, and n -icosane, geranyl acetate and elixene may be major players in tolerance mechanisms of the tested species.