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

Abstract

Eucalypts are important forest resources in southwestern China, and may be tolerant to elevated ground-level ozone (O₃) concentrations that can negatively affect plant growth. High CO₂ may offset O₃-induced effects by providing excess carbon to produce secondary metabolites or by inducing stomatal closure. Here, the effects of elevated CO₂ and O₃ 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 O₃ (< 10 nmol mol⁻¹ and 60 nmol mol⁻¹) and CO₂ (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 O₃ and the countering effect of high CO₂ 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 O₃ and CO₂, singly or in combination, affected only leaf physiology but not biomass of various organs. Elevated CO₂ impacted several leaf traits, including stomatal conductance, leaf mass per area, carbon, lignin, n-icosane, geranyl acetate and elixene. Limited effects of elevated O₃ on leaf physiology (nitrogen, n-icosane, geranyl acetate, elixene) were commonly offset by elevated CO₂. We conclude that E. globulus, E. grandis and the hybrid were tolerant to these O₃ and CO₂ treatments, and n-icosane, geranyl acetate and elixene may be major players in tolerance mechanisms of the tested species.

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