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.
Highlights
High concentrations of tropospheric ozone (O3) can induce damage in plants and reduce forest productivity (Broadmeadow et al 1999; Percy et al 2003; Manning 2005; Proietti et al 2016; Yuan et al 2015, 2016)
Seedlings of Eucalyptus globulus (Glo) and Eucalyptus grandis (Gra) and cuttings of hybrid E. deglupta × E. camaldulensis (Hyb) were used because they are popular for plantations
Gs, Vcmax and Jmax varied significantly among species; and values were greater for the hybrid than for the other two species, except that Vcmax for E. globulus did not differ significantly from E. grandis and the hybrid (Table 1)
Summary
High concentrations of tropospheric ozone (O3) can induce damage in plants and reduce forest productivity (Broadmeadow et al 1999; Percy et al 2003; Manning 2005; Proietti et al 2016; Yuan et al 2015, 2016). As a result of increasing O3 concentrations, 50% (17 million k m2) of the world’s forest may be exposed to O3 levels > 60 nmol mol−1 and, decrease photosynthetic productivities by the year 2100 (Fowler et al 1999; Sitch et al 2007). The considerable genetic variability among plants results in substantially different levels of damage in response to particular O 3 levels (Booker et al 2009). Atmospheric carbon dioxide (CO2) concentrations have increased and are predicted to reach 600 μmol mol–1 near the year 2060 (IPCC 2007). It is imperative to determine the interactive effects of CO2 and O3 on such representative species for future afforestation practices (e.g., Karnosky et al 2003; Kitao et al 2015; Shi et al 2017)
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