Intraspecific diversity of terpenes of Eucalyptus camaldulensis (Myrtaceae) at a continental scale

Plants show a high degree of intraspecific variation in several traits including plant secondary metabolites. This variation can be influenced by genetic and environmental factors that result in geographical structure in their distribution. By growing plants from several populations in a controlled environment, we studied variation in foliar terpenes in Eucalyptus camaldulensis, which is the widest distributed eucalypt, with a large range both latitudinally and longitudinally. We found that the concentration of terpenes is highly variable among subspecies. We identified four chemotypes dominated by 1,8-cineole, γ-terpinene, α-A nd β-phellandrene. While the 1,8-cineole chemotype is abundant in all populations, the other three chemotypes are rare in the central area and the north-east of Australia. The γ-terpinene chemotype is mainly restricted to the north and west of Australia, whereas the α-A nd β-phellandrene chemotypes show an opposite distribution in the north and south of the continent. The annual mean temperature and humidity of the source populations correlate with the abundance of the dominant terpenes. We also tested the effects of elevated CO₂ concentrations on the terpene concentration and found that elevated CO₂ atmosphere reduces the overall accumulation of foliar terpenes. The results suggest that variation in terpene composition in E. camaldulensis can be influenced by environmental variables, mainly favouring the 1,8-cineole chemotype in arid locations.