Eucalyptus species maintain secondary metabolite production under water stress conditions at the expense of growth

Plants have evolved a vast array of secondary metabolites that help defend them against natural enemy attack. The production of these secondary metabolites is influenced by environmental factors, such as temperature, rainfall (soil water availability) and atmospheric [CO₂], which may be altered under climate change. This study aimed to test how two of these environmental factors, namely soil water availability and [CO₂], will affect secondary metabolite and biomass production in Eucalypts. To investigate this, we measured secondary metabolite and biomass production of three Eucalypt species (Eucalyptus grandis, E. moluccana, E. saligna) grown under ambient and elevated [CO₂] (400 and 600 ppm, respectively) and water-stressed and well-watered conditions, in a fully factorial glasshouse experiment. The secondary metabolites measured were foliar phenolic concentration as well as terpene and green leaf volatile emissions. We found that water stress did not alter secondary metabolite production. Unsurprisingly, the water-stressed plants maximised their water uptake potential by reducing shoot biomass but maintaining root biomass. The detrimental effects of water stress on shoot biomass production were not reduced under elevated [CO₂] compared to ambient [CO₂], despite all species experiencing significant reductions in stomatal conductance. Further, [CO₂] as a single factor did not have a significant effect on species’ net carbon gain, with an increase in green leaf volatile emissions under elevated [CO₂] being mitigated by a reduction in shoot biomass production. These results suggest that reductions in carbon gains due to water stress will affect the growth of Eucalypts rather than their chemical defence capabilities.