Effect of limited water availability on foliar plant secondary metabolites of two Eucalyptus species

Plant secondary metabolites (PSMs) have many ecological roles such as influencing decomposition, flammability and herbivory. PSM concentrations are genetically determined, but are also affected by environmental factors. Drought periods are predicted to become more frequent in many regions, and may have widespread impacts on PSM-mediated ecological interactions. We used two dominant tree species to investigate the impact of multiple levels of water availability on leaf PSM concentrations. Juvenile Eucalyptus globulus and Eucalyptus viminalis plants were subjected to one of three glasshouse watering regimes (high [control], moderate or low water availability) and the effect on leaf PSMs was investigated between species, treatments, and within-plant foliage age classes. Moderate and low water availability decreased total phenolic concentrations of both species, and decreased the C:N of E. globulus leaves. Low water availability reduced the concentrations of two terpene compounds, but only in specific foliage age classes. Overall, the majority of terpenes were unaffected by decreased water availability, as were two formylated phloroglucinol compounds and condensed tannins. We conclude that water limitation had little impact on overall leaf PSM concentrations, and that juveniles of these two eucalypt species generally maintained PSM concentrations while plant growth declined, eluding to both direct impacts of water limitation and within-plant resource prioritization.