The effects of soil water deficit on physiological, morphological and chemical traits of Eucalyptus

Drought periods leading to soil water deficit can fundamentally affect plant growth and survival, and alter plant traits such as leaf chemistry. Predictions of increased drought periods in many regions means that widespread plant species may experience soil water deficit that differs in duration and severity over the species natural range. Glasshouse experiments were used to test the effect of water deficit level, water deficit duration and re-watering on juveniles from genetically-distinct Eucalyptus globulus and E. viminalis provenances by quantifying plant morphological, physiological and chemical traits. Eucalyptus globulus provenances vary in genetic-based drought tolerance, and provenances are located across a rainfall gradient. I hypothesised that juveniles of provenances from wet locations would be affected by water deficit to a larger degree than those from dry locations due to intra-specific adaptation to local rainfall patterns. Both species and all provenances responded similarly to all water deficit treatments and to re-watering regardless of genetic-based constitutive trait variation, local rainfall patterns, or in the case of adult E. globulus, field drought susceptibility. The only response to water deficit which varied between provenances was foliar abscisic acid (ABA) levels, and the response pattern was similar for both species at each paired locality, indicating parallel evolution and local adaptation. While many E. viminalis constitutive chemical and morphological traits changed over time due to plant development, the duration of moderate (50%) water deficit had no impact on any trait. Eucalyptus globulus leaf water potential, foliar ABA levels, plant biomass and C:N were affected by different levels of water availability ranging from 90 ‚Äö- 0% of control evapotranspiration. Plant secondary metabolite (PSM) responses to water deficit varied between experiments, and also between the selected PSM traits. Overall, concentrations of individual terpenes and total oil were not influenced by water deficit. Concentrations of phenols were more plastic in response to water deficit than terpenes, as condensed tannin concentrations increased in some experiments and total phenolic and formylated phloroglucinol compound (FPC) concentrations decreased in some experiments. A short re-watering period reversed the effects of water deficit on some chemical traits, but also decreased phenol concentrations. Variation in responses to water deficit between experiments is discussed. Overall, genetic-based quantitative PSM variation was high between species and provenances, with comparably low levels of plasticity due to water deficit. While responses to soil water deficit may largely be similar in juveniles across each species range, the ecological impact of modest changes to Eucalyptus PSM concentrations during drought may be equally modest, and the effect on mature trees is unknown.