Response of 25-day old Eucalyptus globulus to elevated CO2

Increasing [CO₂] may influence commercial crop and timber yield. While selection of genotypes sensitive to elevated [CO₂] (e[CO₂]) appears possible in agricultural crops, there is limited evidence for genotype-by-CO₂ (G×CO₂) interactions in commercial tree species. We examined [CO₂] responsiveness in 124 open-pollinated Eucalyptus globulus subspecies globulus (E. globulus) families with the aim of assessing whether G×CO₂ interactions are detectable in seedlings for early-age screening. Plants were grown in ambient (a[CO₂]; ~405 μmol mol^-1) and e[CO₂] (640 μmol mol^-1) and harvested 25 days after germination. Total, shoot and root dry weights were determined for each plant. Carbon isotopic discrimination against 13C (Δ13C) was determined at the family level. We observed highly significant (p<0.0001) increases in mean total, shoot, and root dry weights. Mixed-model equations were used to estimate the main and interaction effects of the G×CO₂ for each mass trait. The main effects from the mixed model output ([CO₂] and individual-tree effects) were significant for all traits. However, [CO₂]-by-individual tree interactions were non-significant for all traits, indicating little G×CO₂ interaction. A secondary aim was to examine the correlation between greenhouse and mature-age growth from breeding trials that use common families conducted under ambient [C_O2]. These correlations were non-significant, suggesting early growth is not necessarily indicative of later-age responses.  Our results suggest that while early growth of E. globulus is enhanced under e[CO₂], genotypes respond relatively uniformly to e[CO₂] and little opportunity exists for seedling-based selection at the population level based upon the response of plants during the first weeks of growth.