Relationships between climate of origin and photosynthetic responses to an episodic heatwave depend on growth CO2 concentration for Eucalyptus camaldulensis var. camaldulensis

Stressful episodic weather is likely to affect the C balance of trees as the climate changes, potentially altering survival. However, the role of elevated CO ₂ concentration ([CO ₂]) in tolerating off-season episodic extremes is not clear. We tested for interactive effects of elevated CO ₂ and springtime heat stress on photosynthesis for seven genotypes of Eucalyptus camaldulensis Dehnh. var. camaldulensis, representing its widespread distribution across south-eastern Australia. We grew clonal material under glasshouse conditions of ambient (aCO ₂; 400 parts per million (ppm)) or elevated (eCO ₂; 640ppm) [CO ₂], and air temperatures of 25:17°C (day:night), and measured the electron transport rate in PSII (ETR), stomatal conductance to water vapour (gs) and net CO ₂ assimilation (A). Measurements were made before, during and after a four-day temperature excursion of 35:27°C. ETR and A were ∼17% higher for plants grown in eCO ₂ than in aCO ₂. Photosynthesis remained stable for plants in eCO ₂ during the heatwave. Based on the effect size ratio (eCO ₂:aCO ₂), gs and ETR were temporarily affected more by the heatwave than A. A reduction in ETR in eCO ₂ was the only lasting effect of the heatwave. There were no significant differences among genotypes. Correlations between photosynthesis and climate of origin differed for plants grown in aCO ₂ compared with eCO ₂, suggesting potential complex and multiple control points on photosynthesis.