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 CO2 concentration ([CO2]) in tolerating off-season episodic extremes is not clear. We tested for interactive effects of elevated CO2 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 (aCO2; 400 parts per million (ppm)) or elevated (eCO2; 640 ppm) [CO2], 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 CO2 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 eCO2 than in aCO2. Photosynthesis remained stable for plants in eCO2 during the heatwave. Based on the effect size ratio (eCO2 : aCO2), gs and ETRwere temporarily affected more by the heatwave than A. A reduction in ETR in eCO2 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 aCO2 compared with eCO2, suggesting potential complex and multiple control points on photosynthesis. Additional keywords: chlorophyll fluorescence, electron transport, seedling growth, stomatal conductance, temperature stress. Received 21 March 2017, accepted 28 June 2017, published online 18 August 2017