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 (aCO(2); 400 parts per million (ppm)) or elevated (eCO(2); 640ppm) [CO2], and air temperatures of 25:17 degrees C (day:night), and measured the electron transport rate in PSII (ETR), stomatal conductance to water vapour (g(s)) and net CO2 assimilation (A). Measurements were made before, during and after a four-day temperature excursion of 35:27 degrees C. ETR and A were similar to 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)), g(s) 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.