Effects of a heat wave on photosynthesis for Eucalyptus camaldulensis genotypes grown in elevated CO2

Background/Question/Methods

Atmospheric CO₂ has risen 40% in the past 150 years, and heat waves (periods with a minimum of 5 days of air temperatures >5K above average maximum temperatures) have become far more frequent in Australia. There is genetic variation in plant response to elevated CO₂, including within a single species, suggesting that tree plantation selection for CO₂-responsive genotypes is possible. This study tested the effects of a transient heat wave on photosynthetic performance of 14 genotypes of a widespread, economically important Australian species, Eucalyptus camaldulensis, grown in ambient (400 ppm; C_A) and elevated (640 ppm; C_E) CO₂ in a temperature controlled glasshouse at ambient growth day/night air temperatures of 28/15^oC (T_A). The heat wave was imposed for five days at 38/28^oC (T_E) and then air temperature was returned to T_A. Photosynthetic gas exchange and chlorophyll a fluorescence from Photosystem II (PSII) were measured one week before the heat wave, three days into the heat wave, and five days after the heat wave had ended.

Results/Conclusions

For all genotypes combined, photosynthesis (A) was 18% higher in C_ET_A plants compared with C_AT_A plants before the onset of the heatwave. During the heat wave, A averaged 40% higher for the 14 genotypes at C_ET_E compared with C_AT_E, and 20% higher at T_E compared with T_A. In comparison to T_A, plants at T_E had higher quantum yield and electron transport rates in PSII, and higher transpiration rates during the heat wave. Across all 14 genotypes, the range of stomatal conductance (g_s) values was decreased during the heat wave (compared to before the heat wave), and the range remained lower five days after returning to T_A. At the same time, A for the 14 genotypes averaged 20% higher in C_ET_A plants compared with C_AT_A plants. C_AT_E plants had higher quantum yield and electron transport rates in PSII and transpiration rates compared to other treatment combinations, suggesting a lasting effect of T_E under ambient CO₂. For the 14 genotypes combined, there was no difference in the relationship between A and CO₂ concentrations inside leaves (C_i) as a result of exposure to the heat wave, but there was a difference in the correlation of A with C_i at C_A compared to C_E. Results suggest variable responses of A to CO₂ and high-temperatures during the heat wave across the 14 genotypes, and that few aspects of photosynthesis exhibited a lasting impact from the heat wave when grown in C_E.