Drought increases heat tolerance of leaf respiration in Eucalyptus globulus saplings grown under both ambient and elevated atmospheric [CO₂] and temperature

Climate change is resulting in increasing atmospheric [CO₂], rising growth temperature (T), and greater frequency/severity of drought, with each factor having the potential to alter the respiratory metabolism of leaves. Here, the effects of elevated atmospheric [CO₂], sustained warming, and drought on leaf dark respiration (R _dark), and the short-term T response of R _dark were examined in Eucalyptus globulus. Comparisons were made using seedlings grown under different [CO₂], T, and drought treatments. Using high resolution T-response curves of R _dark measured over the 15-65 °C range, it was found that elevated [CO₂], elevated growth T, and drought had little effect on rates of R _dark measured at T <35 °C and that there was no interactive effect of [CO₂], growth T, and drought on T response of R _dark. However, drought increased R _dark at high leaf T typical of heatwave events (35-45 °C), and increased the measuring T at which maximal rates of R _dark occurred (T _max) by 8 °C (from 52 °C in well-watered plants to 60 °C in drought-treated plants). Leaf starch and soluble sugars decreased under drought and elevated growth T, respectively, but no effect was found under elevated [CO₂]. Elevated [CO₂] increased the Q ₁₀ of R _dark (i.e. proportional rise in R _dark per 10 °C) over the 15-35 °C range, while drought increased Q ₁₀ values between 35 °C and 45 °C. Collectively, the study highlights the dynamic nature of the T dependence of R _dark in plants experiencing future climate change scenarios, particularly with respect to drought and elevated [CO₂].