An extreme heatwave enhanced the xanthophyll de-epoxidation state in leaves of Eucalyptus trees grown in the field

Heatwaves are becoming more frequent with climate warming and can impact tree growth and reproduction. Eucalyptus parramattensis can cope with an extreme heatwave in the field via transpiratory cooling and enhanced leaf thermal tolerance that protected foliar tissues from photo-inhibition and photo-oxidation during natural midday irradiance. Here, we explored whether changes in foliar carotenoids and/or the xanthophyll cycle state can facilitate leaf acclimation to long-term warming and/or an extreme heatwave event. We found that leaves had similar carotenoid levels when grown for one year under ambient and experimental long-term warming (+ 3 °C) conditions in whole tree chambers. Exposure to a 4-day heatwave (> 43 °C) significantly altered the xanthophyll de-epoxidation state of carotenoids revealing one mechanism by which trees could minimise foliar photo-oxidative damage. The levels of zeaxanthin were significantly higher in both young and old leaves during the heatwave, revealing that violaxanthin de-epoxidation and perhaps de novo zeaxanthin synthesis contributed to enhancement of the xanthophyll cycle state. In a future climate of long-term warming and increased heatwave events, leaves of E. parramattensis will be able to utilise biochemical strategies to alter the xanthophyll cycle state and cope with extreme temperatures under natural solar irradiation.