Responses to heatwaves of gas exchange, chlorophyll fluorescence and antioxidants ascorbic acid and glutathione in congeneric pairs of Acacia and Eucalyptus species from relatively cooler and warmer climates

Key message: Two species from warmer climates, but not the corresponding congeneric species from relatively cooler ones, decreased stomatal conductance upon heatwaves, and one of them showed significant decrease in the efficiency of open reaction centres of PSII in the light. In contrast, responses of major antioxidants ascorbic acid and glutathione to heatwaves were more similar between congeneric species than between species from similar climates. Abstract: According to climate change predictions, heatwaves will increase in frequency and intensity. This can threaten survival of sensitive tree species. Heatwaves affect photosynthetic capacity and cause an imbalance between light driven electron transport and carbon fixation. This can increase the concentration of reactive oxygen species, and can cause photo-oxidative stress. Heat dissipation and antioxidants are crucial in plant defence against photo-oxidative stress. We hypothesised that stomatal regulations, heat dissipation as measured by chlorophyll fluorescence and responses of major antioxidants ascorbic acid and glutathione to heatwaves will vary according to the climate at the site of origin of tree species. Tree seedlings from warmer (Eucalyptus grandis, Acacia aneura) and cooler climates (Eucalyptus tricarpa, Acacia melanoxylon) were exposed to air temperatures about 5 °C above control levels for 5 days. The two species from warmer climates responded to heatwaves with stomatal closure restricting transpiration and carbon fixation, and E. grandis also significantly increased heat dissipation (as judged by decreased efficiency of open reaction centres of PSII in the light). Species from cooler climates kept stomata open allowing continuing carbon assimilation and transpirational cooling. Heatwaves did not reduce maximum quantum efficiency of PSII in Acacia species, with insignificant decreases in Eucalyptus species. Glutathione concentrations increased significantly upon heatwave in E. tricarpa (from cooler climates), showed a time-dependent response to heatwave in E. grandis (from warmer climates) with an increase at later stages, and showed a tendency to decrease upon heatwave in both Acacia species (non-significant only for A. melanoxylon). Ascorbic acid concentrations increased significantly in E. tricarpa (cooler climates), and did not change significantly in other species. The redox state of ascorbic acid or glutathione only changed significantly and transiently in response to heatwave in A. aneura (warmer climates; GSSG % and ASC %) and A. melanoxylon (cooler climates; GSSG %), but not Eucalyptus species. Therefore, differences in stomatal regulations upon heatwaves aligned with the origin of species in warmer or cooler climates, whereas responses of ascorbic acid and glutathione to heatwaves were more aligned with the two genera.