Acclimation and De-Acclimation of Photosystem Heat Tolerance of Alpine Plants in Response to Multiday Heat Exposure in the Field

Aims and Questions: Climate change is exposing plants in alpine regions to increasing heat, which has been associated with changes in vegetation communities. Our study aims to investigate whether photosystem heat tolerance of alpine plants of different growth forms acclimates to and recovers from exposure to multiday heat stress applied directly in a field manipulation experiment. Location: Mt. Hotham, Victoria, Australia (36°58′37″ S, 147°08′06″ E; 1840 m asl). Methods: We established field plots and selected eight species (graminoids, herbs and shrubs) to compare unheated plots to heated plots in an Australian alpine grassy herbfield. We actively heated chambers with convective heaters to target day/night temperatures of 32°C/22°C. Leaves for photosystem heat tolerance assays were sampled daily for 6 days during heat treatments and 6 days post-heating. We used chlorophyll fluorescence imaging to derive the critical temperature for photosystem II disruption to function (T_crit) by heating leaves up to 70°C. Results: T_crit was inherently high (~47°C before heating) in all species, but was higher among graminoids than other growth forms. Overall, T_crit increased significantly (i.e., acclimated) with cumulative days of heating and increasing heat load, but subsequent recovery to pre-treatment values was slow. We found strong species-specific responses, but not growth form differences, in T_crit acclimation to and recovery from, heat exposure. The effects of heat priming may facilitate understanding plant and community responses to warming in alpine regions. Conclusions: While some variation in T_crit was explained by growth form, strong species-specific acclimation effects were stronger drivers of variation in heat tolerance and its acclimation. The magnitude and duration of heat exposure may contribute to apparent differences in the acclimation of photosystem heat tolerance observed among species and studies. The effects of heatwaves on the composition of alpine vegetation communities will likely depend on highly species-specific responses and may be complex to predict.