Antagonism between elevated CO2, nighttime warming, and summer drought reduces the robustness of PSII performance to freezing events

- PSII performance response to warming, elevated CO2 and precipitation are complex.
- Freeze events are highly decisive for PSII performance robustness in late season.
- Interactive effects (D × CO2, T × CO2, T × D × CO2) influenced PSII performance.
- Reduced robustness of PSII performance may well reduce photosynthetic carbon uptake.
- Freeze events reduced PSII performance via interactions with drought, elevated CO2 and warming.

Plant responses to warming, elevated CO2, and changes in summer precipitation patterns involve complex interactions. In this study we aim to reveal the single factor responses and their interactive effects on photosystem II (PSII) performance during an autumn-to-winter period. The study was carried out in the CLIMAITE multifactor experiment, which includes the combined impact of elevated CO2 (free air carbon enrichment; CO2), warming (passive nighttime warming; T) and summer drought (rain-excluding curtains; D) in a temperate heath ecosystem. PSII performance was probed by the effective quantum yield in light, Fv′/Fm′, using the pulse amplitude methodology, and the total performance index, PItotal, which integrate changes of the chlorophyll-a fluorescence transient including the maximal quantum yield in darkness, Fv/Fm.Decreasing temperature during autumn linearly reduced PItotal, both in the wavy hair-grass, Deschampsia flexuosa, and in the evergreen dwarf shrub common heather, Calluna vulgaris, and following freezing events the PItotal and Fv′/Fm′ were reduced even more. Contrary to expected, indirect effects of the previous summer drought reduced PSII performance before freezing events, particularly in Calluna. In combinations with elevated CO2 interactive effects with drought, D × CO2 and warming, T × D × CO2, were negatively skewed and caused the reduction of PSII performance in both species after occurrence of freezing events. Neither passive nighttime warming nor elevated CO2 as single factors reduced PSII performance via incomplete cold hardening as hypothesized. Instead, the passive nighttime warming strongly increased PSII performance, especially after freezing events, and when combined with elevated CO2 a strongly skewed positive T × CO2 interactive effect was seen. This indicates that these plants take advantage of the longer growing season induced by the warming in elevated CO2 until a winter frost period becomes permanent. However, if previously exposed to summer drought this positive effect reverses via interactive D × CO2 and T × D × CO2 effects immediately after freezing events, causing the full combination of TDCO2 not to differ from the control.In a future warmer climate with high CO2 and summer drought, the occurrence of freezing events thus seem highly decisive for reducing PSII performance in the autumn-to-winter period. Such a reduced robustness of PSII performance may be highly decisive for the magnitude of the late season photosynthetic carbon uptake and reduce the growing season length in these temperate heath plants.