Daily Heat Stress Induces Accumulation of Non-functional PSII-LHCII and Donor-side Limitation of PSI via Downregulation of the Cyt b ₆ f Complex in Arabidopsis thaliana

Ongoing climate change has intensified heatwaves, which suppress photosynthesis and thus threatens agriculture. While acute heat stress damages photosystem II (PSII), detach light harvesting complex (LHC)II from PSII core and linearize thylakoid membrane, the consequences of prolonged recurring moderate heat exposure across the plant life cycle remain unclear. Here we characterized the effects of long-term daily exposure to high temperature on thylakoid membrane ultrastructure, photosynthetic protein complexes and their function, as well as to the redox states of PSI electron donors and acceptors in Arabidopsis thaliana . We show that plants exposed daily to 38°C for four hours exhibit altered thylakoid ultrastructure and structurally intact, but functionally impaired PSII-LHCII. Intriguingly, PSI is limited by the donor side due to dramatically reduced Cytochrome (Cyt) b ₆ f content. We propose that Cyt b ₆ f downregulation represents a protective acclimation strategy, safeguarding PSI at the cost of photosynthetic efficiency under prolonged heat exposures.