Daily Heat Stress Induces Donor‐Side Limitation of PSI Via Downregulation of the Cyt b ₆ f Complex

The rapidly warming climate is driving increasingly frequent and intense heat waves worldwide, posing major challenges to plant metabolism and growth. Despite the central role of photosynthesis in plant growth, the effects of prolonged heat stress on the primary reactions of photosynthesis remain poorly understood. Here, we studied how long‐term heat exposure affects growth and photosynthetic performance in the model plant Arabidopsis thaliana . Plants were exposed daily to high temperature (38°C) for 4 h throughout the growth period. Conflicting to earlier studies, we show that PSII‐LHCII remain relatively intact under prolonged heat despite slightly reduced ∆F/F m ’, relative electron transfer rate (rETRII) and proportion of open reaction centers (qL). Strikingly, the content of the Cytochrome b 6 f complex, which mediates electron transfer between the photosystems, was reduced by 30–40%. This leads to more oxidized downstream electron acceptors and prevents PSI over‐reduction in conditions where carbon assimilation is inhibited.