Drastic Reduction in Cytochrome b 6 /f Complex Confers Robust PSI Photoprotection Under Fluctuating Light at the Expense of Photosynthetic Capacity

ABSTRACT Plants in natural habitats frequently encounter fluctuating light (FL), which can lead to photoinhibition of Photosystem I (PSI), thereby limiting photosynthetic productivity. The cytochrome (Cyt) b 6 /f complex plays a pivotal role in regulating photosynthetic electron flow and influencing PSI stability in plants. However, the precise impact of a substantial reduction in Cyt b 6 /f content on PSI photoprotection under FL and the associated trade‐offs with photosynthetic capacity remain to be elucidated. In this study, we investigated PSI tolerance and photosynthetic performance in transgenic tobacco ( Nicotiana tabacum ) lines with varying Cyt b 6 /f levels, comparing wild‐type (WT) plants to those with drastically reduced Cyt b 6 /f content. Our results show that a marked reduction in Cyt b 6 /f levels conferred substantial PSI photoprotection against FL‐induced damage, even under extremely high light pulses. This enhanced PSI stability was attributed to the restricted electron flow towards PSI, which likely maintained P700 in a more oxidized state. However, this robust PSI protection in the plants with significantly reduced Cyt b 6 /f levels came at a considerable cost to the overall photosynthetic capacity, as evidenced by reduced PSII efficiency (Y(II)), photochemical quenching (qL), and non‐photochemical quenching (NPQ) under both steady‐state and fluctuating light conditions. These findings reveal a critical trade‐off between PSI photoprotection and photosynthetic productivity, which is strongly modulated by the abundance of the Cyt b 6 /f complex. This trade‐off offers key insights into plant adaptive strategies in dynamic light environments and highlights potential targets for improving crop productivity under natural light fluctuations.