Role of Plastocyanin in response to photosynthetic electron transport in Solanum nigrum under Cd stress

• Plastocyanin (PC) over-reduction emerged as the central bottleneck in photosynthetic electron transport under Cd stress. • PC over-reduction triggers photosynthetic control, prioritizing avoiding PSI over-reduction. • Impaired PC function contributed to ΔpH collapse and compromised non-photochemical quenching (NPQ), exacerbating PSII photoinhibition. Plastocyanin (PC), an essential electron carrier located between the cytochrome b 6 f (cyt b 6 f) complex and PSI in photosynthetic electron transport, remains inadequately understood regarding its regulatory role in electron transfer under heavy metal stress. Solanum nigrum L., a Cadmium (Cd)-hyperaccumulating species with remarkable photosynthetic stability, serves as an ideal model to investigate the physiological basis of metal stress adaptation. This study established a gradient Cd stress system (0, 3, 10, 20 mg kg⁻¹) and employed a four-channel dynamic LED-array near-infrared spectrometer (DUAL-KLAS-NIR) combined with chlorophyll fluorescence kinetics to explore the functional role of PC in mediating photosynthetic electron transport and photoprotection under Cd stress. The results demonstrate that Cd stress inhibited leaf photosynthetic electron transport, reduced activities of both photosystem II (PSII) and photosystem I (PSI), accompanied by significant downregulation of PC oxidation (PCox) fraction. Notably, PC over-reduction emerged as a central bottleneck in both linear electron flow (LEF) and cyclic electron flow (CEF) pathways, without markedly impairing downstream acceptors such as P700 or ferredoxin. This bottleneck weakened CEF-dependent ΔpH formation, leading to reduced qE-type non-photochemical quenching (NPQ) while exacerbating PSII photoinhibition. Crucially, PC over-reduction triggered CEF enhancement under moderate Cd stress (3, 10 mg kg⁻¹) and induced photosynthetic control (PCON) at elevated Cd concentration (20 mg kg⁻¹ Cd), maintaining the P700 oxidation and preventing PSI over-reduction. These findings identify PC as a critical regulatory hub coordinating photosynthetic electron partitioning, redox balance, and photoprotection in response to Cd stress.