PSI-mediated NADPH production and enzyme regulation are critical for carbon assimilation in pepper seedlings under cadmium stress

Photosynthetic CO 2 assimilation forms the basis of crop growth and yield formation and is highly sensitive to soil cadmium (Cd) pollution. This study investigated the mechanisms behind Cd-induced inhibition of photosynthetic CO 2 assimilation in pepper ( Capsicum annuum L.) seedlings under hydroponic conditions. Pepper plants were exposed to Cd at 0.3 and 1.0 mg · L -1 , and their photosynthetic performance, photosystem I (PSI) and photosystem II (PSII) activity, Calvin-Benson-Bassham (CBB) cycle enzyme activities, ATP and NADPH content, and expression levels of photosynthesis-related genes were assessed. Results showed that Cd stress significantly reduced the activities of key CBB cycle enzymes, including Rubisco, phosphoglycerate kinase (PGK), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), along with the expression of their corresponding genes. Cd stress also caused severe PSI damage, disrupting electron transport, impairing NADPH synthesis, and triggering excessive reactive oxygen species (ROS) accumulation. These effects decreased the maximum Rubisco carboxylation rate (Vc max ) and ribulose-1,5-bisphosphate (RuBP) regeneration capacity (J max ), leading to a 35.2%-70.7% reduction in the maximum CO 2 assimilation rate (Pn max ). Variance partitioning analysis (VPA) identified PSI activity and NADPH content as the primary contributors to the Cd-induced decline in CO 2 assimilation efficiency. Weighted gene co-expression network analysis (WGCNA) revealed strong positive correlations between the down-regulation of PSI structural genes ( PsaD , PsaE , and PsaF ) and electron transport genes ( PetE and PetF ) and the reduction in CO 2 assimilation efficiency under Cd stress. In conclusion, this study emphasizes the critical role of PSI damage in Cd-induced disruption of photosynthetic CO 2 assimilation in pepper seedlings and provides new insights into the physiological and molecular mechanisms underlying plant responses to Cd stress.