Nitric Oxide Alleviates Low-Light-Induced Photosynthetic Impairment by Improving Photosystem Coordination and Electron Transport in Tomato Seedlings

Low light is a major environmental constraint that limits crop productivity by restricting photosynthetic carbon assimilation and disrupting photosynthetic function. Nitric oxide (NO) is an important signaling molecule involved in plant stress responses; however, its role in regulating photosystem stability under low-light conditions remains unclear. In this study, tomato seedlings were treated with the NO donor sodium nitroprusside (SNP) and the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) to investigate the effects of NO-related treatments on growth, photosynthetic performance, photosystem function, electron transport, ROS accumulation, and antioxidant defense under low-light conditions. Low light markedly suppressed growth and impaired the photochemical activities of both photosystem II (PSII) and photosystem I (PSI), as indicated by decreases in the maximum quantum efficiency of PSII (Fv/Fm), the maximal P700 oxidation capacity (Pm), and electron transport rates, together with enhanced reactive oxygen species (ROS) accumulation. Exogenous NO application significantly improved photosystem performance under low-light conditions. NO enhanced PSII and PSI photochemical performance, improved excitation energy distribution, and restored photosynthetic electron transport, while strengthening antioxidant enzyme activities, thereby reducing excitation pressure and oxidative damage. In contrast, PTIO treatment aggravated photosystem impairment under low light. These findings indicate that NO contributes to maintaining photosystem function and photosynthetic electron transport under low-light conditions, highlighting its potential role in improving low-light tolerance in tomato.