Salt stress severely impairs photosynthesis and development in tomato seedlings. This study investigated the regulatory role of exogenous melatonin (MT) on photosynthetic performance under salt stress by determining chlorophyll content, chlorophyll a fluorescence parameters, Calvin cycle enzyme activities, and related gene expression. Results showed that salt stress significantly reduced chlorophyll content and impaired photosystem II (PSII) functionality, as evidenced by the increased minimum fluorescence (F_o) and decreased maximum quantum efficiency of PSII (F_v/F_m) and effective PSII quantum yield (Φ_PSII). MT application mitigated these negative effects, as reflected by higher F_v/F_m, increased chlorophyll content, and lower non-photochemical quenching (NPQ). In addition, MT-treated plants exhibited improved PSII electron transport and more efficient use of absorbed light energy, as shown by elevated Φ_PSII and qP values. These changes suggest improved PSII functional stability and reduced excess thermal energy dissipation. Furthermore, MT significantly enhanced both the activity and expression of key enzymes involved in the Calvin cycle, including ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), Rubisco activase (RCA), phosphoglycerate kinase (PGK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), fructose-1,6-bisphosphatase (FBPase), fructose-bisphosphate aldolase (FBA), transketolase (TK), and sedoheptulose-1,7-bisphosphatase (SBPase), thereby promoting carbon fixation and ribulose-1,5-bisphosphate (RuBP) regeneration under salt stress. Conversely, inhibition of endogenous MT synthesis by p-CPA exacerbated salt stress damage, further confirming MT’s crucial role in salt tolerance. These findings demonstrate that exogenous MT enhances salt tolerance in tomato seedlings by simultaneously improving photosynthetic electron transport efficiency and upregulating the activity and gene expression of key Calvin cycle enzymes, thereby promoting the coordination between light reactions and carbon fixation processes. This study provides valuable insights into the comprehensive regulatory role of MT in maintaining photosynthetic performance under saline conditions.

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.