Objectives We studied the mechanism of carbon monoxide (CO) and hydrogen sulfide (H₂S) on alleviating the NaCl stress to crop seedlings, to provide a theoretical basis and technology for efficient crop production in the secondary salinized soil.

Methods A soilless cultivation experiment was carried out using a processing tomato strain KT-7 as the test material. The donor for carbon monoxide (CO) was hemin solution (15 μmol/L), and for H₂S was hypotaurine solution (100 μmol/L). In addition, zinc protoporphyrin solution (25 μmol/L) was used for heme oxygenase inhibitor, and H₂S scavenger hypotaurine solution (HT, 100 μmol/L) were used as the requirement of the treatments. There were 6 treatments in the experiment, except for CK₀ (irrigating normal nutrient solution and foliar spraying with clean water), the other five treatments were all irrigated with a nutrient solution containing 200 mmol/L NaCl, and foliar sprayed with clean water (CK₁), CO, CO+HT, H₂S, and H₂S+ZnPPⅨ, respectively. When the seedlings were at age of 4 leaves-one- sprout, NaCl stress and foliar spraying were carried out for consecutive six days then the morphological indicators were investigated, Li-6800 portable photosynthetic system was used to measure the photosynthetic parameters. The Imaging-PAM modulated fluorescence imaging system was used to determine the fluorescence parameters of chlorophyll, and the plant efficiency meter was used to measure the leaf green fluorescence kinetic curve (OJIP).

Results After 6 days of NaCl stress, the net photosynthetic rate (Pn), actual photochemical efficiency (Y(Ⅱ)) and PSⅡ performance indicators (PI_ABS and PI _(CSM)) of the seedling leaves decreased significantly, the OJIP curve was deformed, and the K point appeared. Both CO and H₂S treatments improved the photosynthetic pigment content, photosynthetic parameters, comprehensive performance of PSⅡ and electron transfer efficiency on the receptor side, revised the deformation of the OJIP curve, and make the J point, I point, P point rises, ΔK falls. Among them, Pn, apparent CO₂ utilization rate (CUEapp), Y(Ⅱ) and electron transfer rate (ETR) increased by 75.63%, 100%, 60.00% and 57.72% respectively in CO treatment. In H₂S treatment, they increased by 65.95%, 100%, 55.00% and 52.68% respectively. Foliar spraying of CO and H₂S also reduces the proportion of non-photochemical quenching and heat dissipation, and reduces the damage to the PSII donor side oxygen evolving complex (OEC). The CO+HT treatment were recorded similar parameters with CO treatment, while H₂S+ZnPPⅨ treatment was recorded significantly lower pigment content, Pn, Fv/Fm, Y(Ⅱ), qL, ETR, PI_ABS and PI _{(CSM )}, but significantly higher heat dissipation ratio, compared to those in H₂S treatment.

Conclusions Both exogenous CO and H₂S are conducive to to optimize the PSⅡenergy distribution ratio, and protect the photosynthetic electron transport chain from salt damage. CO maybe act in the downstream of H₂S involving in the photosynthesis, and intensify the effect of H₂S on improving the salt tolerance of tomato seedlings.