Objectives This paper elucidated the photosynthetic physiological mechanism of salicylic acid (SA) and nitric oxide (NO) synergistically alleviate the salinity damage of tomato seedlings.

Methods The effects of single and compound applications of SA and NO donor sodium nitroprusside (SNP) on PSⅡphotochemical efficiency, distribution of excitation energy and absorption and utilization of luminous energy by antenna pigment within tomato (‘Qin Feng Bao Guan’) seedlings under the 100 mmol/L NaCl stress were studied using the method of hydroponic nutrient solution.

Results Single and combined applications of SA and SNP could effectively relieve the NaCl stress on PSⅡ damage. The combined application of SA and SNP gave the best stress relieving effect. After 3–7 days of stress treatment, the net photosynthetic rate (P_n), PSⅡmaximal photochemistry efficiency (F_v/F_m), antenna conversion efficiency (F_v′/F_m′), photochemical fluorescence quenching coefficient (qP), quotient of absorbed luminous energy used for photochemical action (P) and chlorophyll fluorescence decay rate (R_fd) were significantly increased by 25.5%–94.9%, 9.5%–15.3%, 25.7%–34.6%, 38.8%–121.9%, 74.2%–198.6% and 22.2%–25.5%, respectively. The original fluorescence (F_o), non-photochemical fluorescence quenching coefficient (NPQ), excitation pressure (1-qP), quotient of absorbed luminous energy used for non-photochemical dissipation in reaction center (E_x) and antenna heat dissipation (D), and the unbalanced distribution of coefficient deviation on excitation energy between two photosystems (β/α-1) were significantly reduced by 19.8%–23.5%, 22.8%–23.4%, 32.5%–39.9%, 15.1%–19.1%, 27.8%–31.4% and 51.8%–72.8%, respectively.

Conclusions These results above show that exogenous salicylic acid and nitric oxide have synergistic effect in protecting PSⅡ and photosynthetic transport chain from salt damage and improving salt tolerance of tomato seedlings.