Objectives To explore the effect of exogenous Ca²⁺ on photosynthetic system of processing tomato seedlings under salt stress, to provide a theoretical basis for the application of Ca against salt stress in vegetable production.

Methods A hydroponic experiment was conducted using an in-bred line ‘KT-7’ of processing tomato (Lycopersicon esculentum Mill.), with strong salt tolerance, as test material. The Hoagland nutrient solution was added with 100 mmol/L NaCl to make salt stress. The tomato seedlings with four open leaves and one new leaf were transplanted in grown on the salt stress nutrient solution, and foliar sprayed with CaCl₂ of 0, 5, 10, 15 and 20 mmol/L, respectively, on the same day. The fast chlorophyll fluorescence and 820 nm reflection kinetics were measured by M-PEA fluorometer after 3, 6, 9 days of treatment, and the growth indexes were measured after 9 days of treatment.

Results Treatment after 9 days of salt stress significantly reduced the growth indexes of processing tomato seedlings. The morphology, aboveground and belowground biomass of the seedlings treated with 15 mmol/L CaCl₂ were higher than those treated with other calcium concentrations. With the prolongation of salt stress time, the I and P phases of the fast chlorophyll fluorescence induction curves of leaves gradually declined, conversely, the K and J phases gradually rose, and the amplitude of the modulated 820 nm reflection kinetic gradually decreased, primary photochemical efficiency (F_v/F_o), maximum photochemical efficiency (F_v/F_m), light energy absorption performance index (PI_abs), PSⅡ ability to transfer electrons downstream (Ψ_o), electron transfer quantum yield (φE_o), energy transferred by electrons per unit leaf cross-sectional area (ET_o/CS_m), energy transferred by electrons per unit reaction center (ET_o/RC), apparent quantum flux per unit leaf cross-sectional area (ABS/CS_m, TR_o/CS_m, ET_o/CS_m), the number of active reaction centers per unit leaf cross-sectional area (RC/CS_m), and PSI redox properties (ΔMR_fast/MR_o, ΔMR_slow/MR_o, V_ox, V_red) gradually reduced, while the relative variable fluorescence at J point (V_J), quantum efficiency and energy flux of heat dissipation (φD_o, DI_o/CS_m, DI_o/RC) increased. Compared with the treatment without exogenous CaCl₂ under salt stress, the changes of each index amplitude of processing tomato seedlings after application of exogenous CaCl₂ were influenced. Among them, the treatment with 15 mmol/L CaCl₂ significantly improved the F_v/F_o, F_v/F_m, PI_abs, Ψ_o, φE_o and the redox properties of PSI, however significantly reduced V_J and heat dissipation. Principal component analysis was performed on the 19 parameters after treatment of 9 days, and the contribution rates of the first and second principal components accounted 82.996% and 8.591% of the variance, respectively, indicating that these two independent principal components could reflect 91.587% of the information of the 19 parameters. According to the order of comprehensive scores of principal components, the ability of exogenous CaCl₂ to alleviate salt stress in processing tomato ranged from high to low as 15 mmol/L>10 mmol/L>5 mmol/L>20 mmol/L.

Conclusions Salt stress significantly inhibited the growth of processing tomato seedlings, damaged the structure and function of photosystem. Exogenous application of 15 mmol/L Ca²⁺ improves the growth of processing tomato under salt stress, protects photosynthetic apparatus, improves the efficiency of photochemical reaction, optimizes the energy utilization of PSⅡ reaction center, thereby enhances the salt tolerance in the crop.