Background Heritiera littoralis is an endangered semi mangrove tree species. Recently, the community structure, biological characteristics, medicinal value, fruit characteristics and germination technology and genetic diversity of H. littoralis have mainly been analyzed by scholars, but the high CO₂ concentration on photosynthetic characteristics in the leaves of H. littoralis seedlings under salinity treatment has been less investigated. It is important to explore the effects of elevated CO₂ on photosynthesis characteristics in H. littoralis under salinity treatment, which is essential to provide references for the application of H. littoralis to the vegetation restoration in coastal areas.

Methods 1-year-old H. littoralis seedlings were used for comparison test in the green house of Guangdong Academy of Forestry in Guangzhou. Salinity treatment was simulated using salt solution concentration gradients of 0 (CK) and 2.5% (salinity treatment) at different CO₂ concentration (400, 600 and 800 μmol/mol). There were 16 repeats in each concentration, thus totally 96 pots. After 60 days, photosynthetic physiological parameters were measured by LI-6800 photosynthesis instrument (LI-COR, USA). Then modified rectangular hyperbolic model was adopted to fit the photosynthesis light-response curves at different CO₂ concentration and CO₂ response curves of H. littoralis leaves of salinity treatment.

Results 1) After the experiment treatment, with the increase of CO₂ concentration, net photosynthetic rate (P_n), maximum net photosynthetic rate (P_nmax), apparent quantum yield (AQY), light saturation point (LSP), dark respiration rate (R_d), water use efficiency (WUE) and intercellular CO₂ concentration (C_i) in the leaves of H. littoralis seedlings increased, while light compensation point (LCP), transpiration rate (T_r) and stomatal conductance (G_s) decreased. 2) At the same CO₂ concentration, P_n, P_nmax, AQY, LSP, LCP, R_d, T_r and G_s under salinity treatment were lower than those under control, while the difference of WUE and C_i between the two treatment groups were not significant. 3) With the increase of photosynthetic active radiation (PAR), T_r and G_s showed an increasing trend, WUE increased first and then decreased slowly, while C_i decreased linearly and then reached a steady state. 4) Maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), triose-phosphate utilization efficiency (TPU), P_nmax, initial carboxylation efficiency (CE) and photorespiration rate (R_P) under salinity treatment were lower than those under control, while CO₂ saturation point (C_isat) and CO₂ compensation point (Γ) showed an opposite trend. 5) T_r and G_s decreased with the increase of CO₂ concentration, while WUE and C_i presented linear increase.

Conclusions The increase of CO₂ concentration promotes the increase of P_n, P_nmax, AQY, LSP, R_d, WUE and C_i. Higher CO₂ concentration in the environment can increase the photosynthetic efficiency of H. littoralis leaves, especially when the light intensity is high. The salt stress can reduce the photosynthetic rate of H. littoralis leaves significantly. The results of this research indicate that the increase of CO₂ concentration promotes the growth of H. littoralis seedling, while salinity treatment inhibits its growth.