Abstract: Abstract Aims Mangrove plants play crucial ecological roles in coastal wetland ecosystems, yet environmental stressors such as salinity-alkalinity and heavy metal pollution (e.g., copper) may significantly impact their photosynthesis and growth. This study aimed to investigate the photosynthetic physiological and ecological responses of three mangrove species—Kandelia obovata,?Laguncularia racemosa?and?Bruguiera gymnorrhiza—under combined salt and copper stress. The goal was to provide a scientific basis for effective management and restoration strategies to maintain mangrove wetland ecological functions. Methods Controlled field experiments were conducted using combined salt and copper treatments. Principal component analysis and generalized linear mixed models were employed to assess changes in chlorophyll fluorescence characteristics, leaf structure (leaf length, leaf area, specific leaf area), and biochemical components (relative chlorophyll content (SPAD), total copper content, total carbon content, total nitrogen content) across different treatment conditions. Important findings The results demonstrated that under low-concentration salt and copper treatments, the average maximum quantum yield of photosystem II (Fv/Fm) in three mangrove species ranged from 0.764 to 0.866, indicating strong stress resistance. Among them,?Kandelia obovata?and?Laguncularia racemosa?exhibited superior stress tolerance compared to?Bruguiera gymnorrhiza. The performance index (PIabs) and potential activity of PSII (Fv/Fo) ranged from 0.63 to 1.89 and from 3.81 to 6.33, respectively, with an overall declining trend as salt concentrations increased, suggesting that high salinity inhibited photosynthetic activity. Low copper concentrations showed no significant impact on Fv/Fm or Fv/Fo, while high copper levels suppressed photosynthesis. Under combined salt-copper treatments, chlorophyll fluorescence parameters of the three mangroves remained stable, implying a counteracting effect of copper on salt stress. Principal component analysis (PCA) revealed temporal variations in the correlations among chlorophyll fluorescence parameters, leaf structural traits, and biochemical components across treatments. Species differentiation was indistinct in July 2019,?Laguncularia racemosa?showed clear separation in December 2019, and?Bruguiera gymnorrhiza?was distinctly separated in August 2020. Overall, species differentiation across the three sampling periods was not pronounced. Generalized linear mixed model (GLMM) analysis highlighted significant positive correlations between Fv/Fm, Fv/Fo, and SPAD values in?Kandelia obovata, but negative correlations with total nitrogen (N) content. Its PIabs and SPAD values were positively linked to total copper content. In?Laguncularia racemosa, PIabs showed a strong positive correlation with SPAD values. For?Bruguiera gymnorrhiza, Fv/Fm and Fv/Fo were positively correlated with SPAD values and total carbon (C) content, yet negatively associated with total nitrogen content.

Key words: Mangrove, Salt, Copper, Plant functional traits, Chlorophyll fluorescence parameters