Concentration-dependent effects of mixed phenolic acids on growth, photosynthetic traits, and nutrient stoichiometry of Schima superba seedlings

Soil-borne phenolic acids are important components of the belowground chemical environment and can influence plant growth and physiological performance. However, the concentration-dependent responses of tree seedlings to phenolic acids occurring in mixed-stand soil solutions remain insufficiently understood. In this study, we conducted a controlled soil-based experiment to examine the effects of a field-informed ferulic–veratric acid mixture on growth, biomass allocation, photosynthetic traits, and leaf nutrient stoichiometry in Schima superba seedlings. S. superba seedlings showed clear concentration-dependent responses to the fixed-ratio ferulic–veratric acid mixture. The 0.5× treatment promoted biomass accumulation and several growth traits, whereas higher exposure levels reduced total and leaf biomass and increased the root-to-shoot ratio. Height elongation remained relatively stable across treatments, while root-collar diameter was reduced at the highest exposure level, indicating greater sensitivity of biomass production and radial growth than height growth. Higher phenolic-acid exposure also decreased chlorophyll and carotenoid contents and reduced fitted maximum net photosynthetic rate ( P n, max ). Patterns of stomatal conductance and intercellular CO 2 concentration suggested that stomatal closure alone could not fully account for the decline in photosynthesis, although stomatal and non-stomatal contributions could not be quantitatively partitioned. Leaf P concentration declined and C/P increased at higher exposure levels, whereas leaf N and N-related stoichiometric ratios remained comparatively stable, indicating that leaf P status was particularly responsive to phenolic-acid exposure. Together, these responses indicate coordinated changes in growth, photosynthetic performance, and P-related nutritional status under increasing phenolic-acid exposure. S. superba seedlings exhibited clear concentration-dependent responses to the fixed-ratio ferulic–veratric acid mixture, with low exposure promoting biomass accumulation and higher exposure constraining growth and photosynthetic performance. Elevated phenolic-acid exposure was also associated with reduced leaf P concentration and increased C/P, indicating altered P-related nutritional status. These findings indicate coordinated changes in growth, photosynthetic performance, biomass allocation, and nutrient status under increasing phenolic-acid exposure. The results provide a basis for further evaluating how soil-borne phenolic acids may influence seedling performance and belowground chemical interactions under natural mixed-forest conditions.