Carbohydrate allocation strategies and C/N balance shape root plasticity of urban tree species under ozone stress.

Ozone (O3) pollution is a major environmental stressor affecting forest productivity, yet species-specific responses in carbohydrate allocation and their effects on root architectures remain poorly understood. In this study, we investigated carbohydrate allocation, carbon (C) and nitrogen (N) balance, and root traits of three urban greening tree species (Celtis sinensis, Quercus acutissima, and Quercus nuttallii) across five O3 levels. Results showed that C. sinensis with high O3 tolerance maintained relatively stable total leaf C and N contents, while leaf non-structural carbohydrates (NSCs) pools increased, primarily due to starch accumulation; in contrast, the two less tolerant Quercus species exhibited increased starch and NSC contents in roots. Furthermore, root length of C. sinensis was greater than Quercus species but was inhibited by O3 stress, whereas other key root traits, such as specific root length (SRL), specific root area (SRA), and root tissue density (RTD), showed significant fluctuations under elevated O3 concentrations. Regression analyses suggested that in Q. acutissima and Q. nuttallii, SRL and SRA positively correlated with leaf soluble sugar contents and stem C/N ratio, but negatively correlated with leaf and stem N content, implying that excessive N availability may constrain root development by increasing nitrogen assimilation costs. These findings showed that species-specific patterns of carbohydrate allocation among tree organs can potentially determine trees' O3 tolerance, underscoring the crucial role of carbohydrate allocation strategies and C/N stoichiometry in shaping root plasticity under O3 stress, and offering practical implications for tree species selection under atmospheric pollution.