Urban Green Walls under Traffic Stress: Linking the Elemental Composition of Particulate Matter Deposition to Plant Physiological and Gene Expression Responses

Abstract This one-year study integrates multi-elemental fractionation, physiological, and gene expression analyses to investigate PM retention and plant responses in a nine-species green wall at a high-traffic urban site. A chemical fractionation approach was optimized to discriminate between endogenous and exogenous elements, overcoming a major limitation of conventional methodologies. PCA of the insoluble fraction of Al, Cr, Cs, Fe, Mn, Mo, Rb, Sb, Sn, Ti, and Zn identified Photinia × fraseri as the species with the highest and most stable long-term retention of non-exhaust traffic-derived particles, whereas Pteris vittata showed the lowest and most unstable retention capacity. Insoluble elements reliably traced PM deposition, while soluble and leaf fractions were strongly influenced by endogenous pools of both essential nutrients and tracers such as Cs and Rb, potentially leading to overestimation of PM deposition by conventional approaches. SEM–EDS confirmed up to 100-fold higher particle retention in P. × fraseri than P. vittata. Physiological and gene expression analyses revealed pronounced seasonal modulation in P. × fraseri, suggesting greater physiological and molecular plasticity in response to environmental fluctuations, whereas P. vittata maintained comparatively stable photosynthetic and transcriptional profiles. The results provide a robust framework for plant-based biomonitoring and urban green infrastructure design.