¹⁵N-labeled ammonium nitrogen uptake and physiological responses of poplar exposed to PM_2.5 particles

Air pollution caused by particulate matter with aerodynamic diameters less than 2.5 μm (PM_2.5) is a serious environmental problem. Plants can improve air quality by removing PM_2.5 from the atmosphere. However, direct evidence of PM_2.5 absorption and assimilation into plants has not yet been found. In this study, we demonstrate that ¹⁵NH₄⁺ in PM_2.5 was absorbed by poplar leaves in low and high PM_2.5 treatment groups (namely, LPT and HPT). Then, ¹⁵N was subsequently transferred to other parts of the treated seedlings as shown by ¹⁵N tracing and simulated PM_2.5 generation. ¹⁵N and total N contents were the highest in high pollution treatment (HPT), followed by that in low pollution treatment (LPT) and the control. Glutamate dehydrogenase (GDH) contributed more to NH₄⁺ assimilation than glutamine synthetase and glutamate synthase in the leaves of treated seedlings. GDH aminating activity was induced upon NH₄⁺ exposure whereas GDH deaminating activity was repressed in both LPT and HPT, suggesting that poplar seedlings can alleviate NH₄⁺ toxicity by enhancing NH₄⁺ assimilation. At the end of PM_2.5 treatment period, the decreased amino acid content in the treated seedlings was attributed to the probably altered balance of amino acid metabolism. The decline in the net photosynthetic rate (Pn) was accompanied by the decrease in the stomatal conductance in poplar leaves with the extension of PM_2.5 treatment time, indicating that stomatal limitation is a major reason for Pn reduction. This study may provide novel insights into the relationship between PM_2.5 pollution and plants.