Combined μ-XRF and XANES Track the Behavior of Pb from PM2.5 Entering Chinese Cabbage Leaves

Atmospheric fine particulate matter (PM2.5) is the main contributor to Pb accumulation in edible Chinese cabbage leaves in North China. PM2.5-Pb primarily enters leaves via stomatal foliar uptake. However, how PM2.5-Pb is transported and stored within the leaf cells of Chinese cabbage remains unclear. Thus, this study mapped Pb, Ca, and Mg distributions in Chinese cabbage leaves following PM2.5-Pb stress using synchrotron and fast micro-X-ray fluorescence. Findings revealed that PM2.5-Pb was transported and localized in guard cells, the epidermal cell wall, and chloroplasts. X-ray absorption near-edge structure revealed that Pb(CO3)2·Pb(OH)2 in PM2.5 was converted to Pb(OH)2, glutathione-Pb (GSH-Pb), and PbC2O4 in Chinese cabbage leaves. GSH-Pb proportion in the low Pb accumulation (LPA) variety Chinese cabbage leaves was 2.13 times higher than that in the high Pb accumulation (HPA) variety. Glutamate concentration decreased by 44.53% in the LPA variety leaves under PM2.5-Pb stress, increasing GSH-Pb efflux symplasm and reducing Pb accumulation. X-ray fluorescence mapping of Ca and Mg in leaves indicated chlorophyll biosynthesis inhibition in the HPA variety leaves but not in the LPA variety leaves. Pb speciation and distribution vary drastically between the LPA and HPA variety leaves. This study provides guidance for breeding a high-quality LPA variety of Chinese cabbages.