Effects of different cellular and subcellular characteristics on the atmospheric Pb uptake, distribution and morphology in Tillandsia usneoides leaves

Lead (Pb) is a widespread highly toxic and persistent environmental pollutant. Plant leaves play a key role in accumulating atmospheric Pb, but its distribution in different cells and subcellular structures and the factors affecting it have been little studied. Here, Tillandsia usneoides, an indicator plant for atmospheric heavy metals, was treated with an aerosol generation device to analyze Pb contents in different cells (three types of cells in leaf surface scales, epidermal cells, mesophyll cells, vascular bundle cells), subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and cell wall components (pectin, hemicellulose 1 and 2, and cellulose). Results show the different cells of T. usneoides leaves play distinct roles in the process of Pb retention. The outermost wing cells are structures that capture external pollutants, while mesophyll cells, as the aggregation site after material transport, ring cells, disc cells, epidermal cells, and vascular cells are material transporters. Pb was only detected in the cell wall and pectin, indicating the cell wall was the dominant subcellular structure for Pb retention, while pectin was the main component affecting Pb retention. FTIR analysis of cell wall components indicated the esterified carboxyl (C--O) functional group in pectin may function in absorbing Pb. Pb entered leaf cells mainly in the form of low toxicity and activity to enhance its resistance.