Manganese (Mn), a typical heavy metal pollutant, is an essential trace element found in various concentrations in all tissues. Manganese is known to be taken up by soil, water, and plants. Mn-contaminated soils and waters are becoming an environmental concern following increased awareness of the need for environmental protection. Soil Mn in particular accumulates easily and cannot be removed naturally in great quantities. Plants exposed to excess Mn often suffer from Mn poisoning, which has many negative effects. In general, excess heavy metal concentrations can induce specific damage to the ultrastructure of plant cells; such damage is expressed primarily as abnormal changes to cell components including the Golgi body, endoplasmic reticulum, nucleus, chloroplast, mitochondria, vacuole, and plasma membrane. Moreover, the extent of damage imparted to cell ultrastructure by heavy metal toxicity increases with both exposure time and heavy metal concentration.
Polygonum perfoliatum L. is a Mn-tolerant plant that can grow on abandoned Mn tailings. Accordingly, it is thought to be promising for the revegetation of land formed from such tailings. Following hydroponic experiments in a greenhouse environment, transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) was used to investigate changes in the ultrastructure of root, stem, and leaf cells of P. perfoliatum and to study variations in the form of Mn in leaf cells under various Mn concentrations (5, 1000, and 10000 μmol/L). The results demonstrate the following. (1) For an Mn concentration of 5 μmol/L, the ultrastructure of P. perfoliatum was clearly visible, without any obvious damage. The structures of the root, stem, and leaf cells of P. perfoliatum remained intact, with clearly visible and seemingly undamaged organelles, when the Mn concentration was increased to 1000 μmol/L. (2) Organelles were still present at Mn concentrations greater than 1000 μmol/L. However, the number of mitochondria in the root cells decreased, the double-membrane system and cristae of mitochondria became dim, and the chloroplasts in stem cells began to exhibit signs of damage. Moreover, evidence of damage began to appear in chloroplast membrane structures of leaf cells, where the lamellar structure of grana was poorly developed and the number of osmiophilic granules decreased significantly. Although the chloroplasts and chloroplast membrane structures of P. perfoliatum had significant changes under heavy Mn stress, this species could still survive and grow. All these suggested that P. perfoliatum had an extraordinary tolerance to Mn. (3) Black agglomerations were observed in the cells after treatment with Mn concentrations of 1000 μmol/L or 10000 μmol/L for 30 days, with more agglomerations observed at higher Mn concentrations. Such features were not noted in the control experiments. For an Mn concentration of 10000 μmol/L, Mn existed as a solid acicular substance within leaf cells and intercellular spaces, possibly indicating a mechanism employed by P. perfoliatum to allow accumulation of Mn while avoiding poisoning. The present study has furthered understanding of the Mn tolerance mechanisms of plants and laid a scientific foundation for adopting tolerant plants for use in the revegetation of polluted soil over large areas. Thus, the results offer promise for further developments in the ecological restoration of Mn tailings wasteland.