ABSTRACT
Heavy metal contamination of soils became a major global problem in recent years due to an increase in geological and anthropogenic activities. Metal processing, mining activities, uncontrolled dumping of waste in landfills, and industrial waste releases are some of the prominent sources of metal contamination around the world. Phytoremediation, where plants are used to clean up metal-contaminated soils, is a widely accepted method for in situ soil remediation. Plants growing on metal-contaminated soils often show an alteration in their physiological, chemical, and anatomical characteristics due to enhanced metal uptake and accumulation. Consequences of increased metal concentrations on the internal structure of plant leaf, stem, and root is an important determinant of the physiological adoptability and the phytoremediation potential of the plant. The variation in the plant leaf size, changes in the orientation and intercellular spaces of mesophyll cells, orientation, and shape of the vascular bundles in leaf, root, and stem will all impact the metal uptake characteristics of the plant. This chapter pays attention to some of the newly developed plant imaging methods that can be employed for continuous monitoring of the metal accumulation in different plants and quantification of the resulting internal structural and physiological changes during the process of phytoremediation. Due to the fact that large areas are often contaminated with multiple metal pollutants, special attention is being paid to the metal-specific effects on the internal structure of broad leaf and grass leaf plants during the process of metal accumulation. This chapter summarizes the studies that investigate the feasibility of using spectral reflectance to monitor Zn and Cd accumulation in Indian mustard (Brassica juncea) and barley (Hordeum vulgare) plants, and to search for spectral indices sensitive to structural changes caused by metal accumulation during the process of phytoremediation. Zn accumulation at high concentrations results in a decrease in biomass, a decrease in relative water content (RWC), and changes in the internal structure of the leaves in both plants. The structural and spectral results show significant changes in Zn-treated plants, while the changes are minimal in Cd-treated plants when compared with the untreated plants. These studies reflects that the infrared reflectance spectrum of the plant canopy and the derived spectral indices may provide a nonintrusive monitoring method to assess the physiological status of plants grown in heavy metal–contaminated soil. These results also systematically illustrate the physiological implications of structural alterations caused by Zn and Cd at higher concentrations in mustard and barley plants.
