Monitoring plant spectral reflectance and internal structure changes during the phytoremediation processes for selected heavy metals.

摘要

The objectives of the study were to investigate the phytoremediation process of heavy metals and to identify spectral changes and search spectral bands/indices to assess heavy metal phytotoxicity in canopies of selected plants, to relate these spectral reflectance characteristics to changes in leaf internal structure, and to localize and characterize the effect of metal uptake and accumulation on the roots, stem and leaves of selected plants. Hyperspectral reflectance data were acquired over the 350--2500 nm range using a handheld spectroradiometer. Microscopic data was obtained using light microscopy, scanning electron microscopy, and transmission electron microscopy. Structural and ultrastructural changes were correlated with spectral reflectance measurements. Metal accumulation in plant tissues was determined by chemical analysis. Biophysical data included fresh weight, dry weight, relative water content, and plant height. The phytoremediation potential of three metal-accumulating plants (barley, Indian mustard and brake fern) were evaluated using six toxic heavy metals (Cd, Zn, Cr, As, Sr and Cs). The plant-metal combinations were: barley for Cd and Zn; brake fern for As and Cr; and Indian mustard for Cd, Zn, As, Cr, Cs and Sr. The metal accumulation and translocation efficiency in respective plant species were calculated and correlated to structural changes.; The results of this study indicate that heavy metals at higher concentrations had a profound impact on physiology and internal structure of plants which in turn, affected the spectral reflectance. Different spectral indices were computed and their relationship with foliar structural changes, metal accumulation and biophysical attributes were compared. This study found that foliar structural changes can be best predicted by spectral bands around 1110 nm and 810 nm in the near infrared region. A spectral ratio index of R1110/R810 closely correlated to the leaf structural changes. This study determined that the combination of vegetative indices, such as NDVI (Normalized Difference Vegetative Index) and spectral ratio index R1110/R810 in the 800--1300 nm region, can provide a non-intrusive and continuous monitoring method for the impact and content of certain heavy metals in the canopies of living plants grown on contaminated soils.