Abstract
The objectives of this study were (i) to investigate the feasibility of using spectral reflectance for monitoring As and Cr accumulation in Chinese brake fern (Pteris vitatta), and (ii) to search for spectral indices sensitive to structural changes caused by metal accumulation during the process of phytoremediation. Potted Chinese brake fern plants were exposed to As (100 and 300 ppm) and Cr (300 and 600 ppm) treatments for 22 days. The plants were then harvested and analysed for metal accumulation. Diffuse reflectance spectra (350–2500 nm) of the plant canopies were collected regularly throughout the metal treatment period using a portable spectroradiometer. Leaf reflectance is governed by leaf surface properties, internal structure, and foliar pigments and biochemical components. Leaf samples were collected and analysed for structural changes through microscopic observations. Our microscopic studies on changes of leaf structure provide insight into the physical changes that are remotely detected as changes in reflectance, and may permit extrapolation of these results to other plant species. Cr accumulation resulted in a decrease in biomass, relative water content (RWC), and changes in the internal structure of the leaf. The structural and spectral results show significant changes in Cr‐treated plants while the changes were minimal in As‐treated plants compared to untreated plants. Our spectral analysis revealed that a unique ratio index R 1110/R 810 can be used to monitor structural changes in plants due to accumulation of Cr. This index distinguished Cr‐treated plants from untreated and As‐treated plants. The Normalized Difference Vegetative Index (NDVI) distinguished stressed plants, but NDVI cannot distinguish Cr‐stressed plants from As‐stressed plants. Our results show that brake fern can accumulate significant amounts of Cr in shoots (2108 mg kg−1 dry weight), but it is not a hyperaccumulator for Cr because much higher Cr accumulation was found in roots (7686 mg kg−1 dry weight). This study suggests that the infrared reflectance spectrum (800–1300 nm) of plant canopy may provide a non‐intrusive monitoring method to access the physiological status of plants grown in heavy metal‐contaminated soil.
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