Correlating Arsenic-Induced Morphological Changes in Spinach Leaves with Leaf Spectral Characteristics.

Arsenic (As) is a widely spread soil contaminant which can be accumulated into plant parts.  The presence of As in edible portions of plants allows for potentially dangerous ingestion by humans and animals.  The ability to detect As in plants is an important tool to minimize such risks.  Remote sensing of plant metal toxicity has been the subject of numerous studies.  Although, many studies have investigated the spectral characteristics of metal affected plants, the extent to which different toxicities may result in correspondingly different spectral properties has received little attention.  These spectral properties are strongly affected by leaf structural characteristics. Therefore, quantitative analyses of structural changes in the arrangement of mesophyll cells caused by As will help to explain spectral responses to As.  The objectives of this study were to use stereological methods to quantify internal structural changes in leaves with As treatment in spinach plants and to relate these changes to leaf spectral properties.  Hydroponically-grown spinach (Spinacia oleracea L. cv. Babyleaf) was treated with 0, 5, 10 and 20 μmol L−1 for 4 weeks in a growth chamber. Spectral properties of leaves were obtained for visible and infrared frequencies.  Leaf structural properties such as mesophyll thickness and mesophyll surface area were measured using stereological methods.  Quantitative analysis of leaf structure showed that total leaf thickness and intercellular spaces in spongy mesophyll cells decreased with increasing As treatment.  Changes in leaf reflectance (LR) in near infrared wavelengths were strongly correlated with leaf As concentration and leaf structural changes.  Multi linear regression of LR values at the highest correlated wavelengths (1048, 1098, 1081, and 1080 nm) generated an R² value of 0.68.  Results from this study support hypothesis that relationships between leaf structure and reflectance may be useful in the interpretation of spectral data to detect leaf As concentration.