Selected Morphological Characteristics, Lead Uptake and Phytochelatin Synthesis by Coffeeweed (Sesbania exaltata Raf.) Grown in Elevated Levels of Lead-Contaminated Soil

Remediation of lead-contaminated soil is significant due to the inherent toxicity of lead (Pb), and the quantity of Pb discharged into the soil. One of the most cost-effective and environmentally sound technologies for the cleanup of metal-contaminated soils is through the use of plants. While much is known about the ecological evolution of metal tolerance in plants, the physiological, biochemical, and genetic mechanisms of tolerance is not well understood in the majority of resistant ecotypes such as the legume, Sesbania exaltata Raf. This study was therefore conducted to determine the morphological and physiological characteristics of Sesbania that had been grown in Pb-contaminated soil, and to assess phytochelatin synthesis as a way of elucidating its relative Pb tolerance. Sesbania plants were grown in the greenhouse and exposed to various levels of Pb: 0, 1000, and 2000 mg Pb/kg soil. Plants were harvested after 6, 8, and 10 weeks of growth and morphological characteristics (e.g., root and shoot biomass, root length, number of root nodules, shoot height, number of leaves, number of flowers, number and length of pods) were recorded. Generally, there were no statistical differences in morphological characteristics among the treatments. Further, no discernible phytotoxic symptoms, such as chlorosis, wilting, or necrotic lesions, in neither roots nor shoots were observed. We concluded that while Sesbania did not fit the model of a hyperaccumulator, the plant was, nonetheless, tolerant to elevated Pb levels. Our assessment for phytochelatin synthesis as a tolerance mechanism was inconclusive and further investigations of tolerance mechanisms are warranted.