Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Sheila M. Macfie

Abstract

Cadmium (Cd) is a non-essential trace element and its environmental concentrations are increasing due to human activities. Edible plants can accumulate high concentrations of Cd, which could be toxic to humans. Understanding how and where Cd is stored in plants is important for ensuring lower concentration of Cd in the food. In this thesis, the accumulation and distribution of Cd in three agricultural plants, namely lettuce (Lactuca sativa L.), barley (Hordeum vulgare L.) and radish (Raphanus sativus L.), were investigated with a focus on the potential mechanisms involved in the localization of Cd in the root. The main objectives of the study were: (1) to understand the effect of transpiration on Cd accumulation in lettuce, barley and radish, (2) to investigate the role of phytochelatins in Cd distribution in lettuce and barley, and (3) to determine the localization of Cd in the roots of lettuce and barley. The plants were grown hydroponically and analyzed using inductively coupled plasma atomic emission spectrometry, high performance liquid chromatography, and a combination of histochemistry and light microscopy, energy dispersive spectrometry, wavelength dispersive spectrometry, and x-ray fluorescence microscopy. The result showed that radish was sensitive to Cd and did not survive beyond 1.0 µM Cd. Below this concentration, radish accumulated negligible amounts of Cd in the edible organ (tap root) and was considered to have low risk of toxicity to consumers. Of the other species, barley accumulated more Cd in the root compared to lettuce, which was related to the ability of barley to retain more Cd in the root and possible redistribution of Cd from the shoot to the root via a phloem-mediated pathway. Barley provided more effective barriers against radial flow of Cd to the stele in the root and synthesized more phytochelatins and their precursor peptides in the root, which possibly immobilized Cd in the cytoplasm. Lettuce had most of its root Cd bound to the cell wall and the flow of Cd to the stele was less interrupted. This knowledge will be useful in designing or engineering plants with lower concentrations of Cd in the edible organs.

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