Metal fate and sensitivity in the aquatic tropical vegetable Ipomoea aquatica

The aquatic plant Ipomoea aquatica is a widely consumed vegetable in Southeast Asia. It often grows in waters serving as recipients for domestic and other types of wastewater. These waters contain not only nutrients, but also a wide variety of pollutants, including heavy metals, posing a risk to the population. In addition, fertilizers are generally used in the commercial cultivation of I. aquatica for the food market. The aim of this thesis was to estimate the accumulation and toxic effects of mercury (Hg), methyl-Hg, cadmium (Cd) and lead (Pb) in I. aquatica and how these are affected by ambient nutrient concentrations. The potential for these metals to affect plant growth and the hazards they may pose to human health have also been evaluated. In plants from cultivations in the Bangkok area, Thailand, the concentrations of Cd and Pb in the shoots were well beneath recommended maximum values for human consumption, but at some sites the Hg concentrations were high. It was demonstrated that I. aquatica has the capacity to accumulate Cd and Pb in the shoots at concentrations on the order of a hundred times higher than found in field-cultivated I. aquatica from Bangkok, before exhibiting toxic symptoms. The Hg concentrations in field-cultivated I. aquatica, however, occasionally reached levels that are toxic for the plant. At most sites up to11% of total-Hg was in the form of methyl-Hg, the most toxic Hg species, though at one site methyl-Hg made up 50-100% of the total-Hg. To study if methyl-Hg is formed or degraded in I. aquatica, plants were exposed to inorganic Hg through the roots. Methyl-Hg was formed in the shoots of I. aquatica, especially in the young, metabolically active parts, but there was no sign of Hg demethylation activity. A major proportion of the absorbed metals was retained by the roots, which had a higher tolerance than the shoots for high internal metal concentrations. The nutrient level did influence accumulation and effects of Hg, Cd and Pb in I. aquatica. Lower external nutrient levels resulted in increased metal accumulation in the shoots and metal-induced toxic effects in the plant at lower external metal levels. A generous supply of sulphur or nitrogen induced the formation of glutathion and other thiol-rich peptides in I. aquatica, compounds that have a metal detoxifying effect in plants. To conclude, the levels of Cd and Pb in I. aquatica from the Bangkok area do not pose any apparent threat to human health or risk for reduced plant growth. The levels of Hg however, were high at some sites and could be a health threat, for children and foetuses in particular, and especially considering the presence of methyl-Hg. The use of fertilizers in cultivations of I. aquatica is favourable as it reduces the risk for increased metal concentrations in the plants and reduced crop yields.