The uptake and tolerance levels of selected heavy metal ions and their distribution in sweet potato plants (ipomoea hatatas) under in vitro conditions
Ndathe, John K
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Some heavy metals are essential while others may adversely affect human who are water and plant consumers. Pollution by heavy metals is a major problem globally and hence the need to identify sources of the metals and explore effective, yet low cost methods for environmental remediation. Phytoremediation is one such method, which involves uptake and degradation of organic and inorganic pollutants by plants from soil and groundwater. In case of inorganic, such as heavy metals, phytoremediation involves uptake and sequestration of these metals in foliage for easier disposal. Selected plants can therefore be used to restore environment contaminated with pollutants. This research study aimed at investigating the extent to which locally available sweet potato plant varieties (Ipomoea batatas) absorbed and translocated cadmium, zinc, lead and chromium from their respective heavy metal-containing solutions of known concentrations. The influence of the sweet potato plant varieties in the electrical conductivity, pH and temperature of the heavy metalcontaining solutions in which they were immersed was monitored during the experimental period. The study was also investigated the tolerance of the plants in varying concentrations of heavy metal-containing solutions. Five sweet potato varieties namely UP-A, UP-B, UP-C, UP-D and UP-16 were used in the study. The plant cuttings of the size 15-20cm of the plant varieties were obtained from the garden at the University of Nairobi, College of Biological and Physical Sciences (CBPS). Their leaves were removed and then pre-rooted in large open plastic containers containing tap water for 30 days. The cuttings developed new leaves and roots. Stock solutions of 1000ppm cadmium, zinc lead and chromium were prepared from their respective soluble salts using distilled water and further diluted into working solutions of concentrations 10ppm, 20ppm and 50ppm. Distilled water was used as a control in this work. The already pre-rooted plants were then immersed in distilled water and the heavy metal-containing solutions and monitored for 14-21 days. While there were no significant changes in pH and temperature of the solutions over the experimental period, the electrical conductivity of the heavy metal-containing solutions of cadmium, zinc and chromium with immersed sweet potato plant varieties increased over the experimental period compared to those without the plant varieties. However, the electrical conductivity of lead-containing solutions increased for the first week and then reduced towards the end of the experimental period .. The extent to which the heavy metals were absorbed and translocated from the heavy metalcontaining solutions into the plant roots, stem and leaves was analysed using Atomic Absorption Spectroscopy technique. The dry plant tissues were acid digested for metal ion analysis. The degree of uptake varied with the concentration of the metal solution and the dry weight of the tissues. It was observed that in almost all the plant varieties, the roots registered higher amounts of the heavy metals followed by the stem and the leaves Cadmium content obtained from sweet potato plant varieties immersed in IOppm cadmium-a containing solutions ranged from 653.51 ug to 77l.52 ug of dry weight while that from 20ppm W containing solutions ranged from 653.51 solution ranged from 920.18 ug to1032.70 ug of total dry weight. Results of zinc content in the plant varieties immersed in 10ppm zinc-containing solutions gave the range of361.38 ug to 499.79 ug per dry weight and 505.44 ug to 60l.67 ug per dry weight for plants immersed in 20ppm zinccontaining solution. However lower range of between 308.63 ug and 370.42 ug of dry weight was recovered from plants immersed in 50ppm zinc-containing solution. Plant varieties immersed in lead-containing solutions registered the highest lead content compared to the other metals used for the study. The plants immersed in IOppm lead-containing solution accumulated a range of7.14 ug to 108.1Ougof lead per dry weights while the plants immersed in 20ppm lead-containing solution gave a range of 1389.41 ug to 1962.13 ug lead per dry weight. Plant varieties immersed in 50ppm solution had lead content of between 2.80mg and 3.57mg per mean dry weight. Plants immersed in chromium-containing solutions however, showed lower contents compared with those immersed in 10ppm total chromium-containing solution giving a range of 109.23Jlg to 164.64Jlg of dry weight while the plants immersed in 20ppm cadmium-containing solution ranged from 159.13ug to 240.7Jlg per dry weight. Total chromium content of the plants harvested from 50ppm solution ranged from 341.49Jlg to 685.69Jlg of dry weight. From the results it was concluded that sweet potato plants can actually take up cadmium, zinc lead and chromium and distribute them in the different parts of the plants. The plants can be used to remediate heavy metal contaminated sites. The amount of each heavy metal in the roots, stem and leaves increased with the increase of its ions in the solutions. However, the percentage of the metals in each part of the sweet potato plant variety decreased with the increase in metal ions in the solutions an observation attributed to possible toxicity of heavy metal to plants. The variations in metal ions uptake by different varieties could not be fully explained. The recommendations made inciude a detailed study of adsorption of the heavy metal ions on the surface of the roots and the stem of the sweet potato plants should be carried out to provide data. A study of the absorption and translocation paths of the metals in the plant tissues should also be carried out. This study may help in explaining the transport path ways of different metal ions in the sweet potato plant tissues. Further research of the extent of accumulation of these metals in the sweet potato tubers obtained from contaminated sites is recommended. This will help the consumers of the tuber against consuming sweet potato tubers and leaves obtained from heavy metals contaminated sites. After the use of the sweet potato plant in phytoremediation process, the next challenge is to find out the safest way of disposing the vines in order to avoid more contamination. A study to this effect is therefore recommended.
CitationBachelor of Science in chemistry
University of NairobiChemistry