Assessment and Categorization of Metal-Accumulating Potentials of Two Tropical Weeds Cultivated on Asunle Dumpsite, Obafemi Awolowo University, Ile Ife.

This study assessed and categorized the metal-accumulating potential of Tithonia diversifolia Mexican (Sunflower) and Chromolaena odorata (Siamweed) cultivated on Obafemi Awolowo University general dumpsite popularly called Asunle dumpsite in relation to the available heavy metals of interest, which are Pb, Cd, Cu and Hg. Viable seedlings of T. diversifolia and C. odorata were collected within the Obafemi Awolowo University (OAU), Ile-Ife and authenticated at IFE Herbarium. The experiment was laid in Randomized Complete Block Design (RCBD) in three replicates which involved three treatments namely; T. diversifolia, C. odorata and a control plot without weed plant. Growth parameters such as number of leaves, stem girth, plant height and leaf area were measured at two-week intervals for 12 weeks. After harvesting, the plant were separated into roots and shoots using clean razor blade and rinsed with distilled water and were oven-dried at 70oC to constant weight. Heavy metal uptake in the root and shoot samples was determined using standard methods. Results were analysed using analysis of variance and means were separated by Duncan’s Multiple Range Test at p < 0.05. Categorization of the test weeds into accumulators, hyperaccumulators or excluders were carried out by determining the Bio-accumulation and Translocation factors which were also used to determine the metal-accumulating potentials of the test weeds. The results from this research showed that Heavy metal uptake was generally higher in the root than the shoot for both test weeds. However, significantly higher values (p < 0.05) were observed for Cu (2.11 mg/kg) in the shoot of T. diversifolia and Pb (9.32 mg/kg) in the root of C. odorata. Transfer factors for Pb (0.92), Cd (0.83), Cu (1.68) and Hg (1.00) for T. diversifolia; and for Pb (0.68), Cd (2.17), Cu (0.67) and Hg (0.35) for C. odorata were obtained. The bioaccumulation factors of all the metals considered were less than one (1) for both weeds. The study concluded that high clay content and alkalinity of the dumpsite soil could be responsible for the unavailability of heavy metals in the study site. It was also concluded that T. diversifolia and C. odorata are more of excluders than accumulators of some of the heavy metals in the study area. The exceptions are Cu and Hg in T. diversifolia and Cd in C. odorata which were accumulated.