Remediative Abilities of Cowpea (Vigna unguiculata), Bambara Groundnut (Vigna subterranean) and Maize (Zea mays) on Soils Polluted with Lead and Zinc.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos.
Accessions Tvu 3788 and IT 99K-377-1, Tvsu 102 and Tvsu 1685 and ACR.91SUWANI-SRC1 and DMR-LSRW of Cowpea (Vigna unguiculata), Bambara groundnut (Vigna subterranean) and Maize (Zea mays) respectively were subjected to lead (Pb) and zinc (Zn) treatment and screened for their ability to tolerate these metals. Accessions Tvu 3788, Tvsu 102 and ACR.91SUWANI-SRC1 of cowpea, bambara groundnut and maize respectively were chosen and used for the experiment. The toxic effects of lead and zinc on the growth and development and biochemical activities of the three crop plants were carried out. The genotoxic effect of the heavy metals (Pb) and (Zn) on cowpea, bambara groundnut and the maize crop was investigated. The potential of cowpea, bambara groundnut and maize in the remediation of lead and zinc from polluted soils as well as the effects of Ethylenediamine tetra acetic acid (EDTA) and farmyard manure on the remediative abilities of the crop plants were also investigated. The genetic variability for lead and zinc tolerance by the crop plants were also determined. Fresh and dry weights of plants were obtained using a weighing balance and by oven-drying. The chlorophyll content was determined by spectrophotometry. Genotoxicity was determined by chromosomal squash technique with lactic acetic orcein stain. The amount of metals contained in plants’ tissues was determined by Atomic Absorption spectrophotometry. RAPD-PCR analysis for determining genetic variation of plants was also carried out. One-way ANOVA and student’s t-test using Microcal origin 5.0 software procedures were carried out on the morphological and biochemical parameters. A statistically significant difference (P<0.05) between control and treated plants was observed for the morphological parameters, chlorophyll content and enzyme activity of the three test plants. Lead and zinc caused a reduction in the mitotic index of treated plants. A statistically significant difference (P<0.05) between control and treated plants was observed. For instance, the mitotic index for cowpea were 2.70 ± 0.83, 0.50 ± 0.33 and 1.20 ± 0.52 for the control, 50mg/L of Pb and 50mg/L of Zn respectively. At lower concentrations of 25mg/L, bridges, vagrant and laggard chromosomes were observed, whereas at higher concentrations of 100mg/L, sticky chromosomes were the most common especially in plants treated with lead. Maize being able to translocate these metals especially lead through the vascular system act as a phytoextractor while cowpea could be employed for phytoextraction of zinc for translocating it through the vascular system probably by mechanisms such as uptake and metal redistribution to various tissues in the shoot through phloem and xylem transport. Bambara groundnut displayed the property of a metal excluder by immobilizing these metals at its root zone, possibly through mechanisms such as adsorption and accumulation in roots by vacuole sequestration, cell wall binding and complex formation by root exudates. When cowpea, bambara groundnut and maize were treated with 100 mg/kg of Pb, the total percentage of Pb accumulated within their tissues were 54.24%, 49.79% and 62.85% respectively. When treated with lead and augmented with manure, the total percentages of Pb accumulated within plants’ parts were 49.24%, 30.57% and 82.14% for cowpea, bambara groundnut and maize respectively. However, when treated with 100mg/kg of Pb and augmented with EDTA, 65.27%, 58.48% and 70.64 % of Pb was accumulated for cowpea, bambara groundnut and maize respectively. It is therefore suggested that for better removal of Pb, cowpea and bambara groundnut may be assisted with EDTA while maize be assisted with manure. RAPD-PCR analysis, revealed decrease and increase in the total number of bands of treated plants compared to their control. However, the close values obtained from their coefficient of similarity and regions of cluster showed the effects of metal treatment to be minimal on the DNA of treated plants for these concentrations tested. It is suggested that metal treated plants be composted and the compost especially for zinc be applied to Zn-deficient soil. The metal treated plants can also be incinerated.