The genotoxic effect of lead and zinc on cowpea (Vigna unguiculata) and maize (Zea mays Linn.)

Abstract

An investigation was carried out on the treatment effect of lead and zinc on the chromosomes of cowpea and maize. The seeds of cowpea and maize were placed in Petri dishes in three replicates and allowed to  germinate for 5 days in different concentrations: 25mg/L, 50mg/L and 100mg/L of both lead and zinc nitrates respectively, while the control group had distilled water. The total chromosomal aberrations were examined. The mitotic index was calculated and the results were statistically evaluated by the analysis of variance at 5% significant level. The mitotic index decreased as the metal concentration increased (P < 0.05). The highest mitotic index values were 2.70±0.83 and 3.40±0.88 for the cowpea and maize untreated plants (control) respectively, while the least values observed were 0.20±0.13 and 1.80±0.70 for cowpea and maize treated with the 100 mg/L Zn respectively. The results showed the most frequent chromosomal anomalies induced by these heavy metals as stickiness, vagrants and bridges. Pb was found to be more toxic than Zn, as no germination was observed in both plants for the highest concentration of Pb tested. For Zn, there were no aberrations observed in cowpea at the highest concentration tested as well, while 22.22% aberrations were observed for maize. Pb induced greater aberrations having percentage abnormalities of 48.0%, 21.74% while Zn had 35.71%, 28.57% in maize and cowpea respectively for the 25mg/L concentration. As more abnormalities accumulate, gamete formation is affected and this lead to non-viable gametes, which considerably reduces plant fertility. The results from this study showed that Pb and Zn can induce genotoxicity in plants and these suggest potential health risk to human populations.

Keywords: Chromosomes, Aberrations, Genotoxicity, Heavy Metals, Mitotic Index.