EFFECT OF SELECTED HEAVY METALS ON THE GERMINATION AND EARLY SEEDLING GROWTH OF Jatropha curcas (Linnaeus)

摘要

Heavy metal contamination of agricultural soil has become a critical ecological concern. Over the past years there has been increasing interest in the use of live plants and other natural materials to remediate contaminated soils and water. Phytoremediation of this polluted soil with non-edible plants like Jatropha curcas offers an environment-friendly and cost-effective method for remediating polluted soil. This study thus investigated the effect of varying concentration of heavy metals on seed germination and seedling growth of Jatropha curcas. Heavy metals used in this study were nitrate salts of copper (Cu), zinc Zn and lead Pb. Solutions of the salt of each of the metals were prepared using distilled water at concentrations of 0(control), 10, 20, 30 and 40mg/l. The experiment consisted of a total of 45 pots with ten seeds planted per pot and each containing 600g of sterilized and washed river sand thoroughly spiked with the prepared solutions. Seed germination and seedling growth were monitored for three weeks. Data assessed included the mean seed germination (%), mean root length (cm) and mean shoot length (cm) of the Jatropha curcas seedlings. All data obtained were analyzed using descriptive statistics and analysis of variance (ANOVA) using ASSITAT version 7.7 package. The exposure of Jatrophaxurcas seeds to heavy metal salts increased the seed germination at 10mg/l but further increase in heavy metal concentration resulted in a decrease in Jatropha curcas seed germination. Germination index (GI) values of Jatropha curcas seed sown in soil spiked with zinc, lead and copper nitrates were 91-94, 88-82 and 47-87 respectively. It was observed that the GI values of Jatropha curcas at 20, 30 and 40 (mg/l) in Pb spiked soil were lower than 80% recommended value for phytotoxic free and mature compost. Heavy metals (Zn, Cu and Pb) concentration at 10mg/l promoted Jatropha curcas root lenght by 101%, 102% and 131% respectively relative to the root lenght in control experiment. It was noticed that Cu~(2+) values reduced the shoot length when compared with the control at 20 and 40mg/l while Zn~(2+) and Pb~(2+) increased the shoot length Jatropha curcas even at higher concentration. Results also showed that Zn was the only metal that significantly increased the seed germination at the concentrations used in this study (p<0.05) while shoot length reduced significantly for seedlings in Cu~(2+).spiked soils at concentration higher than 10mg/l. Thus, the study showed that Jatropha curcas can germinate and grow efficiently at any zinc concentration assessed in this study; and may be grown directly in the soils that are moderately contaminated with copper and lead. Further studies need to be conducted in order to establish the maximum amount of zinc that the plants may tolerate and its ability to germinate and grow in media containing mixtures of these metals.