Abstract
The present study investigated the accumulation of selected HMs by Chromolaena odorata and the concomitant effects on leaf anatomical features. Top soils were collected from a marked plot and pooled together to obtain a composite sample. The soil was sun-dried to constant weight and measured into experimental pots at 20kg each). The pots were divided into 5 metal groups with 3 sub-groups each. Each group was polluted with Manganese (Mn), Cadmium (Cd), Copper (Cu), Lead (Pb) and Zinc (Zn) in their respective chloride forms. Concentrations of the metals in the soil were initially based on their respective ecological screening value/benchmark (ESV). The ESV values for the 5 HMs were 50, 4, 100, 50 and 50 mg/kg respectively. Within each group, the respective HMs was applied in 3 concentrations of 1ESV, 3ESV and 5ESV. The control experiment consisted of plants grown in soils with no exogenous application of the test metals. The experiment was triplicated. Twenty hours later, equal sized stem cuttings of C. odorata (2.0 – 2.3cm in thickness, 30 cm in length) were planted per experimental pot. Six months later, results showed significant accumulation of metals in plant stem, leaves, and most especially the root. Mn was the most accumulated HM in all plant parts (9.22 – 17.86 mg/kg), compared to Cd (0.85 – 1.66 mg/kg).Significant changes in folial anatomy were reported in HM-impacted plants compared to the control. There were more upper epidermal stomata (270 – 353 mm2) in Mn-exposed plants compared to the others. Increase in vascular bundle thickness (p<0.01) was reported in HM-exposed plants compared to control. Highly significant decrease in stem parenchyma thickness (p<0.01) never the less parenchyma thickness of HM-exposed plants ranged from 46.37 – 49.53 µm in Zn and Pb-exposed plants compared to 79.23 µm in the control.
Highlights
The impact of toxicity of heavy metals on plants has been known to be far-reaching
When plants were exposed to higher concentrations of cadmium, the physiological impact was a significant impairment of calcium equilibrium, which inadvertently resulted in impairment of cell wall elasticity and cell redox balances (Perfus-Barbeoch et al, 2002)
The major concern is the consequences these anatomical changes would present on plants that are used in the remediation of heavy metalcontaminated systems (Li et al, 2006; Katayama et al, 2013)
Summary
The impact of toxicity of heavy metals on plants has been known to be far-reaching These effects include significant changes in plant morphology, anatomy, physiology or biochemistry, which eventually result in either a decrease in the plant’s exposure to the stressor(s) and/or limit damage and enable repair of compromised plant systems. Much as the efficiency of heavy metal clean-up is critical, it is important to know the level of anatomical impairment exposure to heavy metal may present on the plant in question. One of such plants is Chromolaena odorata. This is necessary to enhance knowledge of the plants’ tolerance capacities
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