Bioscience Evidence 2024, Vol.14, No.4, 154-160 http://bioscipublisher.com/index.php/be 158 plants. Where growth reduction was recorded at high concentrations, Zn was generally not significant compared to the control. This is an indication that negative impact of heavy metals on C. odorata growth was minimal, demonstrating remarkable tolerance to lead and zinc contamination, though more in Pb than Zn. This was evident in plant height, stem girth, leaf area, number of leaves as well as number of roots and root length. This resilience aligns with findings by Lambert et al. (2012), indicating that plant responses to heavy metals vary, with some possessing natural remediation abilities. Tolerance of plants to toxicity depends on the species and tissue, element type, and duration of exposure to stress (Ayesa et al., 2018; Tiwari and Lata, 2018). Heavy metals, when occurring at low concentrations, are involved in redox reactions, electron transfers, nucleic acid metabolism, and as an integral part of several enzymes. Some heavy metals such as Cu, Fe, Mn, Zn, and Ni are components of some enzymes and proteins, thereby essential for plant growth and metabolism. Reduction in number of leaves and stem girth by Zn, and reduction in biomass by Pb confirm that heavy metals can negatively impact plant growth, development and reproduction when they are accumulated at concentrations above their optimal levels (Rashid et al., 2023). This can, therefore, be ascribed to metal accumulation beyond permissible level. Pb is an example of potential heavy metal that is neither an essential element nor have any role in the process of cell metabolism but is easily absorbed and accumulated in different parts of a plant. High concentration of heavy metals such as lead can cause a number of toxic symptoms in plants that may be retardation in growth (stunted growth), negative effects on photosynthesis (chlorosis), blackening of roots and different other symptoms. Lead has the ability to inhibit photosynthesis, disturb mineral nutrition and water balance, changes in hormonal status and affects membrane structure and permeability (Nas and Ali, 2018). This corresponds to reports of heavy metals affecting plant growth, causing degradation in crop quality, soil health and yield because of their accumulation in crops (Shah and Daverey, 2020; Wang et al., 2020). Many researchers have also confirmed negative effects of heavy metals on plants. For instance, lead and zinc were reported to have negative impact on seedling growth of alfalfa (Medicago sativa L.) by Yahaghi et al. (2019). Also, cadmium, chromium and lead had negative impact on growth and development of cultivated plants (Madhu and Sadagopan, 2020). Similarly, Nickel (Ni) was found to negatively affect vegetative and reproductive growth parameters of Nigella sativa (Khan et al., 2023). Reduction of fresh and dry weights of plant parts particularly by Zn at 100 mg/kg is similar to the findings of Kekere et al. (2020) who explained biomass reduction in Tithonina diversifolia due to the variations depending on the quantity of metal. Plants, when grown in soils contaminated with heavy metals, are often faced with some changes at physiological levels which include nutrient accumulation, respiration, and gaseous exchange. Heavy metals at high concentrations also affect plant metabolism and physiological events, reduce growth, and contaminate the environment. Heavy metals also cause oxidative stress in plants mainly through excessive production of reactive oxygen species (ROS). Excessive production of ROS increases production of unsaturated lipid peroxidation, fatty acids and disturbs cell membrane function. Cell membrane damage can cause an imbalance in enzymatic activities, disrupts the normal redox balance of the cell, and causes oxidative damage that affects cell metabolism (Wang et al., 2020). Metals accumulation in plants can significantly affect plant viability, carbohydrate level, and respiratory rates. Interestingly, high concentrations of the heavy metals had a positive impact on some plant growth indices, supporting Masindi and Muedi (2018) findings of increased growth rate in Tithonia diversifolia due to heavy metal effects. The rapid growth of C. odorata seedlings, even under high metal concentrations, also aligns with Kupper et al. (2002) suggestion that such growth could result from cell division processes, respiration, and protein synthesis. The experiment results indicated that the overall performance of C. odorata was not severely hampered by increased heavy metal concentrations. Even at high levels of metals, the plant exhibited improved growth with minimal negative effects. This re-echoes the findings of Ayesa et al. (2018) on Tithonia diversifolia, reinforcing the notion that certain plants can thrive in heavy metal-contaminated environments. Srirueang et al. (2021) earlier confirmed this result with studies showing that certain plants succeed in the accumulation of multiple heavy metals in the presence of high concentrations. Plants have developed some mechanisms against heavy metal stress. These mechanisms include immobilization; exclusion of plasma membranes; restriction of absorption and transport; synthesis of specific heavy metal carriers; induction of stress proteins; chelation; and sequestration by certain ligands (Kumar et al., 2019; Yu et al., 2019; Madhu and Sadagopan, 2020; Wang et al., 2020; Islam et al., 2024).
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