Heavy metals or trace element ions, derived from anthropogenic activities and industrial development, represent some of the most severe environmental contaminants today. Soil and aquatic pollution by heavy metals pose significant threats to human health, aquatic ecosystems, and agriculture (Overesch et al., 2007). Additionally, heavy metal contamination of agricultural soil is a major global challenge. The primary visual effects of heavy metal poisoning in plants include chlorosis, reduced growth, impaired respiration and nitrogen metabolism, reduced water and nutrient absorption, and decreased photosynthetic rate (Benavides et al., 2005; Wang et al., 2008). Cadmium (Cd) and lead (Pb) are widely recognized as the most prominent heavy metals (Redovnikovic et al., 2017). The negative effects of Cd include reduced biomass production, decreased enzyme activity, disruption of soluble protein metabolism, and increased generation of reactive oxygen species (ROS), ultimately hindering plant growth (Kieffer et al., 2009; Ali et al., 2014a; EF et al., 2015). Moreover, Cd adversely affects soil microorganisms, leading to reduced organic matter decomposition and decreased soil nutrient availability, which further inhibits plant growth (Qiu et al., 2022).

Pb causes chlorosis and reduces seed germination, growth and biomass, photosynthetic rate, essential element absorption, and water transport. It also alters membrane permeability and increases ROS production in plants, interfering with enzyme activities within plant cells (Ali et al., 2014b; Saxena et al., 2020). According to Wyszkowska et al. (2024), Pb at concentrations of 800 mg/kg significantly impacts soil enzyme activities and alters various physicochemical properties of the soil. Both Cd and Pb are toxic to plants even at low concentrations and lack any known biological functions within plant cells (Khellaf and Zerdaoui, 2010).

To address environmental pollution, various remediation technologies based on mechanical or physicochemical principles have been employed to remediate contaminated soil (Marques et al., 2009). These technologies include in-situ excavation and landfilling, soil flushing and washing, and electrokinetic methods (Wuana and Okieimen, 2011). However, the aforementioned methods are costly and labor-intensive, disrupt soil structure, reduce indigenous soil microflora, and render the soil unsuitable for plant development (Vangronsveld et al., 2009). Therefore, using plants for soil remediation is considered a biological strategy. Phytoremediation is an effective and cost-efficient method that uses the natural abilities of plants to remove pollutants and reduce hazardous substances in the environment (Carbisu and Alkorta, 2001; Polle et al., 2013). Additionally, it does not adversely affect the chemical quality of the soil (Shrestha et al., 2019). Furthermore, phytoremediation can enhance physical soil quality and improve biological processes (Wan et al., 2016).

A key strategy in phytoextraction is identifying plants that possess tolerance mechanisms against heavy metals and can efficiently uptake them while maintaining significant biomass without adverse impacts on growth (Bhargava et al., 2012; Ye et al., 2017). Plants used in heavy metal phytoremediation should have rapid growth, substantial biomass, and extensive root systems; should be easy and cost-effective to plants; and should have an aesthetically pleasing appearance (Brunner et al., 2008; Luo et al., 2016).

The bioavailability of heavy metals in the soil and their various physiological functions in plant cells (e.g., plant growth stage, soil pH, heavy metal levels, and exposure duration) significantly affect the toxicity, adsorption, and translocation of heavy metals in plants (Bhargava et al., 2012; Zarubova et al., 2015). Additionally, soil physicochemical properties, heavy metal bioavailability, plant and microflora secretions, and the plants’ ability to absorb, accumulate, sequester, or detoxify elements are factors that influence remediation efficiency (Wang et al., 2020).

High levels of ROS caused by heavy metals can damage nucleic acids, peroxidize membrane lipids, oxidize proteins, and inhibit enzymes, ultimately leading to plant cell death. Maintaining a balance between ROS and the antioxidant system is crucial for plants, especially under heavy metal stress (He et al., 2011; Sharma et al., 2012). Plants mitigate oxidative stress through antioxidant enzyme activities [e.g., superoxide dismutase (SOD), glutathione S-transferase (GST), guaiacol peroxidase (POD)] and low molecular weight antioxidants (e.g., ascorbic acid, reduced glutathione, carotenoids, phenolics) (Benavides et al., 2005; Hossain et al., 2012).

Phytochelatins, enzymatically synthesized in plants, are produced in response to heavy metal stress as a defense mechanism (Chia, 2021). Numerous studies have identified several plant species as suitable phytoremediators for Pb- and Cd-contaminated soil, including Populus species, Salix species (willow), Jatropha curcas, and Schima superba (Alvarez-Mateos et al., 2019; El-Mahrouk et al., 2020, 2021; Wang et al., 2022). This study focuses on Conocarpus erectus, an evergreen tree from the Combretaceae family, as a potential phytoremediator. Indigenous to tropical and subtropical regions, C. erectus is extensively used in Egypt for landscaping purposes. Despite its widespread use, its suitability for phytoremediation is poorly understood due to limited data [e.g., growth behavior, phytoremediation efficiency, chemical and biochemical compositions under heavy metals (HMs)] on its remediation capabilities.

Understanding the mechanisms by which plants resist specific metals is necessary to expand the list of plants suitable for treating heavy metal-contaminated soil (Sharma et al., 2023). This study particularly emphasizes Cd and Pb, which are common heavy metals frequently found in soils across various regions of Egypt. The contamination primarily results from the use of sewage sludge and the intensive application of chemical fertilizers, particularly calcium superphosphate, as well as from irrigation water mixed with sewage and industrial effluents. These factors contribute to elevated levels of Cd and Pb in the soil, exacerbating environmental risks.

Given the high mobility of Cd and Pb, which can be magnified through food-web mechanisms, they pose significant threats to living organisms. Therefore, this work was conducted to evaluate the growth, enzymatic activities, electrolyte leakage (EL), and remediation efficacy of C. erectus under Cd and Pb stress, individually and in combination.

This research was carried out for 16 months, beginning on 1 April 2021 and ending on 1 August 2022 at the Experimental Farm of the Faculty of Agriculture at Kafrelsheikh University in Egypt. The farm is situated at coordinates 31°61′N, 30°57′E with an elevation of 6 m above sea level. Meteorological data from the nearby Sakha meteorological station recorded a total rainfall of 203.28 mm and 111.68 mm, with relative humidity averages of 53.68% and 57.73% and minimum average temperatures of 15.0°C and 16.6°C during the experimental period. Additionally, the average maximum temperatures were recorded as 28.7°C and 29.6°C for the respective seasons.

Agricultural soil polluted with HMs is a critical issue that poses environmental hazards due to its adverse impacts on various ecological aspects (Masakorala et al., 2013). Certain plants possess significant potential for remediating contaminated soil by absorbing HMs and storing them in their aerial organs (phytoextraction), consequently reducing the bioavailability of pollutants in the soil (Nwoko, 2010). Kaur et al. (2024) observed that the accumulation of Cd and Pb in plants disrupted functional groups such as carboxyl, amino, hydroxyl, and phosphate, leading to unintended alterations in physiological and biochemical processes. For instance, these metals disrupt photosynthesis by impacting chloroplast structure and function, as observed with Cd exposure (Jing et al., 2005), resulting in diminished chlorophyll content. Consequently, the plant’s capacity to harness energy via photosynthesis is compromised, leading to stunted vegetative growth. Furthermore, heavy metals like Pb can induce the generation of ROS, inducing oxidative stress in plants (Kupper, 2017). Pb or Cd can disrupt the uptake and translocation of water and essential nutrients in plants, leading to imbalances and hindering root growth. This disruption can result in decreased growth, abnormalities in phloem and xylem structure, and impairment of biochemical and physiological processes (Kumar et al., 2017; Wu et al., 2018). Furthermore, these metals can modulate the expression of genes associated with growth and development, leading to alterations in hormone production and other signaling molecules that regulate plant growth (Alaraidh et al., 2018). Consequently, stunted vegetative growth and reduced chlorophyll content may occur.

In the current study, the vegetative measurements ( Figures 1A, B ) are consistent with several investigations that have documented the negative impacts of Cd and Pb at various concentrations on vegetative traits. For instance, Tauqeer et al. (2016) observed a significant retardation in the root dry weight of Alternanthera bettzickiana at 0.225 mg Cd/L or Pb at 0.414 mg/L. Similarly, Redovnikovic et al. (2017) found that the dry weights of aerial parts and roots of Populus nigra cv. ‘Italica’ were negatively affected by various combinations of Pb at 400–1,200 and Cd at 10–50 mg/kg soil, but the Cd and Pb combination at a higher rate had so much negative effect. Other studies on various plant species such as P. nigra (El-Mahrouk et al., 2020), Clidemia sericea (Durante-Yanez et al., 2022), S. superba, Chinese sweetgum, and Chinese fir (Wang et al., 2022) have also reported the detrimental effects of Cd and Pb. Furthermore, Yousefi et al. (2023) observed that elevated Cd levels resulted in reduced plant height and diminished growth of Portulaca oleracea. Consistent with these results, Saleem et al. (2024) observed that exposure to 50 and 100 µM of Cd resulted in reduced plant growth and chlorophyll content in Pisum sativum.

The findings of Kumar et al. (2023) and Li et al. (2023) underscore the impact of Cd and Pb toxicity on various aspects of plant physiology, including the inhibition of seedling growth, the synthesis of photosynthetic pigments, and the disruption of biological and physiological activities. This reduction in chlorophyll pigment content under HM stress can be attributed to the inhibition of enzymes responsible for their biosynthesis and the substitution of Mg²⁺ in the chlorophyll molecule by HMs, leading to diminished chlorophyll levels and compromised photosynthetic efficiency (Rastgoo and Alemzadeh, 2011; Sumalan et al., 2023), along with degradation of chloroplast architecture, disruption of photosynthetic enzyme activity, and stomatal closure (Chandra and Kang, 2015). All the aforementioned research aligns with our findings of chlorophyll index ( Figure 1C ). However, our research on C. erectus indicates that it can thrive under conditions of Cd and Pb stress. Similarly, Kaur et al. (2024) found that Salix alba exhibited a survival rate of over 85% when grown in soil with Pb concentrations of 200 mg/kg and Cd concentrations of 15 mg/kg.

According to Wilkins (1978), a TI_b or TI_r value of less than 1 indicates that a plant is under stress, characterized by a decrease in weight and significant depletion of root growth. This observation aligns with our findings ( Figure 1D ). The level of tolerance exhibited by the plants is contingent upon the specific plant species or clones. Similarly, Dos Santos Utmazian et al. (2007) observed variations in the tolerance levels of 20 different willow and poplar clones when exposed to Cd and Zn. Moreover, the TI_b and TI_r of P. nigra were <1 at Cd Cl₂ (20, 40, 60, and 80) and Pb acetate (250, 450, 650, and 850) mg/kg soil (El-Mahrouk et al., 2020). Using lower levels of Cd and Pb, C. erectus showed excellence in tolerance mechanism, reaching over 80% and 90% against Cd and Pb, respectively, and it had a good defense against Cd and Pb at medium levels.

ROS are generated due to the incomplete reduction of molecular oxygen, resulting in the formation of hydroxyl radicals (OH), hydrogen peroxide (H₂O₂), superoxide anions (O²⁻), and ozone (O₃) (Nathan and Ding, 2010). While ROS are essential for signaling pathways and are naturally produced in cells, adverse conditions cause higher plants to generate more ROS, leading to a wide range of physiological changes. Elevated ROS levels can degrade antioxidants, triggering the expression of antioxidative response genes (Mansoor et al., 2023). However, excessive ROS inflict damage on cellular components such as proteins, lipids, and DNA; disrupt normal mitosis; and ultimately culminate in senescence and cell death (Santos et al., 2010; Kupper, 2017; Bashir et al., 2021; Hendrix et al., 2023). To combat ROS, plants possess a sophisticated antioxidative defense system comprising both non-enzymatic and enzymatic components, which serve as a natural defense mechanism against increased concentrations of heavy metals (Li et al., 2023). Enzymatic components such as PPO, POD, and CAT play a crucial role in preventing the overproduction of ROS and subsequent oxidative damage due to heavy metal exposure, thereby safeguarding the plant from oxidative harm (Yang et al., 2009; Sofy et al., 2020). PPO is a terminal oxidase that facilitates the direct transfer of electrons to O₂ during the oxidation of intermediate products in plant respiration. It catalyzes the transformation of compounds like phenol into quinone and is involved in the synthesis of phenol-containing cell wall compounds such as lignin. Under heavy metal stress, PPO activity increases significantly, aiding in plant detoxification by converting phenolic compounds into less reactive forms and thereby reducing ROS accumulation (Rivero et al., 2001). Supporting this notion, Battana and Ghanata (2014) demonstrated in their study on pigeon pea that Cd detoxification occurs through the formation and sequestration of Cd–Ca crystals during phenol polymerization by phenol oxidases. Another category of enzymes involved in the breakdown of hydrogen peroxide (H₂O₂) is POD, which converts H₂O₂ to water by oxidizing various substrates simultaneously (Nianiou-Obeidat et al., 2017). POD exists in numerous isoforms and participates in a wide array of cellular activities, such as growth and development, differentiation, lignification, senescence, and auxin degradation, utilizing a diverse range of phenolic substrates (Passardi et al., 2004). Additionally, CAT is a “heme-containing” enzyme and plays a crucial role in scavenging H₂O₂ (Sharma et al., 2012). CAT directly degrades H₂O₂ without the need for an electron donor, resulting in the formation of H₂O + O₂ (Ahmad et al., 2010). The observed delay in the enhancement of photosynthetic activity relative to the reduction in lipid peroxidation suggests that acclimation mechanisms are essential for minimizing oxidative stress damage and repairing the photosynthetic system after prolonged exposure to Cd. Elevated levels of POD can potentially enhance the elimination of H₂O₂, thereby promoting photosynthesis (Solti et al., 2016). Similarly, Alsherif et al. (2022) documented that Amaranthus retroflexus plants avoid ROS stress by increasing their antioxidant system, inducing lower HM harmful symptoms even at higher accumulation of HM levels.

In this study, PPO, POD, and CAT activities were observed to increase with moderate levels of Cd and Pb exposure, either individually or in combination (see Figures 2A-C ). Wang et al. (2008) provided support for the increase in PPO, POD, and CAT activities relative to Cd and Pb treatments compared to the control. They noted that enzyme activities in metal accumulator plants escalated rapidly with rising levels of Cd. They also observed a swift increase in enzyme activities in metal accumulator plants as Cd levels increased. Increasing exposure time to HM stress in P. nigra L. led to a reduction in ROS, with increment of CAT and POD activities (Jakovljevic et al., 2014). Several studies provide insights into the dynamic responses of enzyme activity to HM exposure. Nagajyoti et al. (2010) observed that enzyme activity under HMs, such as Cu, Cd, and Pb, is concentration-dependent and can be either enhanced or reduced. Dai et al. (2012) investigated the response of Populus canescens and noted a significant increase in CAT activity at lower Cd concentrations (10 μM), while higher Cd levels (50 and 70 μM) led to a decrease in CAT activity across various tissues. Additionally, they observed that CAT activity followed a significant order: leaf > wood > bark > root. This suggests that P. canescens has well-established detoxification mechanisms to tackle metal-induced oxidative stress. Tauqeer et al. (2016) found that the POD and CAT activities in A. bettzickiana were enhanced at lower Cd and Pb (0.5 and 1.0 mM), and then their activities were retarded at higher Cd and Pb (2.0 mM). Zou et al. (2017) reported stimulation of CAT, POD, and SOD activities in Salix matsudana at Cd concentrations (10, 50, and 100 mM) but noted reductions in soluble protein and carbohydrate levels. Emamverdian et al. (2018) also observed enhancing enzymatic activities in Indocalamus latifolius under Pb at 500–2,000 mg/kg soil. Similarly, Bankaji et al. (2019) noted an increase in CAT activity in Atriplex halimus with increasing soil Pb levels from 0 to 600 mM. Mohamed et al. (2019) pointed out that total antioxidant activities were increased by 65.5% in 40 mg/L Pb-treated seedlings of Jatropha curcas, being higher compared to the control. Xu et al. (2019) highlighted variations in enzyme activity, suggesting regulation of Cd and Pb stress resistance by an integrated antioxidative system, with suppression of CAT and POD activities observed with increasing Cd and Pb concentrations in willow genotypes. El-Mahrouk et al. (2020) found that PPO, POD, and CAT activities in P. nigra were stimulated with CdCl₂ at 20 and 40 mg/kg soil and with Pb acetate at 250 and 450 mg/kg soil, then decreased under medium and higher levels of CdCl₂ (60 and 80 mg/kg soil) and Pb acetate (650 and 850 mg/kg soil). Wang et al. (2022) revealed that Cd at 12 mg/kg soil stimulated POD in Chinese sweetgum and POD activity in Chinese fir seedlings was stimulated under 6–24 mg Cd/kg; however, 36 mg of Cd/kg declined POD activities in both species relative to control. Additionally, 6 mg Cd/kg soil did not significantly enhance POD activity in S. superba. They highlighted that 12 mg Cd/kg soil increased CAT activity in S. superba and Chinese sweetgum seedlings, while Cd at 12–36 mg/kg decreased CAT activity in Chinese fir. Furthermore, Saleem et al. (2024) found that exposure to 50 and 100 µM Cd significantly increased H₂O₂ production and enzymatic antioxidant activity, as well as the levels of phenolic compounds, flavonoids, and proline in P. sativum. These collective findings provide valuable insights into the intricate interactions and mechanisms involved in heavy metal accumulation in plants. They serve to reinforce and support our observations on this subject.

Heavy metals cause injuries to cell walls and damage to membrane permeability; consequently, EL increases. Also, EL was increased by soil Cd, and Pb levels increased. EL occurs in various plant species, tissues, and kinds of cells and can be forced with all stressed conditions as HMs (Demidchik et al., 2003). The main cause of EL is pertinent and responsible for K⁺ efflux from plants’ cells through cation that continues from the plasma membrane (Demidchik et al., 2014). They added that programmed cell death (PCD) is caused by ROS that results from stress-induced EL. Under more stress, ROS increases K⁺ efflux through guard cell outward-rectifying K⁺ (GORK) channels, and the cells of plants lose so much K⁺ amount, which increases the activity of proteases and endonucleases and supports PCD. EL increased in Brassica juncea as a result of Cd at 200 mg/kg, elevating upon exposure time (Ahmad et al., 2016).

Moreover, the increment of Cd and Pb from 0.5 to 1.0 mM raised EL in A. bettzickiana (Tauqeer et al., 2016). A similar result was found in two genotypes of P. sativum (AG-10 and AP-3) under Cd treatment (Sager et al., 2020). El-Mahrouk et al. (2020) on P. nigra noticed that increasing CdCl₂ from 20 to 80 mg/kg soil and Pb acetate from 250 to 850 mg/kg soil elevated EL, and it increased by increasing HM levels in the soil. Thus, reduction in growth traits, TI, and enzymatic activities may be due to the increase in EL under HM stress. However, under heavy metal stress, an inverse relationship exists between vegetative traits and TI, with an increase in enzyme activities and EL observed simultaneously. Conversely, the decrease in vegetative traits and TI, coupled with the increase in EL, can be ascribed to the detrimental effects of heavy metals, while the rise in enzyme activities signifies a defensive response to heavy metal stress.

Indeed, there is a relationship between the contents of HMs in the plant organs and their soil contents. The HM accumulation and content in different organs of plants have been affected by plant species, metal ion levels and kinds in the soil, soil pH, bioavailability of HMs, and root exudations (Zarubova et al., 2015). Similarly, Samuilov et al. (2016) found that the concentration of Pb in Populus tremula × P. alba organs is related to its content in soil. Various organs of Salix polaris treated with Cd have different Cd levels (Krajcarova et al., 2016). Furthermore, in P. nigra ‘Italica’ treated with Cd (10, 25, and 50 mg/kg soil) and Pb (400, 800, and 1,200 mg/kg soil), its roots have Cd and Pb more than its leaves and stems (Redovnikovic et al., 2017). Additionally, El Mahrouk et al. (2019, 2020) on Salix mucronata and P. nigra, respectively, concluded that the roots of the two species contained Cd and Pb compared to their leaves and stems when treated with CdCl₂ (40–80 mg/kg soil) and Pb acetate (250–850 mg/kg soil). A study by Durante-Yanez et al. (2022) on C. sericea revealed that Cd and Pb contents in the roots were higher than in the leaves or stems, and Cd and Pb levels among various organs were significant. Accordingly, Rahman et al. (2022) showed that the seasonal elements in the leaves or barks of some tree species reduced in the order of Zn > Pb > Cu > Cd in industrial and residential areas, and the tree species significantly indicated various quantities of HM uptake. In addition, Yousefi et al. (2023) pointed out that a higher content of Cd was in the leaves and stems of P. oleracea treated with 25, 50, 75, and 100 mg/L Cd. Significant differences in nickel (Ni), copper (Cu), Cd, and Pb concentrations were observed between contaminated and uncontaminated plant organs of the genotypes of S. alba (347, NS 37/6, and B-44) and S. viminalis (Urošević et al., 2024). They reported that the roots accumulated the largest amounts of Ni and Pb, while Cd was accumulated in the leaves more than other Salix genotypes’ organs. This is supported by Kaur et al. (2024), who reported that S. alba roots accumulated Cd and Pb more than the stems and leaves, in the order of Cd > Pb > Cd + Pb.

In general, the levels of BCF_s, or BCF_r, are relative to the levels of metal, as well as the other HMs present in the soil. The current study showed a decreasing trend in Cd and Pb BCF with the upheaval of their levels in the soil. This was documented by Redovnikovic et al. (2017). Also, the BCF_s of Cd and Pb, when they were applied individually, were rather higher compared to when they were used in combination, having the same levels, except for the BCF_s of Pb; this may be due to the fact that they are being absorbed at lower levels as a result of the antagonism between them. This is in contrast to Cd or Pb BCF_r, with the exception of Cd BCF_r. Conocarpus erectus Cd BCF_s was >1, which matched with the findings of Dominguez et al. (2008), who showed that the Cd BCF of the leaves of P. alba was >1 (nearly 2) and the Cd BCF_s values of P. alba cv. 6K₃ and P. alba cv. 14 P11 were 2.5 and 4, respectively (Zacchini et al., 2009). Also, the Cd BCF_s of P. alba was >1 and the Cd BCF_r values of P. alba and Morus alba were <1 at 40, 80, and 160 mg Cd/kg soil (Rafati et al., 2011). Previously, Migeon et al. (2009) mentioned that Cd BCF values were 1.39, 2.26, and 1.98 for P. deltoides × P. nigra, P. tremula × P. tremuloides, and P. trichocarpa × P. deltoides, in succession. Additionally, the Cd BCF values of poplar (Jakovljevic et al., 2013) and willow and poplar (Algreen et al., 2014) were >1. This was supported by Durante-Yanez et al. (2022), who concluded that the Cd BCF for C. sericea was >1. Our results indicated that Cd BCF_r, Pb BCF_s, and Pb BCF_r values were <1. The findings are in agreement with those of Redovnikovic et al. (2017), who showed that the Cd and Pb BCF values for poplar under 10, 25, and 50 mg Cd/kg soil and 400, 800, and 1200 mg Pb/kg soil were <1, and the Pb BCF reached 0.23 with the highest Pb content. This was supported by Algreen et al. (2014) for the Pb BCF of willow and poplar trees at Danish polluted sites with HMs. Soil pH, EC, and Ca levels significantly affected the bioavailability of Pb (Zhang et al., 2019). Also, the Cd BCF_r and Pb BCF values of the leaves, stems, and roots of S. mucronata and P. nigra grown in soil containing CdCl₂ (20–80 mg/kg) and Pb acetate (250–850 mg/kg) were <1 (El-Mahrouk et al., 2019, 2020).

The movement of metal ions is more related to the efficiency of plants for metal uptake, the movement of metal ions through the intracellular system, metal accumulation in various plant organs, and soil metal level. The Cd and Pb transfer within the plant can be affected by cellar displacement, which affects the free concentrations (Niu et al., 2007; Fuleker et al., 2009; Pourrut et al., 2011). Plant mechanisms such as adjusting membrane permeability, modifying cell wall capacity, or increasing the secretion of chelating compounds play crucial roles in maintaining optimal physiological levels of essential nutrients. These mechanisms help mitigate the toxic effects of HMs on aboveground organs when plants are exposed to such stresses (Kalaivanan and Ganeshamurthy, 2016). The least soil Cd and Pb levels may be a crucial factor for the roots’ actual accumulation quantities, which helps in the transference of metals through detoxification mechanisms and development in the plant obstructing ions in the vacuole through adhesion and union with ligands (proteins, organic acids, and peptides) leading to higher translocation amounts (Nworie and Lin, 2021; Sharma et al., 2021). Similarly, Rafati et al. (2011) treated P. alba and M. alba with 40–160 mg/kg soil, and they found that the Cd TF of the two species was >1. Also, Redovnikovic et al. (2017) found that P. nigra cv. ‘Italica’ grown in soil polluted with 25 and 50 mg Cd/kg and Pb at 800 and 1,200 mg/kg recorded Cd TF >1 and Pb <1. In addition, the Pb TF was <1 for P. nigra and C. sericea grown under various concentrations of Pb (El-Mahrouk et al., 2020; Durante-Yunez et al., 2022). Additionally, the BCF and TF of Cd were higher than those of Pb in all S. alba organs, and the combined utilization of Cd and Pb affected BCF at each application level, demonstrating a negative interaction between Cd and Pb (Kaur et al., 2024). Based on the data related to TI, Cd contents in plant organs, BCF_s, BCF_r, and TF, C. erectus is a suitable species for Cd phytoextraction under all the Cd treatments used because its BCF_s and TF were >1, and it is a phytoextractor for Pb at (L) Pb, (M) Pb, and (M) Cd (M) Pb treatments because its BCF_r was <1 and its TF was >1. Also, C. erectus is a phytostabilizator for Pb at (H) Рb, (L) Cd (L) Pb, and (H) Cd (H) Pb treatments because its Pb BCF_r and TF were <1. These results were supported by the correlation of data among Cd or Pb parameters.

The issue of heavy metal contamination in agricultural soils is a significant challenge encountered by numerous countries globally. Resolving this issue necessitates the utilization of safe and eco-friendly methods, such as phytoremediation, that do not exert adverse effects on the environment. The identification of novel plant species, like C. erectus, is crucial, as it has exhibited notable tolerance to Cd and Pb at levels reaching 60 mg/kg and 700 mg/kg in soil, respectively. Generally, HMs have a detrimental impact on plant development, biochemical structure, and EL due to their accumulation in plant tissues. Conocarpus erectus is considered a strong Cd accumulator because its Cd BCF_s and Cd TF were >1 in all tested Cd levels. Additionally, C. erectus is suitable for Pb phytoextraction at 400 and 700 mg Pb/kg soil and 60 mg Cd + 700 mg Pb/kg soil, as its BCF of the shoots or roots were <1 and its TF were >1. Furthermore, C. erectus is appropriate for Pb phytostabilization at 1,000 mg Pb, 40 mg Cd + 400 mg Pb, and 80 mg Cd + 1000 mg Pb/kg soil, where the Pb BCF of the shoots or roots and the Pb TF were <1. There is a highly significant positive correlation between vegetative traits and the tolerance index of C. erectus. Conversely, under Cd and Pb levels, the correlation between vegetative traits (which decrease) and enzyme activities or EL (which increased) is highly significantly negative. Moving forward, the forthcoming study should incorporate interventions such as microbe-assisted techniques, organic amendments, biochar application, and natural extracts to enhance the tolerance mechanisms of plant species and to understand the influences of HMs on the foliage and xylem structure as well as the chemical compositions such as tannins and phenols found in the wood of C. erectus. Moreover, the research should ascertain the quantities of cellulose, hemicellulose, lignin, and ash present under the influence of HM stress.