The toxic heavy metals, resulting from human activities and industrial processes, are often released or leached into the environment, causing detrimental effects to the ecosystems [1,2]. Among these heavy metals, lead (Pb) is particularly hazardous as it hinders metabolic processes, impedes plant growth, and reduces productivity [1,3,4]. In mungbean and other plant species, Pb toxicity reduces plant height, root-to-shoot ratio, dry weight, number of nodules per plant, and chlorophyll content [5] due to the inhibition of cell division and differentiation [6]. The Pb toxicity disrupts water balance, mineral nutrition, and membrane structure while impeding photosynthetic process and enzyme activity and inhibiting cell division [7–10]. Additionally, Pb presents significant health hazards to both children and adults through the transmission of contaminated food chains [11].

Plants have developed a range of tolerance strategies to Pb stress, including mechanisms for Pb exclusion from the cytosol, reduction of Pb uptake, and the chelating or binding of Pb to various thiol compounds in the cytosol, such as glutathione, phytochelatins, and metallothioneins [2,12]. These compounds are subsequently sequestered into inactive organelles. Plants also detoxify the Pb-induced reactive oxygen species (ROS) to mitigate the detrimental effects [2].

Various strategies have been proposed for improving or restoring soil contamination with Pb, including physical, chemical, and biological strategies. Phytoremediation is a biological strategy in which hyperaccumulator plants of heavy metals are used, which is a feasible, and and cost-effectiveness strategy. The effectiveness of phytoremediation is limited by several factors [13,14]. For instance, plant biology restricts the remediation process primarily to the rhizosphere at shallow depths. Moreover, the concentration of heavy metals in the soil should be maintained at non-excessive levels due to their toxicity and limited bioavailability [15]. The exploration of alternative approaches to enhance phytoremediation is necessary due to the limitations of traditional methods. These limitations include long remediation times, variations in soil pH that influence the concentration and bioavailability of metal ions, limited understanding of plant behavior and their potential in phytoremediation, and challenges in managing plant crops [1,2,13,15].

Recent studies highlighted the significance of enhancing phytoremediation efficiency by utilizing chelating agents to improve metal removal across different plant species [2,16]. Chelating agents, such as ethylene diamine disuccinic acid (EDDS), ethylene diamine tetraacetate (EDTA), diethelene tri-amine penta-acetic acid (DTPA) and nitrilotriacetic acid (NTA), promote the uptake of heavy metals in plants by increasing their solubility [17,18]. It is important to carefully regulate the application rate of chelating agents to monitor and mitigate potential environmental risks [19]. Furthermore, the promoting effect of EDTA exhibits selectivity towards certain metal elements [17,18]. For instance, EDTA exhibits high complexation constants with metals such as Cu, Ni, Pb, Zn, and Cd, thereby facilitating the activation of heavy metals in the soil [17]. Similarly, the addition of EDTA in the growth medium increases the concentration of water-soluble Cd and Pb in soil, facilitating their uptake by rapeseed, corn, and wheat [2,17,20]. However, high concentrations of EDTA-Pb complex were found to reduce the transpiration rate of Indian mustard (Brassica juncea L.) and shoot water content, attributed to the presence of free protonated EDTA (H-EDTA) [21–23]. Moreover, EDTA poses environmental concerns due to its poor biodegradability and potential soil-related risks. Among naturally occurring organic acids, CA has been extensively used in chemically enhanced phytoextraction processes and is considered a potential alternative to EDTA [24–26]. Similarly, Evangelou et al. [20] also supported the use of EDTA alternatives but biodegradable chelating agents for enhancing the phytoremediation capacity of plants. However, the comparative performance of different chelating agents in different plant species for phytoextraction of Pb is poorly understood. Additionally, the potential environmental risks associated with the use of these agents require further investigation. Therefore, it is imperative to understand the mechanisms of CA- or EDTA-assisted changes in Pb uptake by plants.

Mung bean is an important pulse crop for resource-poor farmers due to its potential ability to enhance soil fertility through symbiotic nitrogen fixation [27]. Mung bean has a short growing period, and it is important in intensive crop production. Given the economic importance of mung bean and the role of chelating agents in enhancing phytoremediation, we hypothesized that different chelating agents with a leguminous crop of mung bean have distinct effects on Pb phytoextraction. The role of CA and EDTA in facilitating Pb uptake through roots and its subsequent transport to different parts of plants of mung beans needs to be assessed. Therefore, the present study aimed to assess and compare the effectiveness of chelating agents (DPTA and citric acid) in enhancing phytoextraction of Pb in mung bean. In addition, the secondary objective of the study was to evaluate the extent to which DTPA and CA modulate the different physiological and biochemical processes, thereby affecting the growth and yield of mung bean. This study will help in the development of effective and environmentally sustainable remediation strategies for heavy metal-contaminated soils.

A pot experiment was conducted to evaluate the potential of two chelating agents (DTPA) and Citric acid (CA) in reducing Pb toxicity in two cultivars of mung bean (Vigna radiata) (07001; 07002). This experiment was carried out in the net house at the Botanical Garden of the University of Agriculture, Faisalabad. Seeds of two cultivars of mung bean were obtained from Ayub Agriculture Research Institute (AARI), Faisalabad. Ten seeds of each cultivar were sown in each pot lined with polyethylene bags (15 cm × 20 cm; diameter × depth) filled with 11 kg soil containing ¼ of green manure. After one week of seed germination, seedlings were thinned to five seedlings per pot. Selected seedlings were equal in size and equally placed in pots. This experiment was laid out in a Completely Randomized Design (CRD) with three replicates. The application of lead and chelating agents was carried out four weeks after seed germination. There were the following six treatments details of which are given below in Table 1.

Two liters of each treatment solution were applied to the respective treatment pot. Plants were allowed to grow for a further three weeks. Three plants from each pot were harvested, separated into shoots and roots and their fresh weights were recorded. Plant parts were oven-dried at 70°C for 48 h and their dry biomass was recorded. Two plants of each cultivar in each pot were allowed to grow till yield and yield attributes, i.e., number of pods per plant, number of seeds per plant, and 1000-seed weight were determined. Before harvesting plants, the following physiological attributes were measured.

The Pb toxicity poses a significant threat to plant growth and crop yield, leading to detrimental effects on final agricultural productivity. The severity of lead toxicity differs among various crop species, with certain crops exhibiting higher tolerance or resistance compared to others. The current study demonstrates that the addition of Pb in the growth medium inhibited the growth of both cultivars of mung bean (Vigna radiata), which is consistent with findings from previous research on different crops [31,32]. Exogenous application of chelating agents has been found to enhance plant capabilities in phytoextraction of heavy metals by improving ion mobility, solubility, and bioavailability during soil solution and root uptake [19,21,24,33]. The present study indicates that the addition of DTPA in Pb-contaminated soil (60 mg Pb + 0.5 g DTPA·L⁻¹) resulted in decreased plant growth and photosynthetic pigments in both mung bean cultivars. Moreover, the adverse effect of DTPA application was more pronounced in mung bean plants compared to those treated only with 60 mg Pb·L⁻¹, which indicates that the addition of DTPA did not enhance the phytoextraction capacity of mung bean cultivars. These results are in contrast with a previous study that reported enhancement of phytoextraction capacity in plants through the use of DPTA as a chelator [34]. The application of DTPA resulted in a 9.5 times increase in Pb accumulation in the shoots of maize plants [35]. Similarly, the addition of DTPA enhanced a 2.09-fold increase in Pb accumulation in the leaves of Lolium perenne [36]. The detrimental impact of DPTA on the growth of both mung bean cultivars can be attributed to the high mobilization of Pb in the soil induced by DPTA, which resulted in decreased plant growth and phytoextraction efficiency [34,36].

Natural organic chelating agents such as citric acid (CA) are biodegradable in soil and can form complexes with metals of varying stability. This property enhances plant tolerance to heavy metals, making them preferable to synthetic chelators for assisted phytoremediation initiatives. In this study, CA application improved the growth and chlorophyll content in both mung bean cultivars. The 07002 cultivars showed better growth compared to that of the 07001 cultivars as a result of CA treatments under Pb stress conditions. These results align with the findings of Ghnaya et al. [37], who demonstrated that CA has the potential to translocate and accumulate metals in the shoot, suggesting its potential for for remediating Pb-polluted soils. In addition, growth improvement and enhancement in chlorophyll content of mung bean cultivars is attributed to the antioxidant properties of CA, which helps in mitigating oxidative stress [38].

Heavy metal stress, particularly Pb stress reduces plant growth and yield by affecting key physiological processes such as photosynthesis. The photosynthetic capacity of plants can be assessed as net CO₂ assimilation rate, stomatal conductance, transpiration rate, and chlorophyll biosynthesis. In the present study, Pb stress reduced gas exchange attributes, including net CO₂ assimilation rate, stomatal conductance, transpiration rate, and water use efficiency in the leaves of both mung bean cultivars. Similar results were reported in a previous study [39]. The net assimilation rate of CO₂ decreased in both mung bean cultivars under pb stress; however, the addition of CA alleviated the adverse effects of Pb, resulting in higher net CO₂ assimilation rates. The addition of CA enhanced the stomatal conductance in both mung bean cultivars under Pb stress, thereby increasing the CO₂ fixation, as previously observed in wheat [40,41].

The mineral nutrition is essential for the normal growth and development of plants. These minerals play crucial roles in essential physiological and biochemical processes, such as photosynthesis, respiration, protein synthesis, and hormone regulation. The Pb stress causes excessive accumulation of Pb in roots, stems, and leaves, which disrupts the uptake and transport of essential mineral nutrients such as K⁺ and Ca²⁺. This disruption results in nutrient imbalances and deficiencies [1,3,4,12]. In the present study, Pb stress reduced the nutrient uptake and accumulation in all plant parts including roots, stems, and leaves. The addition of CA in the growth medium significantly enhanced the mineral nutrition status of both mung bean cultivars. However, the addition of DPTA had minor effects on the mineral nutrition status of both mung bean cultivars. These findings are similar to those of previous studies indicating that Pb stress reduced the concentration of potassium and phosphorus in cowpea, rice, and maize, while the presence of DTPA or CA increased the uptake of Pb in barley shoots, leaves, and stems [42–44].

Grejtovský et al. [45] observed that the presence of DTPA significantly increased the concentration of Pb in barley shoots, leaves, and stems, while the absence of DTPA resulted in a minimal effect of Pb. It was concluded that chelating agents can increase the transfer of Pb to plants and increase its uptake. In mung bean plants, Pb toxicity reduced the physiological and biochemical physio-biochemical activities of plants, such as photosynthetic pigments [46]; however, CA application improved it in Helianthus annuus under similar conditions [47].

The yield and yield attributes of mung bean are important indicators of its productivity. Pb stress reduced the mung bean yield by impairing various yield attributes associated with physiological processes such as floral development, fertilization, number of pods, number of seeds per pod, and size of seeds. In this study, Pb stress reduced the yield of both mung bean cultivars. However, the addition of CA enhanced the yield and yield attributes in mung bean cultivars. However, the application of DPTA to mung bean plants grown in Pb-contaminated soil caused a further decrease in yield. The observed yield reduction was positively associated with a high accumulation of Pb in leaves reduced amount of photosynthetic pigments and lower rate of CO₂ assimilation rates. High concentrations of Pb have been associated with reduced photosynthesis and a significant decline in all yield parameters of rice and other crops, as reported by various earlier studies [1–3,20,46]. Understanding the mechanisms underlying the negative impact of Pb on mung bean is essential for developing strategies to mitigate the harmful effects of Pb stress and improve the yield potential of mung bean crops.

Lead (Pb) stress caused a significant increase in Pb accumulation in all plant parts which caused a reduction in growth and yield. Application of CA modulated the uptake and transport of Pb in seeds and leaves. Application of CA alleviated the Pb toxicity on plant growth of mung bean plants by having a positive effect on nutrient uptake (N, P, and K), photosynthetic pigments, and photosynthetic rate by regulation of stomatal conductance and transpiration rate. However, the application of DTPA showed a more toxic effect on mung bean as it poorly regulates the nutrient uptake and photosynthesis of plants. The cv. 07001 is more susceptible to Pb stress. In addition, cv. 07002 was found to be more tolerant to Pb stress and showed better performance in the CA application. The application of CA can be considered as a potential chelating agent for the remediation of Pb-contaminated soil.