Citric Acid Optimizes Lead (Pb) Phytoextraction in Mung Bean (<i>Vigna radiata</i> (L.) Wilczek) by Regulating Nutrient Uptake and Photosynthesis

The low efficiency of phytoextraction of lead (Pb) from agricultural fields poses a significant agricultural challenge. Organic chelating agents can influence Pb bioavailability in soil, affecting its uptake, transport, and toxicity in plants. This study aimed to assess the impact of citric acid (CA) and diethylenetriaminepentaacetic acid (DTPA) on chelate-assisted phytoextraction of Pb and its effect on growth and physiology of two cultivars (07001; 07002) of mung bean (Vigna radiata). The cultivars of mung bean were exposed to 60 mg·L−1 lead chloride (PbCl2) solution, with or without the addition of 300 mg·L−1 CA or 500 mg·L−1 DTPA, until maturity. The exposure of plants to Pb stress increased the accumulation of Pb in roots (49% of control), stems (58% of control), leaves (67% of control), and seeds (61% of control). Maximum accumulation of Pb was observed in roots and the least accumulation was found in seeds of both mung bean cultivars. The extent of Pb accumulation in different plant parts correlated positively with Pb toxicity and reduced growth of both mung bean cultivars (33% to 40%). The cultivar cv 07001 was more susceptible to Pb stress. The addition of CA and DTPA increased the accumulation of Pb in plant parts of mung bean cultivars-phytoextraction (10.8% to 21.5%). However, the addition of CA partitioned Pb in vegetative parts, i.e., root, stem thus mitigated the toxic effects of Pb on the growth of mung bean cultivars (6.25%–10.5%). In contrast, the addition of DTPA had adverse effects on the growth of mung bean cultivars. The addition of CA facilitated a greater uptake and accumulation of nitrogen, phosphorous, and potassium in the roots and leaves of mung bean cultivars. In addition, CA also improved the photosynthetic pigments (11%–14%) and photosynthetic rate (5%–12%) under both control and Pb stress conditions. The ameliorative effect of CA on the photosynthetic capacity of mung bean cultivars was likely associated with photosynthetic metabolic factors rather than stomatal factors. Furthermore, cv 07002 was found to be more tolerant to Pb stress and showed better performance in CA application. Overall, the application of CA demonstrated significant potential as a chelating agent for remediating Pb-contaminated soil.