Foliar application of citric acid alleviates lead toxicity and enhances physiological resilience in tomato seedlings

Contamination of heavy metals in agricultural soils, particularly with lead (Pb), poses a severe hazard to ecosystems, crop production, and food safety. Although citric acid has been proposed as a potential detoxifying agent, its dose-dependent effects on Pb-stressed tomato plants under controlled conditions are not well understood. A hydroponic experiment was conducted at Khulna Agricultural University, Bangladesh, from January to March 2023 to assess the impact of CA application on tomato seedlings under Pb stress. However, Pb stress significantly impaired plant growth, water content, photosynthetic pigments, and ionic contents (Ca²⁺, Mg²⁺) while increasing water loss, electrolyte leakage, and Pb²⁺ content compared to the control condition. In this study, the CA treatment, particularly HM2 + CA2 treatment, showed the most significant improvements compared to HM2 stress only. Results showed that HM2 + CA2 significantly boosted seedling growth compared to HM2 stress only by increasing root and shoot biomass, plant height, root number, and root volume. It also significantly improved relative water content, total chlorophyll, beta-carotene, carotenoids, and Ca²⁺ and Mg²⁺ accumulation in roots and leaves. Additionally, HM2 + CA2 significantly reduced water loss, electrolyte leakage, and Pb²⁺ content in roots and leaves compared to HM2 stress only, demonstrating its strong protective effects under heavy metal stress. Hierarchical clustering, PCA, and correlation analyses showed clear separation between Pb-only and CA-treated plants, with the latter displaying improved growth, pigment levels, nutrient status, and water balance, especially under the higher CA dose. These results highlight citric acid's strong capacity to mitigate Pb stress. However, the study's hydroponic setup and elevated Pb levels represent limitations that necessitate validation under field conditions, and while higher CA concentrations (CA2) were effective, excessive CA use may pose risks of phytotoxicity or nutrient imbalance, highlighting the need for dose optimization. Overall, the findings support organic acids as promising tools for managing heavy metal contamination.