Exogenous pipecolic acid ameliorates lead-induced phytotoxicity in chili (Capsicum annuum L.) by enhancing antioxidant capacity and physiological homeostasis.

Heavy metal toxicity disrupts plant cellular homeostasis by inducing oxidative stress and impairing the key signaling networks that regulate plant growth and defense. Small signaling metabolites, such as pipecolic acid (PipA), a known regulator of systemic acquired resistance, may play a critical role in modulating plant responses to abiotic stresses. This study investigated the signaling and physiological roles of exogenously applied PipA in mitigating lead (Pb)-induced phytotoxicity in chili (Capsicum annuum L.). Eight treatments comprising an absolute control, Pb stress alone (800  mg kg-1 soil) and single or combined PipA applications (0.5  mM via seed priming, soil drenching, and foliar spray) were administered in homogenized soil media. Lead stress alone drastically impaired plant performance, as the root length decreased by 45.9%, shoot length decreased by 45.8%, total yield decreased by 46.7%, relative water content (RWC) decreased by 32.11%, membrane stability index (MSI) decreased by 42.07%, photosynthetic rate (Pn) decreased by 56.86%, stomatal conductance (gs) decreased by 57.01%, malondialdehyde (MDA) concentration increased by 237.92%, hydrogen peroxide (H₂O₂) accumulation increased by 260.71%, fruit length decreased by 48.38%, vitamin C concentration decreased by 39.92%, and root lead accumulation increased by 73525.21%. In contrast, combined PipA applications significantly reversed these deteriorations, particularly those resulting from seed priming and foliar spray, which increased the root length by 35.94%, enhanced Pn by 105.36%, elevated the gs by 113.76%, and reduced the contents of MDA by 60.54 and also reduced the form of accumulated H2O2 by 63.37% relative to that in stressed plants. Dual delivery systems are synergistic in activating antioxidant enzymes, stabilizing membranes, optimizing gas exchange, and minimizing heavy metal translocation. PipA supplementation showed beneficial effects on reprogramming stress-responsive pathways and holds promise for enhancing crop resilience under chronic metal stress conditions. Reclaiming (polluted) agroecosystems should be accomplished through the adoption of integrated bioremediation protocols using exogenous PipA. Collectively, the evidence suggests that combining both exogenous PipA application methods (initial treatment via seed priming and subsequent treatment via foliage) is a viable, biologically compatible, and practical/flexible way to overcome Pb-induced physiological effects on crop yield in contaminated agricultural ecosystems.