Physiological and biochemical responses of Italian basil to lead stress following salicylic acid and silicon-based treatments

Lead (Pb) pollution imposes severe constraints on the growth, physiology, and metabolic stability of medicinal plants. In this investigation, a silica-salicylic acid nanocomplex (Si-SA NCs) was synthesized and physicochemically characterized using zeta potential analysis, dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM), and its protective effect against Pb stress was assessed using Italian basil (Ocimum basilicum L.). Plants were treated with Si-SA NCs, potassium silicate, and salicylic acid (SA) and subjected to Pb stress (1000 mg kg⁻¹ soil). Lead toxicity significantly decreased growth parameters, biomass production, mineral balance, membrane stability, and photosynthetic pigment concentration, and increased oxidative stress. Application of SA- and silicon-based treatments significantly alleviated Pb-induced damage, with treated plants exhibiting improved plant height, root development, leaf area, fresh and dry biomass, and enhanced uptake of Silicon (Si), calcium (Ca), and potassium (K). Improvements in relative water content and membrane stability were closely associated with increased activities of antioxidant enzymes, including guaiacol peroxidase, polyphenol oxidase, catalase, and ascorbate peroxidase, particularly under Pb stress conditions. In addition, treated plants showed elevated levels of phenolic and flavonoid compounds, along with increased essential oil percentage and yield, accompanied by shifts in major oil constituents. Multivariate analyses revealed coordinated improvements in growth performance and biochemical resilience particularly in response to Si–SA NCs application. In general, the results show that Si-SA NCs, together with individual SA and Si treatments, are effective in reducing lead-induced phytotoxicity by improving physiological performance, mineral homeostasis, and antioxidant biochemical defenses.