Ameliorative effect of green-synthesized zinc oxide nanoparticles in <i>Brassica juncea</i> under lead (Pb) stress

Lead (Pb) is one of the most toxic heavy metals and can disrupt morphological, physiological, and biochemical processes in plants. In this study, for the first time, we present a green-synthesis approach for zinc oxide nanoparticles (ZnO NPs) using Rumex dendatus leaf extract and demonstrate their efficiency in ameliorating Pb-induced toxicity in Brassica juncea (Brown mustard). The green ZnO NPs were spherical, with an average crystalline diameter of ~7.5 nm, showed a characteristic UV‒Vis absorption peak at ~350 nm, and consisted of zinc (58.58%) and oxygen (41.42%) by energy-dispersive X-ray analysis. In the pot experiment, thirteen-day-old B. juncea seedlings were exposed to Pb (30, 300, and 3000 mg L−1) and ZnO NPs (2, 20, and 200 mg L−1), individually or in combination, under controlled conditions. Exposure to Pb at 3000 mg L−1 markedly inhibited plant growth and physiology, reducing root/shoot lengths and chlorophyll content by 22%, 18%, and 29%, respectively, compared with untreated controls. In contrast, ZnO NP treatment at 200 mg L−1 significantly enhanced plant growth and related traits. Notably, combined ZnO NP + Pb treatment mitigated Pb toxicity: co-treated plants showed improved root length (+18%), shoot length (+17%), fresh weight (+19%), and leaf chlorophyll content (+24%) compared with Pb-treated plants. Pb accumulation was also reduced, with root and shoot Pb uptake decreasing by 57% and 43%, respectively, relative to Pb-stressed plants. Antioxidant enzymes activities were elevated; SOD and CAT increased several-fold in ZnO NP + Pb-treated plants compared with Pb-stressed plants, indicating enhanced ROS scavenging. APX and POD levels were similarly boosted, contributing to reduced oxidative damage. Overall, green-synthesized ZnO NPs promoted B. juncea growth under Pb stress by enhancing chlorophyll content, reducing Pb uptake, and suppressing ROS-associated toxicity.