Foliar Application of ZnO Nanoparticles Enhances Plant Growth, Antioxidant Profile, and Zinc Biofortification in Portulaca oleracea L.

Zinc deficiency in agricultural soils limits food security and the nutritional quality of crops. Purslane (Portulaca oleracea L.) is a leafy vegetable of growing interest as a functional food owing to its high content of omega-3 fatty acids, minerals, and antioxidant compounds. Zinc oxide nanoparticles (ZnO-NPs) are an innovative strategy for agronomic biofortification and for stimulating the synthesis of bioactive compounds. This study evaluated the effect of the foliar application of increasing concentrations of ZnO-NPs (0, 25, 50, 75, 100, and 125 mg L−1) under greenhouse conditions on yield, nutraceutical profile, and zinc accumulation in purslane. The 50 mg L−1 concentration increased fresh biomass (+61.6%) and the biosynthesis of bioactive compounds (+61.4%, +179.6%, +32.7%, and +58.6% for phenols, flavonoids, antioxidant capacity, and vitamin C, respectively), whereas 100 mg L−1 maximized zinc concentration (+162.4%) and soluble protein (+42.4%). The ZnO-NPs also reconfigured the antioxidant system: catalase activity increased (+53.2% at 75 mg L−1), while peroxidase decreased (−88.2%), indicating that the plant partially replaces enzymatic scavenging with the accumulation of non-enzymatic antioxidants, a metabolically less costly defense strategy that simultaneously improves nutraceutical value. ZnO-NPs therefore represent a promising strategy to improve both the productivity and the nutraceutical quality of purslane.