Foliar-applied silicon and zinc nanoparticles improve plant growth, biochemical attributes, and essential oil profile of fennel (Foeniculum vulgare) under different irrigation regimes

The comparative efficacy of silicon (Si) and zinc (Zn) nanoparticles (NPs) in mitigating drought stress in fennel (Foeniculum vulgare ) remains largely unexplored. This study evaluated the impact of Si NPs and Zn NPs on enhancing plant growth and physiological-biochemical attributes of fennel under varying irrigation regimes. The 2-year study was a split-pot design with irrigation at three irrigation levels (100, 75, and 50% field capacity, FC) and five treatments of foliar application of Si and Zn NPs (control, 1mM Si NP, 2mM Si NP, 1mM Zn NP, 2mM Zn NP). Results showed that drought stress reduced plant performance. Increases in superoxide dismutase (SOD, 131%) and catalase (CAT, 276%) were seen after a 50% FC drought without the use of Si and Zn NPs. Conversely, biological yield (34%), seed yield (44%), chlorophyll a +b (26%), relative water content (RWC, 21%), and essential oil (EO) yield (50%) were all reduced. However, application of Zn and Si, particularly 1mM Si and 2mM Zn, greatly mitigated drought stress via lowering CAT and SOD activity and enhancing plant yield, chlorophyll content, RWC, and EO. The composition of the EO consisted primarily of anethole, followed by limonene, fenchone, and estragole. During drought conditions, monoterpene hydrocarbons increased while oxygenated monoterpenes decreased. The opposite trend was observed for Si and Zn NPs. Our results suggest that applying Zn NPs at 2mM followed by Si NPs at 1mM improved plant resilience and EO yield in fennel plants under water stress.