Physiological and metabolic responses of dill to nano-silica and nano-zinc under drought stress: implications for yield and essential oil quality

Drought stress severely limits the productivity of medicinal and aromatic plants by disrupting physiological processes and altering secondary metabolism. This study investigated the potential of nano-silica (Si) and nano-zinc (Zn) to improve drought resilience and essential oil quality in dill ( Anethum graveolens L.) across two growing seasons (2023–2024). The experiment was conducted in a split-plot design with two irrigation regimes (well-watered and drought stress at flowering stage) and six nano-fertilizer treatments (control, Si25, Si100, Zn25, Zn50, Zn100). Nanoparticles were synthesized via sol–gel (Si) and precipitation (Zn) methods and characterized using DLS, zeta potential, SEM, XRD, and FTIR analyses. Results showed that drought stress significantly reduced chlorophyll retention, photosynthetic efficiency, root biomass, and seed yield, while increasing oxidative damage indicators such as malondialdehyde (MDA), electrolyte leakage (EL), hydrogen peroxide (H₂O₂), and superoxide radicals (O₂•⁻). Application of nano-silica and nano-zinc markedly mitigated these negative effects, with higher doses (Si100 and Zn100) being most effective. Treated plants exhibited enhanced SPAD index, photosynthetic rate, and root growth, alongside elevated proline accumulation and reduced oxidative stress markers, thereby preserving membrane stability and sustaining photosynthetic function. These improvements translated into significantly higher seed yield and harvest index under both irrigation regimes. In addition to primary physiology, nano-fertilizer treatments reprogrammed secondary metabolism, enhancing the accumulation of key monoterpenes (α-pinene, limonene, dill ether) and phenylpropanoids (carvacrol, apiole) in essential oil profiles, particularly under drought stress. The cumulative effects were more pronounced in the second year, highlighting stable and seasonally reinforced benefits of nano-fertilizer application. Overall, this study demonstrates that nano-silica and nano-zinc act through multifaceted mechanisms—including structural reinforcement, osmotic adjustment, antioxidant activation, and metabolic reprogramming—to alleviate drought-induced damage while improving yield and essential oil quality in dill. These findings underscore the dual agronomic and industrial value of nano-fertilizers as sustainable tools for enhancing resilience and product quality in climate-sensitive crops.