Applications of SiO2 Nanoparticles Enhance Growth and Antioxidant Defense System in Cumin (Cuminum cyminum L.) under Water-deficit Stress.

Drought stress severely reduces cumin (Cuminum cyminum L.) yield. This study investigated the physiological and molecular mechanisms by which foliar silicon dioxide nanoparticles (SiO₂ NPs; 0, 4, 6mM) mitigate drought (60, 40, and 20% field capacity) in two landraces (Isfahan, Semnan). The 4mM dose under 60% FC in Semnan maximized yield (757.6kgha-1), thousand-seed weight (3.31g), and umbels per plant (19.16). Foliar aplication of 4mM SiO2 under 40% field capacity (FC) boosted antioxidant activity (SOD: 16.1 U.mg-1 protein; POD: 21.3 U.mg-1 protein; GST: 50.0 U.mg-1 protein) and reduced oxidative damage (MDA: 2.2 nmol.ml-1). A strong negative correlation was observed between yield and MDA (r ≈ -0.7). At the molecular level, SiO₂ NPs upregulated a coordinated defense network: WRKY TF1 expression increased by 82% under severe drought with NPs, correlating with enhanced antioxidant genes (Cu-Zn SOD 2.2-fold, POD57 3.1-fold, and GST 3.5-fold) and yield. The MAPK1 and MYB48 genes also showed synergistic upregulation of 2.5-fold and 2.1-fold, respectively, under synergistic NP-stress conditions. Crucially, the Semnan landrace exhibited a 40-60% greater fold-change in this gene network than Isfahan, aligning with its superior physiological resilience. The 4mM concentration was optimal for activating this WRKY-MAPK-antioxidant module, particularly in the more responsive Semnan landrace. However, under severe drought (20% FC), 4mM NPs increased MDA by 117% (to 9.7 nmol ml-1), indicating a stress-dependent toxicity threshold. We conclude that SiO₂ NPs mitigate drought by amplifying a genetically encoded transcriptional-antioxidant system, but optimal benefits require precise dose and genotype matching to stress severity.