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Abstract

Silver nanoparticles (AgNPs) have been increasingly investigated as nano-elicitors capable of modulating plant secondary metabolism; however, their role in regulating terpenoid biosynthesis in aromatic grasses remains insufficiently understood. This study evaluated the effects of green-synthesised AgNPs on secondary metabolite accumulation, gene expression, and terpene synthase (TPS) activity in Cymbopogon nardus. Plants were treated with AgNPs (0, 50, 100 and 150 mg), with NPK (150 mg) used as a fertiliser control. GCMS analysis revealed substantial alterations in monoterpenoid composition at 150 mg AgNPs. Geraniol content increased from 1.02% in control plants to 2.68% (+162.7%), while 3,7-dimethyl-1,6-octadien-3-ol markedly increased from 1.92% to 12.97% (approximately 6.7-fold). Limonene (9.81%) was prominently induced under AgNP treatment but was not dominant in control plants. In contrast, NPK treatment showed a different metabolic profile dominated by fatty acid methyl esters. At the molecular level, AgNP application induced concentration-dependent upregulation of key MEP pathway genes. CnDXR showed the strongest induction (+82.5%), followed by CnDXS (+63.2%), CnGES (+59.4%), and CnGPPS (+32.1%) compared with control plants. Consistently, TPS enzyme activity increased from 2.85 ± 0.21 U/mL to 6.12 ± 0.44 U/mL (+114.7%), exceeding NPK treatment (4.10 ± 0.31 U/mL). Collectively, AgNP treatment at 150 mg significantly enhanced monoterpenoid accumulation through coordinated transcriptional activation and enzymatic stimulation of the terpenoid biosynthesis pathway. These findings demonstrate that optimised AgNP application functions as an effective nano-elicitor to improve essential oil productivity in Cymbopogon nardus.

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