Lemon balm (Melissa ofcinalis) is a vital industrial and medicinal plant widely utilized in pharmaceutical, food, and cosmetic industries due to its rich bioactive metabolites, including terpenoids and phenolic compounds, as well as its diverse therapeutic properties. This study examined the effects of silicon nanoparticles (Si-NPs; 300 mg/L foliar spray), superabsorbent polymer (SAP; 1 g/kg soil), and drought stress (75% feld capacity) on the phytochemical, physiological, and growth parameters of lemon balm under greenhouse conditions. Conducted over 2023–2024 at Arak University’s medicinal plants department, the experiment followed a randomized complete block design with eight treatments and three replicates. Drought stress signifcantly reduced chlorophyll *a* (0.74 mg/gFW) and total chlorophyll content (1.00 mg/gFW), while Si-NPs and SAP mitigated these effects by enhancing chlorophyll levels, stem height, and diameter while reducing electrolyte leakage and proline accumulation. Conversely, drought stress elevated electrolyte leakage (32.45%) and proline (1.26 µmol/gFW). The combined Si-NP and SAP treatment yielded the highest flavonoid concentrations (6.76 mg QE/g DW) and antioxidant activity (59.37%). Notably, Si-NPs and SAP alleviated drought-induced morphological and physiological impairments. Essential oil content peaked (0.29%) under drought stress, with geranial and neral identifed as primary constituents. The control and SAP treatments produced the highest geranial content (51.53%), whereas SAP alone maximized neral (34.95%). Si-NPs elevated linalool acetate (5.71%), and drought stress increased isopulegol (11.16%). These fndings demonstrate the efcacy of Si-NPs and SAP in enhancing lemon balm’s drought tolerance and phytochemical yield.
