This research was conducted in order to investigate the effect of superabsorbent polymer, nano silicon, drought stress and their combined effect on the morphological, physiological and biochemical properties of the Melissa officinalis. This research was conducted as a randomized complete block design (RCBD) with three replications in the the Department of Medicinal Plants, Arak University, during the 2023 and 2024. The experimental treatments included control, drought stress, superabsorbent polymer (SNP), silicon nanoparticles (SiNPs), drought stress-superabsorbent, drought stress-nanosilicon, nanosilicon-superabsorbent, nanosilicon-superabsorbent-drought stress. The drought stress was at the level of 75% of the field capacity, spraying SiNPs was used in a concentration of 300 mg/liter and 2650 kg/ha of SNP. The results showed that drought stress caused a decrease in chlorophyll a and total chlorophyll. SiNPs and SNP increased the mentioned traits along with the height and diameter of the stem and reduced the electrolyte leakage and proline. On the other hand, electrolyte leakage and proline increased under the influence of drought stress. The highest amount of flavonoid and antioxidant activity was observed in SiNPs treatment. Application of SiNPs and SNP in drought stress conditions was effective in reducing the negative effects of drought stress in morphological, physiological and biochemical traits. The highest essential oil content (%0.29) was obtained in the drought stress treatment. Geranial and neral were identified as the main components. The highest percentage of geranial (%51.53) was obtained in control and SNP treatment. The highest percentage of neral (%34.95) was obtained in the SNP and the highest percentage of linalool acetate (%5.71) was obtained in the SiNPs treatment. Also, the highest percentage of Isopulegol (%11.16) was observed in drought stress treatment.
