Introduction

Anise hyssop (Agastache foeniculum L.) is a medicinal and perennial herb plant belonging to the Lamiaceae family. Anise hyssop essential oil has antioxidant, anti-inflammatory, antifungal and antibacterial properties and is used in pharmaceutical and food industries. Drought is a major environmental challenge that seriously affects the productivity of medicinal and aromatic plants. Given the severe limitations of water resources in Iran, increasing productivity and optimizing water consumption in the agricultural sector are of great importance. Accordingly, this study investigated the effects of deficit irrigation on the morphophysiological and phytochemical characteristics of the Anise hyssop, aiming to achieve an optimal balance between crop yield, and water conservation.

Materials and Methods

This study was conducted at the Research Farm of Urmia University using a randomized complete block design with four replications. To prepare the land, plots measuring 2 × 2 m were established, with 1.5 m spacing between plots and 3 m between blocks. Seedlings produced in the greenhouse were transplanted to the field in early June and planted in rows spaced 30 cm apart, with 25 cm between plants within each row. Deficit irrigation treatments were applied by omitting one irrigation event at different growth stages as follows: (1) full irrigation during the growth period (control); (2) irrigation cut-off at complete plant establishment stage (20 leaves); (3) irrigation cut-off at inflorescence emergence stage; (4) irrigation cut-off at flowering stage; and (5) irrigation cut-off at both the establishment and inflorescence emergence stages. Plants were sampled at the full flowering stage to measure growth parameters (plant height, stem diameter, leaf number, number of lateral branches, total length of lateral branches, inflorscence length, fresh and dry weight of leaves and stems, fresh and dry herb yield) chlorophyll index (SPAD), total phenol and flavonoid contents, antioxidant capacity, essential oil content and yield, essential oil composition, and leaf nutrient (nitrogen, potassium and phosphorus) concentration. Essential oil was extracted from shade-dried samples by hydrodistillation using a Clevenger apparatus. Essential oil analysis was performed using gas chromatography (GC) and gas chromatography coupled with mass spectrometry (GC/MS).

Results and Discussion

The results showed that the deficit irrigation treatments significantly reduced inflorescence length, fresh and dry weight of leaves and stems, fresh and dry herb yield, chlorophyll and leaf nitrogen content. The highest fresh and dry herb yield (6322.1 and 2054.1 kg ha-1, respectively) and lowest (3039.5 and 973.4 kg ha-1, respectively) were obtained in the control (full irrigation) and irrigation cut-off at both the establishment and inflorescence emergence stages, respectively. The reduction in plant growth indices and yield can be attributed to factors such as decreased cell turgor, impaired cell division and growth, reduced photosynthesis due to stomatal closure, degradation of leaf pigments, hormonal imbalance, and cellular damage resulting from oxidative stress. Total phenolic content, antioxidant activity, and essential oil content increased significantly in response to the deficit irrigation treatments. The highest (7.57 mg GAE g-1 DW) and lowest (6.4 mg GAE g-1 DW) total phenolic content were obtained in the irrigation cut-off at both the establishment and inflorescence emergence stages and full irrigation, respectively. There were no significant differences between the treatments in terms of plant height, stem diameter, leaf number, total flavonoid content, and leaf potassium and phosphorus concentrations. Although irrigation cut-off at different growth stages reduced the essential oil yield, the differences between the full irrigation treatment (47.06 L ha⁻¹) and the irrigation cut-off treatments at the plant establishment stage (38.91 L ha⁻¹) and the flowering stage (40.73 L ha⁻¹) were not significant. According to the results of essential oil analysis, seven different compounds including sabinene, β-pinene, limonene, linalool, methyl chavicol, E-caryophyllene, and germacrene D were identified in the essential oil. Methyl chavicol (92.46-95.02%) and limonene (3.34-5.69%) were identified as the main components of the essential oil, and their concentrations did not change significantly under different levels of deficit irrigation.

Conclusion

In conclusion, although irrigation cut-off caused a decrease in essential oil yield, this reduction was not significant during the plant establishment and flowering stages. Therefore, irrigation cut-off at these stages, particularly during the flowering stage, which coincides with summer water scarcity can be considered as a beneficial strategy for water management and conservation. Additionally, due to the lower growth of plants under water deficit conditions, increasing plant density can be recommended to compensate for the decrease in essential oil yield under water shortage conditions.