Document Type : Research Article
Authors
Department of Plant Production and Genetics, Faculty of Agriculture, University of Maragheh, Maragheh, Iran
Abstract
Introduction
Currently, due to the adverse effects of climate change and rising global temperatures, agricultural yields are declining, with the most severe impacts occurring in the arid and semi-arid regions of the world. Drought stress is one of the most critical abiotic factors limiting plant growth and productivity in many areas, including Iran. With limited water resources and an average annual rainfall of only 250 mm approximately two-thirds less than the global average (Biglari et al., 2019). Iran is particularly vulnerable. The severity and duration of drought stress cause significant alterations in the ecophysiological, biochemical, and molecular characteristics of plants, adversely affecting both the quantity and quality of their growth and yield (Salehi-Lisar & Bakhshayeshan-Agdam, 2020). Arbuscular mycorrhizal fungi (AMF), as biofertilizers, enhance plant tolerance to various biotic and abiotic stresses, including drought, nutrient deficiency, heavy metal toxicity, and salinity. In medicinal plants, the application of AMF has been shown to improve photosynthesis, enhance water uptake, increase chlorophyll and carotenoid levels, and boost antioxidant enzyme activity under stressful conditions (Varma et al., 2018). Within the framework of sustainable agriculture, the use of nanotechnology-based alternatives to conventional fertilizers is considered a promising strategy to increase crop productivity and help meet the food demands of the rapidly growing global population (Liu et al., 2015). Among nanomaterials, titanium dioxide nanoparticles (TiO₂ NPs) have been recognized as effective stimulants of plant growth (Ingle, 2021). Additionally, intercropping one of the most important sustainable agricultural practices plays a crucial role in enhancing biodiversity and improving overall productivity.
Materials and Methods
This study was carried out at the Research Farm of the Department of Plant Production and Genetics, Faculty of Agriculture, University of Maragheh. The experiment followed a split–split plot design within a randomized complete block design (RCBD) with three replications, conducted over two consecutive years. The main factor consisted of three irrigation regimes: 25% (MAD25), 50% (MAD50), and 75% (MAD75) maximum allowable depletion (MAD) of soil available water (SAW), representing normal irrigation, mild water stress, and severe water stress, respectively. The sub-factor was planting pattern, including peppermint monoculture, sage monoculture, and peppermint–sage intercropping. The sub–sub factor involved fertilizer treatments, including: (i) no fertilizer (control), (ii) TiO₂ nanoparticles (100 mg.L⁻¹), (iii) arbuscular mycorrhizal fungi (AMF) inoculation, and (iv) combined application of AMF and TiO₂ (AMF + TiO₂). Each plot consisted of five planting rows, with inter-row and intra-row spacings of 40 and 25 cm, respectively, for peppermint and sage. The AMF inoculum (Funneliformis mosseae) was obtained from Varian Biotechnological Company, Karaj, Iran. At planting, 80 g of soil containing fungal hyphae (equivalent to 1000 spores per 10 g of soil) was applied to each row. Peppermint and sage seedlings were established at a density of 10 plants.m⁻². The first irrigation was applied immediately after transplanting. To facilitate seedling adaptation and optimal establishment, drought stress treatments were initiated one month after planting. Soil moisture content was monitored using a moisture meter to ensure accurate application of drought stress. At full flowering, dry matter yield, essential oil (EO) content and yield, EO composition, and photosynthetic pigments were measured. Data were tested for normality before analysis. A combined analysis of variance was performed using a two-way split–split plot design in SAS software (version 9.4). Mean comparisons were conducted using the least significant difference (LSD) test at the 5% probability level, and graphical illustrations were generated in Microsoft Excel.
Results and Discussion
The results indicated that sage dry matter yield decreased by 27% and 64.1% under mild and severe drought stress, respectively. Considering the interaction effect of cropping pattern and fertilizer sources, the highest sage dry matter yield (172.7 g m-2) was observed in monoculture with the combined application of AMF + TiO2. The combined application of AMF + TiO2 enhanced sage dry matter yield by 36.1% compared to the control (absence of fertilizer). The highest essential oil (EO) content in sage (1.33%) was obtained under intercropping with the integrated application of AMF + TiO2. Notably, EO content under mild drought stress was 41.2% higher than that under normal irrigation. Chemical analysis of EO compounds revealed that the maximum levels of dominant compounds, namely cis-thujone and 1, 8-cineole, were achieved in intercropping with AMF + TiO2 under both mild and severe stress conditions. The enhanced production of secondary metabolites serves as an effective defense mechanism in medicinal and aromatic plants, mitigating the adverse effects of drought stress. Typically, EO formation and accumulation increase under environmental stress, protecting the plant against excessive light and water deficiency. The observed increase in sage EO with AMF + TiO2 application is associated with improved photosynthetic activity, enhanced absorption and availability of key EO-forming nutrients such as nitrogen and phosphorus, and the development of additional EO-secreting glands.
Conclusion
In summary, the integrative application of arbuscular mycorrhizal fungi )AMF) combined with TiO₂ in the intercropping system of sage with peppermint is highly recommended as a sustainable and effective approach to improve both the yield and quality of essential oils, particularly under drought stress conditions. This strategy not only enhances plant resilience against water deficit but also promotes more efficient nutrient uptake, leading to healthier growth and higher production of valuable secondary metabolites. Implementing this combined approach can serve as a practical and eco-friendly solution for farmers seeking to optimize essential oil production while maintaining soil health and long-term agricultural sustainability.
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