Application of Arbuscular Mycorrhizal Fungus (Rhizophagus intraradices) and Nitrogen Fertilizer under Different Irrigation Regimes on Quantity and Quality Characteristics of Peppermint (Mentha piperita L.)

Water deficit or drought stress is one of the critical abiotic stresses and limiting factors in the productivity of plants, especially in arid and semi-arid regions. In these conditions, the nutrient use efficiency by plant decrease due to the lower mobility of these elements. The application of bio-fertilizers is one of the most important and sustainable strategies for soil fertility management and plant nutrition; in addition to reducing chemical pollution, improves plant growth conditions. The application of arbuscular mycorrhizal fungi (AMF), as bio-fertilizer improves plant nutrients and water uptake and enhances plant resistance to stress conditions leading to improving plant growth and productivity. The objectives were evaluating the effect of different fertilizer sources (chemical fertilizer and AMF as biofertilizer) and harvesting time on the quantity and quality characteristics of peppermint under drought-stress conditions.

To evaluate the effects of different fertilizer sources and harvesting times on the quantity and quality of essential oil in peppermint (Mentha piperita L.) under drought stress conditions, a field experiment was carried out at the Faculty of Agriculture, University of Maragheh, Iran, in 2019. The study followed a split-split plot design based on a randomized complete block design (RCBD) with 36 treatments and three replications. The first factor included three irrigation levels: irrigation after depletion of 30% available water as control, depletion of 50% of available water as mild stress, and depletion of 70% of available water as severe stress. The sub-factor included different fertilizer sources, including control (C), 100% nitrogen fertilizer (NF), arbuscular mycorrhizal fungi (Rhizophagus intraradices) (AMF), 75% NF + AMF, 50% NF + AMF, and 25% NF + AMF. The third factor was harvesting time (first and second harvest). The distance between rows was set to be 4 m, with a plant density of 10 plants per m2. In AM fungi treatments, 80 g of the soil containing mycorrhizal fungi hyphae and the remains of the root and spores (1000 g spore.10-1 g soil) was added to the soil during planting. The aerial parts of peppermint were harvested at 50% flowering stage on the first and second harvests. The data were analyzed using analysis of variance and mean comparison based on the least significant difference (LSD) test with SAS 9.3 statistical software.                                                                                                                          

The results showed that plant height, number of nodes per plant, number of leaves per plant, number of lateral branches per plant, SPAD index, dry matter yield, essential oil content, and yield were significantly affected by the interaction of fertilizer sources x irrigation levels, harvesting time, and harvesting time x irrigation levels. The highest and lowest values of morphological characteristics, leaf greenness, dry matter yield, and essential oil yield of peppermint were achieved under non-stress conditions with the application of 75% nitrogen fertilizer + AMF and severe water stress without fertilization, respectively. The maximum (2.4%) and minimum (1.2%) of essential oil content were observed under mild water stress fertilized with 75% nitrogen fertilizer + AMF and severe water stress without fertilization, respectively. The main essential oil compounds were menthol, menthone, 1,8 cineol, and menthofuran. The highest menthol and menthone content were recorded under mild water stress fertilized with 75% nitrogen fertilizer + AMF and non-stress conditions fertilized with 25% nitrogen fertilizer + AMF. The first harvest showed higher values of morphological characteristics, dry matter yield, essential oil content, and yield compared to the second harvest due to the longer growth period and better growth conditions in the first harvest.

The results demonstrated that the plant height, the number of nods per plant, number of leaves per plant, the number of lateral branches per plant, SPAD index, dry matter yield and essential oil yield decreased significantly with increasing stress levels. In contrast, the application of AMF reduced the adverse effects of water stress, so that in severe water stress conditions (irrigation after depletion of 70% available water), individual and integrative application of AMF with nitrogen fertilizer increased the mentioned traits when compared with control. In addition, the mentioned traits in the first harvest increased by 127.8, 194.6, 159.8, 147.7, 17.7, and 37.9% in comparison with the second harvest. Also, the essential oil content of peppermint is enhanced with increasing water stress to mild stress. So, the essential oil content in mild stress increased by 11.4 and 39.7%, respectively, when compared with non-stress and severe stress. The highest essential oil yield was achieved at the first harvest with the integrative application of 75% NF+ AMF. Also, the major essential oil compounds of peppermint (menthol) was recorded in mild stress integrated with 75% nitrogen fertilizer+ AMF. Generally, considering that the economic purpose of cultivating medicinal plants is extracting the maximum content of secondary metabolites, and since the productivity of peppermint essential oil increased significantly by the integrative application of 75% nitrogen fertilizer+ mycorrhiza in mild stress, it can be suggested as a superior treatment.