Effect of Piriformospora indica and Trichoderma harzianum on Morphological Characteristics, Yield and Essential Oil of Peppermint (Mentha piperita) at Different Amount of Phosphorus and Irrigation

In order to investigate the application of Piriformospora indica (Pi) and Trichoderma harzianum (Th) fungi on morphological characteristics, yield and essential oil of Peppermint at different amount of phosphorus fertilizer and irrigation, a split factorial experiment at Research Farm of Sari University of Agricultural Sciences and Natural Resources was conducted. Irrigation treatments as 40 and 80 mm evaporation from the evaporation pan and combination of phosphorus fertilizer (0, 50 and 100% of the plant requirement based on soil analysis) and the application of the fungus (non-application, Pi, Th and Pi + Th) were main and subplots, respectively. Analysis of variance showed that all of the simple effects of three factors were significant on the studied traits (except drought stress on root length, phosphorus and fungi on essential oil percentage). Also, the interaction effects of stress and fertilizer on root length and other interactions only had a significant effect on plant height. Mean comparisons showed that the highest dry weight at levels of drought stress, phosphorous fertilizer and fungus application was observed in 40 mm evaporation, 100 kg ha-1 and combined application of two fungi (1867, 1980 and 1824 kg.ha-1, respectively). The essential oil percentage at 80 mm evaporation was 2.13%, which increased by 33% compared to 40 mm evaporation treatment. Also, the highest yield of essential oil was observed in 80 mm evaporation, 100 kg.ha-1 phosphorus fertilizer and combined application of two fungi (34.1, 37.1 and 34.2 g.m-2, respectively). The value of increasing essential oil yield at considered levels compared with this check treat was 14, 33 and 16%, respectively. Phosphorus fertilizer and single and combined application of two fungi reduced the negative effects of drought stress on plant height of peppermint.