Plant biostimulants such as arbuscular mycorrhizal (AM) fungi and humic substances (HS) can be used as an appropriate alternative to chemical fertilizers, as regards to environmental problems of chemicals. The effects of Funneliformis mosseae as an AM fungus, HS (foliar spray and topdressing application), and chemical fertilizer (NK), separately or interacting, on biochemical responses in Mentha piperita L. plants were investigated under field conditions. Results revealed that the mentioned three factors appear as valid practicable method for improving growth and metabolites of peppermints cultivated in the field even when root colonization of AM fungus does not achieve high rates. The triple interaction between mycorrhizal inoculation, HS application (especially foliar spray), and NK fertilizer treatment induced the maximum accumulation of photosynthetic pigments, starch, soluble sugars, total proteins, proline, total phenolics, and the antioxidants in leaves. However, positive effects of F. mosseae and HS were more than those of the chemical fertilizer. We propose that F. mosseae and foliar spray of HS, particularly in combination can be used as suitable plant biostimulants in peppermint plants under field conditions, which in turn will improve soil health and reduce environmental problems.
