Lemon balm (Melissa officinalis L.), an important medicinal herb belonging to the family Lamiaceae, has been used as far back as the Middle Ages to improve human health and to cure various disease [1]. It has now been proven that a number of the medicinal properties of this plant are due to the presence of polyphenolic constituents, the most prominent of which is rosmarinic acid (RA) [2]. Arbuscular mycorrhizal fungi (AMF) are the oldest and most prevalent mutualistic symbiosis [3]. Association of AMF with medicinally important plants not only promotes the plant growth, but also improves the productivity of valuable secondary metabolites like phenolic compounds [4]. The purpose of this study was to examine the effect of AMF on the content of some nutritional/pharmaceutical phenolic compounds in lemon balm. Experiments were carried out using a randomized complete block design in three replications included four mycorrhizal treatments [un-inoculated plants, plants inoculated with Glomus mosseae, plants inoculated with G. intraradices and plants inoculated with mixed inoculum (G. mosseae + G. intraradices)]. Seeds were sown in a mixture of soil: fungal inoculum (10:1 w/w). The percentage of root colonization was measured in four months old plants. Extraction of phenolic compounds was performed by methanol (80%) in an ultrasonic bath. The total contents of phenols (TP), flavonoids (TF), anthocyanins (TAC) and phenolic acids (TPA) in the extracts were measured using the spectrophotometric method. The concentrations of RA and salvianolic acid B (SAL B) in the extracts were also determined by HPLC method. The highest percentage of colonization (59/697%) was observed in the roots of plants inoculated with G. mosseae. Treatment of the plants with G. mosseae, G. intraradices and mixed inoculum (G. mosseae + G. intraradices) resulted to the increased levels of all examined phenolic compounds. The maximum amounts of TP (51/705 mg GAE/g Dw), TF (14/795 mg QE/g Dw), TAC (21/121 µM/g Fw) and TPA (0/05385 mg RAE/g Dw) as well as RA (5/196 mg RA/g Dw) and SAL B (7/365 mg SAL B /g Dw) were found in the plants treated with G. mosseae. Our results showed that successful colonization of M. officinalis with AMF increased the amounts of health promoting-phenolic compounds that improved the nutritional and medicinal values of the plant for human health.

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