436 Bulgarian Journal of Agricultural Science, 23 (No 3) 2017, 436–442 Agricultural Academy COMPARISON OF SEVERAL ARBUSCULAR MYCORRHIZAL FUNGI AND SWEET MARJORAM (ORIGANUM MAJORANA L.) SYMBIOTIC ASSOCIATIONS IN HEAVY METAL POLLUTED SOIL MARIETA HRISTOZKOVA * ; IRA STANCHEVA; MARIA GENEVA; MADLEN BOYCHINOVA Bulgarian Academy of Sciences, Institute of Plant Physiology and Genetics, Laboratory of Plant Soil Interactions, BG-1113 Sofia, Bulgaria Abstract Hristozkova, M., I. Stancheva, M. Geneva and M. Boychinova, 2017. Comparison of several arbuscular mycorrhi- zal fungi and sweet marjoram (Origanum majorana L.) symbiotic associations in heavy metal polluted soil. Bulg. J. Agric. Sci., 23 (3): 436–442 Mycorrhizal fungi participated in plant nutrient assimilation, root development and metal absorption from the soil depending on the resistance and stress tolerance of the involved strain. The overview of several mycorrhizal and marjoram symbiotic as- sociations represented the effect of the studied stress over different strains and their interactions with the plants. We tested four mycorrhizal isolates, derived from various rhizospheres: Claroideoglomus claroideum (Cc1), Rhizophagus clarum, Claroideo- glomus claroideum (Cc2), Funneliformis mosseae. In order to get insight into the role of mycorrhizal symbiosis in protecting Origanum majorana L. against the excess of metals (Cd and Pb), we investigated growth, uptake and distribution of heavy metals in the plant parts, mycorrhizal colonization status, glomalin- related soil proteins and acid phosphatase activity (in root and soil). Pb and Cd partitioning in non-mycorrhizal plants were soil>shoots>root while in symbiotic associations variants changed to soil>root>shoots. Bio concentration factor (BCF) and translocation factor (TF) values confirmed by the less heavy metals shoot uptake in inoculated than control plants. The root dry weight increased in plants inoculated with C. claroideum strain (Cc1), which is isolated from industrially metal-contaminated sites. The highest shoot biomass correlated with the percentage of mycorrhiza- tion, relative mycorrhizal dependency, glomalin production and acid phosphatase activity was determined by C. claroideum (Cc2) and Funneliformis mosseae. Both strains derived from natural metalliferous sites. The findings in this study are essential to get the most benefits of mycorrhizal association in unfavorable conditions connected with plant development and herbal products free of harmful ingredients. Key words: Arbuscular mycorrhizal fungi; Origanum majorana L.; heavy metals Abbreviations: AMF – Arbuscular mycorrhizal fungi, HM – heavy metal, NM – non-mycorrhizal plants, APA – Acid phosphatase activity, Rc – Rhizophagus clarum, Cc – Claroideoglomus claroideum, Fm – Funneliformis mosseae *Corresponding author: mhristozkova@abv.bg Introduction Heavy metals in contaminated environments stand for a potential hazard to humans and animal health, and they have gradually become one of the major ecological concerns worldwide (Yoon et al., 2006). The contaminated soils char- acterized by negative properties such as poor nutrient avail- ability, lack of structure, low organic content, high salinity and/or acid pH. Accumulated lead (Pb) and cadmium (Cd) in the growth’ medium cause biological toxicity by affecting different physiological and biochemical processes in plants and soil microorganisms (Adriano, 2001).