The dual symbiosis between arbuscular mycorrhiza and nitrogen fixing bacteria benefits the growth and nutrition of the woody invasive legume Acacia cyclops under nutrient limiting conditions

Background and aims: Acacia cyclops is an invasive species within Mediterranean ecosystems, characteristically low in soil nutrients. Thus associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhiza (AM) may provide an advantage to these legumes. This study investigated the role of AM and NFB in the growth and nutritional physiology of A. cyclops. Methods: Seedlings were inoculated with naturally occurring NFB, Glomus mosseae or both, and grown under glasshouse conditions for 5 months. Plants were cultivated in sand and supplied with a 20 % strength nutrient solution. Xylem sap nutrients, photosynthetic rates, biomass and chemical compositions, were recorded. Results: The dual inoculation decreased the colonization of both symbionts, compared to a single symbiosis with either symbiont. Despite low colonization levels, the dual symbiosis increased host biomass and relative growth rates. This was associated with increased photosynthetic rates and enhanced nutrition. Additionally, dual symbiotic plants had enhanced N and P acquisition and utilization rates. Xylem sap analysis showed higher levels of NH4+ being exported from the roots to the shoots in the dual symbiotic plants compared with other treatments. Conclusions: These findings suggest the dual symbiosis is an important factor in the growth and development of A. cyclops under nutrient limiting conditions. © 2012 Springer Science+Business Media B.V.