Selection of infective arbuscular mycorrhizal fungal isolates for field inoculation.

Arbuscular mycorrhizal (AM) fungi play a key role in host plant growth and health, nutrient and water uptake, plant community diversity and dynamics. AM fungi differ in their symbiotic performance, which is the result of the interaction of two fungal characters, infectivity and efficiency. Infectivity is the ability of a fungal isolate to estab- lish rapidly an extensive mycorrhizal symbiosis and is correlated with pre-symbiotic steps of fungal life cycle, such as spore germination and hyphal growth. Here, different AM fungal isolates were tested with the aim of selecting infective endophytes for field inoculation. Greenhouse and microcosm experiments were performed in order to as- sess the ability of 12 AM fungal isolates to produce spores, colonize host roots and to perform initial steps of sym- biosis establishment, such as spore germination and hyphal growth. AM fungal spore production and root colo- nization were significantly different among AM fungal isolates. Spore and sporocarp densities ranged from 0.8 to 7.4 and from 0.6 to 2.0 per gram of soil, respectively, whereas root colonization ranged from 2.9 to 72.2%. Per- centage of spore or sporocarp germination ranged from 5.8 to 53.3% and hyphal length from 4.7 to 79.8 mm. The ordination analysis (Redundancy Analysis, RDA) showed that AM fungal isolates (used as explanatory variables) explained about 60% of the whole variance and their effect on fungal infectivity variables was significant (P = 0.002). The biplot clearly showed that variables which might be used to detect infective AM fungal isolates were hyphal length and root colonization. Such analysis may allow the detection of the best parameters to select effi- cient AM fungal isolates to be used in agriculture.