Effect of arbuscular mycorrhizal fungal inoculation on the growth of Welsh onion in soil rich in available phosphate, and characterization of indigenous arbuscular mycorrhizal fungi isolated from the soil

The objective of the present study was to explore the arbuscular mycorrhizal (AM) fungi adapted to soil rich in available phosphate. (1) An inoculation experiment with the AM fungus Glomus sp. R - 10 (R - 10) on Welsh onion was conducted over 2 years under field conditions. Welsh onion was pre - inoculated during a nursery period and transplanted to a field rich in available phosphate. Growth, yield and AM colonization of the inoculated Welsh onion were compared with those of non - inoculated Welsh onion. To examine the indigenous AM fungi, DNA was extracted from the roots of non - inoculated Welsh onion, and rRNA genes of AM fungi were amplified and analyzed. (2) AM fungi were isolated from an area adjacent to the experimental field and identified based upon spore morphology and partial rRNA gene sequences. (3) Effects of the isolated AM fungus on Welsh onion were examined in comparison with those of R - 10 by an in - pot experiment in a growth cabinet. The results were as follows: (1) the growth and yield of the plants inoculated with R - 10 were not significantly different from those of non - inoculated ones. Although the phosphate availability in the soil was very high, indigenous AM fungi colonized the roots in the non - inoculated plants. Sequences of Gigaspora margarita and Claroideoglomus etunicatum were detected. (2) An isolated AM fungus was identified as C. etunicatum. (3) The isolated C. etunicatum increased the growth of Welsh onion to a greater extent than did R - 10 and also showed a high colonization rate under high phosphate conditions. Thus, the ability of the indigenous AM isolate to colonize Welsh onion plants under high - phosphate conditions of soil was confirmed, suggesting the presence of AM fungi adapted to soil rich in phosphate. The isolated C. etunicatum was effective even in soils rich in available phosphate.