Communities of Arbuscular Mycorrhizal Fungi in Pyrus pyrifolia var. culta (Japanese pear) and an Understory Herbaceous Plant Plantago asiatica

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in most terrestrial plant communities and form mutualistic associations with the vast majority of plant species, including many cultivars (1, 36). This symbiosis is usually effective for growth promotion of host plants through improved mineral nutrients, particularly phosphorus (P). It is well known that most orchard tree species have symbioses with AMF (4, 5, 12, 29, 30, 32). Orchard soil often contains an excess amount of fertilizer components, especially phosphorus, because of repeated fertilization (7, 25). Many studies have shown that the AMF colonization rate decreases with the addition of P to the soil (16, 17). In orchard environments, Youpensuk et al. (42) found that AMF root colonization rates and spore densities were significantly decreased in a tangerine (Citrus reticulate) orchard containing more than 500 mg P kg−1 of available P in soil. Rutto et al. (32) analyzed the effects of some cultural practices on AMF in various orchards and showed that excessive fertilization had adverse effects on AMF colonization and their spore populations. Thus, it is considered that AMF symbiosis is not fully utilized in plant cultivations in many orchards with excessive fertilization. In orchards, various herbaceous plants are usually found around the trees, and many of them are known as AM plants (24). Sod culture is a cultivation method to manage orchards with herbaceous plants (35). Rutto et al. (32) found that the AMF colonization level and spore population in peach (Prunus persica Batsch) trees in sod culture were higher than those without cover plants, which suggested that sod culture may be effective to increase AMF colonization in orchard trees. In this study, we examined AMF community in an orchard tree and a surrounding herbaceous plant. As the orchard tree species examined, we selected Pyrus pyrifolia var. culta (Rosaceae), known as Japanese pear. This is a well-known fruit tree species cultivated in various regions of Japan. In our previous study, it was found that AMF diversity and colonization were reduced by the accumulation of soil phosphorus in this orchard tree species (41). As the herbaceous plant species, we selected Plantago asiatica (Plantaginaceae), which is an herbaceous perennial that is often found in the understories of orchards. The Plantaginaceae are also known as hosts of AMF (22, 28). In our previous observation, it was found that fine roots of Py. pyrifolia were sparsely distributed in soil, and new fine roots often arose from lignified small roots with no AMF colonization; therefore, it is considered that AMF colonizing roots of herbaceous plants such as Pl. asiatica could be important inocula for orchard trees such as Py. pyrifolia. Many studies have shown that host plant species are determinants of AMF communities (38, 39), Meanwhile, it has also been known that host specificity in AMF symbiosis is usually low (36). According to these studies, we developed a hypothesis that some AMF communities can be shared between the two host plants, Py. pyrifolia and Pl. asiatica, while they are diversified among the hosts. In order to test the hypothesis, we investigated AMF communities in the roots of both plants, Py. pyrifolia and Pl. asiatica, collected from the same soil cores to examine the overlap of AMF between the two host plants as well as to compare the AMF communities.