Abstract
Functional differences between the arbuscular mycorrhizal fungi Glomus intraradices Schenk and Smith and Scutellospora heterogama Nicolson and Gerdemann as they affect Persea americana Mill. growth, glomalin, and fungal sporulation were examined by varying the composition and relative density of the two fungi over a gradient of available phosphorus (P). The plant benefit provided by these mycorrhizal fungi together was not a simple sum of the benefits provided by each fungus in monoculture at its respective density. Glomus intraradices and S. heterogama interacted to reduce plant growth rates and uptake of P, zinc (Zn), and iron (Fe) relative to plants inoculated with G. intraradices alone. Thus, for plant growth and nutrition, no evidence for functional complementarity was detected. Instead, interspecific interactions between mycorrhizal fungi resulted in a negative feedback on plants. Under high available P, fungal functional differences were reduced, whereas the overall difference between mycorrhizal and nonmycorrhizal plants was greatest. Overall, S. heterogama produced more glomalin than did G. intraradices. In a mixture, sporulation of the inferior mutualist, S. heterogama, was lower than that of the superior mutualist, G. intraradices, but interspecific fungal interactions increased the sporulation of both fungi. Despite the negative impact of interspecific interactions on plants, supporting multiple arbuscular mycorrhizal fungi was of greater benefit than being nonmycorrhizal.
Highlights
Mycorrhizal plants regulate their interactions with arbuscular mycorrhizal fungi (AMF) (Smith and Smith 1996, 1997; Barker et al 1998), the symbiosis can range from mutualistic to parasitic (Johnson et al 1997; Smith and Read 1997)
Plant growth When plant growth rates are adjusted for % leaf P, only plants inoculated with G. intraradices monocultures were significantly faster growing than control plants (Fig. 1a)
The negative G Â S interaction can be attributed to the lower plant growth rates associated with mixtures (G. intraradices and S. heterogama) relative to what was predicted for plant growth rates associated with G. intraradices monocultures of the same infectivity (Table 1; Fig. 1a)
Summary
Mycorrhizal plants regulate their interactions with arbuscular mycorrhizal fungi (AMF) (Smith and Smith 1996, 1997; Barker et al 1998), the symbiosis can range from mutualistic to parasitic (Johnson et al 1997; Smith and Read 1997). Smith et al (2004) suggested that a lack of correlation between plant P uptake and plant growth in response to AMF might be due to differences in carbohydrate needs of specific AMF species or isolates. Other studies demonstrated that AMF exhibit different P uptake abilities for mycorrhizal plants (Dickson et al 1999). The interplay between AMF carbohydrate needs and the ability of specific AMF to enhance plant P nutrition is important to predicting the cost of the symbiosis to plants. In individual plant root systems, functionally different AMF can, and do, coexist, and the outcome of interspecific interactions between these AMF most certainly impact the overall cost of the symbiosis to plants
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