Spore germination, penetration, and root colonization of six species of vesicular–arbuscular mycorrhizal fungi on soybean

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Abstract
Six species of vesicular–arbuscular (VA) mycorrhizal fungi, each at 500 spores per kilogram of pasteurized soil, were compared for spore germination, hyphal root penetration, and root colonization on soybean in a repeated greenhouse experiment. Generally, the six species were categorized into two groups. Those species in group I (Glomus mosseae, Gl. intraradices, and Gigaspora heterogama) had higher spore germination, root penetration, percentage root colonization values, and fungal growth rates in the roots than those species in group II (Gl. etunicatum, Gi. margarita, and Entrophospora sp.). However, when we calculated the fungal growth rate (centimetres per day) per penetration point or used Smith and Walker's equation, the highest values were assigned to group II. Fungal growth rate per penetration point was inversely proportional to the number of penetration points, indicating growth "interference" among hyphae originating from different penetration points in the root. The number of penetration points was correlated significantly with root length but not with percentage of root colonization. Dry root weight was significantly correlated with length of colonized roots in both experiments, while shoot weight was correlated with colonized root length in only one of the two experiments. Glomus mosseae was the only species which induced a significant increase both in dry shoot weight and numbers of Rhizobium nodules as compared with those for nonmycorrhizal plants.

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