MYCORRHIZAE. In one-species cultures with Zea mays L. as the plant host, Gl. gibbosum formed mycorrhizae with arbuscules, vesicles, as well as intra- and abundant extraradical hyphae staining intensively in 0.1% trypan blue. The arbuscules and coils were numerous and evenly distributed along the root fragments examined.

In roots of Z. mays

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DISTRIBUTION. Glomus gibbosum has originally been found in soil samples collected from the root zone of plants colonizing maritime sand dunes adjacent to Swinoujscie (53º55’N, 14º14’E) located in north-western Poland (Błaszkowski 1997). The plant species associated in the field with spores of Gl. gibbosum were Ammophila arenaria (L.) Link, Helichrysum arenarium (L.) Moench., Hieracium umbellatum L., and Petasites spurius (Retz.) Rchb.

Błaszkowski et al. (2001) revealed spores of Gl. gibbosum in maritime sand dunes adjacent to Tel Aviv (32º4’N, 34º46’E), Israel. No other report exists of the finding of Gl. gibbosum in the world.

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NOTES. A unique feature of Gl. gibbosum is the formation of the hyphal mantle completely enclosing a cluster of spores (Błaszkowski 1997).

The hyphal mantle is composed of a continuous layer forming a transparent structure resembling an envelope without an opening. The hyphal layer is first thin, hyaline and flexible. At times, it becomes thicker, coloured and rigid. The hyphal mantle encloses from 2 to 8 (mean 4) spores.

An arbuscular mycorrhizal fungus most closely related to Gl. gibbosum is Gl. albidum. Both species form spores similar in shape, size and colour with a narrow and thin-walled, straight or slightly funnel-shaped subtending hypha. The main feature distinguishing Gl. gibbosum from Gl. albidum is the spore wall composition. Glomus gibbosum has a spore wall composed of four layers: an outermost sloughing layer 1 adherent to a permanent, semiflexible layer 2, a laminated layer 3, and a thin flexible layer 4. Glomus albidum has been described to produce spores with three layers: a sloughing outermost layer adherent to a laminated layer and an inner, thin membranous layer formed after initial spore development (Walker et al. 1995). If present, the sloughing spore wall layer of Gl. gibbosum remains thin and does not separate from layer 2 with age, whereas that of Gl. albidum spores crumbles and expands in mature specimens (Błaszkowski, pers. observ.; Walker and Rhodes 1981). Additionally, spores of Gl. albidum stain pink to orange-red in Melzer’s reagent (Walker and Rhodes 1981), and those of Gl. gibbosum are nonreactive in this reagent. Finally, formation of a hyphal mantle enclosing a spore or a cluster of spores is a unique property of Gl. gibbosum.

Many species of arbuscular fungi form spores inside spores of other arbuscular fungi, plant debris or living roots (Koske 1984; Koske and Gemma 1990; Schenck and Smith 1982), structures superficially reminiscent of the hyphal mantle of Gl. gibbosum. However, spores of arbuscular fungi with internal other arbuscular fungi usually possess most properties enabling their identification and are dead (Koske 1984). There is no resemblance between the morphological properties of the hyphal mantle of all the Gl. gibbosum specimens examined and those of spores of known arbuscular fungi. Both plant debris and living root fragments frequently consists of many, coloured cells. In contrast, the hyphal mantle of Gl. gibbosum always is a one-celled structure. Additionally, the mantle consists of a very thin, hyaline layer in some specimens to a thick, rigid, and coloured layer in others. This suggests that the rigidity of the layer of the hyphal mantle is imparted as additional material is synthesized in this layer during its development. Thus, the hyphal mantle is a living structure.

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