Key words : Glomus viscosum, Western Ghats, germination, soil binding, mucilage. Glomus viscosum ... Morphological investigations were made on at least 150.
J. Mycol. Pl. Pathol. Vol. 35, No. 1, 2005
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Glomus viscosum - An Arbuscular Mycorrhizal Fungus From Western Ghats, Southern India T. Muthukumar and K. Udaiyan Microbiology Laboratory, Department of Botany, Bharathiar University, Coimbatore - 641 046, Tamil Nadu, India.
Abstract Morphological features of spores of Glomus viscosum Nicolson are described and illustrated. The fungus was isolated from the root zones of Desmodium triflorum (L.) DC, Oxalis corniculata L., Tephrosia purpurea Pers. and Vico indica L. DC of Western Ghats, Southern India, forms hyaline to white spores with four walls arranged in two groups. None of the walls, reacts to Melzer’s reagent. Scanning electron microscopic studies revealed the binding of soil particles by G. viscosum fungal hyphae. Key words : Glomus viscosum, Western Ghats, germination, soil binding, mucilage
Glomus viscosum Nicolson was described from a potted substrate of unknown origin from a nursery in Italy (Walker et al., 1995). It was found producing hyaline to white spores in loose clusters with two walls. The outer wall produces an apparently mucilaginous outer coating resulting in the adherence of soil particles rendering the spore’s surface rough. During investigations on the occurrence of arbuscular mycorrhizal fungi in Western Ghats of southern India, spores of G. viscosum were found (Muthukumar and Udaiyan, 2000). The aim of this paper is to describe and illustrate the morphological features of G. viscosum from India and to characterize the worldwide occurrence of this species.
Materials and Methods Soils were sampled from a depth of 5-30 cm using a small shovel. Spores were extracted by wet sieving and decanting method (Gerdemann and Nicolson, 1963). Morphological investigations were made on at least 150 spores mounted in polyvinyl alcohol lactophenol (PVL) with or without Melzer’s reagent (1:1,v/v). Spore colour was determined under a dissecting microscope on fresh
specimens immersed in water. Specimens preserved on slides mounted in PVL have been deposited in the Department of Botany Culture Collection (DBCC), Bharathiar University, Coimbatore. Scanning electron microscopy (SEM) was used to examine the spore and hyphal surface in more detail. Specimens for SEM were fixed in glutaraldehyde (3%) in 0.02 M phosphate buffer (pH 6). They were passed through an ethanol dehydration series, dried, mounted on slides using double sided adhesive tape and coated with gold. For germination studies, spores were placed between Whatman No.1 filter papers and buried in sterilized wet sand and incubated at 22±2 °C. The filter papers were taken out every alternate day and observed under a dissecting microscope. Germinated spores were transferred to microscopic slides for examinations at higher magnification under a compound microscope.
Results and Discussion Description. Spores globose to subglobose, (46-) 61.7 (-78) x (40-) 60.7 (-80) µm in diam, hyaline to white often pale yellow to pale brown due to adherent soil
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particles, formed in tight or loose, white clusters (500- ) 753.3 (-930) x (550- ) 606.7 (-750) µm (Plate 1A, C). Spore consisted of 4 walls (W1 – W4) in two groups. Group A consist of a hyaline, unit wall (W1) (0.5-) 1.1 (-2) µm thick when free from debris, but can be >10 µm due to attachment of soil particles to the exuding mucilage-like substance (Plate 1D). A thin < 0.5 µm hyaline layer (W2), is adherent tightly to W3 but separates on vigorous crushing of spores. Wall 3 is laminated and (0.9-) 2.09 (-3) µm thick. Group B consists of a single unit wall (W4) of 0.5 – 1 µm thick (Plate 1B). Subtending hyphae single, straight, concolorous with the spore wall (4-) 6.5 (-8) µm and (6) 8.7 (-10) µm at the point of attachment and covered with mucilage like substance incorporating soil particles. Pore usually open or occluded by hyphal wall thickening at the spore base. Reaction of wall layers to Melzer’s reagent not distinct. Collections examined, Coimbatore. From the rhizosphere of Desmodium triflorum (L.) DC 18 September 1995, T. Muthukumar, DBCC 120-122, Vico indica L., 21 August 1995, T. Muthukumar, DBCC 123126, Tephrosia purpurea Pers. 23 November 1995, T. Muthukumar, DBCC 127 & 128 and Oxalis corniculata L., 23 November 1995, T. Muthukumar, DBCC 129 & 130. Distribution and habitat. Glomus viscosum has been found in the semi-arid grasslands at the foot hill of Maruthamalai an off shoot of Western Ghats (11°04’ N & 76° 93’ E, 550 M above MSL). The spores of G. viscosum were associated with the rhizospheres of Desmodium triflorum (L.) DC, Tephrosia purpurea Pers. (Papilionaceae), Oxalis corniculata (Oxalidaceae) and Vico indica L. (Compositae). The other AM fungi occurring with G. viscosum were G. mosseae (Nicol. & Gerd.) Gerd. & Trappe, G. geosporum (Nicol. & Gerd.) Walker, G. sinuosum (Gerd. & Bakshi) Almeida & Schenck and Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders. The chemical properties of the soil in which G. viscosum present were 0.12% total N, 0.09% available P, 0.82% exchangeable K and 2.3% organic matter. The spore density of G. viscosum ranged from 3-20 (av. 9.3) in 100 g of dry soil. However, the spore numbers of G. viscosum might be higher than observed as they tend to sediment with soil particles to which they were bound. Germination. Germination of more than 50 per cent of the spores occurred within 1 week. Germination was mainly through the subtending hyphae. The germ tube arising through the subtending hyphae formed a balloon shaped swelling at the germination point, before continuing growth (Plate 1E). In addition, direct germination through spore wall was also observed.
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However, the frequency of direct germination was relatively low (
