Mycorrhiza (1992) 1: 147-152 ycorrhzza Illl[lill 9 Springer-Verlag 1992 The propagules of vesicular-arbuscular mycorrhizal (VAM) fungi capable of initiating VAM infection after topsoil disturbance S. E. Bellgard Department of Biology,Universityof Wollongong,Wollongong,N. S. W. 2500, Australia Summary. The removal and storage of topsoil decreases the infectivity of vesicular-arbuscular mycorrhizal (VAM) fungi. The propagules of VAM fungi include spores, root fragments containing hyphae and vesicles, and soil hyphae. The viability of each type of propagule after disturbance will determine the initiation of VAM associations with plants recolonizing the disturbed site. This study aimed to examine which of the propagules of VAM fungi are capable of initiating VAM infection aft- er soil disturbance. Soil from an open woodland site of low soil fertility, in southeastern Australia was wet- sieved through a tier of three sieves (1 ram, 250 gm and 106 ~tm), and the following fractions were extracted: (i) root fragments, (ii) fungal hyphae, and (iii) VAM spores. Each fraction was tested to determine its poten- tial to initiate VAM. Hyphae of VAM fungi grew from root fragments within 14 days. The VAM spore fraction initiated VAM infection after 28 days. VAM hyphal fragments did not produce any VAM infection even aft- er 42 days. Key words: Topsoil disturbance - VAM fungi - Colo- nized root fragments - VAM hyphae - Spores Introduction The removal and storage of topsoil decreases the infec- tivity of vesicular-arbuscular mycorrhizal (VAM) fungi (Moorman and Reeves 1979; Abbott and Robson 1991). The propagules of VAM fungi are blastospores, chlamy- dospores or azygospores (resting spores), soil-borne ves- icles and mycelia or colonized root fragments containing hyphae and vesicles (Daniels and Skipper 1982). These propagules have been shown to have different suscepti- bilities both to the direct impacts of topsoil disturbance, and also to the associated changes in the soil environ- ment (e.g. Stahl et al. 1988; Jasper et al. 1989a). The viability of each type of propagule after soil disturbance will determine in part the number of infective propa- gules available to initiate VAM with plants recolonizing the disturbed site. Soil-borne spores have been considered to be the most important type of propagule of VAM fungi (Brun- drett 1991). However, soils in a range of ecosystems oft- en contain low numbers of living spores (e.g. Read et al. 1976; Janos 1980; Gay et al. 1982; Visser et al. 1984; Brundrett and Kendrick 1988; Bellgard, in preparation). Some species of VAM fungi apparently do not produce spores (e.g. Johnson 1977; McGee 1989). The successful germination of VAM spores is dependent upon interac- tions with a range of soil and environmental factors (e.g. Slankis 1974; Schenck et al. 1975; Black and Tink- er 1979; Daniels and Trappe 1980; Tommerup 1983a, 1984; McGee 1989), but living spores of VAM fungi will not function as propagules if they are quiescent (Tom- merup 1983b, 1985). Blastospores of VAM fungi, be- cause of their lipid content and thick walls, are consid- ered to be more resistant to adverse environmental con- ditions than other VAM propagules (Daniels Hetrick 1984; Abbott and Robson 1991). Tommerup and Kidby (1979) demonstrated that some species of Glomus and Gigaspora can remain infective after lyophilization. In comparison, Scutellospora calospora did not recover after one wet/dry cycle (McGee 1989). Consequently, it is reasonable to conclude that the importance of spores as a source of inoculum varies between sites and is de- pendent upon a range of variables, including the species of endophyte, the abundance of the endophyte, and the local soil and environmental conditions. Non-spore propagules, such as roots colonized by VAM fungi (containing hyphae and vesicles), can ini- tiate VAM, provided they are in close proximity to an actively growing root (e.g. Rives et al. 1980; McGee 1987). Tommerup and Abbott (1981) demonstrated that root pieces colonized by several species of VAM fungi retained their infective potential even when stored in dry soil at - 50 MPa. It is not known whether storage under humidities more conducive to root decomposition or to root desiccation might result in loss of viability of hy- phal fragments contained in root pieces (Daniels Hetrick 1984; Miller 1987). Thus, the ability of colonized root