GERMINATION
ORB. Hyaline to pale yellow (3A3); circular or somewhat elliptic; 60-65 x 60.0-75.0 µm when seen in a plane view; formed by a centrifugally rolled hypha, (7.5-)10.5(-19.0) µm wide.
SPORIFEROUS
SACCULE hyaline; globose to subglobose; 80 - 100 µm diam; neck 140-300 µm long, tapering from 12-18 µm diam at the saccule to 8-10 µm diam at the point of spore attachment; distance from saccule to spore 120-185 µm. Saccule usually collapses or falls off in mature spores.
CICATRIC. A scar with slightly raised border, circular, 6-10 µm diam.
MYCORRHIZAE. Associated in the field with roots of, e. g., Ammophila arenaria (L.) Link (Błaszkowski, unpubl. data), Elymus arenarius L. (Greipsson et al. 2002), Pyrus malus (Estrada-Torres et al. 1992b), and Triticum aestivum L. (Schalamuk et al. 2006). Formed mycorrhizae with arbuscules, vesicles, and intraradical hyphae staining variably in trypan blue (Morton 2002).
PHYLOGENETIC POSITION. Unknown.
DISTRIBUTION. In Poland, isolated from only one pot trap culture containing a mixture of the rhizosphere soil and root fragments of Ammophila arenaria growing in dunes of the Baltic Sea adjacent to Swinoujscie (Błaszkowski, unpubl. data).
Originally found in a greenhouse bed used for the propagation of various ornamental plants at the University of Arizona, U.S.A. (Walker et al. 1986). Also revealed in maritime dunes of Iceland (Greipsson et al. 2002), in cultivated soils of Mexico (Estrada-Torres et al. 1992b), Argentina (Schalamuk et al. 2006), China (Wang et al. 2008), and soils of the El Palmar National Park in Argentina (Velazquez et al. 2008).
NOTES. The distinctive morphological characters of A. delicata are its relatively small and pale-coloured spores with a light greenish tint. Additionally, this species is one of a few members of the genus Acaulospora whose the inner layer of the second germinal wall stains faintly or not at all in Melzer's reagent.
The species of the genus Acaulospora forming spores somewhat resembling in colour and size those of A. delicata are A. gedanensis, A. mellea, and A. morrowiae. However, spores of all these species may be markedly darker-coloured [pale yellow (3A3) to lemon yellow (3B8), pale yellow (3A3) to orange (5B8), and pale yellow (3A3) to yellow (3A6) in A. gedanensis, A. mellea, and A. morrowiae, respectively vs. subhyaline to pale yellow (3A3) in A. delicata; Błaszkowski 2003; Morton 2002] and never have the light greenish tint of spores of A. delicata. Moreover, even the largest spores of A. gedanensis (75 µm diam when globose; Błaszkowski 2003) do not attain the lower size range of globose spores of the fungus discussed here (80 µm diam; Błaszkowski 2003; Morton 2002; Walker et al. 1986).
The most evident differences between the species listed above reside in the structure of their spore wall and inner germinal walls and the phenotypic and biochemical properties of the components of these walls.
Only spores of A. delicata and A. gedanensis have a spore wall and a first inner germinal wall consisting of two and one layer(s), respectively (Błaszkowski 2003; Morton 2002). In the other species compared here, their spore wall comprises three layers, and first germinal wall – two layers (Błaszkowski 2003; Morton 2002). However, while the first germinal wall of spores of A. delicata is flexible, that of spores of A. gedanensis is rigid and easily cracks in even slightly crushed spores. According to Morton (2002), the germinal wall 1 of A. delicata spores may be bi-layered as in most other Acaulospora species, but only one layer is visible.
The second germinal wall in all these species is 2-layered and, except for A. gedanensis, the upper surface of the outer layer is ornamented with granular excrescences (Błaszkowski 2003; Morton 2002). In A. gedanensis, the upper surface of the first layer of this wall is smooth and adheres to a flexible layer, similarly to the first layer of the second germinal wall of spores of A. delicata. However, in A. gedanensis, this layer does not stain at all in Melzer's reagent, whereas it generally stains light pink in this reagent in A. delicata. In contrast, the inner layer of the second germinal wall of spores of A. mellea and A. morrowiae is plastic and stains intensively red in Melzer's reagent (Błaszkowski 2003; Morton 2002).
REFERENCES
Błaszkowski J. 2003. Arbuscular mycorrhizal fungi (Glomeromycota), Endogone and Complexipes species deposited in the Department of Plant Pathology, University of Agriculture in Szczecin, Poland. http://www.agro.ar.szczecin.pl/~jblaszkowski/.
Estrada-Torres A., Varela L., Hernandez-Cuevas L., Cavito M. E. 1992. Algunos hongos micorrizicos arbusculares del estado de Tlaxcala, México. Rev. Mex. Mic. 8, 85-110.
Greipsson S., El-Mayas H., Vestberg M., Walker C. 2002. Arbuscular mycorrhizal fungi in sandy soils in Iceland. Arctic, Antarctic, Alpine Res. 34, 419-427.
Morton J. B. 2002. International Culture Collection of Arbuscular and Vesicular-Arbuscular Mycorrhizal Fungi. West Virginia University. http://www.invam.caf.wvu.edu/.
Schalamuk S., Velazquez S., Chidichimo H., Cabello M. 2006. Fungal spore diversity of arbuscular mycorrhizal fungi associated with spring wheat: effect of tillage. Mycologia 98, 16-22.
Velazquez M. S., Cabello M., Irrazabal G., Godeas A. 2008. Acaulosporaceae from El Palmar National Park, Entre Rios, Argentina. Mycotaxon 103, 171-187.
Walker C., Pfeiffer C. M., Bloss H. E. 1986. Acaulospora delicata sp. nov. – an endomycorrhizal fungus from Arizona. Mycotaxon 25, 621-628.
Wang Y. Y., Vestberg M., Walker C., Hurme T., Zhang X., Lindström K. 2008. Diversity and infectivity of arbuscular mycorrhizal fungi in agricultural soils of the Sichuan Province of mainland China. Mycorrhiza 18, 59-68.
