Fourwing saltbush, Atriplex canescens (Pursh) Nutt., is an ecologically important range plant in arid south-western U.S.A. rangelands. Native populations of this chenopodiacious shrub are more extensively colonized by melanized dark septate fungi (DS) than by conventional mycorrhizal fungi. Seedling radicles of A. canescens are colonized at germination by a DS fungus identified as Aspergillus ustus that cannot be removed by heat or sterilization. The association of A. canescens with A. ustus was evaluated by comparing naturally colonized control seedlings receiving no P (0P) or adequate plant available P (AAP) receiving 30 p.p.m. supplied as KH2PO4in the root zone to seedlings whose roots were separated from plant unavailable P (as rock phosphate (RP) or tricalcium phosphate (TCP)) by a barrier that only allowed access by the fungus. A. ustus penetrated A. canescens roots with hyaline septate hyphae, formed melanized runner hyphae at the root surface, and extended through the root exclusion barrier into RP and TCP. In these treatments A. ustus obtained plant carbon, increased shoot and root biomass, and phosphorus use efficiency. A. ustus grew well in culture on RP and TCP and internally colonized in vitro Ri-T DNA D. carota roots. The mutualistic association of DS fungi in arid ecosystems is discussed. © 2002 Elsevier Science Ltd.
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