The Ecophysiology of Arbuscular Mycorrhizal Fungi: Phosphatase Activity Associated with Extraradical and Intraradical Mycelium

Abstract: Arbuscular mycorrhizal (AM) fungi colonise a wide range of plant species and are especially common in soils with a relatively high pH, where phosphorus availability is a limiting factor for primary production. The aim of this thesis was to study the ecophysiology of arbuscular mycorrhizal (AM) fungi when exposed to different environments. Development of AM fungi and their host plants were investigated with focus on the activity and the location of phosphatases associated with the AM fungal extraradical and intraradical mycelium. Plants and AM fungal mycelium were grown in substrates varying in soil origin, phosphorus content, or pH. The use of the Enzyme-Labelled Fluorescence (ELF) phosphatase substrate in AM fungal research was investigated. The studies in this thesis have led to the following findings. The ELF substrate method is a sensitive method in the determination of acid and alkaline phosphatase activity associated with mycorrhizal mycelium. The AM root colonisation is positively related to the biomass of the extraradical mycelium, however, locally the extraradical mycelium biomass production can respond to the conditions of certain patches. I found no active release of phosphatase activity by AM fungi, indicating that mineralisation of organic P in soil by AM fungi is not through the release of phosphatases. Staining for phosphatase activity is more intense in the intraradical than in the extraradical mycelium. In the intraradical mycelium the proportion of acid phosphatase-active mycelium seems greater than the alkaline phosphatase-active proportion, while in the extraradical mycelium it seems smaller. The high proportion of phosphatase-active arbuscules indicate the importance of phosphatases for the P metabolism in arbuscules and thus for the P transfer to the plant. A tubular vacuole system is present in the extraradical mycelium of a Glomus isolate and a high P content in the growth substrate decreases its motility. Phosphatases associated with the extraradical and intraradical mycelium of AM fungi seem to be related to the phosphorus uptake by the fungi and phosphorus transfer to the host plant. In this thesis some possible roles of these phosphatases in phosphorus metabolism of AM fungal plants are discussed.