We investigated element accumulation in the arbuscular mycorrhizal fungus Glomus intraradices. Fungal spores and mycelia growing in monoxenic cultures were analyzed. The elemental composition was quantified using particle-induced X-ray emission (PIXE) in combination with scanning transmission ion microscopy. In the spores, Ca and Fe were associated mainly with the spore wall, while P and K showed patchy distributions and their concentrations were correlated. Excess of P in the hyphal growth medium increased the P and Si concentrations in spores and increased the K/Ca ratio in spores. Increased P availability decreased the concentration of Zn and Mn in spores. We concluded that the availability of P influences the uptake and accumulation of several elements in spores. It is demonstrated that PIXE analysis is a powerful tool for quantitative analysis of elemental accumulation in fungal mycelia.Arbuscular mycorrhizal (AM) fungal spores are asexual lipidrich survival organs (32) formed on the soil mycelium. After germination, they provide the energy carbon (C) for the fungus before carbohydrates can be taken up through symbiotic exchange with plants. In addition to lipids, the spores are potential storage organs for mineral nutrients essential for the formation of a germ tube. However, the nutrients stored in spores may reflect not only the need for formation of new fungal biomass but also that of the plants that are being colonized. The establishment of new symbiotic relations could thus depend on the accumulation of plant-limiting nutrients in spores and their transport into the host roots for recognition of a nonparasitic intruder.The nutrient accumulation and distribution in AM fungal spores have not been studied in as much detail as carbon accumulation has (32), although there are clear links between the nutrient availability and C metabolism (16,17,18). Quantitative analysis of elemental composition in AM fungal mycelia is crucial for our understanding of the distribution of nutritional resources.Electron-based methods, such as energy-dispersive X-ray spectroscopy or electron probe microanalysis (5, 30), can be used to obtain elementary maps of thin sections of fixed biological tissues. However, quantification of intact structures is not possible with these methods.Particle-induced X-ray emission (PIXE) analysis is based on the analysis of characteristic X-rays emitted from specific elements bombarded with protons from an accelerator (10). PIXE can be used to analyze elemental distributions in three-dimensional structures, and no chemical fixation is needed. Use of this technique, in combination with scanning transmission ion microscopy (STIM), enables quantification of the amounts and distribution of elements in structures as small as AM fungal spores and hyphae. PIXE is analogous to energy-dispersive X-ray spectroscopy, but its sensitivity is much higher, allowing quantification at levels of g g Ϫ1 . Limitations of the PIXE method are that it requires an extensive amount of time and the number of samples tha...