02804nam a2200241 a 450000100080000000500110000800800410001910000250006024500890008526000410017430000100021550000170022552022100024265000110245265000160246365000150247965000090249465000120250365000130251565000140252865000090254265300110255115550231999-11-23       1985    bl uuuu  m   00u1 u    #d1 aMIRANDA, J. C. C. de  aEffects of soil phosphorus on vesicular-arbuscular mycorrhiza in a Brazilian oxisol.  aReading: University of Readingc1985  a227p.  aPhD. Thesis.  aThe direct effects of soil P on VA mycorrhizal fungi and VA mycorrhiza development were investigated. An attempt was made to identify whether soil P or plant P is the primary factor controlling VA mycorrhiza formation, by means of a split-root experiment with sorghum plants inoculated with Glomus macrocarpus and three phosphate levels. At low soil P, low plant P did not affect VA mycorrhizal root infection and external mycelium content. However, high plant P depressed root infection, but did not affect  the external mycelium content to the same extent. At high soil P both parameters were similarly reduced. The direct effects of soil P on the VA endophyte Glomus macrocarpus were investigated in a cellophane-soil agar plate experiment. Spore germination and mycelium growth measured at ten different phosphate levels and three periods of incubation were independently affected by the P status of the soil. Both parameters were stimulated when 12.5 ug P.g-1 was applied to the soil and decreased with further soil P increments. At high soil P treatments spore germination became less sensitive to changes in P concentrations. The direct effects of soil P on different species of VA mycorrhizal fungi were also investigated. The VA endophytes, Glomus macrocarpus, Gigaspora heterogama and Glomus fasciculatus had different sensitivities to the P status of the soil. Spore germination was higher and mycelium growth more profuse for Gigaspora heterogama. However, the latter was more sensitive to phosphorus fertilization than the Glomus spp. The effect of soil P levels and time of their application on the soil phase of establishment of VA mycorrhizal root infection of sorghum plants by Glomus macrocarpus was studied. The external mycelium content, quantified by the soil-agar film technique, and root infection were stimulated at low soil P levels. High soil P depressed hyphal growth and root infection. Late additional P applications had a negative effect on mycelium growth. A comparison  was made between Glomus macrocarpus and the VA endophytes Gigaspora heterogama and Glomus fasciculatus. The three VA mycorrhizal fungi species were different  in their response to phosphorus in the soil.  aBrazil  amycorrhizae  aphosphorus  asoil  aCerrado  aFósforo  aMicorriza  aSolo  aBrasil