The direct effects of soil phosphorus on spore germination and hyphal grov, th of the arbuscular mycorrhizal fungi (AM), Glomus etunicatum Beck. & Gerd., E3 and Scutellospora heterogama (Nicol. & Gerd.) Walker & Sanders, were investigated by nicatis of two soil-agar plate experiments. In the first experiment, the percentage spore germination and h\phal length of G. eturiuaUim, measured at ten different P levels and three pertods of incubation, were affected by the P status of the soil. The best correlation with P level in the soil occurred at 7 d of incubation for spore germination and at 14 d for hyphal growth and hyphal growth increment. Spore germmation was stimulated when 125//gPg ' was applied to tbe soil and decreased with further soil P increments. Hyphal growth decreased significantly with soil P amendments above 37'5//gPg"'. ln the second experiment including three different P levels atid the three AM endophytes, soil P affected spore germination and hyphal growth, and both decreased at the high phosphate treatment for G. etunicatuni and S. heterogama. Tbe E3 spore germination was similar to the other two species, but the hyphal growth was poorer, S. heterogama was more sensitive to P fertilization than the Glomus spp. and for the three species the hyphal production uas not proportional to spore size.

Sl'M M Am-The cflfct of soil niul plant phosphorus {]') on iiiycorrhi/,a was studied usiiif; a sphf-root icclinitiue. In this expc-rinicnl, sorghum {Sdrglniiii tiicalor (L.) Moench.) plants inoculated with the \ esieular-arhuscular (\'A) m\corrhizal IUIIKUS CHOIIIUS iiKicyocarpuin (Tul. tv TLIL), were subjected siiiuihaneoitsly to low and hiyh phosphate coticetittatiotis applied to a phosphorus delieietit Htazilian DarU Ked Latosol. Phosphorus was ti anslocateil frotii one halt of the toot systetn to the other. \\ luti stnall atklttiotis of 1' were tnade to the soil, tnycotthizal infectioti, spoie tuinibets and external h\phae iticreased cotnpareti with the ittisupplenn-nted soil. Translocation ot phosphorus itito roots j.;row in« in itnsup)-ilenietited soil luid little etteet on percenta,t,'e root infection, spore nuniber or external h>|-)hae in that eonijiarttiient althoit^h inteeted root lenyth iticreased sli)j;htl\\ In the tteattiient where .sf)il P and platit P wete lii>,'h, percetitaKe root itilection aiul extertial Inplial growth wete hoth sitiiilarK reilueed.In the hi^h phospliate treatnients although percentage root infection was reduced, the total lenj^th of niyeorrhizal root was not affected by phosphate fertilization.

Resumo -Rotação de culturas e variações sazonais podem promover alterações quantitativas e qualitativas na comunidade de fungos micorrízicos arbusculares nativos e na formação da micorriza arbuscular. Essa dinâmica foi avaliada, em campo, num Latossolo Vermelho, em relação ao tempo de cultivo e variação sazonal, em sistemas de rotação de culturas. Em casa de vegetação, avaliou-se, em solo proveniente da área experimental, a contribuição da micorriza arbuscular no crescimento de soja e capim-andropógon utilizados na rotação. O número de esporos dos fungos aumentou no solo cultivado. O número de esporos e o porcentual de colonização radicular, inicialmente maiores sob pastagem, variaram de acordo com o tempo de cultivo, as estações seca e chuvosa, a cultura e o sistema de rotação utilizados. O número de gêneros e espécies aumentou com o tempo de cultivo e manejo de culturas e foi maior sob culturas anuais em rotação. A presença dos fungos no solo contribuiu no crescimento da soja e do capim-andropógon em 53% e 95%, respectivamente. A cultura e o sistema de cultivo são fatores determinantes para o enriquecimento do sistema com micorriza arbuscular.Termos para indexação: fungos micorrízicos arbusculares, esporos, colonização radicular, culturas anuais, pastagens. Dynamics and contribution of arbuscular mycorrhiza in culture systems with crop rotationAbstract -Crop rotation and seasonal variations can promote quantitative and qualitative changes in the indigenous arbuscular mycorrhizal fungi population in the soil and arbuscular mycorrhiza establishment. These fungi dynamics were evaluated in the field, in a Red Latosol, in relation to cropping time, seasonal variation and rotation systems. The contribution of arbuscular mycorrhiza to the growth of andropogon grass and soybean, which were used in the systems, was evaluated in a greenhouse experiment using soil from the experimental area. The number of spores of the fungi increased in the cultivated soil. The spores number and percent root colonization varied according to cropping time, soil moisture, crops and rotation system and were, initially, higher under pasture. The number of genus and species increased according to cropping time and culture management and was higher under annual crops in rotation. The presence of these fungi in the soil contributed 53% and 95% to the growth of soybean and andropogon grass, respectively. Crops and cultivation system are crucial for the system's enrichment in arbuscular mycorrhiza.

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