Diversity and colonization levels of naturally occurring arbuscular mycorrhizal fungi (AMF) in onion roots were studied to compare organic and conventional farming systems in the Netherlands. In 2004, 20 onion fields were sampled in a balanced survey between farming systems and between two regions, namely, Zeeland and Flevoland. In 2005, nine conventional and ten organic fields were additionally surveyed in Flevoland. AMF phylotypes were identified by rDNA sequencing. All plants were colonized, with 60% for arbuscular colonization and 84% for hyphal colonization as grand means. In Zeeland, onion roots from organic fields had higher fractional colonization levels than those from conventional fields. Onion yields in conventional farming were positively correlated with colonization level. Overall, 14 AMF phylotypes were identified. The number of phylotypes per field ranged from one to six. Two phylotypes associated with the Glomus mosseae–coronatum and the G. caledonium–geosporum species complexes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional farming systems had similar number of phylotypes per field and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, including phylotypes associated with the genera Glomus-B, Archaeospora, and Paraglomus. This suggests that farming systems as such did not influence AMF diversity, but rather specific environmental conditions or agricultural practices.Electronic supplementary materialThe online version of this article (doi:10.1007/s00572-009-0237-2) contains supplementary material, which is available to authorized users.

Root endophyte fungi of ericaceous plants were compared with those obtained from the dominant grass in Dutch heathlands, Deschampsia flexuosa. We investigated the phylogenetic relatedness of these fungi and their effects on nutrient uptake in both Calluna vulgaris and D. flexuosa seedlings in synthesis trials in vitro. Molecular analysis based on nuclear ribosomal internal transcribed spacer (ITS) region sequences revealed that four grass root endophytes belonged to the Helotiales (Ascomycetes). The majority of the ericaceous root isolates (68 %) also clustered within the Helotiales and showed a remarkably high diversity. Other important fungal groups included Phialocephala fortinii-like fungi, making up 22 % of isolates, and Cryptosporiopsis species, making up 8 %. Results of the synthesis trials showed that both grass root and ericaceous isolates colonized roots of both test host species successfully and could be seen to significantly enhance nitrogen uptake of inoculated D. flexuosa and C. vulgaris seedlings when these were compared to the uninoculated controls. We conclude that beneficial, helotialean fungi associate with roots of D. flexuosa and that these form a group potentially overlapping in phylogeny and function with endophytes from Ericaceae

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Appropriate selection and well-timed measurement of plant developmental, morphological and physiological parameters are essential to maximize efficacy and minimize time consumption of experiments. To select for the most sensitive indicators of drought or salt stress, three independent pot experiments with diverse setups were analysed with 20-20 measured parameters. Parameters of plant growth, phenology and symbiotic interactions, visual stress symptoms, photosynthetic activity, nutrient composition and vitality were studied and the result matrices were evaluated with principal component analysis (PCA). Stress effects manifested in PC1 of two experiments and in PC2 of the third one. Traits assumed to be adequate for stress indication were characterized by high PC1 or PC2 loading values. Beside parameters of biomass production, growth and visible stress symptoms, less evident traits e.g. root electrical capacitance, membrane stability index in roots and leaves, relative water content of leaves and SPAD units were identified.