Arbuscular mycorrhizal fungal communities associated with date palms in a traditional and a modern experimental plantation and with desert plants in the adjacent natural habitats in Southern Arabia

The ubiquitous symbiosis between plants and arbuscular mycorrhizal (AM) fungi is multifunctional. In this symbiosis, plants exchange photosynthates for phosphorus (P) and other mineral nutrients, and they gain increased resistance to soil borne diseases, drought and extreme temperature. All of these benefits might be crucial for plants growing in extreme environments. The aim of this thesis was to shed light on the diversity and dynamics of AM fungal communities in Southern Arabia, known for its particularly arid conditions and low fertility of soils. AM fungal communities in two agricultural sites were compared with those in adjacent natural habitats. The agricultural sites were cultivated with date palms (Phoenix dactylifera) and managed according to “traditional” and “modern” farming systems. The natural sites contained native plant species (among those Zygophyllum hamiense, Salvadora persica, Prosopis cineraria and Heliotropium kotschyi). Soil was sampled from the rhizosphere of plants and from these samples, AM fungal spores were isolated and morphologically identified. Furthermore, “trap cultures” were established in the green house, using the soil samples from the field as AM fungal inocula. The results showed that the AM fungal community composition at the agricultural sites differed from that at the natural habitats. Agricultural sites had a much higher AM fungal spore abundance, species richness and inoculum potential supposedly due to the land-use change from natural to agricultural with irrigation and fertilizer application. A molecular approach was used to identify the AM fungi colonizing the roots of the date palms at the two agricultural sites. Nine phylogenetic taxa were revealed, eight of which could be attributed to the Glomus group A, the most diverse group in the Glomeromycota, and one to the Scutellospora group that occurred at the traditional agriculture site only. Two of the nine taxa could be associated to AM fungal species already described. These were Glomus sinuosum and Glomus proliferum. Three phylotype groups were associated with AM fungal sequences previously detected in environmental samples. The other 4 phylotype groups were not associated with any of the sequences in the GenBank nor in large database of the Botanical Institute and, therefore, we assume that they are new to science. The communities of these fungi were found to differ between the two agricultural sites and consisted of both site-specialist and site-generalist groups. This was in accordance with spore morphospecies differences found between the two sites. The composition of the detected phylotypes was quite unique because it lacked certain groups commonly occurring in most habitats around the world investigated so far. Trap cultures inoculated with rhizosphere soils of date palms growing on a modern agricultural plantation showed an AM fungal community consisting of Glomus aurantium, Glomus intraradices, Diversispora spurca, Acaulospora sp. and five different Glomus phylotypes which presumably new to science. Based on morphological identification of AM fungal spores, a total of 36 morphospecies were detected at the five sites investigated in Southern Arabia. Twenty two of them belonged to the genus Glomus, six to Scutellospora, four to Acaulospora, two to Archaeospora and one to each genus of Paraglomus and Ambispora. This is a quite high richness considering that so far only around 200 AM fungal species have been described worldwide in the phylum Glomeromycota. The composition of AM fungal communities detected in this study was compared with communities found in other habitats of the world to seek for biogeographical patterns. It was found that the agricultural sites in the present study have a composition most similar to those 2 found at sites with sandy soils around the world. The natural sites, however, seem to maintain a unique species composition, which might have emerged due to unique local biotic and abiotic environmental factors of Southern Arabia. To my knowledge, this is the first report on AM fungal communities in Arabian Peninsula and the first molecular investigation ever on AM fungi associated with date palm, a socioeconomically important plant in many dry lands of the world. On a global scale, I believe that this work is a significant contribution to the knowledge on diversity, phylogeny and ecology of AM fungi.