JOURNAL OF PURE AND APPLIED MICROBIOLOGY, Sept. 2012. Vol. 6(3), p. 1119-1125 * To whom all correspondence should be addressed. E-mail: alqarawi@ksu.edu.sa Diversity of Structural Colonization and Spore Population of Arbuscular Mycorrhizal Fungi in Some Plants from Riyadh, Saudi Arabia A.A. Al-Qarawi*, M.A.U. Mridha and O.M. Alghamdi Plant Production Department, Faculty of Food and Agricultural Sciences, King Saud University, P. O. Box. 2460 Riyadh 11451, Saudi Arabia. (Received: 18 May 2012; accepted: 10 August 2012) The percentage infection in the roots of different species with the arbuscular mycorrhizal fungi varied widely and independently, irrespective of plant species. The overall highest infection was recorded in Petunia hybrida and Gaillardia pulchella (97%), which was followed by Calendula officinalis (90%), Cynodon dactylon and Ocimum sanctum (87%), Convolvulus arvensis (70%), Phoenix dactylifera (53%), Tagetes patula (43%) and the lowest infection was found in Sesuvium portulacastrum (33%). The maximum vesicles were found in P. hybrida (97%), which was followed by G. pulchella (80%), C. officinalis (67%), O. sanctum (60%) and the minimum was in P. dactylifera (7%). In case of total infection with arbuscules, again the highest percentage of infection was recorded with P. hybrida (73%) and arbuscules were not found with C. arvensis, S. portulacastrum. The second and third highest percentage infection of arbuscule was recorded with C. officinalis (63%) and O. sanctum (57%). The intensity of infection also varied widely and independently in each individual plant species and it was not always comparable to the percentage infection with different structure of the AMF. Spore population also varied irrespective of plant species. Highest number was recorded from the rhizosphere soils of P. hybrida and the lowest number was found with S. portulacastrum. So far, this is the first report of extensive study on structural colonization and spore population study of AM fungi from a large number of plant species from Riyadh, Saudi Arabia. Key words: Arbuscular mycorrhizal fungi, structural colonization, spore population, Saudi Arabia. The Kingdom of Saudi Arabia comprises of 225,000sq. kilometer area and occupies four fifths of the Arabian Peninsula. The climate of Saudi Arabia is hot but not humid for the greater part of the year, relative humidity is low except along the coastal zones where it reaches over 90%. The average annual temperature is 33.4°C in summer (a maximum of 50°C ) and 14°C in winter (below zero at night). Rainfall in the greater part of Saudi Arabia is scanty, unpredictable and irregular 1 . Saudi Arabia has diverse vegetation because of the diverse physiographic features coupled with diverse climatic differences 2,3 . Desert (semi- and arid lands) covers about 40% of land surface 4 . Desert land is characterized by little precipitation, hence there is limited moisture, and high temperature at day time at least part of the year, and moreover high incidence of light are prevailing. In many areas of the desert, the soil is sandy and poor in organic nutrients. Arbuscular Mycorrhiza Fungi (AMF) are widespread throughout the world and found in