Drought stress is one of the most important limiting factors for field crops in arid and semi-arid regions. Yield assessments under drought stress conditions, using soil microorganisms to reduce the damage from drought stress, demonstrate that soil microorganisms are sustainable solutions for crop production in such climates. Therefore, we undertook a two year experiment to understand the effects of root colonization by two species of mycorrhizal fungi (Glomus mosseae and Glomus hoi), under different drought stress conditions, quantifying sunflower growth, nutrient uptake, yield, yield components, oil percentage and irrigation water productivity. The experiment was conducted at Tarbiat Modares University, located in the semi-arid region of Iran, during the 2006 and 2007 growing seasons. Drought stress was induced at two stress levels by irrigating after 60 and 80% water depletion, which were defined as mild and severe drought stress, respectively. Irrigating after 40% water depletion was considered to be normal irrigation (no stress). The results indicated that irrespective of the mycorrhizal species and the drought stress intensity, inoculated plants produced more dry matter, heavier seeds and greater seed and oil yields than did non-inoculated plants. Interestingly, the seed yields of plants inoculated with G. mosseae under each irrigation regime were higher than those of plants inoculated with G. hoi, or of the non-inoculated plants. Although drought stress reduced the N percentages in the leaves and seeds, mycorrhizal fungi enhanced the N percentages, particularly when the plants were inoculated with G. mosseae. In addition, seed P percentages were not affected by mycorrhizae in 2006. In contrast, the highest P percentages in both leaves and seeds were obtained from plants inoculated with G. mosseae in 2007. Overall, our results show that G. mosseae is more efficient under drought stress, and better supports sunflower plants. The application of these microorganisms could be critical in the cultivation of sunflowers under arid and semi-arid conditions, where water is the most important factor in determining plant growth and yield.
