province in the plan to plant one billion trees, it was chosen as the tree species studied in this research. This research is factorial in the form of randomized complete block design with two factors of inoculation with arbuscular mycorrhizal fungi(AMF) at two levels of Rhizophagus irregularis, Funneliformis mosseae and control (without inoculation) and drought stress at four levels of level 100 (control), 75, 50 and 25% of crop capacity (FC) were done in four replications. The morphological characteristics of Fraxinus angustifolia seedlings (height, collar diameter, etc.) were measured at the beginning and end of the experiment period. The results showed that with increasing drought stress, vegetative traits such as height, collar diameter, dry weight of roots and aerial parts of Fraxinus angustifolia seedlings decreased significantly. The application of mycorrhiza significantly increased the vegetative growth and morphological traits of seedlings under drought stress conditions, the fungal species F. mosseae in all morphological traits except dry weight and root length was more favorable than the fungal species R. irregularis created Fraxinus angustifolia for one-year-old seedlings, Also, the fungal species F. mosseae showed a higher abundance and a better coexistence with Fraxinus angustifolia seedlings on average 40%. The main root length of the seedlings showed an increasing trend with the increase of drought stress up to FC equal to 50%, which indicated an increase in the root length under water deficit conditions. In general, Fraxinus angustifolia seedlings exhibit a drought avoidance mechanism by increasing root length and symbiosis with arbuscular mycorrhizal fungi(AMF) under stress conditions.
Introduction

Approximately 90% of the world's arid regions are concentrated in 27 countries, including Iran (Giri et al., 2005). From a biological perspective, stress can be described as a condition induced by an increased demand on the plant, leading to instability in its activities, with the plant returning to its initial state once the condition is removed (Laxa et al., 2019). Ecological and physiological studies have proven that mycorrhizal symbiosis often enhances water uptake from the soil. Mycorrhizal fungi increase the root absorption area, aiding the host plant in absorbing more water from the soil (Auge et al., 2001). Fungal hyphal branches can play a significant role in water absorption and transport. One of the key reasons for mycorrhizal support to the host plant under drought stress conditions is the increased uptake of soil nutrients and improved plant nutrition (He et al., 2017). Given Iran's climatic position and location in arid and semi-arid zones, the use of adapted plants and biological processes that enhance plant drought resistance is of particular importance.

Materials and methods
The effect of two species of arbuscular mycorrhizal fungi, Rhizophagus irregularis and Funneliformis mosseae, on the growth characteristics of one-year-old narrow-leaved ash (Fraxinus angustifolia) seedlings under water stress was evaluated. Seedlings with an average height of 50-70 cm, a minimum diameter of 0.5-1 cm, and at least 30 leaves were selected from the nursery affiliated with the Department of Natural Resources of Isfahan Province.
The treatments included:
1. Inoculation with arbuscular mycorrhizal fungi at three levels: R. irregularis, F. mosseae, and a control (no inoculation).
2. Drought stress at four levels: 100% field capacity (FC) as the control (optimal irrigation), mild stress (75% FC), moderate drought stress (50% FC), and severe drought stress (25% FC).
After seedling establishment, the irrigation treatments of 100%, 75%, 50%, and 25% field capacity were applied. At the end of the experiment, all morphological traits of the seedlings under the influence of the treatments were measured.

Results and discussion
Among the studied fungal species, F. mosseae had a greater impact on the morphological characteristics of narrow-leaved ash seedlings across different drought treatments. Regardless of the mycorrhizal type used, drought stress reduced the growth of the seedlings, with optimal irrigation showing the highest growth and severe drought stress showing the lowest growth in most treatments. The one-year-old narrow-leaved ash seedlings exhibited tolerance to moderate (50% FC) and severe (25% FC) drought stress through increased root length and a well-developed, robust root system, indicating a drought avoidance mechanism. In general, inoculating narrow-leaved ash seedlings with mycorrhizal fungi can improve this species' resistance under drought stress conditions.

Conclusion
In general, it can be concluded that inoculating one-year-old narrow-leaved ash seedlings with mycorrhizal fungi under drought conditions improves seedling nutrition, enhances nutrient uptake, and ultimately mitigates the negative effects of drought on seedling growth. Mycorrhizal fungi, by increasing root fungal colonization, enhance plant growth characteristics and water absorption, thereby increasing the plant's tolerance to adverse environmental conditions, including drought.
Introduction

Approximately 90% of the world's arid regions are concentrated in 27 countries, including Iran (Giri et al., 2005). From a biological perspective, stress can be described as a condition induced by an increased demand on the plant, leading to instability in its activities, with the plant returning to its initial state once the condition is removed (Laxa et al., 2019). Ecological and physiological studies have proven that mycorrhizal symbiosis often enhances water uptake from the soil. Mycorrhizal fungi increase the root absorption area, aiding the host plant in absorbing more water from the soil (Auge et al., 2001). Fungal hyphal branches can play a significant role in water absorption and transport. One of the key reasons for mycorrhizal support to the host plant under drought stress conditions is the increased uptake of soil nutrients and improved plant nutrition (He et al., 2017). Given Iran's climatic position and location in arid and semi-arid zones, the use of adapted plants and biological processes that enhance plant drought resistance is of particular importance.

Materials and methods
The effect of two species of arbuscular mycorrhizal fungi, Rhizophagus irregularis and Funneliformis mosseae, on the growth characteristics of one-year-old narrow-leaved ash (Fraxinus angustifolia) seedlings under water stress was evaluated. Seedlings with an average height of 50-70 cm, a minimum diameter of 0.5-1 cm, and at least 30 leaves were selected from the nursery affiliated with the Department of Natural Resources of Isfahan Province.
The treatments included:
1. Inoculation with arbuscular mycorrhizal fungi at three levels: R. irregularis, F. mosseae, and a control (no inoculation).
2. Drought stress at four levels: 100% field capacity (FC) as the control (optimal irrigation), mild stress (75% FC), moderate drought stress (50% FC), and severe drought stress (25% FC).
After seedling establishment, the irrigation treatments of 100%, 75%, 50%, and 25% field capacity were applied. At the end of the experiment, all morphological traits of the seedlings under the influence of the treatments were measured.

Results and discussion
Among the studied fungal species, F. mosseae had a greater impact on the morphological characteristics of narrow-leaved ash seedlings across different drought treatments. Regardless of the mycorrhizal type used, drought stress reduced the growth of the seedlings, with optimal irrigation showing the highest growth and severe drought stress showing the lowest growth in most treatments. The one-year-old narrow-leaved ash seedlings exhibited tolerance to moderate (50% FC) and severe (25% FC) drought stress through increased root length and a well-developed, robust root system, indicating a drought avoidance mechanism. In general, inoculating narrow-leaved ash seedlings with mycorrhizal fungi can improve this species' resistance under drought stress conditions.

Conclusion
In general, it can be concluded that inoculating one-year-old narrow-leaved ash seedlings with mycorrhizal fungi under drought conditions improves seedling nutrition, enhances nutrient uptake, and ultimately mitigates the negative effects of drought on seedling growth. Mycorrhizal fungi, by increasing root fungal colonization, enhance plant growth characteristics and water absorption, thereby increasing the plant's tolerance to adverse environmental conditions, including drought.