Effects of growth-promoting bacteria and humic acid on certain morphological traits of oak under drought stress

Background and Objective: This study aims to explore the effects of humic acid and the growth-promoting bacterium Pseudomonas putida on specific morphological characteristics of Iranian oak (Quercus brantii) seedlings under drought stress. Oak trees are vital for maintaining biodiversity, stabilizing soils, and providing ecological resources. By offering shade and retaining moisture, they help sustain plant and animal diversity. Drought stress is a major limiting factor for the growth and development of these trees, leading to adverse consequences. In drought conditions, plants experience a reduced ability to absorb water and nutrients, resulting in stunted growth and decreased productivity. Therefore, it is crucial to adopt innovative and effective approaches to improve the drought resilience of oak seedlings. Humic acid, an organic substance known for its soil-enhancing properties, and growth-promoting bacteria such as Pseudomonas putida, can significantly mitigate the detrimental effects of drought by enhancing nutrient absorption and plant growth. Humic acid improves soil structure and boosts its water retention capacity, thus alleviating the negative impacts of drought. Additionally, Pseudomonas putida enhances plant stress resistance by producing growth hormones and boosting enzymatic activities within the plant.
Material and Methods: This study was conducted as a factorial experiment within a completely randomized block design, featuring three replications and four treatments, under greenhouse conditions. The treatments consisted of two levels of humic acid (0 and 2.5 grams per kilogram of soil) and two levels of Pseudomonas putida (with and without inoculation). Humic acid, an organic substance known for its soil-enhancing properties, was incorporated into the soil, while Pseudomonas putida was applied to the seedling roots as an inoculant. Drought stress was induced by implementing three irrigation regimes (control, irrigation every four days, and irrigation every eight days). After a three-month vegetative growth period, various morphological traits, including seedling height, root length, leaf count, and dry weight of both aerial parts and roots, were measured and recorded. Data analysis was conducted using statistical software, with analysis of variance (ANOVA) to examine the independent and interactive effects of each treatment on the measured traits.
Results: The results of this study revealed that drought stress significantly diminished all the measured traits. As the intensity of drought stress increased, there was a notable reduction in seedling height, root length, leaf count, and the dry weight of both aerial parts and roots. These reductions were attributed to decreased water and nutrient uptake under drought conditions, hindering the plants' ability to grow optimally. On the other hand, the application of humic acid and Pseudomonas putida significantly enhanced the morphological traits of the seedlings. Seedlings inoculated with Pseudomonas putida and treated with humic acid exhibited the most substantial increases in height, root length, and leaf count. Specifically, seedlings treated with humic acid showed a significant boost in the dry weight of their aerial parts and roots compared to the control group, indicating improved growth and enhanced drought resistance. Moreover, inoculation with Pseudomonas putida independently led to significant improvements in all the studied traits. These positive effects are likely due to enhanced nutrient uptake, improved enzymatic activities, and stronger root systems. Pseudomonas putida plays a key role in boosting plant resistance to environmental stress by producing growth hormones and promoting enzymatic activities within the plant. Additionally, the combination of humic acid and Pseudomonas putida produced the most favorable results, with seedlings treated with this combination demonstrating the highest resistance to drought stress. These findings suggest that humic acid, by improving water retention and enhancing the soil’s physical and chemical properties, supports plant growth and resilience. This organic compound improves soil structure and microbial activity, facilitating better nutrient absorption and increased resistance to environmental stresses. Overall, these results highlight the potential of using humic acid and Pseudomonas putida as effective strategies to mitigate the negative effects of drought stress on Iranian oak seedlings and improve the growth and development of these trees in arid and semi-arid regions. These insights underscore the value of organic substances and growth-promoting bacteria in enhancing plant resistance to adverse environmental conditions, offering new strategies for more effective oak forest management.
Conclusion: The results of this study revealed that the application of humic acid and Pseudomonas putida significantly improved the morphological traits of Iranian oak seedlings under drought stress. These beneficial effects were attributed to enhanced nutrient absorption, increased enzymatic activity, and strengthened root systems. The combination of humic acid and Pseudomonas putida produced the most favorable outcomes, effectively boosting the drought resistance of oak seedlings. Future studies are recommended to explore the long-term impacts of these compounds on the physiological and biochemical processes of oak seedlings. Additionally, evaluating their effects under field conditions would provide more comprehensive insights into the practical efficacy of these treatments. Further research on the influence of these treatments on other aspects of seedling growth and development could enhance forest management strategies and yield more practical applications.