The effect of mycorrhizal fungi and growth-promoting bacteria on yield, water use efficiency, and biochemical stress-related indices of wheat under different irrigation regimes

Background and Objectives: Bread wheat (Triticum aestivum L.), as one of the world's most vital cereals, plays a decisive role in ensuring food security. However, its production faces serious challenges, including drought stress. Drought, as one of the primary limiting factors, causes reduced yield and disruption of physiological processes in plants. In this regard, the use of bio-fertilizers such as mycorrhizal fungi and Plant Growth-Promoting Rhizobacteria (PGPR) has been proposed as a sustainable strategy to enhance plant resilience against stresses and improve water use efficiency. These microorganisms contribute to improved plant growth and performance through various mechanisms, including facilitating water and nutrient uptake, producing growth hormones, and inducing stress resistance. The objective of this research was to investigate the individual and combined effects of mycorrhizal fungus (Rhizophagus intraradices) and plant growth-promoting bacteria (Pseudomonas fluorescens and Azospirillum brasilense) on physiological processes, biochemical resistance, and yield of wheat under different irrigation levels.
Materials and Methods: This research was conducted during the 2021-2022 agricultural year at the Jahad Keshavarzi research farm in the Arshaq region (Meshginshahr, Ardabil province) as a factorial experiment based on a randomized complete block design with three replications. Experimental treatments included four levels of bio-fertilizer (control, mycorrhizal fungus, plant growth-promoting bacteria, and combination of both) and three levels of irrigation regime (no irrigation, irrigation at 50% of water requirement, and full irrigation), totaling 12 treatments. Wheat seeds were inoculated using the coating method with a bacterial suspension (concentration of 10⁸ CFU/ml) and fungal inoculant (containing 100 live spores per gram). Various traits were measured, including root colonization percentage, electrolyte leakage, malondialdehyde (MDA) content, total soluble sugars, leaf area index (LAI), root dry weight, number of grains per spike, 1000-grain weight, grain yield, gluten index, and water use efficiency (WUE).
Findings: Results of variance analysis showed that the main effects of bio-fertilizers, irrigation regime, and their interaction were significant on all measured traits. The highest root colonization percentage (79.6%), grain yield (4891.6 kg/ha), leaf area index (6.08), number of grains per spike (36.7), 1000-grain weight (43.5 g), and gluten index (45.72%) were observed in the combined bio-fertilizer treatment under full irrigation. In contrast, the highest values of electrolyte leakage (42.53%), malondialdehyde (27.73 nmol/g), and total soluble sugars (18.37 mg/g) were recorded in the control treatment (no fertilizer and no irrigation). Water use efficiency under 50% irrigation (1.38 to 1.61 kg/m³) was significantly higher than under full irrigation (0.84 to 0.98 kg/m³), and under both conditions, bio-fertilizer treatments, especially the combined treatment, improved this index. The highest root dry weight (5.51 g) was observed in the combined bio-fertilizer treatment under 50% irrigation, indicating root system development for better access to limited water resources.
Conclusion: The combined application of mycorrhizal fungus and plant growth-promoting bacteria was the most effective treatment, increasing plant resilience against drought stress by coordinately improving physiological, biochemical, and performance traits of wheat. These bio-fertilizers, through synergistic mechanisms such as increased root colonization, reduced oxidative damage, development of root system and leaf area, and improved water uptake, provided the basis for realizing wheat yield potential even under water-limited conditions. Therefore, the use of these bio-fertilizers can be recommended as an effective and sustainable strategy in wheat cultivation management to reduce the negative effects of drought stress and increase yield and water use efficiency.