Effect of biopriming with plant growth-promoting bacteria on seed germination indices of kavir wheat under salinity stress

Background and Objectives: Soil salinity is a major challenge for global agriculture, especially in arid and semi-arid regions, threatening crop yields and food security. Wheat (Triticum aestivum L.), one of the world's most important staple crops, is particularly sensitive to salinity stress, which hampers seed germination, seedling establishment, and subsequent growth and productivity. The detrimental effects of salinity include reduced germination rates, stunted root and shoot growth, disrupted nutrient uptake, and decreased photosynthetic efficiency. However, plant growth-promoting rhizobacteria (PGPR) present an eco-friendly solution to mitigate these adverse effects. PGPR enhance plant growth through mechanisms such as nitrogen fixation, phytohormones production, ethylene reduction, improved nutrient acquisition (e.g., phosphorus solubilization), and systemic resistance induction. This study aimed to explore the potential of seed bio-priming with selected PGPR strains to enhance germination and seedling growth of the salt-tolerant wheat cultivar 'Kavir' under different salinity levels. The objectives included assessing the effects of single and combined (consortium) inoculations of Azospirillum (As), Azotobacter (Az), and Pseudomonas (Ps) on key seedling growth parameters and identifying the most effective bacterial combinations for improving salt tolerance in the 'Kavir' cultivar.Materials and Methods: A factorial experiment was conducted in a completely randomized design (CRD) with three replications at the Soil Biology and Biotechnology Research Department of the Soil and Water Research Institute of Iran. The experiment consisted of two factors: salinity levels (control: 0.35 dS/m (tap water), 6, and 14 dS/m) and seed bio-priming treatments. The bacterial treatments included three isolates: Pseudomonas fluorescens Strain 169 (Ps), Azotobacter chroococcum Strain 5 (Az), and Azospirillum brasilense Strain OF (As), applied as single, dual (As+Az, As+Ps, Az+Ps), and triple (Az+As+Ps) inoculations, along with a non-inoculated control. The experiment was carried out in 72 Petri dishes containing sterile filter paper moistened with the respective saline solutions. Seeds of 'Kavir' wheat were surface-sterilized and inoculated with the bacterial treatments before being placed in the Petri dishes. The Petri dishes were incubated under controlled conditions: a temperature of 25 ± 2°C, a 16-hour light/8-hour dark photoperiod, and 70–80% relative humidity. Measured traits included germination percentage and rate (calculated using Maguire’s formula), root and shoot length (mm), seedling water content (%), seedling vigor index (calculated as (Germination % × Seedling Length (cm)), and root-to-shoot ratio. Data were analyzed using analysis of variance (ANOVA), and means were compared using Duncan’s multiple range test (DMRT) at the 5% probability level (P ≤ 0.05). Data analysis was performed using SAS software version 9.1. Prior to conducting mean comparisons, the normal distribution of the data was assessed and confirmed.Results: The results showed that the main effects of salinity and bacteria were significant at the 1% probability level (p<0.01), and their interaction effect on germination percentage, germination rate, root length, shoot length, and seedling vigor index was significant at the 5% probability level (p<0.05). The interaction effect of salinity and bacteria on shoot fresh weight was not significant (p>0.05). Increasing salinity led to a significant decrease in all indices. For example, in the control condition (without salinity), the seedling vigor index in the control treatment was 20342, which decreased to 7296 (approximately 64% reduction) with increasing salinity to 14 dS/m. However, bacterial treatments had a positive effect on all traits, and combined (consortium) treatments showed a significant improvement in the indices compared to single inoculations. For instance, in the control condition, the Az+As+Ps combined treatment increased the seedling vigor index to 31670.6 (approximately 56% increase compared to the control). At 14 dS/m salinity, the As+Az combined treatment also increased the seedling vigor index to 12757.3 (approximately 75% increase compared to the control of the same salinity level). Regarding root length, in the control condition, the As+Az treatment showed approximately a 36% increase (to 186 mm) compared to the control (137 mm). At 14 dS/m salinity, the same treatment showed approximately a 66% increase (to 106 mm) compared to the control (64 mm). Regarding germination percentage, in the control condition, the Az+Ps treatment showed approximately a 14% increase (to 96%) compared to the control (84%), and at 14 dS/m salinity, the As+Az, Az+As+Ps, and Ps treatments similarly showed approximately a 31% increase (to 67%) compared to the control (51%).Conclusion: The findings of this study revealed that salinity stress significantly impairs seed germination and seedling growth in the salt-tolerant wheat cultivar 'Kavir,' leading to a reduction in germination percentage and rate, as well as root and shoot length. However, seed bio-priming with plant growth-promoting rhizobacteria (PGPR), particularly combined treatments, effectively mitigated the adverse effects of salinity and improved key parameters associated with seedling establishment and growth. The impact of PGPR varied across different salinity levels, indicating that the optimal bacterial combination changes depending on the intensity of stress. In general, triple combinations (e.g., Az+As+Ps under non-saline and mild salinity conditions) and dual combinations (e.g., As+Az or As+Ps under severe salinity) demonstrated superior performance in enhancing seedling growth. This highlights the dependency of PGPR efficacy on salinity levels, emphasizing that no single bacterial combination can be universally recommended for all conditions. Therefore, selecting the appropriate bacterial combination should consider soil salinity levels and environmental conditions. Another significant finding is that 'Kavir,' recognized as a salt-tolerant wheat cultivar, showed further enhancement in its inherent salt tolerance through the use of PGPR. This underscores the potential of PGPR as a tool to boost the resilience of even salt-tolerant genotypes, providing additional protection and improved growth under saline conditions.