Introduction

The use of saline water for irrigation has increased extensively in arid and semi-arid regions (Shrivastava and Kumar 2015; Yousefi et al. 2020). Salinity is a severe problem that significantly reduces plant growth, development, and yield potential, and induces reactive oxygen species (ROS) accumulation (Zhan et al. 2019; Zahra et al. 2020; Ali et al. 2021). Salinity stress is related to nutritional imbalance and oxidative stress via osmotic stress and ion toxicity (Liu et al. 2015). It drastically affects the uptake of key mineral ions, including K, thereby triggering hindrances in plant growth (Ali et al. 2021). Salinity stress downregulates the photosynthesis rate by increasing chlorophyll degradation (Alinia et al. 2021). It also limits enzyme activity and induces ROS accumulation, which can cause lipid peroxidation of the membrane (Parihar et al. 2014).

Biofertilizers are viewed as key components of sustainable agriculture that can be employed to raise the tolerance of stressed plants (Bal et al. 2012; Arnao and Hernández-Ruiz 2019; Abd El-Ghany and Attia 2020). Plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) are critical for reducing stress symptoms (Begum et al. 2019; Ali et al. 2021; Motamedi et al. 2022). They are considered the most important biofertilizers that regulate stress tolerance in plants by improving cellular redox homeostasis, mitigating oxidative stress, increasing photosynthesis, and promoting mineral accessibility (Begum et al. 2021). The modes of action of biofertilizers have been classified into direct and indirect mechanisms. The direct impacts of biofertilizers include enhancing biological nitrogen fixation, regulating phytohormones, improving nutrition efficiency and plant growth, and reducing ethylene synthesis. The indirect effects of biofertilizers include tolerance to abiotic and biotic stresses (Begum et al. 2019, 2021; Motamedi et al. 2022). Recently, Begum et al. (2019) and Bouskout et al. (2022) reported that AMF facilitate stress tolerance by improving nutrient uptake and water absorption. According to Su et al. (2017) and Jia-Dong et al. (2019), PGPR and AMF regulate the production of antioxidant enzymes and alter hormonal homeostasis in plants. Bal et al. (2012) illustrated that PGPR application dramatically improved the growth parameters of rice, including shoot and root growth and chlorophyll content, as compared to the uninoculated control. Based on the information reported by Abd El-Ghany et al. (Abd El-Ghany and Attia 2020), bacterial inoculation can affect salinity tolerance through different mechanisms, including enhancing N, P, and K concentrations and reducing Na+ concentrations.

Rapeseed (Brassica napus) is a source of vegetable oil for human consumption (Zangani et al. 2021; Batool et al. 2022). Although rapeseed is a moderately salt-tolerant species, its growth, photosynthesis, and nutrient uptake can still be affected by excess salinity through osmotic stress and ionic imbalances (Shahzad et al. 2021; Mohamed et al. 2022). Members of the Brassicaceae family are exceptions that create low AMF colonization (Valetti et al. 2016). Many authors have shown that PGPR application can improve AMF colonization in plants (Veiga et al. 2013; Petrić et al. 2022).

The impacts of Micrococcus yunnanensis’s on the development and yield of rapeseed plants have been investigated under different environmental stresses, but there is no information on their application with Glomus versiforme on rapeseed plants in saline conditions. In the present study, we hypothesized that co-application of Glomus versiforme and Micrococcus yunnanensis could regulate redox status and ion homeostasis to improve salt stress tolerance in rapeseed. Hence, the research specific objectives were: (1) explore the interactive effect of Glomus versiforme and Micrococcus yunnanensis to suppress salinity stress by enhancing redox status and ion homeostasis in rapeseed. Such knowledge enables us to use the potential of Micrococcus yunnanensis in combination with Glomus versiforme to improve plant tolerance to salinity.

Materials and Methods

Materials

A completely randomized design with a factorial arrangement was used to assess the effect of Glomus versiforme and Micrococcus yunnanensis on improved tolerance of rapeseed (Brassica napus L., cv. Neptune) in response to salinity stress. This experiment was performed with three replicates in a research greenhouse under natural light conditions at 28 °C, located at Department of Plant Production and Genetics, School of Agriculture, Shiraz University, Shiraz, Iran (52°35′ E, 29°43′ N, 1810 m above mean sea level). The physicochemical characteristics of the soils are shown in Table 1. Four salinity levels of 0, 5, 10, and 15 dS m−1 NaCl, two AMF (non-inoculated and inoculated with Glomus versiforme (Accession AJ504642, (Weisany et al. 2015)), and two PGPR (non-inoculated and inoculated with Micrococcus yunnanensis (Accession KP090345, (Estrada et al. 2013)) were used as experimental treatments. Rapeseed seeds were obtained from the Agricultural and Natural Resources Research Center (Fars, Iran). Healthy rapeseed seeds were sterilized for 2 min with 70% ethanol followed by 5% NaOCl for 5 min, washed with double-distilled water three times, and then grown in Petri-dishes (10-cm diameter) for three days. After germination, the uniform seedlings were aseptically transplanted into plastic pots (33 × 36 cm, inner diameter × depth) with 4.5 kg of soil. The soil was autoclaved at 121 °C for 2 h to remove soil microorganisms. The physicochemical properties of the soil used in the pots were determined according to a conventional method (Selvakumar et al. 2018).

Table 1 Physicochemical properties of soil
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Glomus versiforme (AMF) inoculants were obtained from the Department of Soil Science, Shiraz University. The AMF inoculants were prepared by the trap culture of maize (Zea mays L.). Autoclaved soil/quartz-sand (< 1 mm) (1: 4, v/v) was used as the trap culture medium. The trap culturing process was prolonged for 8 weeks. After this period and removing the shoots, the contents of pots (roots in addition to soil containing fungal spores and mycelia) were kept in polyethylene bags at 4 °C until inoculation. AMF was inoculated by placing 30 g of AM fungi 1 cm beneath the transplanted plant roots, and the same autoclaved amount was applied to the control pots. The PGPR strain Micrococcus yunnanensis was obtained from the Department of Soil Sciences, Shiraz University. The Micrococcus yunnanensis bacterium had the ability to dissolve the insoluble inorganic and organic phosphates and also produce siderophore (Data not shown). Micrococcus yunnanensis strains were cultured in 100 ml of nutrient broth medium for 24 h at 28 ± 2 °C and 200 rpm in a shaker incubator. The seedlings were then inoculated with 2 mL of the cultures of isolates (Weisany et al. 2015) in the treated group.

Measurement of growth and yield

At the physiological maturity stage, plants were harvested i.e. plant height and yield were measured in each pot.

Gaseous Exchange and Photosynthetic Pigments

To measure gas exchange (net photosynthesis rate (Pn), stomatal conductance (gs) and transpiration rate (Tr)) with a portable photosynthesis system (LCi, ADC Bioscientific Ltd., Hoddesdon, UK) in the fully expanded terminal leaflets, the method presented by Alinia et al. (Alinia et al. 2021) were used.

The leaf concentrations of chlorophyll (Chl) and carotenoids were measured spectrophotometrically, according to a previous study (Parihar et al. 2014). Leaf tissue was frozen in liquid nitrogen and stored at -80 °C. The leaf concentrations of chlorophylls (Chl) and carotenoids were measured spectrophotometrically, and leaf tissues (0.1 g fresh weight [FW]) were homogenized in 10 mL of 80% acetone in the dark until the residue was colorless. The homogenate was centrifuged at 500 rpm for 5 min at room temperature (RT) and A646, A663, and A470 were measured (7315 UV/VIS, Jenway, Staffordshire, UK).

Antioxidant Enzymes Activity

Antioxidant enzyme activities were measured using 0.5 g of leaf tissue in five mL of ice-cold phosphate buffer (pH 7.6), and all determinations were made spectrophotometrically (7315 UV/VIS, Jenway) using phosphate buffer. The peroxidase (POD) activity was determined from the changes in A470 using an assay mixture containing 50 mM buffer (pH 7.0), 16 mM guaiacol and 0.2 mL of sample, after adding 10 mM H2O2 (Rojas-Tapias et al. 2012). Superoxide dismutase (SOD) activity in leaf extracts was measured in a solution containing 50 mM buffer (pH 7.6), 750 mM NBT, 4 μM riboflavin, 13 mM methionine, 0.1 mM EDTA and 0.2 mL of extract (Dodd and Perez-Alfocea 2012). Cuvettes containing the assay solution were illuminated with fluorescent lamps for 15 min, and photochemical reduction of NBT was followed by changes in A560. Catalase (CAT) activity was measured at A240 using an assay solution containing 50 mM buffer (pH 7.0) and 12.5 mM H2O2, mixed with 0.2 mL sample (Zarei et al. 2008).

Ion Concentrations

Oven-dried shoot and root samples (0.5 g) were milled using a grinder, ashed in an electric oven (at 450 °C for 4 h) and then digested with 1 N HCl. Tissue concentrations of K+ and Na+ were determined using flame photometry (Ghavami et al. 2016).

Seed Oil Determination

Oil content was determined by continuous extraction using a Soxhlet apparatus. For each treatment, 30 g of rapeseed seed was pulverized into fine powder using an electric blender and used to extract the oil with 300 mL of n-hexane (50–60 °C) as solvent. The solvent was evaporated under low pressure using a rotary evaporator at 70 °C until the solvent was completely removed. Oil content (%) was calculated according to Hosni et al. (Redondo-Gómez et al. 2021) and the following formulae were used to calculate the oil content values:

$$\boldsymbol O\boldsymbol i\boldsymbol l\boldsymbol\;\boldsymbol c\boldsymbol o\boldsymbol n\boldsymbol t\boldsymbol e\boldsymbol n\boldsymbol t(\boldsymbol{\mathit\%})=\frac{(\mathbf w\mathbf e\mathbf i\mathbf g\mathbf h\mathbf t\boldsymbol\;\mathbf o\mathbf{ff}\boldsymbol\;\mathbf l\mathbf a\mathbf s\mathbf k\boldsymbol\;\mathbf w\mathbf i\mathbf t\mathbf h\boldsymbol\;\mathbf o\mathbf i\mathbf l-\boldsymbol w\boldsymbol e\boldsymbol i\boldsymbol g\boldsymbol h\boldsymbol t\boldsymbol\;\boldsymbol t\boldsymbol o\boldsymbol\;\boldsymbol o\boldsymbol f\boldsymbol\;\boldsymbol e\boldsymbol m\boldsymbol p\boldsymbol t\boldsymbol y\boldsymbol\;\boldsymbol f\boldsymbol l\boldsymbol a\boldsymbol s\boldsymbol k)}{\mathbf w\mathbf e\mathbf i\mathbf g\mathbf h\mathbf t\boldsymbol\;\mathbf{of}\boldsymbol\;\mathbf g\mathbf r\mathbf o\mathbf u\mathbf n\mathbf d\boldsymbol\;\mathbf s\mathbf e\mathbf e\mathbf d\mathbf s}$$
(1)

Determination of the AMF Colonization

To determine the impact of mycorrhizal colonization, at least 1 cm of root tissue was obtained, fixed in 4% paraformaldehyde, and then incubated for 48 h at room temperature. Root segments were washed three times with distilled water and then incubated in 10% KOH for 15 min at 120 °C. Alkaline hydrogen peroxide solution was added and incubated for 20 min at room temperature, after which 0.05% methylene blue (Sigma-Aldrich) was added and incubated for 24 h at room temperature. The samples were observed under a microscope (M5LCD Velab, Co., Pharr, TX, USA) using a 40 × objective lens (Cakmak and Marschner 1992). The mycorrhizal colonization percentages were calculated using the following formula: Percentage of root colonization (%) = no. of infected segments/no. of examined root segments × 100.

Statistical Analysis

Data were statistically analyzed by one-way analysis of variance with three replicates using SAS v. 9.1 software (SAS Institute 2003). Treatment differences were determined using the least significant difference (LSD) test (p < 0.05). A heat map correlation analysis was performed using XLSTAT.

Results

Plant Height

Salinity stress negatively affected plant height (Fig. 1); this parameter was significantly lower in plants subjected to 5, 10, and 15 dS m−1 salinity levels than in those subjected to 0 dS m−1 conditions in all inoculation treatments. AMF and PGPR treatments increased this parameter in plants subjected to salinity compared to non-inoculation. In contrast, the F1 and B1 treatments significantly increased plant height compared with the other inoculation treatments.

Fig. 1
figure 1

Impact of AMF and PGPR on plant height under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Photosynthetic Pigments

Dynamic changes in photosynthetic pigments are shown in Fig. 2. AMF and PGPR significantly affected Chl a, Chl b, and carotenoid contents under salinity stress (Figs. 2a, b, and c). Higher Chl a, Chl b, and carotenoid contents were detected in the combination of F1 and B1 compared with the other inoculated treatments at 0, 5, 10, and 15 dS m−1. The results showed that the F1 plus B1 treatments were significantly increased Chla 1.52, 1.59, 1.65 and 1.74 times, Chlb 1.57, 1.58, 1.62 and 1.84 times and carotenoid 1.65, 1.84, 2.16 and 2.33 times compared to the F0 plus B0 treatments under 0, 5, 10 and 15 dS m−1 levels, respectively (Figs. 2a, b, and c).

Fig. 2
figure 2

Impact of AMF and PGPR on photosynthetic pigments: chlorophyll a (a), chlorophyll b (b), and carotenoid (c) in rapeseed under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Oil Percentage

All treatments had a significant positive effect on oil percentage under saline and non-saline conditions (Fig. 3). In this regard, the combination of F1 and B1 was proven to be a significantly efficient tool for increasing the oil content of rapeseed plants during salinity stress. The F1 plus B1 treatments had a more significant effect on the oil percentage than the combination of F0 and B0 treatments under salinity stress. A significant difference was observed between the inoculated and non-inoculated plants, which indicated their effectiveness in salinity stress.

Fig. 3
figure 3

Impact of AMF and PGPR on oil content of rapeseed under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Antioxidant Enzyme Activities

The activities of antioxidant enzymes such as CAT, POD, and SOD were significantly affected when subjected to salinity treatments compared to non-saline conditions (Figs. 4a, b, and c). The combination of F1 and B1 significantly improved the plant activity of antioxidant enzymes in all salinity treatments compared with the non-inoculated plants. The F1 plus B1 treatments remained efficient in increasing CAT activity at salinity levels over non-saline conditions. Furthermore, the combination of F1 and B1 differed significantly for the enhancement of POD under 15 dS m−1 than under other salinity levels. The combined F1 and B1 treatments were significantly different for SOD under salinity stress compared to non-saline conditions. Under 0, 5, 10, and 15 dS m−1 salinity levels, the F1 plus B1 treatments increased the activity of SOD 1.5, 1.3, 1.2, and 1.2 times, respectively, compared to the combination of F0 and B0 treatments. These results indicate that the AMF plus PGPR enhances the activity of antioxidant enzymes and salt tolerance of rapeseed.

Fig. 4
figure 4

Impact of AMF and PGPR on the activity of CAT (a), POD (b) and SOD (c) in rapeseed under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Photosynthetic System

The photosynthetic system of rapeseed plants was significantly affected by salinity stress compared to that under non-saline conditions (Fig. 5). Both the single and combined applications of AMF and PGPR significantly increased plant Pn, gs, and Tr compared with the non-inoculated plants. The combined F1 and B1 treatments remained efficient in increasing Pn, gs, and Tr under saline conditions compared with non-inoculation. The highest Pn (14.4 µmol CO2 m−2 s−1), gs (31.32 mmol H2O m−2 s−1) and Tr (12.5 mmol H2O m−2 s−1) were found under non-saline conditions, F1, and B1 (Figs. 5a, b and c). Under 5, 10, and 15 dS m−1 salinity levels, the F1 plus B1 treatments increased Pn 1.6, 1.7 and 2.0 times, gs 1.5, 1.9 and 2.0 times, and Tr 1.5, 2.4 and 2.6 times, respectively, compared to the combination of F0 and B0 treatments (Figs. 5a, b, and c).

Fig. 5
figure 5

Impact of AMF and PGPR on the (a) net photosynthesis rate (Pn), (b) stomatal conductance (gs) and (c) transpiration rate (Tr) in rapeseed under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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AMF Colonization

The F1 treatment increased AMF colonization (Fig. 6). The increase was significant in the combination of F1 and B1 treatments at 0 dS m−1, which produced the highest AMF colonization (6.1%) (Fig. 6). Additionally, the maximum AMF colonization was found in the F1 plus B1 treatments, with an average of 6.1%, representing 1.4, 1.6, 2.4, and 3.8 times increase colonization than the combination of F1 and B0 treatments at 0, 5, 10, and 15 dS m−1 salinity levels, respectively (Fig. 6).

Fig. 6
figure 6

Impact of AMF and PGPR on the AMF colonization under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Na+ and K+ Concentrations and K+/Na+ Ratio

The Na+ and K+ concentrations in shoots and roots were significantly affected by AMF, PGPR, and salinity stress (Table 2). Salinity decreased the K+ concentration in all AMF and PGPR treatments, and inoculated plants showed higher K+ concentrations than non-inoculated plants (Table 2). Plant inoculation with the combination of F1 and B1 treatments improved K+ concentration at 5, 10, and 15 dS m−1 salinity levels in both shoots and roots when compared with the corresponding combinations of F0 and B0 treatment plants. With increasing salinity, the Na+ concentration steadily increased in the shoots and roots of all inoculation treatments (Table 2). However, under salinity stress, Na+ concentration was significantly lower in shoots of (1.4, 1.1, and 1.1 times at 5, 10, and 15 dS m−1, respectively) inoculated plants with F1 and B1 as compared to the F0 plus B0 treatments. Moreover, inoculation with F1 and B1 significantly decreased the Na+ concentration in the roots of salt-stressed plants.

Table 2 Impact of AMF and PGPR on concentrations of K+ and Na+ and K+/Na+ ratios in shoots and roots of rapeseed
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Yield

The inoculated treatments had a significant positive effect on yield under saline and non-saline conditions (Fig. 7). The F1 plus B1 treatments proved to be a significantly efficient method for increasing the yield of rapeseed plants under salinity stress. The combination of the F1 and B1 treatments resulted in higher yields (20 g) than the other treatments. Under 0, 5, 10, and 15 dS m−1 salinity levels, the F1 plus B1 treatments increased the product yield 2.2, 2.9, 3.2, and 3.5 times, respectively, compared to the combination of F0 and B0 treatments (Fig. 7).

Fig. 7
figure 7

Impact of AMF and PGPR on the yield under different levels of salinity. Data present the mean ± standard deviation of three replicates. Different letters above bars indicate significant differences (p < 0.05) among treatments by LSD test. F0B0, non-inoculated; F0B1, inoculation of PGPR; F1B0, inoculation of AMF; F1B1, inoculation of AMF and PGPR

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Pearson Correlation

In this experiment, a significant positive correlation was observed between oil content and Pn and photosynthetic pigments (Fig. 8). This might, in turn, improve the salt tolerance of rapeseed and consequently enhance the plant height. In addition, negative correlations were found between Na+ and K+ concentrations.

Fig. 8
figure 8

Heatmap representation of the Pearson correlation of different rapeseed attributes

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Discussion

Salinity stress negatively affects plant growth and yield (Przybyłko et al. 2021). One strategy to flexibly adapt to stress conditions is the establishment of microorganism symbiosis (El-Sawah et al. 2021). In the present study, the reduction in AMF colonization under salinity stress was likely due to increased osmotic stress and ion toxicity, decreased water uptake and carbon availability in the host plant, and lower fungal spore germination. Similar results are reported in Colocasia esculenta (L.) Schott (Przybyłko et al. 2021), Gleditsia sinensis Lam. (Shahzad et al. 2021) and Triticum aestivum L. (Poveda et al. 2022; Baltazar-Bernal et al. 2022).

The combination of the F1 and B1 treatments improved rapeseed growth and yield under both saline and non-saline conditions. Studies have shown that salinity stress significantly decreases the growth and yield of Triticum aestivum (Baltazar-Bernal et al. 2022), Gleditsia sinensis (Shahzad et al. 2021), and Colocasia esculenta (Przybyłko et al. 2021). Improved plant growth under salinity stress by AMF and PGPB has been reported in maize (Dąbrowska et al. 2014; Hashem et al. 2019; Baltazar-Bernal et al. 2022; Chaichi et al. 2022). It has been suggested that salinity stress can suppress plant growth and yield by enhancing osmotic and ionic imbalances, and decreasing the uptake of water and nutrients. Furthermore, to decrease osmotic stress, plants respond to osmotic imbalances by closing their stomata, which leads to a decrease in CO2 availability for photosynthesis, thereby reducing plant growth and yield (Baltazar-Bernal et al. 2022). Studies have demonstrated that symbiosis with AMF and PGPB can reduce nutrient uptake imbalances and enhance plant growth and yield by improving external hyphae, hormonal adjustments, the solubilization of minerals and mineral nutrition uptake (Poveda et al. 2019; Karimmojeni et al. 2021; Janah et al. 2021; Ndiate et al. 2022; Meddich 2022).

In the present study, the combination of F1 and B1 treatments improved the antioxidant enzyme activity under saline and non-saline conditions. These results are consistent with those reported in Dracocephalum moldavica L. (Li et al. 2020; Karimmojeni et al. 2021) and Gleditsia sinensis (Shahzad et al. 2021), in which inoculation with PGPR and AMF improved the salt tolerance of the plants by modifying antioxidant enzyme activities. Enhanced antioxidant enzyme activities have also been observed in alfalfa (Ben-Laouane et al. 2020), wheat (Baltazar-Bernal et al. 2022), common bean (Alinia et al. 2021) and lettuce (Ouhaddou et al. 2022, 2023) plants inoculated with PGPB under saline conditions. These results emphasize the positive impact of co-inoculation with AMF and PGPB in improving salt tolerance by increasing antioxidant genes, enhancing antioxidant enzyme activity, scavenging ROS, and reducing oxidative damage.

The results of our experiments showed that inoculation of rapeseed with AMF and PGPB under salinity stress improved gas exchange and photosynthetic pigment content. The results presented for Piper nigrum L. (Moreira et al. 2020) and Cucurbita pepo L. (Amiri et al. 2017; Pagnani et al. 2018) are consistent with those obtained in the present study. In those investigations, symbiosis with AMF improved water absorption and retention by increasing mycelial density and stomatal conductance to maintain photosynthetic capacity under salinity stress. In addition, PGPR inoculation enhances gas exchange and the contents of photosynthetic pigments by increasing nutrient uptake through the root, which maintains the osmotic balance in cells, thereby improving metabolism (Ghanbarzadeh et al. 2019). It is generally agreed that reduced photosynthetic pigment content and stomatal conductance, and the resulting damage to the photosynthetic apparatus, decreases plant growth and yield under saline conditions. Decreased nutrient uptake through the root during environmental stresses can cause a reduction in photosynthetic pigment synthesis (Khaleghnezhad et al. 2021). On the other hand, the increase in gas exchange and the content of photosynthetic pigments in inoculated plants under salinity stress can be considered as a salinity tolerance strategy. This strategy reduces the effects of salinity stress by increasing the pigment biosynthesis pathway by the action of specific enzymes and maintaining osmotic balance through improved water circulation and nutrient uptake (Abdel Latef and Chaoxing 2014; Ouhaddou et al. 2023).

Consistent with reports on Anethum graveolens L. (Harris-Valle et al. 2018), this study indicated that salinity stress significantly decreased the oil percentage in rapeseed seeds, but the combination of AMF and PGPR increased oil content. This increase in oil percentage may be related to the absorption of P owing to an improvement in soil microbial activity (Harris-Valle et al. 2018).

In the present study, the K+ concentration and K+/Na+ ratio in the shoots and roots of rapeseed significantly increased following inoculation with AMF and PGPB. Similar results have been reported in maize (Mia et al. 2010; Biareh et al. 2022) plants inoculated with AMF and maize (Dąbrowska et al. 2014; Chandrasekaran et al. 2019) inoculated with PGPR under salinity stress. The mechanisms involved in enhancing the K+ concentration and K+/Na+ ratio in plants inoculated with AMF and PGPB are not yet understood, but they might be related to the selective absorption of K+ and inhibition of Na+ transport by inoculated plants (Mia et al. 2010; Dąbrowska et al. 2014). Ait-El-Mokhtar et al. (2020) pointed out that AMF was increased salt tolerance by improving the water and nutrient status of host plants via the boosting of nutrient and water uptake by the hyphal structure effect and the activity of ion and water transporters. It has been suggested that absorption of K+ and exclusion of toxic Na+ are important for maintaining cytosolic ion homeostasis in terms of the K+/Na+ ratio (Adhikari et al. 2020; Chaichi et al. 2022; Biareh et al. 2022). Adequate K+ plays a key role in protein synthesis, enzyme activation and photosynthesis, turgor maintenance, and stomatal movement (Mia et al. 2010).

Conclusions

The results of the present study showed that the symbiotic association of rapeseed with the combination of F1 and B1 treatments would augment salt tolerance and AMF colonization by improving the activity of antioxidant enzymes, K+ concentration in shoots and roots, and photosynthesis capacity, and decreasing the Na+ concentration in shoots and roots. Furthermore, the oil percentage of rapeseed changed under salt-stress conditions and symbiosis with F1 and B1. The F1 plus B1 treatments led to a significant increase in the oil percentage. It could be concluded that symbiotic associations can be considered an important tool for reducing the negative impacts of salt stress conditions on plant growth and yield. Furthermore, further investigations on different species of AM and PGPR and various cultivars of rapeseed in greenhouse and field experiments are required to confirm the present results.