Effects of T. asperellum on salt ion content, sodium adsorption ration, and pH of maize seedlings under saline–alkaline stress
After applying spore suspensions of T. asperellum at different concentrations, we observed significant increases in the soil contents of Ca2+, Mg2+, and K+ relative to those in the control, whereas, Na+, HCO3−, Cl−, and SO42− contents significantly decreased (Table 1). Thus, increasing T. asperellum spore densities in suspension effectively regulated the soil ion balance in the rhizosphere of maize seedlings, and all ions showed significant differences under treatment T3. Compared with those in the control, T3 significantly reduced the Na+ and HCO3− contents by 19.46% and 35.87% in XY335, and 20.02% and 36.29% in JY417, respectively, with an effect more pronounced than that with treatments T1 and T2. Although the Cl− and SO42− contents were low, their variation patterns were similar to that of HCO3− content. Overall, however, the composition of ions in the rhizosphere of maize seedlings was improved by the T. asperellum treatment.
As shown in Table 1, compared with those in the control, T. asperellum treatment significantly reduced the soil pH and SAR values, although with no significant cultivar × treatment interaction effects (P < 0.05). Under saline–alkaline stress, pH was 9.26 and 9.15 in soils planted with XY335 and JY417, respectively, and we found that for both maize cultivars, the pH and SAR values in the rhizosphere soil decreased with an increasing concentration of T. asperellum spores. Under treatment T3, pH was 8.73 and 8.66, while SAR was 3.45 and 3.15 for XY335 and JY417, respectively, which were all significantly different from those recorded under treatments T1 and T2. However, although soil pH and SAR for XY335 did not differ significantly between treatments T1 and T2, we detected significantly different responses between these two treatments in JY417. Further, pH and SAR values in JY417 were lower than those in XY335, indicating that JY417 showed a certain degree of tolerance to saline–alkaline stress.
Effects of T. asperellum on the nutrient contents of maize seedlings in rhizosphere soil
As shown in Table 2, compared with those in the control, T. asperellum treatment significantly increased soil nutrient parameters, although without any significant cultivar × treatment interaction effects (P < 0.05), and with the rhizosphere soil nutrient content for JY417 being higher than that for XY335. Similarly, soil chemical parameters in the rhizosphere soil were significantly improved, being higher after treatment with T. asperellum than those in the control. Furthermore, nutrient contents increased with increasing T. asperellum spore concentration, showing notable differences between treatments T1 and T2. Compared with those in the control, there were significant increases in the soil contents of OM, AN, AP, and AK: 65.32%, 23.80%, 123.60%, and 46.09% in XY335, and 67.42%, 21.14%, 109.94%, and 48.50% in JY417, respectively, in response to treatment T3.
Effects of T. asperellum on the enzyme activity of maize seedlings in rhizosphere soil
As shown in Table 3, there was a significant increase in soil enzyme activities in XY335 and JY417 rhizosphere soil after T. asperellum application compared to those in the control, although no significant cultivar × treatment interaction effects were detected, except for alkaline phosphatase activity (P < 0.05). Soil enzyme activities in JY417 rhizosphere soil tended to be generally higher than those in XY335 soil. Moreover, we noted that soil enzyme activities were differently enhanced by an increase in T. asperellum concentration from T1 and T2. However, the highest soil enzyme activities were recorded under treatment T3 in each case, which were significantly higher than those in the other treatments. Compared with those in the control, the T3 treatment significantly increased soil urease, phosphatase, sucrase, and hydrogen peroxidase activities by 38.24%, 43.48%, 38.13%, and 55.40% in XY335, and 37.83%, 51.89%, 29.43%, and 52.79% in JY417, respectively. Thus, T. asperellum treatment can significantly increase soil enzyme activities and improve the soil physical and chemical environment in the rhizosphere of maize seedlings.
Growth promotion in maize seedlings treated with T. asperellum under saline–alkaline soil stress
Compared to those in the control, T. asperellum treatment significantly increased the maize seedling growth under saline–alkaline stress by significantly enhancing all measured seedling variables, although we detected no significant cultivar × treatment interaction effects (Table 4, Table S2). Moreover, the dry weight of the root system and root relative water content, volume, superficial area, and activity in seedlings treated with T. asperellum were significantly higher than those in the control, and the effect was concentration dependent.
The recorded values for all variables increased with an increasing concentration of T. asperellum, with the growth-enhancing effects of the fungus on XY335 being more apparent than those on JY417 seedlings.
Effects of T. asperellum on non-enzymatic system contents in the roots of maize seedlings cultured in a saline–alkaline soil
Compared with those control, the soluble sugar and proline contents significantly increased in the roots of T. asperellum-treated maize seedlings (P < 0.05), particularly in those receiving the T3 treatment, which promoted the largest accumulation of these substances. These increases in osmoregulatory substances may explain how treatment with T. asperellum induces systemic resistance to saline–alkaline stress in maize seedlings (Table 5).
To elucidate the effect of the AsA–GSH cycle on alleviating oxidative stress in maize seedling roots under saline–alkaline stress, we examined the GSH/GSSG and AsA/DHA ratios (Table 5), showing that these two ratios were significantly reduced under saline–alkaline stress, but significantly increased in response to treatment with increasing concentrations of T. asperellum spores. We also detected significant cultivar × treatment interaction effects on AsA/DHA. Thus, the AsA–GSH cycle appeared to play an important role in controlling the saline–alkaline tolerance induced by T. asperellum, with XY335 performing better than JY417 in this regard.
Effects of T. asperellum on antioxidant enzyme activities in the roots of maize seedlings cultured in saline–alkaline soil
To determine whether the antioxidant enzyme system plays a role in the T. asperellum-induced saline–alkaline tolerance of maize seedlings, we determined the activities of enzymes involved in the AsA–GSH cycle (Table 6). APX, MDHAR, DHAR, and GR significantly increased in the XY335 and JY417 maize seedling roots with increasing concentration of T. asperellum under saline–alkaline stress, with a significant cultivar × treatment interaction effect on APX. Under treatment T3, APX, MDHAR, DHAR, and GR activities increased by 75.02%, 37.66%, 40.26%, and 76.09% in XY335, and 67.18%, 34.41%, 38.95%, and 58.38% (P < 0.05) in JY417, respectively, compared with those of the control. Thus, overall, T. asperellum can promote the seedlings of both cultivars by enhancing antioxidation enzyme activities, which is beneficial in terms of coping with the generation of excess ROS associated with saline–alkaline stress.
Effects of T. asperellum on the reactive oxygen species accumulation and oxidation parameters in the roots of maize seedlings grown in saline–alkaline soils
When analyzing the effect of T. asperellum on ROS clearance, we found that the contents of H2O2, O2−, and MDA in the roots of maize seedlings growing under control conditions were significantly higher than those in the other treatments, with significant cultivar × treatment interaction effects (P < 0.05; Table S3). After 27 days of treatment, H2O2, O2−, and MDA contents in the roots of maize seedlings had decreased significantly in response to treatment with increasing concentrations of T. asperellum, indicating that T. asperellum can enhance ROS clearance in the roots of maize seedlings by affecting both enzymatic and non-enzymatic systems (i.e., antioxidant enzyme activity and osmoregulation, respectively).
Relationships between the AsA–GSH cycle enzyme activity and soil characteristics
Pearson’s correlation analysis was used to evaluate the relationships between the AsA–GSH cycle enzyme activity and soil characteristics (Table 7). The AsA–GSH cycle enzyme activity was significantly correlated with soil characteristics, exhibiting a positive correlation with soil nutrient and a negative correlation with soil pH and SAR.
Source: Ecology - nature.com
