The effect of mycorrhizal fungi and different levels of salt stress on nutrient uptake and root colonization percentage in Lycium depressum

Background and Objective:
Salinity stress is a major challenge in natural and rangeland areas, particularly in arid and semi-arid regions, with significant negative impacts on plant growth and performance. This stress limits plant growth by disrupting the absorption of water and essential nutrients such as phosphorus, nitrogen, and potassium, thereby reducing plant growth and productivity. Arbuscular mycorrhizal fungi (AMF) play a crucial role as root symbionts in enhancing nutrient uptake, increasing plant resistance to environmental stresses, and reducing the absorption of toxic ions like sodium. The medicinal plant Lycium depressum is important for soil stabilization and biodiversity conservation in arid and semi-arid regions due to its relative tolerance to salinity and drought. However, there is limited information on the impact of AMF symbiosis on nutrient uptake and salt tolerance in this plant. The objective of this study is to investigate the effects of different salinity levels and inoculation with various AMF species (including Funneliformis mosseae, Rhizophagus intraradices, and a combination of F. mosseae + R. intraradices) on nutrient absorption (sodium, potassium, calcium, magnesium, and phosphorus) and root colonization percentage in L. depressum. This research aims to develop effective biological strategies for improving the establishment and sustainability of this valuable species in saline ecosystems and providing solutions for the optimal management of degraded rangelands.
Materials and Methods:
This study was conducted as a pot experiment in a factorial design using a completely randomized block design with four replications, totaling 64 experimental units in the greenhouse of Gorgan University of Agricultural Sciences and Natural Resources in 2022. The experimental treatments consisted of two factors: 1) Inoculation with arbuscular mycorrhizal fungi (AMF) at four levels: treatments with Funneliformis mosseae (F1), Rhizophagus intraradices (F2), a combination of F. mosseae + R. intraradices (F1+F2), and a control without fungi (F0); 2) Different salinity levels induced by sodium chloride at four levels: a control (soil salinity of the native habitat, equivalent to 6 dS/m) and levels of 10, 14, and 18 dS/m. Cuttings of Lycium depressum were collected from the Qaraqara large rangeland hills and, after rooting in a washed sandy bed, were transferred to pots containing sterilized soil from the native habitat. Fungal inoculum (60 grams, containing 60 spores per gram) was applied as a thin layer 1 cm below the root in each pot, and salinity levels were applied gradually. Plants were maintained under controlled greenhouse conditions for two years, with a mean temperature of 21°C to 27°C, 8 hours of darkness and 16 hours of light, and humidity maintained at 70% to 80% of field capacity. The concentration of nutrients (sodium, potassium, calcium, magnesium, and phosphorus) was measured in leaves and roots. Root colonization percentage was determined using the trypan blue staining method and counting fungal structures with a light microscope. Data were analyzed using the General Linear Model (GLM) in Minitab software version 19, and mean comparisons were performed using Tukey's test at a 5% probability level. Graphs were drawn using Excel 2016.
Findings:
The results of this study showed that increasing salinity levels significantly reduced growth and nutrient uptake in L. depressum. However, inoculation with arbuscular mycorrhizal fungi (AMF), particularly the combined treatment (F1+F2), had a positive effect on nutrient absorption. In mycorrhizal treatments, the concentration of potassium, calcium, magnesium, and phosphorus in leaves significantly increased (p < 0.05). The highest absorption of these elements was observed at a salinity level of 6 dS/m (S1). At this salinity level, the combined treatment F1+F2 increased phosphorus absorption by up to two-fold and calcium absorption by more than two-fold compared to the control. Additionally, at this salinity level, root colonization percentage in the F1+F2 treatment reached 26.325%, which was 39 times higher than the control. As salinity increased to levels of 10, 14, and 18 dS/m (S2, S3, and S4), the positive effect of the combined treatment F1+F2 on nutrient absorption and root colonization decreased, but it still outperformed the control. For example, at the S4 salinity level, potassium absorption in the F1+F2 treatment increased by more than two-fold compared to the control. Furthermore, the F1+F2 treatment significantly improved calcium and magnesium absorption across all salinity levels, such that at the S4 level, calcium absorption in this treatment was more than three and a half times that of the control. The fungal combination F1+F2 also reduced sodium absorption in roots (by up to 50%) and leaves (by up to 64.3%), especially at higher salinity levels, and enhanced plant tolerance to salinity through synergistic effects. These findings suggest that the fungal combination F1+F2 can be used as an effective strategy to improve nutrient uptake and enhance the tolerance of L. depressum to salinity stress.
General Conclusion:
This study demonstrated that salinity stress significantly affects the absorption of nutrients (sodium, potassium, calcium, magnesium, and phosphorus) and root colonization by arbuscular mycorrhizal fungi (AMF) in Lycium depressum. However, inoculation with a combination of two fungal species (F1+F2), particularly at lower salinity levels, effectively mitigated the adverse effects of salinity. The combined treatment F1+F2 played a crucial role in enhancing the absorption of essential elements like calcium and magnesium (in some salinity levels, several times higher than the control) and significantly increasing root colonization (up to 39 times at the lowest salinity level and notably at other levels). Additionally, the F1+F2 treatment reduced sodium absorption in roots by up to 50% and in leaves by up to 64.3%, especially at higher salinity levels, and improved plant tolerance to salinity through synergistic effects. These findings suggest that using this fungal combination can be an effective strategy for managing salinity stress in L. depressum and potentially similar species. This approach is particularly important for projects aimed at rehabilitating saline rangelands and sustainably developing land use in arid and semi-arid regions facing soil salinity issues.