Drought represents one of the most significant and pervasive natural disasters worldwide. Due to global warming, the frequency, intensity and spatial distribution of droughts have increased markedly, posing a substantial threat to food and ecological security and becoming a primary factor limiting the sustainable development of social and ecological systems (Cheng et al., 2024; Feng et al., 2024). The role of vegetation in mitigating climate change is significant, because of its capacity for carbon sequestration (He T. et al., 2023). Nevertheless, the accelerated pace of large-scale construction projects inevitably exerts considerable influence on the surrounding ecological environment (Gan et al., 2018). In particular, the damage to slopes created by earthmoving excavation has resulted in the destruction of original topography and vegetation. This results in a significant loss of habitat conditions, which in turn leads to a reduction in biodiversity and an increase in soil erosion and other problems. It is challenging to restore the original appearance by relying only on natural restoration techniques. Traditional slope protection methods tend to prioritize mechanical stability, but they lack the ecological functions of landscape and self-repair. This makes it difficult to achieve the restoration or reconstruction of the vegetation cover (Chen et al., 2022; Fang et al., 2022; Ganapathy et al., 2024; Tudorica and Bob, 2024). Consequently, there has been a notable increase in the utilization of ecological slope protection methods, including the vegetation concrete technique (Shen et al., 2023). This involves the mixing of soil, cement, organism of habitat material, amendment of habitat material and vegetative seeds in specific proportions that are determined, by the characteristics of the slopes in the restoration area and the landscape needs. The resulting mixture is then laid on the slopes using barbed wire and fixed anchors and sprayed onto the slopes using sprayers. This process facilitates regreening and slope protection (Yang et al., 2019). This approach has facilitated the large-scale application of vegetation concrete, which provides engineering protection and supports ecological restoration, and has the potential to be sustainable and to sequester carbon (Cheng et al., 2020; Zhao et al., 2022; Xu et al., 2024). However, at present, slope reinforcement and vegetation restoration cannot be conducted concurrently in dry areas.

In recent years, the majority of research conducted on vegetation concrete has focused on enhancing its physico-mechanical strength through the incorporation of external admixtures and improving its fertility characteristics (Huang et al., 2021; Liu D. et al., 2023). This research has predominantly been concentrated in alpine areas (Yang Y. et al., 2023) and regions that experience heavy rainfall (Li M.Y. et al., 2022), with comparatively less research in arid zones. In the initial phase of vegetation concrete restoration of steep slopes, the elevated and precipitous topography gives rise to the formation of surface runoff on the slope surface due to insufficient infiltration of water. Furthermore, plants are susceptible to drought stress due to the evapotranspiration effect of the substrate (He J. et al., 2023). Drought impedes plant root growth and development. In response to drought stress, plants reduce their root and stem lengths and decrease their biomass (Bhandari et al., 2023). The persistence of drought stress has the capacity to alter the morphological structure of plants, as well as the structure of plant chloroplasts. This results in a reduction in chlorophyll content and the closure of plant stomata, in addition to a decline in the net photosynthetic rate, transpiration rate and stomatal conductance of plant leaves. This, in turn, has a direct impact on photosynthetic efficiency (Luan et al., 2023; Zhang et al., 2023; Wang et al., 2024). Concurrently, drought exerts a detrimental impact on soil structure, reducing soil productivity and restricting plant water and nutrient uptake. Additionally, it impairs reactive oxygen species metabolism and membrane lipid fluidity in plants, which in turn precipitates peroxidation of plant cell membrane lipids, osmosis, and redox imbalance, ultimately leading to plant death (Wang et al., 2022; Lei et al., 2023; Yang W. et al., 2023). These issues are particularly prevalent in the ecological restoration of slopes. The implementation of input control methods, such as the establishment of irrigation systems and replanting measures, can temporarily alleviate the aforementioned stresses (Tamura et al., 2017; Strack and Stoll, 2022). However, these measures lack long-term efficacy in improving and restoring plant nutrient utilization and ecosystem self-sustainability. Consequently, the deterioration of ecosystems persists, often accompanied by high maintenance costs and significant resource inputs (Guo et al., 2024). Recently, plant–microbe symbiosis has received widespread attention and compared with traditional ecological restoration techniques, microorganisms have the potential to establish beneficial symbiotic relationships with plants, thereby mitigating the adverse effects of environmental stressors on plant health (Gehring et al., 2020; Abideen et al., 2022; Chieb and Gachomo, 2023). Thus, this approach provides a promising new strategy for improving the drought tolerance of plants used in the ecological restoration of steep slopes and for improving the efficiency and sustainability of ecological restoration projects on steep slopes.

Symbiotic fungi play a significant role in ecological processes (He et al., 2024), as evidenced by the effectiveness of root-associated endophytic fungi in enhancing plant performance and drought resistance (Lu et al., 2021; He et al., 2022a). The term “dark septate endophytes” (DSE) is used to describe a group of small soil fungi that colonize plant roots. These fungi have dark-colored mycelia and distinctive septa, which widely colonize plant root cells and are capable of forming “microsclerotia” structures (Liu and Wei, 2019). The extended mycelia of DSEs are capable of modifying the morphology and structure of plant roots, expanding the root network and enhancing the water uptake of host plants under drought stress (Li et al., 2024). Liu N. et al. (2022) observed that the incorporation of a DSE inoculant led to a notable increase in seedling biomass and the activities of drought-resistant enzymes. Under drought stress, DSE inoculation has been demonstrated to reinforce the resilience of plant root cells; safeguard the structural integrity of cell membranes, nuclei, mitochondria, and other organelles; and mitigate the adverse effects of drought stress on these organelles (Liu and Wei, 2019). Furthermore, the colonization of plants by DSEs has been show to stabilize photosynthetic parameters, reduce stomatal conductance and decrease overall water loss (Li M. et al., 2022). Numerous studies have demonstrated that DSEs play a role in regulating the endogenous hormone content of host plants, as well as increasing the secretion of proline, superoxide dismutase and catalase. Furthermore, it has been observed that DSEs support the synthesis of melanin under stressful conditions by binding free oxygen, thereby protecting cell membrane stability and enhancing plant resistance to environmental stresses, including drought (He et al., 2022b; Bi and Xue, 2023; Li W. et al., 2023).

The impact of drought on the ecological restoration of slopes is significant, resulting in adverse effects on the normal growth and development of plants used for slope protection. DSEs have been demonstrated to increase drought resistance in host plants, thereby conferring resilience to plants growing in areas experiencing desertification and drought (Li X. et al., 2022) and increasing the productivity of medicinal and agricultural crops (Han et al., 2021; Qu et al., 2022). Nevertheless, DSEs are less frequently employed for slope ecological protection; thus, this study proposes the introduction of DSEs into the domain of vegetation concrete. By simulating drought stress, this study investigated the effects of DSEs on the physiological and ecological characteristics of Cynodon dactylon in vegetation concrete under drought conditions. The aim was to clarify the mechanisms through which DSEs increase the drought tolerance of slope-stabilizing plants to provide new solutions and theoretical foundations for improving the drought tolerance of these plants and advancing the ecological restoration process of vegetation concrete.

Drought is one of the most significant environmental factors limiting the growth and development of plants. The development of a community of endophytic fungi and host plants represents a significant strategy for enhancing the resilience of host plants (Pang et al., 2020; Lu et al., 2023; Gu et al., 2024). The present study revealed a significant reduction in both the height and biomass of C. dactylon under drought conditions. However, compared to the CK, inoculation with DSEs, promoted the plant height and biomass of C. dactylon. This may be attributed to the inhibition of plant cell division under drought, which ultimately results in the retardation of plant growth and development (Ganie and Ahammed, 2021). The mycorrhizal symbionts formed by inoculation with DSEs function as bridges between the host plant and the soil, increasing the contact area between the plant root system and the soil and facilitating water uptake by the plant (Li M. et al., 2022). Furthermore, DSEs facilitate the mineralization of organic matter and the conversion of insoluble phosphates in the soil through the secretion of particular enzyme metabolites, including proteases and xylanases (Luo et al., 2023). The mycelium of DSEs around the root system of a plant assists in the retention of water and the transportation of nutrients to the interior of the plant, promote the uptake of nutrients by the plant (Ruotsalainen et al., 2022). They assist host plants by increasing their capacity to endure drought stress and further promote plant growth.

The process of photosynthesis serves as the foundation for plant growth and development, as well as the accumulation of materials (Wang et al., 2022). Photosynthesis is a fundamental process that directly affects plant growth, with water being one of its primary influencing factors. The present study demonstrated that the Pn of C. dactylon significantly decreased under drought conditions, with the magnitude of this decrease increasing in conjunction with the intensification of drought stress. Concurrently, the Ci of the plant leaves increased as the Pn and Tr decreased. Consequently, this study concluded that non-stomatal factors played a pivotal role in limiting photosynthesis in response to drought These findings are consistent with those reported by Liu et al. (2024). At this juncture, the chloroplast structure was irreversibly compromised, which directly resulted in the weakening of the light reaction process and a decrease in the photosynthetic rate (Li Y. et al., 2023). Compared with the Pn values of plants that had not undergone DSE inoculation, the Pn of the treated C. dactylon plants was sustained at a high level following inoculation with DSE, and the Gs and Tr also increased, which indicated that inoculation with DSE increased the photosynthetic capacities of the plants. The increase in leaf Gs and Tr under DSE inoculation facilitated the transport and conduction of plant nutrients, thereby enabling the plant to obtain a greater quantity of nutrients (Miranda et al., 2023). Furthermore, the increase in Tr regulated the surface temperature of the plant leaves, preventing overheating and reducing the damage caused by drought stress. These findings indicate that inoculation with DSEs may regulate plant water use efficiency and increase resistance to drought. Furthermore, evidence suggests that melanin in the fungal cell wall may contribute to enhancing host plant resilience. Additionally, melanin in DSEs has been demonstrated to increase the mechanical strength of the cell wall, protect DSE hyphae under drought conditions, and safeguard organelles to ensure that photosynthesis occurs (Zuo et al., 2022).

In response to drought-induced osmotic stress, osmotic adjustment represents a crucial mechanism through which plants regulate water potential, thereby enabling them to adapt to drought stress (Hanif et al., 2021). Pro, SS and SP are highly sensitive to alterations in soil water levels and are capable of reducing the cellular osmotic potential when plants are subjected to drought stress. The accumulation of these compounds is frequently employed as a marker of drought resistance in plants (Liu F. et al., 2023). The results of this study indicate that the contents of Pro and SS increased rapidly in plants subjected to drought stress. This suggests that C. dactylon can increase the contents of Pro and SS through its own metabolic processes, thereby enabling plant cells to retain water and preventing the normal development of plants from being affected by dehydration. Notably, the change in SP content observed in this study differed from the findings of Wang et al. (2019) who reported that the SP content increased with increasing drought intensity. In this study, there was a significant reduction in the SP content under severe drought stress. This discrepancy may be attributed to the loss of proteins due to severe damage to the cell membrane under severe drought stress, with some of the proteins being diverted for cell membrane repair (Liu H. et al., 2022). The contents of Pro, SS, and SP in C. dactylon were significantly increase following DSE inoculation in this study. Notably, the increase in the plant SP content caused by DSE inoculation was more pronounced under severe drought stress. These findings indicate that DSE inoculation can facilitate the accumulation of osmoregulatory substances in plants and reduce the osmotic potential of host cells, thereby mitigating the dehydration loss caused by drought stress and further promoting the response of plant defense mechanisms against drought stress.

The cell membrane is a crucial interface between plant cells and organelles and the surrounding environment. It plays a pivotal role in maintaining cellular stability and regulating material exchange, energy flow and information transfer (Zhang H. et al., 2022). The tolerance of plants to stress is closely related to the stability of the biofilm structure and its function (He et al., 2020). Drought stress frequently results in the extravasation of organic molecules and electrolytes within cells, which in turn exacerbates membrane lipid peroxidation and ultimately leads to destruction of the structure and integrity of plant cell membranes (Wang et al., 2023). MDA which is produced because of cellular membrane lipid peroxidation, provides a measure of the extent of damage to plant cells caused by adversity (Batool et al., 2020). The results indicated that MDA content increased in direct proportion to the severity of drought stress, reinforcing the hypothesis that severe drought stress leads to significant damage to cell membranes. Conversely, the MDA content of C. dactylon inoculated with DSE under drought conditions exhibited a notable decline compared to that of C. dactylon in the CK. These findings suggest that treatment with DSE may mitigate the accumulation of MDA in C. dactylon and reduce the formation of reactive oxyradicals, thereby limiting the extent of cell membrane lipid peroxidation and mitigating the adverse effects of drought stress on C. dactylon.

When plants experience drought stress, elevated levels of reactive oxygen species (ROS) are typically produced; these ROS contribute to the breakdown of lipids, proteins, and nucleic acids within plant cells. Furthermore, ROS function as pivotal signaling molecules throughout the cell death pathway (Liu N. et al., 2022). The antioxidant system is crucial for plants and microorganisms to cope with oxidative damage (Li X. et al., 2022). A substantial accumulation of ROS activates antioxidant enzymes in plants, with various enzymes acting in concert to safeguard plants from stress (Akhtar et al., 2022). Furthermore, some scholars have demonstrated that endophytes can increase antioxidant enzyme activities in plants by influencing gene expression during stress and increasing the capacity of plants to withstand adverse stress (Zhang Y. et al., 2022). The findings of the present study indicate that the antioxidant enzyme activities in plants first increased, followed by a subsequent decrease in response to an increase in the degree of drought stress. SOD is considered a crucial enzyme in the scavenging of reactive oxyradicals. Specifically, plant SOD activity was elevated in response to the DSE treatments, potentially due to the promotion of its synthesis by DSEs. This facilitates the conversion of the more toxic O₂ ^·− to the less toxic H₂O₂. The observed increase in SOD activity may, therefore, positively influence the conversion of ROS to H₂O₂ in plants (Singh et al., 2020). The inoculation of DSEs under drought stress increased the activities of POD and CAT in the plants. This may be attributed to the fact that H₂O₂ produced by the disproportionation of ROS by SOD requires further conversion into non-toxic H₂O and O₂ by the synergistic action of the more active POD and CAT (Song et al., 2024). These findings suggest that the excess ROS produced by the plants under drought stress were effectively scavenged, alleviating the drought-induced membrane lipid peroxidation of C. dactylon cells. Therefore, treatment with DSEs helped the plants to increase their antioxidant enzyme activities in vivo and maintain them at high levels, effectively scavenging the excess ROS produced by drought stress in the plant and ensuring the stability and integrity of the cell membrane.

According to the results of principal component analysis and correlation analysis, the photosynthesis, osmoregulation and antioxidant enzymes of plants play significant roles in resistance to adverse stress. When C. dactylon is subjected to drought stress, increased photosynthesis positively promotes the accumulation of C. dactylon biomass. Moreover, drought stress activated the antioxidant mechanism in C. dactylon and further increased the activity of antioxidant enzymes after inoculation with DSEs, which helped eliminate the excessive amounts of ROS produced due to drought stress. In addition, the accumulation of osmotic adjustment substances such as Pro, SS and SP helps maintain the imbalance of osmotic pressure inside and outside the cells caused by drought and reduces lipid peroxidation of cell membranes. The coordinated work of various systems in plants helps C. dactylon resist drought stress and maintain its normal growth.

The objective of this study was to evaluate the impacts of three distinct DSEs on the growth of C. dactylon in vegetation concrete subjected to drought stress. Despite the inhibitory effects of drought stress on the normal physiological mechanisms of C. dactylon, which resulted in a reduction in both plant height and dry weight, C. dactylon exhibited robust drought tolerance when inoculated with DSE. Compared with those of the CK, the Pn, Gs and Tr of C. dactylon increased by 33.85%, 46.17% and 33.64%, respectively, following inoculation with DSE. Compared with the uninoculated plants, C. dactylon plants in the treatments with DSE strains had a greater accumulation of osmoregulatory substances and higher antioxidant enzyme activities. Consequently, DSE has the capacity to mitigate the adverse effects of drought on C. dactylon in vegetation concrete to a certain extent and has the potential for further development in the application of ecological restoration technology for vegetation concrete slopes under adverse conditions.