Comparative Mechanisms of Silica Nanoparticles and Cellulose Nanocrystals in Mitigating Drought Stress in Tall Fescue Roots

• SiO₂ NPs and CNCs enhanced the drought tolerance of tall fescue. • SiO₂ NPs and CNCs enhanced root growth, osmoregulation, and antioxidant defense. • SiO₂ NPs conferred root drought tolerance by upregulating lignin/sucrose metabolic genes. • CNCs promoted root stele development and upregulated glutathione metabolism genes. Tall fescue ( Festuca arundinacea Schreb.) is a herbaceous plant widely used in ecological restoration in China. However, its roots are highly sensitive to soil moisture fluctuations, and drought conditions easily lead to decreased root hydraulic conductivity, imbalance in redox homeostasis, and decline in carbon metabolism function, along with significant structural degradation. Together, these impairments ultimately inhibit plant growth and severely affect ecosystem stability. To reveal the underlying mechanisms by which different types of nanomaterials alleviate drought stress in tall fescue roots, this study systematically compared the responses of roots at morphological, anatomical, physiological, biochemical, and transcriptomic levels through soil application of inorganic silica nanoparticles (SiO 2 NPs: 1500 mg/kg) and organic cellulose nanocrystals (CNCs: 600 mg/kg). The physiological status of leaves was also simultaneously analyzed to assess the overall effects. The results showed that both SiO₂ NPs and CNCs effectively mitigated drought-induced root structural degradation and functional impairment by promoting root development, strengthening osmotic adjustment and antioxidant defense, thereby improving the physiological status of the shoot and ultimately leading to a significant increase in biomass. Specifically, SiO₂ NPs increased biomass by 32.11% compared to drought-stressed control plants ( n = 3, p < 0.05), while CNCs increased it by 21.21% ( n = 3, p < 0.05). Further analysis revealed that SiO₂ NP treatment enhanced leaf photosynthetic and carbon assimilation capacity, increasing carbon supply to the roots, and significantly promoted the accumulation of starch and sucrose in roots. This treatment concomitantly upregulated key genes involved in lignin biosynthesis and sucrose metabolism, thereby establishing a slow-release drought tolerance mechanism centered on root structural reinforcement and carbon reserve regulation. In contrast, CNCs significantly alleviated aboveground water deficit by promoting the development of the root stele, and activated a multi-enzyme antioxidant network centered on glutathione metabolism, thereby establishing a rapid-response drought tolerance mechanism characterized by root structural optimization and swift reestablishment of redox homeostasis. These findings elucidate the distinct mechanisms by which the two types of nanomaterials enhance drought tolerance in tall fescue by systematically remodeling root structure and function, providing new insights for plant stress management.