Vascular bundle adaptation to water-salt stress in sweet sorghum: An integrated anatomical and physiological pathway...

As an energy crop, sweet sorghum depends on vascular transport of water and assimilates for stalk yield and sugar accumulation. How water and salt stress influence plant growth, stem vascular bundle microstructure, and sugar accumulation, as well as the underlying regulatory pathways, remains unclear. In this study, pot experiments were conducted to assess the effects of salt and water on sweet sorghum. Specifically, to elucidate the regulatory pathways involved, we conducted a comprehensive analysis that integrated assessments of plant growth, stem sugar accumulation, and vascular bundle anatomy with a structural equation model (SEM). The results showed that water-salt stress notably suppressed plant height, leaf area, stem biomass, and both the concentration and accumulation of soluble sugars in sweet sorghum stems. With intensifying stress, stem maximum hydraulic conductance (K_max) decreased significantly (p < 0.001); the diameter of secondary xylem and the number of vascular bundles declined by 6.7%-20.0% and 10.7%-28.3%, respectively, whereas vascular bundle density increased significantly by 0.46-1.31 times. Correlation analysis indicated that salt stress was the primary environmental factor driving growth performance and sugar quality (r > 0.65, p < 0.001). Among the vascular bundle anatomical structures, vascular bundle density correlated most strongly with leaf area (r = 0.812), followed by stem cross-sectional area (r = 0.764) and K_max (r = 0.656). Structural equation modeling further demonstrated that soil salinity and water treatments exerted indirect effect on stem biomass and soluble sugar accumulation by modulating the vascular anatomy and maximum hydraulic conductance of sweet sorghum stems. Moreover, vascular bundle density serves as the primary limiting factor in vascular anatomy, whereas maximum hydraulic conductance acts as the key functional mediator; Collectively, stem biomass and soluble sugar concentration explained 95.7% of the total variance in soluble sugar accumulation. This study clarifies the quantitative structure-function response pathways of the stem vascular system under water-salt stress, and providing a theoretical basis for improving energy crop performance under suboptimal conditions.