Role of stele and cortex in understanding and predicting root tensile properties

The mechanical properties of plant roots are crucial for soil stabilization and vegetation restoration. To effectively employ bioengineering methods, understanding the tensile properties of plant roots is essential. In most studies, root diameter is used as a predictor of tensile strength but this fails to accurately describe root mechanical behaviour. The stele and cortex are two anatomical parts of the root whose actual mechanical behaviour and specific contributions to root biomechanisms remain unclear.

Tensile tests and scanning electron micrography were performed on roots of four typical species (Robinia pseudoacacia, Pinus tabuliformis, Vitex negundo and Syzygium aromaticum) in the Loess Plateau of China to investigate the roles of the stele and cortex in explaining the root's tensile strength. Then, based on the 'same strain' principle, a tensile strength prediction model was developed and validated using experimental data from plant root.

The stele and cortex of roots exhibited distinct mechanical behaviours: elastic plasticity and linear elasticity, respectively. Tensile strength was negatively correlated with diameter and stelar diameter and cortical thickness were positively correlated with diameter. The cortex had lower tensile strength, strain at maximum stress and thickness compared with the stele. The observed increase in scatter of tensile strength with decreasing root diameter was attributed to the higher coefficient of variation in cortical tensile strength compared with the stele. Notably, predicted results of intact root tensile strength fell within the 95 % prediction interval of the measured intact root tensile strength and could be enhanced 30-80 % by strengthening dataset quality.

Our results demonstrated the actual mechanical behaviour characteristics of cortex and stele, and provide a new perspective for addressing the mechanical properties of roots using composite materials mechanics. The findings of this study will provide a theoretical foundation for implementing plant-based ecological restoration and disaster prevention measures.