Structural adaptations of curved-stem moso bamboo: tissue specialization for resilience under mechanical stress

Reaction tissues in plants develop in response to external stresses. In bamboo, the absence of secondary thickening has been the cause of an ongoing debate over the role of reaction tissues in stress responses. Herein, we evaluated the structural and mechanical basis underlying the curved-stem moso bamboo (Phyllostachys edulis) to determine whether reaction tissues are involved. Our findings reveal that curved-stem moso bamboo exhibits adaptive structural features across multiple hierarchical levels. At the macroscopic level, the convex side of the culm, subjected to greater compressive stress, shows a growth strain magnitude 3.9 times higher than that of the concave side. This side also develops a thicker culm wall, resulting in an eccentric elliptical cross-section geometry that reduced internal stress by up to 8.8 %. At the cellular level, underdeveloped fibers with poorly layered cell walls form a "hinge-like" interface between fiber and parenchyma tissues, alleviating stress concentration in fibers. Additionally, the lignin content in the cell walls is significantly higher by 9 % compared to that of upright-stem bamboo, while the microfibril angle increases markedly (exceeding 20 °). This combination facilitates synergistic load-bearing between the cellulose skeleton and the surrounding matrix under tensile stress. These structural adaptations on both convex and concave sides lead to reduced stiffness and strength, as evidenced by axial tensile moduli that are 27.3 % and 54.7 % lower than those of upright-stem bamboo. However, they enhance stress transmission efficiency and effectively mitigate the bending effects induced by gravitational forces. This challenges the notion that monocots cannot form reaction tissues and has significant implications for heterogeneous high-performance biomimetic design. STATEMENT OF SIGNIFICANCE: It is questioned whether reaction tissue exist in curved-stem moso bamboo (Phyllostachys edulis). Our analysis has revealed adaptive structural features of curved-stem moso bamboo across multiple hierarchical levels. At the cellular level, underdeveloped fiber cells with fewer cell wall layers create a "hinge" structure between the fibers and parenchyma tissues. Lignin concentration increases in cell walls, and microfibrils alignment angle increased (> 20°). These adaptations reduce stiffness and strength on both the convex and concave sides, enhancing stress transfer and mitigating the effects of gravitational forces.