Mechanical damage characteristics and mesoscopic scale simulation of leaf sheath vascular bundles in large rice seedlings during the seedling separation process

Mechanical transplanting is a key approach to large-scale and high-efficiency rice production. However, during the high-speed seedling separating process, bending and squeezing are prone to occur in seedling stems, resulting in seedling damage and subsequently affecting seedling establishment rate and yield. With the promotion and application of mechanical transplanting using large seedlings, although large seedlings exhibit strong stress resistance, their leaf sheath tissues are still subjected to significant impact loads during the separating stage. As the primary load-bearing tissue in the stem, the damage mechanism of the leaf sheath vascular bundles remains unclear, leading to a lack of reliable mesoscopic-scale biomechanical evidence for optimizing the operating parameters of the separating-planting mechanism. Therefore, the mechanical damage characteristics of leaf sheath vascular bundles during the separating process of large rice seedlings were investigated in this study, and the relationship between vascular bundle damage and machine parameters was clarified. 35 d large rice seedlings were used as research material, and leaf sheath vascular bundles were isolated by enzymatic maceration to reduce the influence of mechanical peeling on tissue structural integrity. Typical loading forms during the separating process were simulated by stem bending tests on large rice seedlings. Two bending modes, along the short axis and the long axis of the elliptical cross-section, were set, and bending displacements of 1.5, 3.0, 4.5, and 6.0 mm were applied. After bending-induced damage, tensile tests were performed on the isolated vascular bundles, and the effects of bending mode and bending displacement on the mechanical parameters of the vascular bundles were analyzed. The experimental results showed that the material properties of the vascular bundles were not altered after damage, whereas tensile strength decreased markedly, and the tensile-strength reduction induced by short-axis bending was more pronounced. Accordingly, the critical bending displacements for vascular bundle damage were determined to be 3.0 mm and 4.5 mm under short-axis and long-axis bending, respectively. A biomechanical model and a bending finite element model of leaf sheath vascular bundles in large rice seedlings were constructed, and mesoscopic-scale mechanical simulations were performed. The simulation results showed that stress was mainly concentrated in the vascular bundle sheath region, with more severe stress concentration under short-axis bending. Meanwhile, the stress transmission pattern within vascular bundle tissues remained consistent across different bending modes, indicating that the bending-mode dependence was mainly reflected in the intensity of stress concentration rather than in the stress transfer pathway. Based on the damage thresholds under different bending modes, the maximum operating speed was optimized in combination with the kinematics of the separating-planting mechanism, and the maximum speeds were obtained as 181 r/min under short-axis bending and 243 r/min under long-axis bending. These results provide a theoretical basis for optimizing the operating parameters of the transplanting mechanism, and they also offer a referable methodology for biomechanical modeling studies of plant tissue damage.