The compatibility between the morphological characteristics (MCs) of rapeseed bare-root seedlings and transplanters directly affects planting quality. To improve the adaptability of transplanters to the MCs of different rapeseed cultivars, this study focused on six winter rapeseed cultivars (A1: Huyou 17, A2: Huayouza 9, A3: Fengyou 520, A4: Zhongyou 108, A5: Zheyou 50, and A6: Huayouza 62). Five MCs—root length (RL), seedling height (SH), root diameter (RD), stem thickness (ST), and seedling width (SW)—were measured during the seedling ages (25-40 d). Multiple comparisons were conducted to identify cultivars with no significant differences (NSD, α=0.05) in MCs, while skewness and kurtosis were analyzed to assess temporal variations in MC distributions. Quadratic polynomial regression was employed to model the growth trends of MCs for each cultivar. The results showed that the skewness and kurtosis ranges of MCs were –0.65 to 1.16 and 1.62 to 6.28, respectively, indicating significant variability in growth symmetry and concentration both within and among cultivars. As seedling age increased, the number of cultivars with NSD in MCs progressively decreased (25 d: 5; 30 d: 4; 35 d: 3; 40 d: 2), but A3, A5, and A6 maintained consistent stability before 35 d. Based on MC statistical analysis, the key design parameters for the transplanter were determined as follows: flat belt width was 180 mm, positioning bar spacing was 70 mm, flexible belt width was 150 mm, clamping distance was 8 mm, seedling drop height was 180 mm, and planting depth was 40 mm. Field tests demonstrated that the transplanter exhibited superior compatibility with cultivars A3, A5, and A6, achieving seedling delivery success rates of 93.75% (A3), 93.23% (A5), and 92.71% (A6), along with significantly higher planting success rates compared to the control group (A1, A2, A4). This study provides a theoretical basis for optimizing the compatibility between transplanters and rapeseed bare-root seedlings, as well as guiding the structural design of transplanting machinery.

Transplanting offers several advantages, such as alleviating the conflict of crop rotation, shortening the growth cycle of rapeseed, and increasing the unit yield of rapeseed. However, in the rice-rapeseed planting pattern, the growth period of rapeseed is short, manual transplanting is labor-intensive, and there is a lack of mechanized transplanting technology and equipment for rapeseed bare-root seedlings. To address these issues, this study proposed a mechanical transplanting process scheme of "leveling the ridge surface + furrowing and soil gathering + aligning and placing seedlings + soil covering and compacting" based on the agronomic requirements for rapeseed transplanting and the morphological characteristics of rapeseed bare-root seedlings. Meanwhile, a transplanter for rapeseed bare-root seedlings had been developed. The structure and working principle of the transplanter were elaborated. Structural design and theoretical analysis were carried out for key components such as the spiral soil homogenizer, trenching soil aggregating device, row-aligned seedling placement device, and soil covering and compaction device. According to the degree of freedom constraints during the conveying process of rapeseed bare-root seedlings, morphological characteristics, compatibility analysis with each key component, and the quality requirements for each transplanting operation, the key structural and working parameters of the transplanter were determined. The spiral soil homogenizer, with a pitch of 180 mm, a pitch angle of 64°, and a blade diameter of 200 mm was designed. When the furrowing disc inclination angle, center distance, and penetration depth were 42.1°, 350 mm, and 70 mm respectively, the requirements for preparing a seedling furrow with a width of 170 mm can be met, and at this time, the height of the gathered soil in the middle was 107.2 mm. A mechanical and kinematic model for the lateral sequential feeding and vertical flexible clamping conveying process of rapeseed bare-root seedlings was established. The conditions for stable seedling feeding and "zero-speed seedling dropping" were analyzed. The internal spacing of the partition, initial clamping spacing, clamping conveying inclination angle, and seedling dropping height were determined to be 45 mm, 30 mm, 70°, and 165 mm respectively. The rotational speeds of the first and second driving pulleys were 15-32 mm/s and 155-230 mm/s respectively. The covering disc inclination angle and soil turning depth were analyzed and determined to be 40° and 160 mm respectively, and the compaction wheel thickness and installation inclination angle were 60 mm and 10° respectively. A method for field tests and performance evaluation was proposed, and field transplanting performance tests had been completed. The results of the field tests showed that when the machine advances at a speed of 0.80-1.20 km/h, the inclination angles of the furrowing disc and covering disc were 42° and 40° respectively, and the rotational speeds of the first and second driving pulleys were 20 and 160 r/min respectively, there was no missed transplanting during the transplanting operation. The seedling placement success rate was 93.63%, the seedling injury rate was 3.21%, the planting qualification rate was 90.25%, and the coefficient of variation of plant spacing was 9.01%. All key components of the transplanter operated stably and can completed functions such as ridge surface leveling, seedbed preparation, seedlings alignment and placement, and soil covering and compaction in a single pass. All operational performance indicators met the basic requirements for the mechanized transplanting of rapeseed bare-root seedlings. The growth of rapeseed after transplanting was tracked. The survival rate was counted three days after transplanting and reached 96.52%. This study provided a new approach for the development of mechanized transplanting equipment for bare-root seedlings of leafy and stem vegetables. The findings and the developed transplanter could potentially enhance the efficiency and quality of rapeseed transplanting operations, contributing to agricultural productivity and sustainability in regions where the rice-rapeseed planting pattern is prevalent.