Injury mechanisms in high-speed transplanting of over aged rice seedlings

The mechanical transplanting of over aged seedlings (OAS) of rice poses several challenges, including seedling mortality and a significant delay in the greening period, which severely restrict yield. The causes of these issues remain unknown. Through theoretical analysis, mechanical tests, bench tests, and growth experiments were conducted to explore the reasons for seedling mortality and the significant delay in the greening period from the perspectives of seedlings, transplanting machines, and post-transplant growth. The results indicated that OAS transplanting causes combined root and stem injuries. For 40-day-old seedlings, the energy required for stem elasto-plastic deformation ranged from 1.37 to 7.19 N mm. Within this deformation range, the stem sustains internal injury but is not considered as injured seedling. The main factors influencing stem injury were the stem's major axis length and elastic modulus, whereas root injury was primarily affected by root diameter. Root and stem injuries significantly inhibited seedling growth, as demonstrated by significant structural changes in the stem 3 days post-transplantation, along with partial seedling mortality. New roots emerged only 9 days post-transplantation, and the heart leaf growth rate and SPAD value at 12 days were only 49% and 77% those of uninjured transplanted seedlings, respectively. Based on these findings, it was concluded that the seedling mortality and significant delay in the greening period observed in OAS transplantation are due to inhibited nutrient absorption and transport, caused by stem elasto-plastic deformation and weakened root vitality. These results may serve as a basis for improving transplanting machine design and optimising rice cultivation practices.