Investigation on Outcrossing Seed-Setting Characteristics of Machine- Transplanted Hybrid Rice Seed Production

The objective of this research is to clarify the panicle seeding characteristics of sterile lines in machine- transplanted seed production and explore the reasons for the lower seed yield of hybrid rice in mechanized seed production mode.

Two combinations of Chenghui 727×Shu 21A and Yahui 2115×Yixiang 1A were selected as the materials, and the field experiment was conducted in Chongzhou from 2020 to 2021. Two methods of machine and hand-transplanted were utilized. During the heading to flowering stage, the state of flower synchronization was judged by the heading condition of restorer line and sterile line, and panicle seeding condition of sterile line was investigated at maturity stage. Statistical analysis of seeding characteristics of sterile line in different rows, panicle positions and different branches under two transplanting methods.

(1) Under the condition of flower synchronization, the hand-transplanted restorer line and sterile line had a complete coincidence of flowering period with a flower synchronization index of 100%. On the other hand, the machine-transplanted restorer lines and sterile line had a flowering deviation of 3-4 days and 6-8 days, respectively, resulting in poor flower synchronization with a flower synchronization index of 33.33%-55.56%. (2) The overall setting rate was lower in the machine-transplanted group compared to the hand-transplanted group, with a 33.45% reduction in the setting rate of machine-transplanted. (3) The setting rate of panicles in rows 1-6 of sterile line showed a gradual decrease trend, and a higher setting rate was observed in the flower synchronization treatment in the same line. (4) The upper part panicles of the sterile line had a good pollination posture with a small shielding effect on the pollen. Under normal pollination conditions, the panicle setting rate of sterile line was significantly shown as upper> middle > lower part, while the empty grain rate was lower> middle > upper part. (5) The setting rate of primary branches of sterile line was significantly higher than that of secondary branches under different transplanting methods. The machine-transplanted treatment further reduced the setting rate of primary and secondary branches of sterile line. The primary and secondary branch ustilagomaydis grains of sterile line showed no significant difference under the machine-transplanted treatment, whereas the secondary branch ustilagomaydis grains were significantly higher than the primary branches under the hand-transplanted treatment. (6) The flower synchronization of the restorer line and sterile line was poor under the machine-transplanted treatment, resulting in lower actual seed production yield than the hand-transplanted group. The seed production yield decreased by 39.34% and 41.48% in two years.

The flowering deviation of the sterile line was found to be 3-4 days longer than that of the restorer line, which resulted in poor flower synchronization. This is the primary reason for the lower setting rate of machine-transplanted compared to hand-transplanted. Additionally, the low setting rate of sterile line in the middle and lower parts of the panicle, as well as those further away from the restorer line, further contributes to the low overall setting rate in machine-transplanted. However, the effects of blighted and infected grains on the setting rate and yield were found to be relatively small. To improve the setting rate of hybrid rice in machine-transplanted seed production mode, we suggest the following measures. Arrange the sowing of machine-transplanted parents at different intervals during the full flowering stage to improve the degree of flower synchronization. Standardize the pollination operation to improve the effectiveness of pollination. Strengthen field management to reduce the occurrence of blighted and infected grains. By implementing these measures, we can improve the yield of hybrid rice in machine-transplanted seed production mode.