The aim of this study was to clarify the effects of sowing dates on the eating quality of indica hybrid rice in sub-suitable area of ratoon rice, so as to provide the theoretical and practical basis for the adjustment of planting structure and high quality cultivation in sub-suitable area of ratoon rice.

Field sowing dates experiments were conducted in the two sub-suitable area of ratoon rice in Sichuan (Longchang and Qianwei) with three indica hybrid rice varieties, namely Chuanyou 6203, Yixiangyou 2115, and Fyou 498. The effects of sowing dates on eating quality of indica hybrid rice in sub-suitable area of ratoon rice were studied by the determination of amylose and protein content, and the analysis of rice aroma, appearance, palatability, flavor, cold rice texture, as well as comprehensive score following the national standard sensory evaluation method.

(1) The eating quality of indica hybrid rice was affected by location, sowing date, variety, and their interactions. (2) The effects of sowing dates on the eating quality of different rice varieties were different to the study locations in the sub-suitable area of ratoon rice. The amylose content, protein content, palatability, taste, and flavor in the two study years, as well as the palatability and comprehensive score in 2018 in Longchang were significantly lower than that in Qianwei. Compared wtih the conventional sowing date, suitably delayed sowing date could improve the amylose content, palatability, and taste of rice by decreasing the temperature stress during grain filling stage, which resulted in the increase in comprehensive score of rice. This made the taste quality of rice closer to that of the ratoon rice. (3) Correlation analysis showed that amylose content, palatability, and flavor had significantly or extremely significantly negative correlation with the average daily maximum, minimum, and average temperatures, and sunshine hours from the 20 days after heading to mature stage, while comprehensive score was significantly and negatively related to the average daily minimum temperature from the 20 days after heading to mature stage. (4) The GGE-bioplot double plot analysis showed that the third sowing date in Longchang and second and third sowing dates in Qianwei had higher score and better stability of comprehensive score.

On the basis of ensuring the yield of rice, Longchang ecological point was sowed at the third sowing date (early May), and Qianwei ecological point was sowed at the second sowing date (March 20-March 25), which could avoid high temperature stress during the grain filling period of rice and improve the eating quality of hybrid indica rice. Furthermore, the selection of high-quality eating varieties as Yixiangyou 2115 and Chuanyou 6203 with suitable delaying of sowing date possessed higher taste quality in the sub-suitable area of ratoon rice.

The aim of this study was to clarify the effect of post-anthesis shading stress on the eating quality of indica rice in Chengdu Plain, so as to provide the theoretical and practical basis for the selecting of shade-tolerant rice varieties in low light rice region.

Field light control experiments were conducted in 2019 and 2020 in Wenjiang, Sichuan by using indica rice varieties with different amylose contents as materials. The effect of shading stress on eating quality of rice was studied by measuring the components and RVA spectrum values of rice, as well as the texture properties and eating quality of cooked rice.

(1)Shading stress significantly decreased amylose, amylopectin, and total starch contents of rice, but significantly increased the contents of protein and fat. (2)Shading stress significantly decreased the peak viscosity and breakdown of rice, while increased the pasting temperature and setback. (3)Shading stress also significantly increased the hardness of cooked rice, but decreased the stickiness and elasticity, which contributed to a significant reduction in the comprehensive score of cooked rice by decreasing both appearance and taste. (4)The results of principal component analysis showed that the rice components, RVA spectrum values, and texture properties could explain 81.2% of the total variation of eating quality of cooked rice. The comprehensive score of rice was significantly and positively correlated with the amylose, starch content, peak viscosity, breakdown, elasticity and stickiness, but negatively related to the protein and fat contents.

The protein and fat contents possessed greater influence on the eating quality of cooked rice than that of amylose and starch contents under shading stress. The shading stress leaded to significant variations in starch, protein, and fat components in rice, which contributed to a decrease in peak viscosity and breakdown, but an increase in pasting temperature and setback. And then, those variations contributed to the significantly decrease in eating quality of cooked rice by increasing the hardness, but decreasing both stickiness and elasticity of cooked rice.

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.