In the face of agricultural labor shortages, reducing labor and costs in rice production while meeting demand or increasing yield is crucial for sustainable agricultural development.  Using crop straw boards and raising seedlings at a high-density can reduce labor demand and enhance rice yield.  This study investigated the effects of seeding density and transplanting age on tillering patterns, panicle formation rates, and yield to determine the optimal cultivation practices for maximizing rice yield.  Two-year field experiments were conducted in Sihong County, Jiangsu Province, China, using the japonica rice variety Nanjing 5718.  Five seeding densities (150–350 g/tray) and four transplanting ages (10–25 days) were evaluated to assess their impacts on tillering patterns, panicle formation rates, and yield.  Innovative crop straw boards were employed to enhance planting efficiency and reduce dependence on soil for raising seedlings.  This approach also lessened tillage layer destruction, promoting sustainable practices.  The results indicated that increasing seeding density significantly altered tillering and panicle formation patterns by reducing the occurrence and panicle formation rates of lower-position tillers.  Although the occurrence of middle- and high-position tillers increased, the overall number of panicles per hill decreased, especially at higher densities, negatively affecting yield.  Reducing the transplanting age promoted the emergence and panicle formation of lower-position tillers, thus mitigating these negative effects.  Specifically, compared to traditional methods (150 g/tray, 20-day seedlings), the higher seeding density (300 g/tray) and reduced transplanting age (15-day seedlings) increased total panicle number by 3.79–4.73% and yield by 3.38–5.05%.  Combining higher seeding densities with reduced transplanting ages offers significant advantages over conventional practices by enhancing resource utilization and improving tillering efficiency.  These findings provide actionable recommendations for optimizing rice cultivation practices and contribute to sustainable agricultural development.

The high labor demand during rice seedling cultivation and transplantation poses a significant challenge in advancing machine-transplanted rice cultivation.  This problem may be solved by increasing the seeding rate during seedling production while reducing the number of seedling trays.  This study conducted field experiments from 2021 to 2022, using transplanting seedling ages of 10 and 15 days to explore the effects of 250, 300, and 350 g/tray on the seedling quality, mechanical transplantation quality, yields, and economic benefits of rice.  The commonly used combination of 150 g/tray with a 20-day seedling age in rice production was used as CK.  The cultivation of seedlings under a high seeding rate and short seedling age significantly affected seedling characteristics, but there was no significant difference in seedling vitality compared to CK.  The minimum number of rice trays used in the experiment was observed in the treatment of 350–10 (300 g/tray and 10-day seedling age), only 152–155 trays ha–1, resulting in a 62% reduction in the number of trays needed.  By increasing the seeding rate of rice, missed holes during mechanical transplantation decreased by 2.8 to 4%.  The treatment of 300–15 (300 g/tray and 15-day seedling age) achieved the highest yields and economic gains.  These results indicated that using crop straw boards can reduce the application of seedling trays.  On that basis, rice yields can be increased by raising the seeding rate and shortening the seedling age of rice without compromising seedling quality.

Recently developed ‘super’ rice cultivars with greater yield potentials often suffer from the problem of poor grain filling, especially in inferior spikelets.  Here, we studied the activities of enzymes related to starch metabolism in rice stems and grains, and the microstructures related to carbohydrate accumulation and transportation to investigate the effects of different water regimes on grain filling.  Two ‘super’ rice cultivars were grown under two irrigation regimes of well-watered (WW) and alternate wetting and moderate soil drying (AWMD).  Compared with the WW treatment, the activities of ADP glucose pyrophosphorylase (AGPase), starch synthase (StSase) and starch branching enzyme (SBE), and the accumulation of non-structural carbohydrates (NSCs) in the stems before heading were significantly improved, and more starch granules were stored in the stems in the AWMD treatment.  After heading, the activities of α-amylase, β-amylase, sucrose phosphate synthase (SPS) and sucrose synthase in the synthetic direction (SSs) were increased in the stems to promote the remobilization of NSCs for grain filling under AWMD.  During grain filling, the enzymatic activities of sucrose synthase in the cleavage direction (SSc), AGPase, StSase and SBE in the inferior spikelets were increased, which promoted grain filling, especially for the inferior spikelets under AWMD.  However, there were no significant differences in vascular microstructures.  The grain yield and grain weight could be improved by 13.1 and 7.5%, respectively, by optimizing of the irrigation regime.  We concluded that the low activities of key enzymes in carbon metabolism is the key limitation for the poor grain filling, as opposed to the vascular microstructures, and AWMD can increase the amount of NSC accumulation in the stems before heading, improve the utilization rate of NSCs after heading, and increase the grain filling, especially in the inferior spikelets, by altering the activities of key enzymes in carbon metabolism.