Abstract: 【Objective】The effects of reducing hills and stabilizing basic-seedlings (RHSB) on population canopy quality characteristics of mechanical transplanting indica hybrid rice were investigated in this study, which can provide theoretical support for the promotion and application of mechanical transplanting technology of hybrid rice in low-light paddy region of southwest China.【Method】Two-factor random block designs with different rice varieties (Fyou 498 is a  loose plant type, and Yixiangyou 2115 is a compact upper and drooping lower plant type at middle and late stages) and different  basic seedlings rate (42×10⁴/hm² and 63×10⁴/hm²) were conducted to study the effects of different field collocation patterns (conventional field collocation (CFC) and RHSB) on the canopy architecture, photosynthetic characteristics, and microclimate environment (e.g. canopy temperature, humidity, and light transmittances) of mechanical transplanting hybrid rice population in 2016 and 2017, respectively. 【Result】(1) Compared to CFC, RHSB with a greater specific leaf weight of the upper three leaves (F_{second leaf}=23.67^** and F_{third leaf}=16.91^** in 2017), could maintain a similar green leaf area per stem and grain/leaf ratio at heading stage. Compared to CFC, RHSB also leaded to the 23.84%, 23.53%, and 13.79% significant increases in photosynthetic rate, stomatal conductance, and transpiration rate of flag leaf at the heading stage, respectively. (2) RHSB markedly decreased the canopy convergent index, but increased the canopy amplitude, which increased in light transmittance and ventilation of canopy. RHSB increased the angle of primary tillerings of Fyou 498 at the booting stage and heading stage in 2016, but decreased the angle of primary tillerings of Yixiangyou 2115 in both years..  (3) Correlation analysis indicated that daily average temperature and diurnal variations of temperature and humidity at booting stage were significantly or extremely significantly positively correlated with the specific leaf weight of the flage leaf and the 2nd leaf at the heading stage, but significantly negatively correlated with plant convergent index at the heading stage. Furthermore, the daily average temperature and diurnal variations of humidity at the booting stage was significantly positively correlated with canopy amplitude at the heading stage. Daily average temperature and diurnal variations of temperature at the heading stage were significantly or extremely significantly negatively correlated with the angle of primary tillering at the tilling stage, jointing stage, and 20 d after heading stage. However, the inverse relation was observed by daily average relative humidity. RHSB could optimize the  canopy architecture of rice, which contributed to the increase in canopy temperature and diurnal the variations of temperature at booting and heading stage，and increase diurnal the variations of humidity at booting, heading and 20 d after heading stage, but decrease in the relative humidity.【Conclusion】As one of the main technology that improved the mechanization level of rice production in the low-light region of Southwest China, RHSB optimized the population canopy structure and light distribution of mechanical transplanting hybrid rice, which resulted in the increase in the temperature and humidity difference between day and night within the population and reduced the relative humidity. This contributed to the improvement in the population quality and photosynthetic rate, and contributed to the increase in grain yield of rice.

Key words: rice, mechanical transplanting, field collocation pattern, microclimate environment, reducing hills and stabilizing basic-seedlings