Abstract: We explored the effects of machineplantation densities on yield and quality formation of indica hybrid rice based on the distribution characteristics of photosynthetically active radiation (PAR) inside the canopy. A field experiment was conducted in 2017 and 2018 with three rice cultivars, Huiliangyou-898, Fengliangyouxiang-1, and Y-Liangyou-900. There were six density treatments, including 17 ×10⁴ hills·hm^-2 (D17), 19 ×10⁴ hills·hm^-2(D19), 21 ×10⁴ hills·hm^-2 (D21), 23 ×10⁴ hills·hm^-2 (D23), 24 ×10⁴ hills·hm^-2 (D24), and 25 ×10⁴ hills·hm^-2 (D25). The results showed that the densities significantly affected yield. Generally, the maximum value of grain yield was observed in the D19 treatment in 2018 and in the D21 treatment in 2017, and then followed by high density treatments (D24 and D25) and low density treatment (D17) across both cultivars each year. The fitted equation indicated that the highest grain yield of middle-seasonindicahybrid rice was 9.91×10³ kg·hm^-2 when the density was 20.52×10⁴ hills·hm^-2 in machine transplanted condition in this region. Higher daily crop growth rate during grain filling, greater PAR interception, and larger PAR use efficiency were key factors driving higher grain yield in medium density treatments than that in high density treatments and low density treatment across cultivars. For the three tested cultivars, both processing qualities and appearance qualities were continuously deteriorated with increasing planting density. There was no significant difference among D17 treatment, D19 treatment, and D21 treatment (P>0.05). Meanwhile, there was little difference among the D19, D23, and D24 treatments for eating quality of cultivar Huiliangyou-898 and cultivar Y-Liangyou-900. Densities mainly affected the PAR of 20-40 cm and 40-60 cm from top of canopy, with PAR at 20-60 cm from top of canopy being negatively correlated with planting density. The PAR inside of 20-60 cm canopy was positively correlated with processing qualities, but negatively correlated with appearance qualities (P<0.05). However, only 40-60 cm of PAR was significantly negatively related to grain yield (P<0.05). Theseresultssuggest that the optimum density harvested the highest yield and good qualities should be recommended for 19×10⁴-21×10⁴ hills·hm^-2. Improving 20-40 cm PAR from the canopy top coordinating moderately lower canopy PAR 40-60 cm may be the crucial point for the formation of good quality and high yield of indica hybrid rice.

Key words: middle-seasonindicahybrid rice, mechanical cultivatio, photosynthetically active radiation, yield, quality.