Dense planting has been recommended as a promising practice for achieving greater grain yield. However, whether this practice is suitable for hybrid rice (Oryza sativa L.) is unknown. To evaluate the response of grain yield and canopy structure traits to planting density in hybrid cultivars, field experiments were conducted with inbred-indica (Huanghuazhan), inbred-japonica (Xiushui09), hybrid-indica (Tianyouhuazhan), and hybrid-japonica (Chun-you84 or Yongyou12) cultivars at five planting densities (40, 26.7, 20, 16.7, and 13.3 hills m -2 , designated as D1, D2, D3, D4, and D5) in 2015 and 2016. Results showed that hybrid cultivars had 16.6 to 48.9% greater grain yield compared with inbred cultivars under the dense planting condition (D1 and D2), except for Tianyouhuazhan in 2015. The grain yield increased with increasing plant density in hybrid cultivars (except for Tianyouhuazhan in 2015), with the increase rates of 76.6, 108.0, and 138.6 kg Δhill -1 ha -1 for hybrid-japonica-Chunyou84 (2015), hybrid-indica-Tianyouhuazhan (2016), and hybrid-japonca-Yongyou12 (2016), respectively. However, the yield performance was significantly affected by the climate condition under dense planting. The grain yield of hybrid cultivars in 2015 averaged 8.3 t ha -1 , which was 36.3% lower than that in 2016 under dense planting (D1 and D2). We speculate that unfavorable climate would suppress the yield performance more under the dense planting conditions. Furthermore, results showed that greater plant density leads to an increase in plant radiation use efficiency, except for indica-hybird-Tianyouhuazhan. This might be attributed to the upright top leaves when the planting density increased, which resulted in smaller leaf angles and extinction coefficient of the canopy.
