Evaluating the Effect of Population Density and the Contribution of Early Canopy Closure to Grain Yield of Hybrid Rice

Although increased biomass production and radiation use efficiency (RUE) can effectively improve grain yield, information under nitrogen (N) and density conditions remains limited. A 2-year field experiment was conducted to compare the grain yield, interception percentage (IP), intercepted photosynthetically active radiation (IPAR), and RUE of hybrid rice (varieties: Y-liangyou 900 and Quanliangyou 681) and inbred rice (varieties: Huanghuazhan and Yuenongsimiao) grown under two N and two density treatments. Compared to the inbred varieties, the hybrid varieties had a higher average yield (by 8.1%), spikelets per panicle (by 20.7%), dry matter accumulation (by 15.4%), and IP (by 4.1%) and IPAR (by 10.0%) at the mature stage (MA), but lower IP (by 8.1%) and IPAR (by 14.3%) at the mid-tillering stage (MT). Additionally, the RUE of hybrid rice was 6.5% lower at MT but 19.0% higher at MA than inbred rice. The yield and IP and IPAR of rice were more highly correlated in inbred varieties at MT but were more highly correlated in hybrid varieties at MA. Compared to inbred rice, hybrid rice yield was more strongly correlation with RUE in both MT and MA, and the higher RUE of hybrid rice was due to higher IP and IPAR values. Our study revealed that a higher plant density with reduced N fertilization contributed to early canopy closure, and ultimately a higher biomass production and grain yield in hybrid rice.