Different mechanisms underlying the yield advantage of ordinary hybrid and super hybrid rice over inbred rice under low and moderate N input conditions

The adoption of ordinary hybrid rice and super hybrid rice significantly increased the grain yield under non-limited nitrogen (N) fertilizer condition, however, the information on direct comparison of grain yield and nitrogen use efficiency (NUE) among the high-yielding super hybrid, ordinary hybrid, and inbred rice under low and moderate N rates is limited. Therefore, field experiments with two representative cultivars of each these types were conducted under N0 (0 kg N ha−1) and N90 (90 kg N ha−1) treatments in 2014 and 2015 in Wuxue County, Hubei Province, China. Grain yield of the super hybrid rice was higher than that of the inbred rice by 3.33-7.41% at N0 and 5.94-10.87% at N90. Similarly, the ordinary hybrid rice outyielded the inbred rice by 4.53-6.17% and 4.35-4.69% at N0 and N90, respectively. The yield advantage of the ordinary hybrid rice over the inbred rice was mainly due to higher aboveground total dry weight (TDW), while for the super hybrid rice, higher grain yield was mainly attributed to higher harvest index (HI) at N0, and both high HI and TDW at N90. Larger leaf area index (LAI) at heading stage and higher crop growth rate (CGR) contributed to higher TDW of the ordinary hybrid rice. Total N uptake at maturity was consistently higher in the ordinary hybrid rice, while the super hybrid rice had significantly higher nitrogen use efficiency for grain production (NUEg) than the inbred rice, which was associated with higher N harvest index (NHI) and lower leaf N concentration of the super hybrid rice. In conclusion, it was found that the super and ordinary hybrid rice exhibited higher yield over the inbred rice at low and moderate N rates, but the physiological mechanisms underlying the yield advantage were different.