Yield Attributes Response to Nitrogen Fertilization in Low-Nitrogen Tolerant Hybrid Rice

The development of low-nitrogen (N) tolerant varieties is an effective way to improve the use efficiency of N in rice and is crucial to the sustainable rice production in China. Recently, we observed that higher grain yields were obtained in the hybrid rice variety Deyou 4727 (DY4727) than in the hybrid rice variety Y-liangyou 900 (YLY900) under zero-N fertilization conditions, suggesting that DY4727 has higher low-N tolerance compared to YLY900. In this study, field experiments were performed in 2020 and 2021 to compare the responses of yield attributes to N fertilization (180 kg N ha(-1)) relativized to zero-N fertilization between DY4727 and YLY900. Results showed that the response of grain yield to N fertilization was 13-55% for two varieties in two years. DY4727 had lower response of grain yield to N fertilization than YLY900 by 47% in 2020 and by 70% in 2021. There were no significant or consistent differences in responses of spikelets per m(2), spikelet filling percentage, aboveground biomass, intercepted radiation, radiation use efficiency, and aboveground N uptake to N fertilization between DY4727 and YLY900. DY4727 had 32% and 26% lower positive response of grain weight to N fertilization compared to YLY900 in 2020 and 2021, respectively. The response of harvest index to N fertilization was negative (-3% in 2020 and -9% in 2021) for DY4727 but positive (1% in 2020 and 12% in 2021) for YLY900. DY4727 had higher negative response of internal N-use efficiency to N fertilization (-41% in 2020 and -24% in 2021) than YLY900 (-11% in 2020 and -10% in 2021). This study suggests that grain weight, harvest index, and internal N-use efficiency are key traits determining the difference in response of grain yield to N fertilization between DY4727 and YLY900.