Grain yield,nitrogen use efficiency and physiological performance of indica/japonica hybrid rice in response to various nitrogen rates

Utilizing the heterosis of indica/japonica hybrid rice (IJHR) is an effective way to further increase rice grain yield.  Rational application of nitrogen (N) fertilizer plays a very important role in using the heterosis of IJHR to achieve its great yield potential.  However, the responses of the grain yield and N utilization of IJHR to N application rates and the underlying physiological mechanism remain elusive.  The purpose of this study was to clarify these issues.  Three rice cultivars currently used in rice production, an IJHR cultivar Yongyou 2640 (YY2640), a japonica cultivar Lianjing 7 (LJ-7) and an indica cultivar Yangdao 6 (YD-6), were grown in the field with six N rates (0, 100, 200, 300, 400, and 500 kg ha^–1) in 2018 and 2019.  The results showed that with the increase in N application rates, the grain yield of each test cultivar increased at first and then decreased, and the highest grain yield was at the N rate of 400 kg ha^–1 for YY2640, with a grain yield of 13.4 t ha^–1, and at 300 kg ha^–1 for LJ-7 and YD-6, with grain yields of 9.4–10.6 t ha^–1.  The grain yield and N use efficiency (NUE) of YY2640 were higher than those of LJ-7 or YD-6 at the same N rate, especially at the higher N rates.  When compared with LJ-7 or YD-6, YY2640 exhibited better physiological traits, including greater root oxidation activity and leaf photosynthetic rate, higher cytokinin content in the roots and leaves, and more remobilization of assimilates from the stem to the grain during grain filling.  The results suggest that IJHR could attain both higher grain yield and higher NUE than inbred rice at either low or high N application rates.  Improved shoot and root traits of the IJHR contribute to its higher grain yield and NUE, and a higher content of cytokinins in the IJHR plants plays a vital role in their responses to N application rates and also benefits other physiological processes.