Mid-stage nitrogen application timing regulates yield formation, quality traits and 2-acetyl-1-pyrroline biosynthesis of fragrant rice

The main purpose of the present study was to explore the optimum timing for mid-stage nitrogen (N) application to improve fragrant rice performance. The field experiment was conducted in 2019 and 2020 with two fragrant rice cultivars, i.e., YNX28 and NG9108, and five timings of mid-stage nitrogen application: at the emergence of the top-fifth-leaf (TL5), the top-fourth-leaf (TL4), the top-third-leaf (TL3), the top-second-leaf (TL2), and the top-first-leaf (TL1) of the main stem. The control condition received no mid-stage nitrogen application (CK). The results revealed that the TL5, TL4, TL3 and TL2 treatments significantly increased grain yield of both varieties in 2019 and 2020, compared with CK. Grain yield reached a maximum value under the TL3 treatment of YNX28 and the TL4 treatment of NG9108. Moreover, the TL4, TL3 and TL2 treatments significantly increased the brown rice rate, milled rice rate, head rice rate and protein content of both varieties in 2019 and 2020, compared with CK, whereas the TL2 and TL1 treatments of YNX28 and the TL3, TL2 and TL1 treatments of NG9108 significantly increased the chalky rice rate and chalkiness. Further, appropriate mid-stage nitrogen application timing also improved the grain 2-AP content due to regulation of the contents of synthesis-related precursors (proline, GABA) and enzymatic activities (ProDH, OAT, P5CS, BADH2). Compared with CK, the TL4 and TL3 treatments of YNX28 significantly increased the 2-AP content in the rice grains by 21.8–27.4%, while the TL5 and TL4 treatments of NG9108 significantly increased the 2-AP content in the rice grains by 15.6–31.6%. These results suggest that the TL3 treatment of YNX28 and the TL4 treatment of NG9108 are the optimum timings of mid-stage nitrogen application for fragrant rice production. The optimal mid-stage nitrogen application timing of NG9108 is earlier than that of YNX28.