Characteristics of nitrogen absorption and utilization of super hybrid rice grown under two yield levels

This study aims to investigate the characteristics of nitrogen (N) uptake and utilization in super hybrid rice grown under two yield levels to provide a theoretical and practical basis for super high-yield rice cultivation and super hybrid rice breeding in southwest China. Field and pot experiments were conducted in Deyang City (a high-yielding site, HYS) and Luzhou City (a medium-yielding site, MYS) between 2018 and 2020. Each year, two super-hybrid rice cultivars (‘Deyou4727’ and ‘Luyou727’) and two high-yielding inbred rice cultivars (‘Jinnongsimiao’ and ‘Huanghuazhan’) were planted at each site. Differences in N uptake, translocation, and utilization characteristics between the two yield levels and the four rice cultivars were studied. The results showed marked differences in grain yield, N uptake, and utilization between the two sites in the field experiment. Super-hybrid rice produced higher grain yield and partial factor productivity of applied N (PFPN) in HYS in comparison to those in MYS by 8.3%−23.2% and 8.3%−23.1%, respectively. Super-hybrid rice exhibited higher N uptake and rate of N uptake from sowing (SO) to panicle initiation (PI) and PI to full heading (HD) in HYS in comparison to MYS (except for the duration from PI to HD in 2018). However, HYS demonstrated higher N accumulation during the grain-filling period of super-hybrid rice. Consequently, super-hybrid rice demonstrated 15.6%−33.7% higher total N uptake in HYS in comparison to MYS. On average, despite HYS showing a 4.6% higher N harvest index in super-hybrid rice compared to MYS (except in 2018), a substantial amount of N was still retained in the straw at the maturity stage. Consequently, HYS demonstrated 11.3% lower N use efficiency for grains production (NUEGP) than MYS. Analysis of variance revealed that the effects of soil, the interactions of year (Y)×soil (S), S×location (L), and S × cultivar (C) were not significant for grain yield, N uptake, or NUEGP of super-hybrid rice grown in the pot experiments. The trends in grain yield, N uptake, and NUEGP of super-hybrid rice grown in the pot experiments were similar to those in the field experiments. HYS experienced a higher mean temperature during the SO-PI phase, whereas the mean temperature was lower in HYS compared to MYS. The cumulative solar radiation during SO-PI, PI-HD, and HD-MA was higher in the HYS than in the MYS (except for PI-HD in 2018). Further analysis indicated that rice grain yield was significantly and positively related to N uptake accumulation during PI-HD, as well as the N harvest index in the HYS. There was a significant negative relationship of NUEGP with N uptake accumulation, and mean temperature during the SO-PI in the HYS. There was a significant positive relationship between rice grain yield and NUEGP and total N uptake in the MYS. Additionally, NUEGP was significantly negatively correlated with mean temperature during the SO-PI and HD-MA in the MYS. Our findings indicated that the differences in grain yield, N absorption, and utilization characteristics of super-hybrid rice at different ecological sites are primarily related to the climatic factors (including mean temperature and solar radiation during the growing season) of the planted site.