Optimum total nitrogen application is required to reduce the yield loss of hybrid rice to high temperature

The grain yield and quality of rice (Oryza sativa L.) face great challenges associated with increasing temperatures, thus threatening global food security. The wheat, maize and barley yield loss magnitudes imposed by increasing temperatures reportedly interact with the nitrogen (N) application level under field conditions. However, little is known about the interactive effect between high daytime temperatures and the N supply on the rice grain yield or quality or the influence of this effect on nitrogen use efficiency in the field. In this work, two hybrid rice varieties (Liangyoupeijiu-LYPJ and IIyou602-IIY602) were grown under three N application levels (N75–75 kg ha⁻¹, N150–150 kg ha⁻¹, and N225–225 kg ha⁻¹) in paddy fields in 2019 and 2020 under control and higher-temperature (HT) conditions from the heading stage to maturity. HT exposure significantly decreased the grain yields, grain filling, grain weight, grain-leaf ratio, total biomass at maturity and harvest index (HI) of the two rice varieties at all N levels across both years, while the spikelet m⁻² was only affected in 2019. Moreover, the grain yield reduction caused by HT were least at N150 (57.0%) in LYPJ (68.7% and 68.8% in N75 and N225, respectively) and N75 (60.7%) in IIY602(66.0% and 64.5% in N150 and N225, respectively) on average of two years. Grain filling percentage, biomass and HI also had least decreases under N150 in LYPJ and under N75 in IIY602. Regarding the grain quality, HT exposure decreased the head rice yield and grain length, and increased chalkiness in the two varieties in both years while decreasing the amylose content and increasing the protein content in 2019 and 2020, respectively. Relatively optimal N application rates (N75 or N150 for both varieties) improved grain quality indicators such as the brown rice, head rice, grain length, chalkiness degree and amylose content under HT. After HT exposure, nonsignificant changes in N accumulation at maturity and significant reduction in N translocation and N use efficiencies for grain and biomass production were observed in both varieties. Compared to the relatively higher N applications, applying N150 to LYPJ (by 24.3%) and N75 to IIY602 (by 33.7%) induced lower N translocation reductions. Thus, our results suggest that increasing N supply cannot minimize the damage caused by HT in the realistic fields.