Elevated CO2 cannot compensate for japonica grain yield losses under increasing air temperature because of the decrease in spikelet density

Extensive evidence shows that elevated carbon dioxide (CO2) stimulates rice yield, but increasing global surface temperature decreases it. However, few studies have been conducted to evaluate whether elevated CO2 compensates for the rice yield losses induced by increased air temperature under field conditions. Here, we report the effects of four treatments, namely, ambient condition (ACAT), CO2 enrichment (590 ppm, ECAT), canopy air warming (1 degrees C above the ambient temperature, ACET), and combined CO2 enrichment and warming (ECET) on leaf photosynthesis, nitrogen (N) uptake, spikelet architecture and yield components over two rice growing seasons using a free-air CO2 enrichment facility. We found that elevated CO2 cannot compensate for the negative impacts of increased air temperature on rice yield, especially in the warmer season. Compared to ACAT, ECAT increased the rice grain yield by 14.8% in 2015 and 12.9% in 2016. ACET decreased the rice grain yield by 8% in 2015 and 21% in 2016. Similarly, ECET decreased the rice grain yield by 4% in 2015 and 14% in 2016. Spikelet density was the dominant factor accounting for the yield losses under increased temperature alone or combined with elevated CO2, and spikelet density was mainly affected by the dry matter per unit area before heading. The decrease in the number of spikelets per panicle in ECET was attributed to the differentiation of primary branches and the corresponding spikelets, which was caused by the reduction in nitrogen uptake per culm before the booting stage. However, the decrease in the number of panicles per hill under ECET was related to the reduction in dry matter per culm before jointing. Overall, the dry matter per unit area before the heading stage will be important for alleviating rice yield loss under future climate changes. These results provide a better understanding of the rice growth responses to future climate conditions.