Panicle temperature explains contrasting yield responses of rice genotypes to elevated CO2 and increased temperature in T-FACE environments

Elevated CO2 increases, while high temperature decreases, rice yield. We hypothesized that the interplay between these opposite effects varies across genotypes and these variations are associated with the ability of genotypes to avoid and tolerate stress. We evaluated a Japonica genotype (Changyou5) and an Indica genotype (Yangdao6) under combinations of two CO2 levels (ambient and enriched to 590 μmol mol-1) and two canopy temperatures (ambient and warmed by 2.0 °C) in a temperature by free-air CO2 enrichment (T-FACE) system over two seasons. The elevated CO2 fully offset the adverse effects of the elevated temperature on grain yield of Yangdao6 but failed to do so for Changyou5. In Yangdao6 yield increased by 20.0%, while in Changyou5 it decreased by 7.8% under the combined elevated CO2 and elevated temperature. This genotypic difference was partly due to higher leaf-nitrogen content of cv. Yangdao6, resulting in superior light conversion efficiency. However, it was explained more by a smaller decrease in spikelet fertility (and thus harvest index) in Yangdao6, mainly resulting from lower panicle temperature during flowering. The lower panicle temperature in Yangdao6 was due to earlier flowering hours as well as to higher panicle nitrogen content that presumably led to greater transpirational cooling. The above key genotypic traits could be explored in rice breeding programs to improve yield resilience to climate change.