Mitigation of Elevated CO₂ Concentration on Warming-Induced Changes in Wheat Is Limited under Extreme Temperature during the Grain Filling Period

Elevated CO₂ concentration (<i>e</i>CO₂) generally increases plant growth by improving photosynthesis, but it is unclear whether <i>e</i>CO₂ can alleviate the negative effects of elevated temperatures, especially in high-temperature years. Manipulative experiments with elevated [CO₂] and temperature were conducted in North China to understand the effect of elevated CO₂ concentration and temperature on wheat. The photosynthesis, <i>A</i>_n–<i>PAR</i> and <i>A</i>–<i>C_i</i> curve parameters, growth period, biomass, yield component, and yield of wheat were investigated under different [CO₂] (around 400 and 600 ppm) and temperatures (ambient temperature and ambient temperature +2 °C) for 3 years by using controlled chambers. Results showed that elevated temperature significantly shortened the growth period and decreased the yield and biomass of wheat. Elevated [CO₂] significantly increased the maximum net photosynthetic rate (<i>A</i>_nmax) but reduced the maximum carboxylation rate (<i>V</i>_cmax) and the maximum electron transport rate (<i>J</i>_max). The extremely high temperature during the grain filling period in 2019 exerted a serious negative impact on wheat production. Elevated [CO₂] stimulated photosynthesis, increased kernel number per spike, and extended the duration of the grain filling period, which consequently increased biomass and grain yield under elevated temperatures in normal years (2018 and 2020). Although the combination of CO₂ and temperature reduced photosynthesis and biomass, it also alleviated the negative impact of elevated temperatures on grain yield to some extent under extremely high temperature during the grain filling period in 2019. The mitigative effect of <i>e</i>CO₂ under extreme high temperature is limited, and planting early-maturing cultivars or increasing the genotypes of kernel number per spike help to escape the extreme high temperature of the critical growth period.