Individual grain mass of inbred rice cultivars does not benefit from elevated [CO2]

Climate warming has led to a reduction of global crop yields. Elevated atmospheric [CO₂] is believed to promote crop production by increasing photosynthesis, and thus partly offset the yield losses due to climate warming. However, photosynthetic acclimation may occur when the plant is exposed to long-term high [CO₂] conditions, suggesting that elevated [CO₂] (e[CO₂]) might not bring benefits for cereal crops as the growing season proceeds. To assess the effect of long-term e[CO₂] on rice yield, particularly on individual grain mass determined post-heading, we synthesized existing data from FACE (free-air CO₂ enrichment) experiments across four locations in Japan and China. We also conducted a five-year field experiment with different [CO₂] treatments using OTC (open-top chamber) facility. A novel experiment of pot replacement at heading was conducted in 2018 to evaluate the effect of post-heading e[CO₂] on individual grain mass of rice. Meanwhile, we measured net photosynthetic rates under ambient [CO₂] (a[CO₂]) and e[CO₂] (ambient + 200 μmol mol⁻¹) at different developmental stages to identify the occurrence of photosynthetic acclimation. We show that FACE condition did not increase individual grain mass across thirty-six inbred rice cultivars at various rates of nitrogen application, and that the replacement of potted plants either from a[CO₂] to e[CO₂] or from e[CO₂] to a[CO₂] did not impact the individual grain mass. The yield benefit from e[CO₂] was primarily attributed to an increase in the spikelet density determined pre-heading. We conclude that the individual grain mass of inbred rice does not benefit from e[CO₂], which is most likely attributed to a loss of the advantage in CO₂ gain induced by photosynthetic acclimation. Our findings suggest the necessity for selective breeding strategies to make better use of post-heading e[CO₂] and for crop modelers to incorporate updated knowledge into models in the context of elevated [CO₂].