Do rice growth and yield respond similarly to abrupt and gradual increase in atmospheric CO₂?

Crops have been well studied at abruptly elevated CO₂ (e[CO₂]). In fact, atmospheric CO₂ concentration is rising gradually, but its ecological effect is little known. Thus, rice growth and yield were investigated under gradual e[CO₂] (GE) and abrupt e[CO₂] (AE) using open-top chambers. Gradual e[CO₂] involved an ambient CO₂ (a[CO₂]) + 40 μmol mol^-1 per year in 2016 until a[CO₂] + 200 μmol mol^-1 in 2020, while AE maintained a[CO₂] + 200 μmol mol^-1 from 2016 to 2020. We found that steady-state photosynthetic rates responded similarly and increased significantly under GE and AE, however, photosynthetic induction time in dynamic photosynthesis was reduced by AE. Gradual e[CO₂] had little effect on biomass before the grain filling stage, while AE significantly stimulated biomass because of the stronger tillering ability and faster photosynthetic induction rate. Neither e[CO₂] increased biomass at maturity, however, a significant increase in panicle density was observed under AE. Surprisingly, rice yield was not promoted by both e[CO₂], possibly resulting from the reduced carbon assimilation caused by accelerated phenology from grain filling to maturity. These results promote a new understanding of the CO₂ fertilization effect with small and slow increases in CO₂ concentration, closer to what happens in nature. This may partly challenge the classic view of elevated CO₂ fertilization effects from AE.