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

Crops have been well studied at abruptly elevated CO (e[CO]). In fact, atmospheric CO concentration is rising gradually, but its ecological effect is little known. Thus, rice growth and yield were investigated under gradual e[CO] (GE) and abrupt e[CO] (AE) using open-top chambers. Gradual e[CO] involved an ambient CO (a[CO]) + 40 μmol mol per year in 2016 until a[CO] + 200 μmol mol in 2020, while AE maintained a[CO] + 200 μmol mol 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[CO] 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[CO] increased biomass at maturity, however, a significant increase in panicle density was observed under AE. Surprisingly, rice yield was not promoted by both e[CO], possibly resulting from the reduced carbon assimilation caused by accelerated phenology from grain filling to maturity. These results promote a new understanding of the CO fertilization effect with small and slow increases in CO concentration, closer to what happens in nature. This may partly challenge the classic view of elevated CO fertilization effects from AE.