Effects of elevated CO2 on rice grain yield and yield components: Is non-flooded plastic film mulching better than traditional flooding?

The atmospheric carbon dioxide (CO2) concentration has been increasing rapidly since the Industrial Revolution. The responses to elevated CO2 of rice (Oryza sativa L.) growth and yield have been widely reported, but the majority of these studies investigated rice grown under traditional flooding at two contrasting CO2 levels. The effects of a range of CO2 concentrations (CO2 gradient) on the yield and its components of rice grown under non-flooded vs. flooded conditions remain unclear. Using a CO2 Gradient Tunnel (CGT), we investigated the effects of elevated CO2 (450, 500, 550 and 600 μmol mol−1) on rice yield and yield components under two cultivation practices, viz. traditional flooding (TF) and non-flooded plastic film mulching (PM). Elevated CO2 increased rice yield by 25% under the TF treatment at 450-500 μmol mol−1, but had no effect or decreased the rice yield under the PM treatment. The number of panicles per square meter was decreased by 4-26% under progressive elevation of CO2 concentration, regardless of cultivation practice. Elevated CO2 increased the spikelet number per panicle and filled spikelet percentage under the TF treatment, but had no effect on these parameters under the PM treatment. Specifically, elevated CO2 decreased the number of degenerated spikelets on secondary rachis branches of rice grown under the TF treatment by 75%, but increased that of filled spikelets by 43%. This was the major reason for the CO2-induced increase in rice yield under the TF treatment. The 1000-grain weight and Harvest Index (HI) under the two cultivation practices was increased only when CO2 concentration was elevated to 550-600 μmol mol−1. The CO2 × cultivation interaction was detected for grain yield. When CO2 concentration was increased to 600 μmol mol−1, the rice yield of the PM treatment was 2% higher than the TF treatment. This study demonstrated that improved management practices are needed to maximize the benefits of non-flooded plastic film mulching cultivation in a CO2-rich world. Our results provide major implications for water management of rice production systems and global food security under future higher CO2, and potentially drier, environments.