Effects of free air CO2 enrichment (FACE) on grain yield and quality of hybrid rice

Elevated atmospheric CO2 concentration (eCO2) impacts grain yield and quality of rice, but whether grains at different positions on a large panicle of hybrid rice respond differently to eCO2 is unclear. A 2-year field experiment was conducted to study eCO2-induced changes in grain yield and quality of hybrid rice, with emphasis on the differences between superior spikelets (SS) and inferior spikelets (IS). A hybrid indica cultivar YLY900 was exposed to ambient and elevated CO2 (+200 μmol mol−1) at a large free air CO2 enrichment (FACE) platform in two successive rice seasons. Grain yield, yield components, grain filling capacity and quality traits of SS and IS were measured. Averaged across two years, eCO2 increased grain yield of YLY900 by 15%, which was mainly due to enlarged spikelet density, especially that of secondary branches of panicles. Compared with rice season of 2017, low air temperature, overabundant rainfall and lack of light in the early growth stage in 2016 inhibited tillering formation, resulting in lower panicle density and reduced CO2 fertilization effect on grain yield (22% in 2017 vs. 9% in 2016). In general, grain filling capacity of SS was superior to that of IS, as shown by higher fully-filled grain percentage, larger grain size and grain weight. Grain quality of SS was better than that of IS in terms of less grain chalkiness, fewer green grains, and better cooking quality. However, the concentrations of P, K, Ca, Mg, S and Mn were lower in SS than IS. For both SS and IS, eCO2 decreased head rice percentage, grain N and S concentrations, but increased cooking quality as indicated by the changes in RVA profiles. No grain position by CO2 interactions on grain quality traits were detected except the grain appearance: SS had few green grains at either CO2 condition, but IS had a great portion of green grains, which was reduced by approximately 50% at eCO2. Weather conditions at the key growth stage of sink formation had a strong impact on the CO2 fertilization effect. Although grain quality of SS and IS differed significantly, their responses to CO2 elevation were similar except the eCO2-increased grain ripeness of IS. Elevated CO2 may not be able to offset the yield loss caused by unfavorable weather conditions for rice growth, a proper adjustment in crop management is needed to improve rice yield and quality in future high CO2 environments.