Impact of Elevated CO and Reducing the Source-Sink Ratio by Partial Defoliation on Rice Grain Quality - A 3-Year Free-Air CO Enrichment Study.

Evaluating the impact of increasing CO on rice quality is becoming a global concern. However, whether adjusting the source-sink ratio will affect the response of rice grain quality to elevated CO concentrations remains unknown. In 2016-2018, we conducted a free-air CO enrichment experiment using a popular cultivar grown at ambient and elevated CO levels (eCO, increased by 200 ppm), reducing the source-sink ratio cutting leaves (LC) at the heading stage, to investigate the effects of eCO and LC and their interactions on rice processing, appearance, nutrition, and eating quality. Averaged across 3 years, eCO significantly decreased brown rice percentage (-0.5%), milled rice percentage (-2.1%), and head rice percentage (-4.2%) but increased chalky grain percentage (+ 22.3%) and chalkiness degree (+ 26.3%). Markedly, eCO increased peak viscosity (+ 2.9%) and minimum viscosity (+ 3.8%) but decreased setback (-96.1%) of powder rice and increased the appearance (+ 4.5%), stickiness (+ 3.5%) and balance degree (+ 4.8%) of cooked rice, while decreasing the hardness (-6.7%), resulting in better palatability (+ 4.0%). Further, eCO significantly decreased the concentrations of protein, Ca, S, and Cu by 5.3, 4.7, 2.2, and 9.6%, respectively, but increased K concentration by 3.9%. Responses of nutritional quality in different grain positions (brown and milled rice) to eCO showed the same trend. Compared with control treatment, LC significantly increased chalky grain percentage, chalkiness degree, protein concentration, mineral element levels (except for B and Mn), and phytic acid concentration. Our results indicate that eCO reduced rice processing suitability, appearance, and nutritional quality but improved the eating quality. Rice quality varied significantly among years; however, few CO by year, CO by LC, or CO by grain position interactions were detected, indicating that the effects of eCO on rice quality varied little with the growing seasons, the decrease in the source-sink ratios or the different grain positions.