Effects of elevated CO2 concentration gradients on rice grain quality under two cultivation practices in cold region

Background and Objectives: The effect of elevated atmospheric CO2 (eCO2) on rice (Oryza sativa L.) grain quality has been widely studied, but mostly under two contrasting levels of CO2 concentration. Our study was conducted in a new version of CO2 Gradient Tunnel (CGT) to study the responses of grain quality of japonica rice cv. Wuyoudao no.4 (WYD4) and Songjing no.9 (SJ9) to five CO2 concentrations (390-600 mu mol center dot mol-1) gradient levels at two cultivation practices: traditional flooding (TF) and non-flooded plastic film mulching (PM). Findings: The results showed that the response of SJ9 to eCO2 was more pronounced under TF cultivation. When the CO2 concentration increased to 450 mu mol center dot mol-1, the brown rice ratio and milled rice ratio of SJ9 decreased by 4.2% and 17.7%, respectively. However, when CO2 concentration increased to 600 mu mol center dot mol-1, the brown rice ratio increased by 3.7%. These results reveal that different gradients of eCO2 have different effects on rice quality. Besides that, eCO2 decreased the nutritional quality of SJ9 by decreasing the protein content and amylose content, while eCO2 improved rice appearance and eating quality. Different from SJ9, when CO2 concentration increased to 500-600 mu mol center dot mol-1, the milled rice ratio and head rice ratio of WYD4 increased by 5.9%-7.5% and 16.7%-19.5% under PM cultivation. Furthermore, eCO2 decreased the chalky grain rate or chalkiness by 34.5%-80.5% and 53.1%-77.1%, respectively. Conclusions: Under PM cultivation, the rice milling quality and appearance quality of WYD4 were promoted by eCO2. Rice nutritional quality was significantly decreased in both cultivation practices. In the comparison of rice quality between the two cultivations, except for rice eating quality, the other grain quality parameters of SJ9 under PM cultivation were better than those under TF cultivation. The opposite was true for WYD4. Significance and Novelty: Under future production conditions with eCO2, the high-yielding SJ9 cultivar is more suitable for water-saving cultivation with mulching film, while the high-quality WYD4 is suitable for TF cultivation.