Differences and similarities between japonica and indica rice cultivars in the response of grain quality to free-air CO2 enrichment

Selecting high-yielding rice cultivars with superior quality under a changing climate is of particular importance for ensuring future food security. In this field experiment, japonica rice Wuyunjing27 (WYJ27) and indica rice Yangdao6 (YD6) displaying low and high yield enhancement at elevated CO₂ (eCO₂), respectively, were compared in their grain quality responses to free-air CO₂ enrichment (FACE). Grains located at apical primary rachis (superior spikelets, SS) and at proximal secondary rachis (inferior spikelets, IS) were separately investigated in their responses to eCO₂ because of the asynchronous grain development in rice panicles. Significant quality declines were found in SS of WYJ27, including increased chalky grains and decreased protein and amino acid concentration; in contrast, YD6 was less affected by eCO₂ in these traits. Grain quality of IS of both cultivars was less affected by eCO₂, which might be associated with improved grain ripening, as shown by the reduced proportions of immature grains at harvest. Gel consistency and peak, hot, and final viscosities in the starch rapid visco analyzer profile were increased by eCO₂ when averaged across SS and IS of the two cultivars, indicating enhanced stickiness of cooked rice. For nutrient compositions, only grain sulfur concentration was reduced by eCO₂, while the concentrations of other mineral elements and phytic acid were unchanged when averaged across SS and IS of the two cultivars. These results indicate that indica rice with higher yield increase from eCO₂ displayed less quality deterioration, but the underlying mechanisms need further investigation in order to breed rice with both high yield and good quality in eCO₂ environments.