Effects of elevated atmospheric CO2 on hybrid rice (Oryza sativa L.) starch granules: Structural, functional changes and their underlying causes.

Elevated atmospheric CO2 (E-CO2) intensifies climate change impacts on crops. Hybrid rice, valued for high yield, is crucial for starch production, yet the influences of E-CO2 on its starch properties remain unclear. Using a free-air CO2 enrichment system, we investigated two hybrid rice cultivars (YY538 and YLY900) to understand E-CO2-induced changes in starch physicochemical properties and formation mechanisms. E-CO2 significantly increased the proportion of large-volume (+3.1 %) and large-surface-area (+3.0 %) starch granules, the long-branch-chain proportion (+6.3 %) in amylopectin, double helices (+4.1 %), and crystallinity (+4.7 %), greatly altering starch morphology and molecular structures. Consequently, starch functionality changed accordingly: breakdown, ΔHgel, ΔHret, and transition temperature rose by 26.3 %, 8.5 %, 74.5 %, and 1.04 °C, respectively, while setback decreased by 39.7 %, yielding softer rice (-21.1 % hardness) and better taste. These changes aligned with higher GBSS (+32.8 %), α/β-amylase (+12.3 %-13.5 %), SSS activity (+27.0 %), and a 47.3 % upregulation of OsGBSS1. Indica-hybrid YLY900 responded more strongly than japonica-hybrid YY538 in most cases, highlighting cultivar-specific adaptation to E-CO2-rich climates. Analysis indicates E-CO2-induced changes in starch structure and function stem from enhanced gene expression and enzyme activities, with most traits significantly correlated with ethylene production rate. These findings suggest that despite improved taste, future E-CO2 climates may require more energy for starch processing.