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

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.