Elevated atmospheric [CO₂] and nitrogen (N) availability are critical determinants of plant growth. This study investigated the underlying mechanisms of hormones in mediating elevated [CO₂]-promoted shoot growth and leaf elongation under different N conditions in tall fescue (Festuca arundinacea). Plants were grown under low N (LN, 0.25 mM) and moderate N (MN, 4 mM) conditions. Subsequently, the plants from each N treatment were divided and immediately transferred to ambient (400 μmol mol−1) or elevated [CO₂] (800 μmol mol −1). Elevated [CO₂] promoted plant growth under both LN and MN conditions through affecting cell division and cell elongation, with a more pronounced effect under MN supply levels. Elevated [CO₂]-induced shoot growth and leaf elongation were associated with increased cytokinin level under LN and with enhanced cytokinin and auxin under MN conditions. Exogenous cytokinin inhibitor (lovastatin) and auxin inhibitor (2,3,5-triiodobenzoic acid) altered elevated [CO₂]-enhanced growth in tall fescue regardless of N conditions. Elevation of [CO₂]-enhanced growth by modulating cell growth-related genes OsCycD2, OsPCNA, and OsEXPA10 was counteracted and reduced in FaCKX11-OE lines under LN and MN conditions, respectively. However, this enhancement was counteracted in FaDAO-OE lines under MN but not under LN conditions. These results demonstrated that elevated [CO₂]-enhanced shoot growth in perennial grass species could be primarily mediated by cytokinin under LN conditions, while both cytokinin and auxin were involved in regulating elevated [CO₂]-enhanced growth under MN conditions.

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