Effects of elevated [CO₂] on leaf elongation under different nitrogen conditions mediated by auxin in tall fescue

Leaf elongation is a crucial factor controlling forage production and is sensitive to environmental factors including CO₂ concentration ([CO₂]) and nitrogen (N). The objectives of this study were to examine the effects of elevated [CO₂] on leaf elongation and clarify the underlying regulatory mechanism related to auxin metabolism in tall fescue (Festuca arundinacea. cv. 'Barlexas') under different N levels. Plants treated with low N level (0.25 mM) and moderate N level (4 mM) were exposed to ambient [CO₂] (400 μmol⋅mol^-1) or elevated [CO₂] (800 μmol⋅mol^-1) in environment-controlled growth chambers. Elevated [CO₂] increased leaf elongation by stimulating cell division and elongation, regardless of the N conditions. The promotion of leaf elongation by elevated [CO₂] was more pronounced under moderate N supply. Further results indicated that exogenous naphthylacetic acid treatment and endogenous overexpression of auxin biosynthesis gene FaYUCCA11 counteracted elevated [CO₂]-induced promotion of leaf elongation under different N levels. The results demonstrated that auxin mediated the differential effects of elevated [CO₂] on leaf elongation in tall fescue under different N availability regimes. Our results provide novel strategies for breeding new varieties of Poaceae grasses that will produce high biomass with minimal fertilizer inputs.