Light quality-dependent carbon allocation in tall fescue: Integrated transcriptomic and metabolomic evidence

Light quality regulates plant carbon assimilation and carbon fraction composition, but how turfgrass coordinates photosynthetic capacity with leaf carbon-fraction characteristics remains unclear. We examined tall fescue ( Festuca arundinacea ) under white, red, blue, green, and red–blue light by integrating growth, photosynthetic, carbon-fraction, transcriptomic, and metabolomic analyses. Blue light enhanced photosynthetic capacity and was associated with relatively high whole-plant carbon stock, but these responses were not accompanied by corresponding increases in leaf non-structural carbon or in cellulose, hemicellulose, and lignin concentrations. Red light was associated with relatively high turf quality and greater accumulation of starch and structure-related fractions, particularly hemicellulose and lignin. Red–blue light showed the highest sucrose content and relatively strong antioxidant responses, whereas single-green light resulted in comparatively lower photosynthetic and growth performance. Multi-omics analyses revealed distinct treatment-dependent molecular responses rather than a common UDP-glucose-centered pathway. Red and blue light induced positive enrichment of photosynthesis- and carbon fixation-related pathways at the transcriptomic level, although corresponding carbon-related metabolic responses were not consistently enhanced. Green light showed coordinated negative responses across the phenylpropanoid–flavonoid axis, whereas red–blue light was characterized by positive transcriptional responses in oxidative phosphorylation and glutathione metabolism and concordant enhancement of unsaturated fatty acid biosynthesis across both omics layers. Overall, different light qualities produced distinct relationships among photosynthetic capacity, carbon accumulation, leaf carbon fractions, and metabolic regulation, providing physiological and multi-omics evidence for light-quality-dependent carbon metabolism in tall fescue.