Hydrogen sulfide (H2S) functions as a signaling molecule affecting plant growth, development, and stress adaptation. Tall fescue (Festuca arundinacea Schreb.), a bioenergy crop, encounters significant challenges in agricultural production owing to low light by shading. However, the influence of H2S on tall fescue under low light stress (LLS) remains unclear. To examine the role of H2S in acclimation of tall fescue to low light, we conducted combined analyses of physiological traits, metabolomics, and transcriptomics. These results showed that H2S mitigated LLS-induced inhibition of photosynthesis and maintained normal chloroplast ultrastructure by boosting the expression of photosynthesis-related genes, including PsbQ, PsbR, PsaD, PsaK, and PetH, thereby enhancing the synthesis of carbohydrates (sucrose, starch). H2S upregulated the expression of key genes (PFK, PK, IDH, G6PD) connected to glycolysis, the tricarboxylic acid cycle, and the pentose phosphate pathway to promote carbon metabolism and ensure the supply of carbon skeletons and energy required for nitrogen metabolism. H2S application reverted the LLS-induced accumulation of nitrate nitrogen and the changes in the key nitrogen metabolism enzymes glutamate synthase (GOGAT, EC 1.4.1.13), nitrate reductase (NR, EC 1.6.6.1), glutamine synthetase (GS, EC 6.3.1.2), and glutamate dehydrogenase (GDH, EC 1.4.1.2), thus promoting amino acid decomposition to produce proteins involved in nitrogen assimilation and nitrogen use efficiency as well as specialized metabolism. Ultimately, H2S upregulated the C/N ratio of tall fescue, balanced its carbon and nitrogen metabolism, enhanced shade tolerance, and increased biomass. These results provided new insights into enhancing plant resilience under LLS.
