Integrated Physiological and Transcriptomic Profiling of Tall Fescue in Response to Low-Nitrogen Stress

Low nitrogen (LN) availability is a major constraint limiting plant vegetative growth and reproductive development. To investigate the physiological and molecular mechanisms underlying LN stress in tall fescue (Festuca arundinacea), a comprehensive analysis of phenotypic, physiological, and transcriptional responses was conducted. The results showed that LN stress significantly reduced shoot height, tiller number, leaf length, and shoot biomass. In addition, root system architecture was markedly altered, with increased total root length but decreased root number and root volume. Physiological assays revealed that total chlorophyll content, photochemical efficiency, and net photosynthetic rate were significantly reduced. Concurrently, nitrate reductase activity, NO3− concentration, and N content were significantly reduced, while soluble sugar content was significantly enhanced. Transcriptomic analysis identified a total of 1185 differential expression genes (585 upregulated and 600 downregulated), with significant changes in genes involved in N transport, N assimilation, glycolysis, and the tricarboxylic acid cycle. In addition, 27 transcription factor genes across seven distinct families were significantly differentially expressed. Three bHLH transcription factors, FaIRO2, FabHLH69, and FabHLH062, were identified as candidate regulators of the LN response in tall fescue based on their expression patterns under LN stress. Collectively, this study provides integrated physiological and transcriptional insights into tall fescue responses to LN stress and highlights potential target genes for genetic improvement of perennial forage and turf grasses with high N use efficiency.