Low nitrogen level improves low-light tolerance in tall fescue by regulating carbon and nitrogen metabolism

Low-light (LL) stress is a primary factor contributing to turf deterioration. Previous studies have recommended the use of reduced nitrogen (N) fertility for maintaining turf grasses and improving their LL tolerance. However, the mechanism for turf grass susceptibility to LL-induced stress is still unclear. The present study investigated the role of low N in improving tall fescue photosynthesis under LL stress by regulating carbon and N metabolism. The LL stress significantly reduced tiller numbers, biomass, chlorophyll content, net photosynthetic rate (Pn), the maximum quantum yield of photosystem photochemistry (Fv/Fm), actual photochemical efficiency of photosystem II (ΦPSII), and the carbon and N contents in roots and leaves. The stress also reduced the sugars contents but increased the C/N ratio in the roots and leaves. However, low N alleviated the effects of LL stress by significantly increasing the tiller numbers, root length, root-to-shoot ratio, and sugars contents in roots. Under low light and low nitrogen (40 μmol m−2 s−1 PPFD + 1.0 mM N, LN-) conditions, the tiller number, Pn, soluble sugar content and protein content in the roots increased by 24.72%, 19.43%, 71.91%, and 16.05%, respectively, compared to the low light and normal nitrogen (40 μmol m−2 s−1 PPFD + 6.0 mM N, LN) conditions. Additionally, the activities of sucrose synthase (SS), glutamate dehydrogenase (GDH), and total amylase (AMY) in leaves and the activity of glutamate synthase (GOGAT) in roots were also remarkably enhanced by the low N level. Subsequently, a total of 514 metabolites were detected by non-targeted metabolomics analysis. There were 83 and 173 differential metabolites in the LL stress and control leaves and roots, respectively. Meanwhile, 54 and 67 differential metabolites were also detected in the leaves and roots under LN treatment and LN- treatment, respectively. The KEGG analysis showed that the LN- treatment significantly affected carbon and amino acid metabolism in tall fescue. Conversely, organic acids, amino acids, nucleic acids, and their respective derivatives were reduced in response to LL-induced stress. Furthermore, the LN- treatment enhanced the citrate cycle and increased the glucose and sucrose contents compared to the LN treatment. In addition, the asparagine level was significantly reduced in the leaves but significantly increased in the roots under LN- treatment. Thus, these results indicated that low N level increases photosynthesis by regulating the balance between carbon and N metabolism in the roots and leaves, thereby improving tolerance to LL-induced stress.