Reduced Application of Nitrogen Fertilizer Affects Nitrogen Metabolism of Leaves and Maintains the Production During the Harvesting Stage in Coreopsis tinctoria

Coreopsis tinctoria , as a medicinal plant, has become popular in China in the past decade. However, basic data on NO 3 − supply related to its production and nitrogen (N) metabolism are lacking. In this study, all plants were grown to four leaves in substrate (vermiculite:perlite = 2:1), which were subsequently transferred to different reducing NO 3 − solutions. Appropriate N fertilizer application reductions (1/8N-1/2N) maintained the flower number and biomass of Coreopsis tinctoria during 2020–2021. N fertilizer reduction maintained the N assimilation levels of C. tinctoria and increased N use efficiency (NUE), one of the important reasons was that the reduction of N fertilizer did not damage the chlorophyll fluorescence system and photosynthetic system. 1/2N-1/8N treatment also maintained the activities nitrate reductase (NR), nitrite reductase (NIR), glutamine synthase (GS) and glutamate synthase (GOGAT) and the content of total amino acid and soluble protein in leaves. N reduction treatment also increased the levels of tyrosine (Thr), serine (Ser), leucine (Leu), isoleucine (Ile), lysine (Lys) and arginine (Arg) of leaves in C. tinctoria , and further enhances N assimilation ability. Leaf N content was significantly correlated with the activities of NR, NIR, GS, GOGAT and GDH, indicating that N metabolism was effectively regulated by N levels. In conclusion, the 1/8 N level prior to harvest could be one of the strategies for C. tinctoria cultivation because it minimizes planting costs and maintains nitrogen assimilation capacity and productivity.