Low NO 3 − Levels Regulate Nitrogen Transport and Maintain the Productivity of Coreopsis tinctoria Nutt.

The use of Coreopsis tinctoria as a medicinal plant has become popular in China in the past decade; however, basic data on NO 3 − supply related to yield and nitrogen (N) metabolism in this plant are lacking. This study examined the effects of N reduction on N metabolism and the number of flowers of C. tinctoria . A 2-year (2020–2021) hydroponic experiment was conducted in accordance with a randomized complete block design with five N levels [0-mM Ca(NO 3 ) 2 (N1), 0.625-mM Ca(NO 3 ) 2 (N2), 1.250-mM Ca(NO 3 ) 2 (N3), 2.500-mM Ca(NO 3 ) 2 (N4), and 5.000-mM Ca(NO 3 ) 2 (CK)] and three replications. The results showed that appropriate N reduction levels, especially those in N2, affected N metabolism in C. tinctoria and maintained the number of flowers. N reduction treatments (N2–N4) did not damage the photosynthetic system or chlorophyll fluorescence system of C. tinctoria leaves. Suitable N reduction treatment also led to maintenance of the activities of nitrate reductase (NR), nitrite reductase (NIR), glutamine synthase (GS), and glutamate synthase (GOGAT), as well as the levels of total amino acids and protein in the leaves. In addition, the increase in tyrosine (Tyr), glycine (Gly), leucine (Leu), isoleucine (Ile), lysine (Lys), proline (Pro), and arginine (Arg) levels under reduced N conditions is likely to benefit N storage and transport in leaves. This research showed that low NO 3 − levels prior to harvest could represent a potential strategy for productivity maintenance in C. tinctoria.