Metabolomics Combined with Photosynthetic Analysis Reveals Potential Mechanisms of Phenolic Compound Accumulation in Lonicera japonica Induced by Nitrate Nitrogen Supply

Mineral nutrition is of vital importance in plant growth and secondary metabolites accumulation, and thereby in the nutritional value of plants. In Lonicera japonica, a preference to nitrate (NO3−−N) in comparison to ammonium (NH4+−N) was found in our previous study, which can be revealed from the rapid growth rate of L. japonica under NO3−−N. This study assessed whether a preference for nitrogen sources could invoke metabolic reprogramming and interrelationships between factors. NO3−−fed plants exhibited substantial enhancement of carbon stimulation, which was strongly and positively correlated with mesophyll conductance. As a result, the elevated carbon flux by NO3− supplement was shuttled to phenolic metabolites synthesis, including flavones and caffeoylquinic acids compounds. Notably, the stimulation was triggered by changes in the NO3− and C/N ratio and was mediated by the induction of several enzymes in the phenylpropanoid pathway. On the contrary, NH4+ plants showed an increment in the content of nitrogen, carbohydrates, and amino acids (mainly a strong increase in citrulline and theanine). Within secondary metabolism, NH4+ may involve active lignin metabolism, showing a dramatic increment in hydroxy−ferulic acid and lignin content. This work provides significant insights regarding the mechanisms of L. japonica in response to diverse nitrogen regimes and effective strategies of nitrogen fertilizer input for L. japonica.