Nitrogen Source Affects the Composition of Metabolites in Pepper ( Capsicum annuum L.) and Regulates the Synthesis of Capsaicinoids through the GOGAT–GS Pathway

<b> </b>Phytochemical analyses of pepper fruit metabolites have been reported; however, much less is known about the influence of different forms of nitrogen (N), which is critical for plant growth and fruit quality formation. The &#8220;Longjiao No. 5&#8221; variety (<i>Capsicum annuum</i> L.) grown in Northwestern China was profiled using liquid chromatography&#8722;mass spectrometry (LC&#8722;MS) coupled with multivariate data analysis to explore the composition of different metabolites in pericarp and placenta, and to investigate the effect of three ammonium (NH₄⁺)^-to-nitrate (NO₃^&#8722;) ratios (0:100, 25:75, and 50:50). A total of 215 metabolites were obtained by qualitative analysis, where 31 metabolites were the major differential metabolite components of pepper fruits between placenta and pericarp, and 25 among N treatments. The addition of ammonium up-regulated carbohydrates, such as &#206;&#177;-lactose and sucrose, as well as phenylalanine lyase (PAL) of placenta tissue. The supply of 25% NH₄⁺&#8722;N and 75% NO₃^&#8722;&#8722;N exhibited a relatively higher levels of ascorbic acid in pericarp and amino acids, capsaicin, and dihydrocapsaicin in placenta, and led to higher fruit weight among the ammonium-to-nitrate ratios. The expression and activities of glutamic acid synthetase (GOGAT) and glutamine synthetase (GS) that are involved in ammonium assimilation were affected by adjusting the ammonium&#8722;N proportion, and they were significantly positively correlated with capsaicin, dihydrocapsaicin contents, capsaicinoid synthetase (CS), as well as the relative expression levels of genes related to capsaicinoid biosynthesis, such as acyltransferase 3 (<i>AT3</i>) and acyl-ACP thioesterase<i> </i>(<i>FatA</i>).

Abstract licence: CC BY

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