The Mechanism Of Ammonium And Nitrate Nitrogen Ratio On The Growth And Fruit Metabolism Of Capsicum Annuum L

Ammonium?NH₄⁺?and nitrate?NO₃^-?are two kinds of inorganic nitrogen that higher plants can absorb directly from the agricultural ecosystem.When these two main forms of nitrogen are supplied in a appropriate proportion,the optimal growth of many species can be achieved.However,there are few studies on the effect of NH₄⁺:NO₃^-?A:N?ratio on the growth and fruit metabolism of capsicum.In this study,pepper?Capsicum annuum L?was used as the material in a pot experiment and five ammonium to nitrate ratios were set with the same amount of nitrogen,which were 0:100,12.5:87.5,25:75,37.5:62.5 and 50:50,respectively.The effects of different nitrogen sources and ratios on plant growth,nitrogen metabolism,photosynthetic capacity and fruit quality were determined,and the metabolite composition and the mechanism of ammonium nitrogen induced metabolite changes in pepper fruit were studied from the level of metabolome and transcriptome.The main conclusions of this study are as follow:?1?The growth of pepper plants were promoted by applying the appropriate nitrogen ratio of ammonium to nitrate.When A:N was 25:75,it promoted the dry matter of shoot and root of the plant,increased length,surface areas,volume and tips of root and also resulted in highest nitrogen?N?,phosphorus?P?,and potassium?K?accumulation.Activity of nitrate reductase?NR?and nitrite reductase?NiR?were decreased with the decrease of nitrate.Activity of glutamine synthetase?GS?,glutamate synthases?GOGAT?,glutamine dehydrogenase?GDH?and the levels of relative expression of genes coding enzymes were superior when the A:N ratio of 25:75 were applied.Therefore,an appropriate ratio of A:N?25:75?in nitrogen application can stimulate root development,promote ammonium assimilation related enzymes activity,and improve the N,P,and K accumulation capacity in pepper.?2?Promoting pepper photosynthesis via chloroplast ultrastructure and enzyme activities by optimising the ammonium to nitrate ratio.The results showed that A:N ratio of 25?:75 increased the content of chlorophyll a,leaf area and optimized the thylakoid structure of the chloroplast,it also enhanced the actual photochemical efficiency,photochemical quenching and maximum photosynthetic rate.The addition of 25%and 37.5%NH₄⁺were beneficial for net photosynthetic rate of plant.However,the ratios of 0?:100 and 50?:?50 resulted in higher values for nonphotochemical quenching.An inhibition of maximal photochemical efficiency was found when 50%NH₄⁺was supplied at the later stage of plant growth.Compared with total nitrate treatment,12.5-50%NH₄⁺promoted the Rubisco,1,6-bisphosphatase?FBPase?,fructosylase?FBA?activity and relative expression level of corresponding genes.?3?The appropriate ammonium:nitrate ratio is suitable for the formation of pepper fruit quality and regulates the synthesis of capsaicinoids through the GOGAT–GS pathway.The results showed that compared with the total nitrate treatment?A:N=0:100?,the A:N ratio of 25:75 and 37.5:62.5 increased the yield per plant,fruit width and fruit length,the ratio of 25:75 increased the weight of single fruit,vitamin C,soluble protein,total phenol and flavonoids.The supply of two forms of nitrogen are also conducive to the accumulation of soluble sugar.Compared with total nitrate treatment,the nitrate content of pepper fruit was significantly reduced by adding 25-50%ammonium.A total of 215 metabolites were obtained by liquid chromatography–mass spectrometry?LC–MS?analysis,where 25 metabolites were the major differential metabolite components of pepper fruits among N treatments.The addition of ammonium up-regulated phenylalanine lyase?PAL?as well as carbohydrates,such as?±-lactose and sucrose.The pepper treated with A:N ratio of 25:75 exhibited a relatively higher levels of ascorbic acid in pericarp and amino acids,capsaicinoids in placenta.The expression and activities of glutamic acid synthetase?GOGAT?and glutamine synthetase?GS?which were involved in ammonium assimilation were affected by nitrogen form and ratio,and they were significantly positively correlated with capsaicin,dihydrocapsaicin contents,capsaicinoid synthetase?CS?,as well as the relative expression levels of structure genes which related to capsaicinoid biosynthesis,such as acyltransferase 3?AT3?and acyl-ACP thioesterase?FatA?.?5?The results of transcriptome analysis showed that the application of ammonium nitrogen could significantly regulate the metabolic process of pepper fruit.After screening,there had 557,874,and 1189 differentially expressed genes that found in three comparison groups of 0:100 vs.25:75,0:100 vs.50:50 and 25:75 vs.50:50,respectively.According to GO functional annotation,the difference genes of the three groups were significantly enriched in the metabolic process of molecular functional items,the catalytic activity of biological process items,and the cell and cell parts of cell component items.KEGG analysis showed that the synthesis and metabolism of secondary metabolites and amino acids were significantly regulated by ammonium nitrogen.The up-regulated genes caused by 25%ammonium were significantly enriched in the pathways of zeatin biosynthesis,betadine biosynthesis,amino sugar and nucleotide sugar metabolism;the up-regulated genes caused by 50%ammonium were significantly enriched in the pathways of zeatin biosynthesis,sesquiterpene and triterpene biosynthesis;the up-regulated genes caused by the application of ammonium nitrogen were also annotated in the pathways of alanine,aspartate and glutamate metabolism.In addition,by activating the plant signal transduction mechanism,25%ammonium nitrogen improved the plant's ability to process environmental information.