Analysis Of The Effects Of Different Nitrogen Levels On Capsaicin Synthesis In Capsicum Annuum Var. Conoides Based On Transcriptome

Pepper is an important cash crop in China.Nitrogen is one of the indispensable elements in the growth and development of chili peppers.The unreasonable application of nitrogen fertilizer causes a decrease in chili yield and quality,as well as environmental pollution.This study used’gd-75-3-10’as the experimental material and conducted a pot experiment through soilless cultivation..Six nitrogen levels of 0,5,10,15,20,and 25 mmol·L^-1were set.Study the effects of different nitrogen fertilizer application rates on the growth and quality of pepper fruits,and investigate their regulatory mechanisms.Determining the appropriate amount of nitrogen fertilizer for hot pepper has certain guiding significance for the rational application of nitrogen fertilizer for hot pepper and the development of pepper industry.The results obtained from the experiment are as follows:1.Changes in growth indicators of pepper fruits under different nitrogen levels.Through different nitrogen levels of treatment,the fruit length,fruit width,flesh thickness,fruit shape index,and single fruit weight of pepper fruits in the red ripening stage were higher than those in the green ripening stage.As the nitrogen concentration increases,the overall fruit length and single fruit weight show a trend of first increasing and then decreasing.Reached maximum at 15 N.The fruit width increases with the increase of nitrogen concentration,and reaches its maximum value at 25 N.Nitrogen treatment had no significant effect on fruit pulp thickness.2.Changes in nutritional indicators of pepper fruit under different nitrogen levels.The content of capsaicin in placenta,seeds,and pericarp is sorted according to the developmental stage as follows:red ripening stage>color transformation stage>green ripening stage>green fruit stage,red ripening stage>color transformation stage>green ripening stage>green fruit stage,green ripening stage>green fruit stage>green fruit stage>color transformation stage>red ripening stage.The order of capsaicin content in different parts of chili pepper fruit is:placenta>pericarp>seed.Through nitrogen treatment of 0～25 mmol·L^-1,the content of capsaicin substances in the placenta increased with increasing nitrogen levels during the green fruit and green ripening stages.During the color transition and red ripening stages,there is a trend of first increasing and then decreasing with the increase of nitrogen levels.The highest total capsaicin content was 17.05 g·kg^-1in the 15 N treatment during the red ripening period.There is a positive correlation between the capsaicin content in the placenta and the capsaicin content in the fruit peel.The content of capsaicin in the fruit peel is positively correlated with VC and VE.The capsaicin content in seeds is positively correlated with the capsaicin content in placenta.Glucose,fructose,sucrose,soluble sugar,protein content,total amino acids,vitamin E,total phenols,and total flavonoids were all higher in the red ripening stage than in the green ripening stage,and showed an overall trend of increasing first and then decreasing with the increase of nitrogen concentration.According to comprehensive evaluation and analysis,the most suitable nitrogen concentration for pepper fruit quality is 15 mmol·L^-1.3.The spatiotemporal expression changes of capsaicin synthesis related genes under different nitrogen levels.The relative expression levels of PAL,C4H,4CL,HCT,ACS,C3H,COMT,BCAT,FatA,Pun1 genes are higher in the placenta than in the fruit peel.The expression levels of ACL and BCKDH in the late fruit development stage of the peel are higher than those in the placenta.The expression levels of PAL,C4H,pAMT,and Pun1 are relatively high during the green fruit stage.The expression levels of HCT,BCKDH,and FatA are relatively high during the green ripening stage.The expression levels of 4CL and BCAT genes are relatively high during the color transition period.ACS and C3H have the highest expression levels during the red ripening stage.As nitrogen levels increase,the relative expression levels of 4CL,C3H,BCAT,BAKDH,KAS,and ACS genes are higher at 15 N.The expression level of ACL was higher after 25 N treatment.During the green ripening stage of fruit development,the expression level of FatA was higher after 5 N treatment.The expression level of HCT was higher after 0 N treatment.4.Transcriptome analysis of pericarp and placenta under different nitrogen levels.1552,892,1559,2354,2445,2972,and 4945 differentially expressed genes were screened from seven treatment groups:Pe-0 vs Pe-15,Pe-5 vs Pe-15,Pe-25 vs Pe-15,Pl-0 vs Pl-15,Pl-5 vs Pl-15,Pl-25 vs Pl-15,Pe-15 vs Pl-15.After GO functional annotation,it was found that most differentially expressed genes were significantly enriched in the responses to organic nitrogen compounds,cell responses to reduced oxygen levels,photosynthesis,and chitin responses.KEGG enrichment analysis showed that the pathways for DEGs enrichment between different treatment groups mainly include the biosynthesis of various alkaloids,biosynthesis of phenylpropanoid compounds,plant hormone signal transduction,biosynthesis of secondary metabolites,and photosynthesis antenna proteins.5.Analysis of differential genes in the synthesis pathway of capsaicin.Analysis of the correlation between capsaicin content and capsaicin synthesis genes after treatment with different nitrogen levels.The content of capsaicin is positively correlated with the relative expression levels of genes such as LOC107843098,LOC107878383,and LOC107851983.The expression trends of these genes are similar to those of capsaicin accumulation,suggesting that they play an important role in capsaicin synthesis and accumulation.