Study On The Mechanism Of Salinity Level Regulating The Quality Of Hydroponic Chinese Chive(Allium Tuberosum Rottler)

Chinese chive（Allium tuberosum Rottler ex Spreng.）is one of the main leafy vegetables grown in northern China,which is rich in nutrients and has a unique flavor that is loved by many people.Soil cultivation is a traditional cultivation method for Chinese chives,but in recent years,hydroponic Chinese chives have gradually attracted people’s attention.The main reason is that hydroponic Chinese chives have fewer pests and diseases,which can reduce pesticide residues.Sodium chloride（Na Cl）,as an inorganic salt,can cause changes in nutrient solution salinity,and moderate salinity can promote the accumulation of metabolites by stimulating related metabolic pathways,thereby improving the quality of crops.This study investigated the effects of salinity levels on the growth and physiology,photosynthesis,endogenous hormones,nutrients,and antioxidant capacity of hydroponic Chinese chives by adding different concentrations of Na Cl（0m M,5 m M,10 m M,20 m M,and 30 m M）to the nutrient solution of hydroponic Chinese chives,and analyzed the differences in the composition of volatile flavor compounds of Chinese chives at different salinity levels of the nutrient solution.At the same time,the response of genes and metabolites to different salinity levels was analyzed from the level of transcriptome and metabolome.The main results of this study are as follows:（1）Moderate salinity（5 m M and 10 m M Na Cl）in the nutrient solution had no inhibitory effect on plant height,leaf length,leaf width,leaf number,aboveground dry and fresh weight and belowground fresh weight of hydroponic Chinese chives,and likewise had no significant effect on chlorophyll a,chlorophyll b,total chlorophyll,transpiration rate and stomatal conductance of Chinese chive leaves,but increased the content of GA₃ and IAA in leaves,and the net photosynthetic rate was significantly increased under 10 m M Na Cl.（2）The level of nutrient solution salinity had a significant effect on the accumulation of nutrients and antioxidant capacity of hydroponic Chinese chives.Salinity treatments（5-30 m M Na Cl）significantly reduced the nitrate content of Chinese chives and contributed to their safe quality.Moderate salinity treatments（5 m M and 10 m M Na Cl）differentially increased the content of EAAs,NEAAs,total amino acids,and mineral elements,and enhanced the accumulation ofα-carotene,lutein,zeaxanthin,and total carotenoids,while significantly increasing the content of total phenolic acids and total polyphenols.Salinity treatments（5-30 m M Na Cl）enhanced the ABTS radical scavenging ability,DPPH radical scavenging ability,and oxygen radical uptake ability of Chinese chives.（3）Seven key parameters（SPME fiber,sample weight,Na₂SO₄ weight,extraction temperature,equilibration time,extraction time,and desorption time）were optimized in the headspace solid-phase microextraction（HS-SPME）of Chinese chive volatile flavor compounds using a single-factor test.A total of 75 volatile flavor compounds were identified in Chinese chives using the optimized extraction method,including 19 ethers,14 aldehydes,11 alcohols,7 ketones,6 hydrocarbons,9esters,2 phenols,2 furans,and 5 other compounds,with the highest content of ethers.Moderate salinity treatments（5 m M and 10 m M Na Cl）increased the total amount and content of volatile compounds and the content of precursor S-alk（en）yl cysteine sulfoxides,among which 10 m M Na Cl had the best effect,promoting the production of more ethers,including 2,4-dimethyl thiophene,methyl propyl disulfide,methyl propyl trisulfide,and 1-allyl-3-propyl trisulfide.（4）According to the odor threshold values and odor activity values（OAVs）of volatile flavor compounds,there were 14 aroma active compounds（OAVs>1）among 75 volatile flavor compounds,mainly aldehydes and ethers.Based on the odor descriptions of the compounds,the aroma of Chinese chive volatile flavor compounds could be classified into eight categories,including"green and grassy","floral","fatty","sweet","garlic and onion","fresh","spicy",and"fruity".Among them,"garlic and onion"was the strongest odor,mainly from dimethyl disulfide,dimethyl trisulfide,and diallyl disulfide in the ether group.In comparison among treatments,5 m M Na Cl enhanced the"fruity","floral"and"fatty"odors of Chinese chives,while 10 m M Na Cl enhanced the"green and grassy","sweet","garlic and onion","fresh"and"spicy"odors of Chinese chives.（5）Full-length transcriptome analysis was performed on Chinese chive leaves at different salinity levels,and a total of 61.37 Gb Clean Data were obtained from the 9 c DNA libraries constructed,all with Q30 above 90.6%,and 17957 Unigenes were obtained by assembly.Through differential expression analysis,250,287 and 245 differentially expressed genes were identified in the 0-m M vs 10-m M comparison group,0-m M vs 30-m M comparison group and 10-m M vs 30-m M comparison group,respectively.Further analysis of related genes revealed that salinity treatment could modulate the biosynthesis of Chinese chive flavor precursors by serine O-acetyltransferase（SAT）,cysteine synthase（CS）,glutamate-cysteine ligase（GCL）,glutathione synthase（GS）,gamma-glutamyl transpeptidase（GGT）,flavin-containing monooxygenase（FMO）and phenolic acid biosynthesis pathways by expression of phenylalanine ammonia lyase（PAL）,cinnamate-4-hydroxylase（C4H）,acetylserotonin-O-methyltransferase（ASMT）,and ferulate-5-hydroxylase（F5H）genes in the synthesis pathway promoted the accumulation of CSO and phenolic acids.Based on widely targeted metabolome,a total of 812 metabolites were identified from 9 samples,mainly phenolic acids（16.01%）and lipids（15.52%）.36,86 and 33 differential metabolites were identified in the three comparison groups,respectively.The combined analysis of differential expressed genes and differential metabolites indicated that metabolic pathways,biosynthesis of secondary metabolites,carbon metabolism,amino acid biosynthesis,cysteine and methionine metabolism,and starch and sucrose metabolism were the main biological processes associated with salinity-induced metabolite accumulation in Chinese chive leaves.In summary,moderate salinity treatment（5 m M and 10 m M Na Cl）can improve the nutritional and flavor quality of Chinese chive leaves by inducing the accumulation of metabolites,while having no inhibitory effect on the normal growth of Chinese chives.This study provides a theoretical basis for the high-quality cultivation of hydroponic Chinese chives.