Mechanism Of Different Boron Levels On Tomato Nutrition And Flavor Quality

Tomato is the highest value fruit and vegetable crop worldwide,and an important source of micronutrients in the human diet.In recent years,tomato production is mostly devoted to improving yield,the product quality is uneven,some even appear quality reduction and flavor loss.Boron is an essential trace element for the growth and development of higher plants,plays important role in maintaining the structure and function of plant cell walls,and has an impact on the metabolism of a large number of essential elements Ca,Mg,P,and proteins,amino acids,glucose,and phenolic substances.At present,some studies have shown that boron fertilizer plays an important role in the growth,development,and nutritional quality of horticultural plants,but the effect and mechanism of improving the flavor and quality of tomatoes are still unclear.In this study,two varieties"Kaifeng"and"Hongli",which are widely used in tomato production in Chongqing,were used as experimental materials,and different boron levels(0,1,2,4,and 8 mg·L^-1 H₃BO₃)on the nutrition,flavor quality and boron uptake and transport of tomato.Transcriptomics and genomics were used to further study the metabolic information of tomato fruit in response to boron-induced metabolism and the expression characteristics of key enzyme genes of sugar metabolism and lycopene synthesis and metabolism.The main findings are as follows:（1）Appropriate boron concentration improved the photosynthetic system parameters of tomato plants and increased tomato yield.The photosynthetic system parameters of plants increased most obviously under the B1 treatment(1 mg·L^-1),but the B4(8 mg·L^-1)treatment showed an inhibitory effect.Boron treatment significantly increased the plot yield of the two tomato varieties,and the fruit shape index of tomato increased first and then decreased.The fruit shape index of"Kaifeng"and"Hongli"reached the maximum under B1 and B3(4 mg·L^-1)treatment respectively.（2）Appropriate boron concentration improved the nutritional quality of tomato fruit.Under the boron treatment,the vitamin C content of tomato showed a gradually increasing trend as a whole,the nitrate content showed a gradually decreasing trend,and the organic acid content showed a trend of decreasing first and then increasing.)down to the minimum value.At the same time,B2 treatment significantly increased the soluble solids content and sugar-acid ratio in tomato fruit.For tomato fruit sucrose,fructose,total soluble sugar and starch content,B2 treatment had the best overall effect on"Kaifeng",and B3（4 mg·L-1）treatment had the best overall effect on"Hongli".（3）Appropriate boron concentration improved the flavor quality of tomato fruit.The amount of volatile substances in the tomatoes of"Kaifeng"and"Hongli"both reached the maximum value under the B1 treatment,which were 18 and 23,respectively.At the same time,the total content of volatile substances reached the maximum value under B1 and B2 treatments,respectively,and the total content of characteristic volatile substances in tomato was the same.The total amino acid content of"Kaifeng"showed a trend of first increasing and then decreasing,and"Hongli"showing a trend of first decreasing and then increasing,but the ratio of essential amino acids to total amino acids（EAA/TAA）showed a trend of increasing first and then decreasing.All reached the maximum value under B2 treatment.The content of lycopene increased first and then decreased at the green ripening stage and the ripening stage,and reached the maximum value under the B2 treatment.According to the multi-level fuzzy comprehensive evaluation analysis,the comprehensive evaluation of"Kaifeng"and"Hongli"both reached the maximum value under the B2 treatment,which were 0.24 and 0.28,respectively.（4）The boron content and accumulation capacity of each part of the tomato plant of the two varieties were leaf>stem>fruit>root.The proportion of semi-bound boron in the tomato fruits of the two varieties was the largest,exceeding 50%,followed by the proportion of bound boron,and the content of water-soluble boron was the least.With the increase of boron concentration,the ability of tomato boron utilization decreased gradually.Appropriate boron treatment（B1～B3）could improve the TF（stem/root）,TF（fruit/stem）and TF（fruit/leaf）values of"Kaifeng"and"Hongli".（5）Transcriptome analysis found that the differentially expressed genes of the two tomato varieties were mainly enriched in biological processes,involving flavonoid biosynthesis,phenylpropane biosynthesis and amino acid metabolism,and some differentially expressed genes were also enriched in lycopene.anabolic pathways and sugar anabolic pathways.（6）Under boron treatment,the gene expression levels of key enzymes AA,BA,NI,AI,SPS and SS in tomato fruit sugar synthesis and metabolism increased at first and then decreased at green ripening stage.In the mature stage,the gene expression level tended to be stable,and the change was not significant.Correlation analysis showed that total soluble sugar was positively correlated with the expression levels of BA,NI,AI,and SPS genes;the key enzyme genes PSY,PDS,and ZDS of lycopene synthesis and metabolism were up-regulated in the green maturity stage,ZISO was up-regulated in the mature stage,and LYCB and The expression level of LYCE was down-regulated in the green ripening stage and the mature stage,which increased the lycopene synthesis rate,decreased the lycopene conversion rate and increased the lycopene content.Correlation analysis showed that lycopene content was positively correlated with the expression levels of PSY and Crt ISO genes.