Effects Of Leaf Spraying Methyl Jasmonate On Fruit Quality Of Loquat

Loquat（Eriobotrya japonica Lindl.）is a traditional cultivated fruit in southern China.Therefore,a method to improve the quality of loquat is of great significance to solve the current pain point of loquat industry.At present,the relevant researches on improving the quality of loquat fruit mainly focus on the physical cultivation measures and the preservation of postharvest fruits.However,the related researches on the effects of exogenous hormones on the quality of loquat fruit are relatively lacking.In this study,the loquat‘Zaozhong No.6’from the orchard located in subtropical area of Fujian,China was used as the experimental material.The leaf surface was sprayed with different concentrations of methyl jasmonate（MeJA）i.e.,0 mmol/L,5 mmol/L,10 mmol/L and 15 mmol/L MeJA solution.The contents of soluble sugar,titratable acids and antioxidant activity in loquat fruits treated with different concentrations of MeJA were determined.The contents of malic acid,citric acid,fumaric acid,fructose,ABA,GA₃,IBA,6-BA,rutin,quercetin and luteolin were determined by high performance liquid chromatography-mass spectrometry（HPLC-MS）.Malic acid metabolism pathway and antioxidant key enzyme activities were analyzed.The key genes of sugar and organic acid metabolism were downloaded from NCBI online software and sequentially compared with loquat genome to obtain the key genes of sugar and organic acid metabolism in loquat.The qRT-PCR was used to analyze the differential expression of related genes in sugar and organic acid metabolism pathway between different treatments.The main research results are as follows:1.Leaf spraying methyl jasmonate（MeJA）had a significant effect on the appearance and quality of loquat fruits.The color transformation occurred 15 days earlier and caused the changes in related endogenous hormones.Fruit appearance quality after MeJA treatment was significantly higher than that of control group.Fruit weight per fruit treated with 10mmol/L MeJA at 125 days was 71 g,about 23.6%higher than 57.4 g of control group.MeJA treatment increased the average pulp availability to76.9%（P≤0.001）,which was significantly higher than that of control group（56.7%）.At 125 days after anthesis,fruit yellowish blue（b*）value in control group was 35.7 and significantly higher than 31.4 in 15 mmol/l MeJA group.Correlation analysis showed that b*value was negatively correlated with MeJA concentration（P≤0.001）.At 125 days after the treatment,the ABA content in MeJA group was significantly higher（P≤0.001）than that in control group,and the ABA content in 15 mmol/L MeJA group was 2402 ng/g and that in control group was 1109 ng/g.2.Foliar spraying of MeJA could significantly regulate the sugar and acid quality of loquat fruits.After 15 mmol/L MeJA treatment,the soluble sugar content and titratable acid content of loquat fruits at maturity stage were 51.5 mg/g and 0.84%,respectively,that of control group was 46.1mg/g and 0.5%,and that of 15 mmol/L MeJA group were significantly higher than those of control group.After 5 mmol/L MeJA treatment,the soluble sugar and titratable acid contents reached the level of maturity early,and the soluble sugar content was 33.8 mg/g and titratable acid content was0.8%at 110 days after the treatment.Soluble sugar content increased to35.5 mg/g and titratable acid content decreased to 0.68%at 125 days after anthesis,which increased by 5%and decreased by 0.12%compared with110 days after anthesis,respectively.Among the changes of fructose and organic acid components,fructose content showed an increasing trend,and the highest fructose content was 16.6 mg/g in ripe fruits in 10 mmol/L group.Three organic acids,malic acid,citric acid and fumaric acid were detected in loquat fruits,among which malic acid was the main organic acid in loquat,and the change trend was consistent with titratable acid content.The maximum malic acid content in 15 mmol/L MeJA group was5456 ug/g（P≤0.001）,and that in control group was 3021 ug/g.The correlation analysis showed that the titratable acid content and NADP-ME enzyme activity were significantly（P≤0.01）positively correlated with MeJA concentration at 125 days after anthesis,suggesting that the activity of NADP-ME enzyme was affected by MeJA concentration and organic acid metabolism in fruits.3.Leaf spraying methyl jasmonate significantly increased the content of antioxidant active substances in loquat peel.On 125 days after anthesis,the total flavonoid content and polyphenol content in 15 mmol/L MeJA group were 2.53 mg/g and 0.96 umol/g,which were significantly higher than that in control group,which were 1.03 mg/g and 0.3 umol/g,respectively.The general trend was first up and then down.On 125 days after anthesis,the contents of rutin,quercetin and kaempferol in loquat peel in 15 mmol/l MeJA group were 5206.7 ng/g,6505.1 ng/g and 302.2 ng/g,respectively.The content of rutin（1350.7 ng/g）,quercetin（2241.9 ng/g）and kaempferol（132.6 ng/g）in pericarp of 15 mmol/l MeJA group was significantly higher（P≤0.001）than that of CK group.Correlation analysis in Loquat fruits showed that the contents of polyphenols,flavonoids in peel were positively correlated with MeJA concentration（P≤0.05）.4.Leaf spraying with different concentrations of MeJA had significant effects on the expression of genes related to glucose metabolism in some loquat fruits.The acid metabolism differential genes EjNADP-ME3,EjNAD-MDH7,EjNAD-MDH9,EjNAD-MDH10 and EjPEPC1 in the MeJA treatment group were all found at 80 days after fruit development.The higher the concentration of MeJA was,the more significant the up-regulated expression was（P≤0.001）.However,the relative expression level of EjNADP-ME3 in treatment group was down-regulated after 80days of development（compared with control）.Compared with control,EjFK2,EjFK4,EjFK6,EjHK1,EjSPS2,EjSPP2,EjSPP3 and EjSS5 were significantly different in relative expression levels after MeJA treatment.The relative expression levels of these genes were higher in the treatment group during most periods of development.It may positively regulate sugar synthesis.The correlation analysis between glycoacid metabolites and related genes showed that:the content of citric acid was positively correlated with the expressions of EjNAD-MDH3,EjSS1,EjFK6,EjPEPC4 and EjFK4 genes（P≤0.05）.It was negatively correlated with the expression of EjALMT2,EjFK5,EjNAD-MDH8,EjSPS3,EjSPS1,EjNAD-MDH15 and EjSPP2 genes.On the contrary,the relative expression levels of EjNAD-MDH3,EjSS1,EjFK6,EjPEPC4 and EjFK4 genes were negatively correlated with the contents of soluble sugar and fructose in loquat fruits.The relative expression levels of EjALMT2,EjFK5,EjNAD-MDH8,EjSPS3,EjSPS1,EjNAD-MDH15 and EjSPP2 genes were positively correlated with the contents of soluble sugar and fructose in loquat fruits（P≤0.01）.In conclusion,foliar spraying with different concentrations of MeJA has a great effect on the fruit quality of loquat,significantly improving the fruit availability.Among them,5 mmo L/L MeJA can significantly promote the sugar,organic acids and flavonoids of loquat fruits to reach ripening level earlier,but has no significant improvement on fruit quality.10mmol/L MeJA significantly increased the external indicators such as early color change and fruit size,but did not significantly improve the internal nutrients.Foliar spraying with 15 mmol/L MeJA can regulate the content of Sugars and acids,improve the fruit flavor,promote the synthesis of flavonoids in the peel,enhance the fruit edible rate and improve the quality of loquat fruit.Therefore,15 mmol/L MeJA is the appropriate concentration for loquat foliar spraying.