The Study On The Mechanism Of High Calcium Inhibiting Sugar Accumulation Of Litchi Chinensis Soon.cv.Feizixiao Pulp

Feizixiao litchi（Litchi chinensis soon.cv.Feizixiao）is an important mid early maturing variety,which is widely planted in China.Previous studies have shown that spraying calcium fertilizer on Feizixiao leaves can increase the content of the soluble sugar in pulp,but it is very easy to overdo it in specific application.Therefore,it is necessary to explore the inhibition mechanism of excessive calcium fertilizer on sugar accumulation in Feizixiao litchi pulp.In this experiment,Feizixiao litchi was treated with 0.6%CaCl₂ solution on the leaves and water as the control（CK）.The sugar and acid content,mineral nutrient content and related enzyme activities were observed once a week among 35-77 days after anthesis.The transcriptome analysis and important gene screening were also carried out.The main results are as follows:（1）During the course of fruit growth and development,the content of soluble sugar under treatment and CK increased first and then decreased.The content of glucose and fructose increased continuously throughout the whole process.The content of sucrose increased first and then decreased.Both the treatment and CK showed the phenomenon of"sugar regression",and the content of soluble sugar under treatment was lower than that of CK,indicating that high concentration of Ca inhibited the accumulation of soluble sugar in pulp.77 days after flowering,the contents of glucose,fructose and sucrose were significantly lower than that of CK,indicating that high concentration of Ca also inhibited the accumulation of three sugar components.（2）During the course of fruit growth and development,the content of Vc under treatment and CK increased first and then stabilized.The content of acid decreased first and then stabilized.The ratio of sugar/acid under CK increased continuously,and the ratio of sugar/acid under treatment increased first and then stabilized.57 days after flowering,the content of Vc under treatment was significantly higher than that of CK,which indicated that high concentration of Ca could promote the accumulation of VC before"sugar removal".The acid content of CK decreased to the lowest 77 days after flowering,while that of the treatment decreased to the lowest 71 days after flowering,indicating that the acid content of high concentration Ca treatment decreased one week in advance.57 days after flowering,the ratio of sugar/acid under treatment was significantly lower than that of CK,which indicated that the comprehensive flavor of Feizixiao litchi pulp was decreased by high concentration of Ca.（3）During the course of fruit growth and development,the content of K under treatment and CK showed an"M"trend of increasing,decreasing,increasing and decreasing.The contents of water-soluble Ca and total Mg in treatment and CK decreased continuously and then stabilized.The content of total Ca in CK and treatment showed a"W"trend of decreasing,increasing,decreasing and increasing.The content of water-soluble Mg in treatment and CK decreased first and then increased.There was no significant difference in the contents of K,water-soluble Ca,water-soluble Mg and total Mg between treatment and CK during fruit development.That is to say,high concentration foliar spraying Ca did not change the accumulation of these mineral elements.57 days after flowering,the total Ca content of the treatment was significantly higher than that of CK,indicating high concentration foliar spraying of Ca promoted the accumulation of Ca before the"sugar regression"of pulp.（4）During the growth and development of fruit,the change trend of enzyme activities between treatment and CK was different.50 days after flowering,the activity of AI,NI,SS-Ⅱ and SPS were significantly lower than CK,SS-Ⅰ activity was significantly higher than CK.57 days after flowering,SS-Ⅱ and SPS activity of treatment were significantly lower than CK.71 days after flowering,the activity of NI and SS-Ⅰ was significantly lower than CK,SS-Ⅱ was significantly higher than CK.NI activity of treatment 77 days after flowering was significantly higher than CK.The results showed that sucrose was decomposed into reducing sugar by increasing the activities of sucrose hydrolases and reducing the activities of sucrose synthetases in the early stage of treatment,while sucrose hydrolases were weakened and sucrose synthetases were enhanced,resulting in the conversion of reducing sugar to sucrose after the"sugar regression"of pulp,The high concentration of foliar spraying Ca strengthened the influence of SPS activity on the content of sugar,weakened the influence of NI,strengthened the positive correlation between AI and fructose,NI and glucose content,weakened the correlation between SS-Ⅰ and reducing sugar content,and made NI replace AI play a more important role in regulating sugar content change,and also strengthened the linear correlation between AI,NI and SS-Ⅱ and other enzyme activities.These results indicated that NI and SS-Ⅰ played a more critical role in the regulation of CK sugar content,while SPS,AI and NI played a more critical role in the regulation of CK sugar content.（5）The transcriptome sequencing analysis of Feizixiao Litchi at three time points and control showed that the sequencing results were ideal,and 55352 unigenes could be annotated into four databases.Compared with the NR library,the homology of genes related to 23 species was more than 1%.Fourteen unigenes were identified to be related to the expression of enzymes related to glucose metabolism,and they were supposed to be the key enzyme genes in starch and sucrose metabolism pathway.Eight unigenes were randomly analyzed by real-time fluorescent quantitative PCR（q RT-PCR）.There was a significant positive correlation between their gene expression,indicating that the transcriptome results were true and reliable.High concentration foliar spraying of Ca inhibited the accumulation of sugar components by affecting related genes,such as inhibiting the down-regulation of VIN gene to reduce sucrose accumulation,and down-regulation of CHS family gene to inhibit chalcone accumulation to reduce flavonoid content.Foliar spraying of high concentration of Ca fertilizer caused the changes of K,Ca and Mg metabolism in pulp,and then caused the changes of gene expression,resulting in the decline of comprehensive flavor quality of pulp.The possible mechanism is that,on the one hand,it can inhibit the accumulation of sugar components by inhibiting the up regulation of NI gene and down regulation of VIN gene,on the other hand,it can also reduce the content of flavonoids by inhibiting the accumulation of chalcone,resulting in the damage of reactive oxygen species and the inhibition of sugar accumulation.