| Organic acid is one of the key indicators of fruit quality. A lot of research has been carried out in fruit organic acid accumulation, but this study in pear is limited. Pear is cultivated in China with great area and rich varieties, and the growth and bearing habit among cultivars are quite different. However, the differences of organic acid accumulation and its regulation in different species need to be studied systemly. Therefore, in this study, we have studied the fruit organic acid accumulation, changes in fruit organic acid metabolism enzymes, organic acid inheritance and regulation, and to explore the molecule markers on the high acid character of pear fruit. The main purpose of this study was to provide the theoretical basis of organic acid accumulation and its regulation for the production of the high quality pear fruit. Main results as following:1. The ripe fruit of40varieties of four major pear cultivars, Pyrus ussuriensis, Pyrus bretschneideri, Pyrus pyrifolia, and Pyrus communis, were collected as study samples. The contents of10organic acids in the fruit were examined using HPLC. The results showed that8-9organic acids were detected in the fruits of most examined varieties. The total content of the organic acids in the fruits ranged between3.04and9.13mg·g-1FW in P. ussuriensis,1.74and4.88mg·g-1FW in P. bretschneideri,1.84and3.46mg·g-1FW in P. pyrifolia, and0.86and3.51mg·g-1FW in P. communis. The major components of organic acids present in the pear fruit were malic acid and citric acid. A high level of malic acid was found in the fruit of most varieties. Malic acid, citric acid, and quinic acid exhibited significant and strong positive correlation with the total organic acid content,oxalic acid, acetic acid, shikimic acid, and succinic acid contents were also positively correlated with the total organic acid content. Among the ten organic acids examined, both malic acid and citric acid exhibited highly significant positive correlation with quinic acid, whereas extremely significant negative correlation was observed between acetic acid and lactic acid, and between succinic acid and fumaric acid. Furthermore, significant positive correlation was observed between malic acid and citric acid, and between quinic acid and shikimic acid, whereas significant negative correlation was found between quinic acid and tartaric acid. The differences in the contents of the10individual organic acids and the total organic acid content in the fruit reached a significant and strong level among the40pear varieties. Malic acid, citric acid, and quinic acid displayed extremely significant differences.2. The contents of organic acids of’Xinping’pear in developing fruit and leaf were examined using HPLC(High Performance Liquid Chromatography). The results showed that the contents of the total organic acid in fruit decrease from7.67mg·g-1to5.03mg·g-1, the contents of citric acid, malic acid and lactic acid increase2.75mg·g-1,0.5mg·g-1and0.001mg·g-1, respectively; while the contents of quinic acid, oxalic acid, shikimic acid, acetic acid and succinic acid decrease4.19mg·g-1,0.002mg·g-1,0.29mg·g-1,0.09mg·g-1and0.052mg·g-1, respectively. The contents of the total organic acid in leaf decrease from6.18mg·g-1to5.36mg·g-1; the contents of citric acid, quinic acid, oxalic acid and lactic acid increase1.34mg·g-1,1.09mg·g-1,0.43mg·g-1and0.01mg·g-1respectively; while the contents of malic acid, shikimic acid and succinic acid decrease1.61mg·g-1,0.26mg·g-1and1.82mg·g-1respectively. The content of citric acid in fruit exhibited significant and strong positive correlation with the content of it in leaf, the content of acetic acid exhibited significant positive correlation with the content of it in leaf.3. Changes in the activities of enzymes involved in the metabolism of the organic acids were examined during the process of fruit and leaf development in’Xinping’pear. The activities of CS, NAD-IDH and NAD-MDH showed an overall increasing trend and the activities of Cyt-ACO, PEPC and NADP-ME exhibited an overall decreasing trend in developing fruit. During the leaves’developmental processes, the activities of CS, Cyt-ACO, NAD-IDH, PEPC, NAD-MDH, and NADP-ME decreased gradually.4. Changes in the content of organic acid and the activities of enzymes involved in the metabolism were examined after the regulations of foliar application of N, P, K, Ca or RE (rare earth) fertilizer, bagging, girdling, yield, double fruits in one inflorescence, light, dwarf rootstock, tree age, environment during the tree growing season in ’Xinping’ pear and ’Nanguo’ pear. We observed they affected the total organic acid content and the activities of the enzymes.5. The contents of7kinds of main organic acids in the pear fruits of parents and67lines of F1generation of ’Xinpingli’×’94-08’ were determined by HPLC, and the genetic, correlation and diversity of the organic acids contents were analyzed. The results showed that:The content of7kinds main organic acids in67lines of Fi generation fruits displayed continuous distribution, closing to normal distribution. The order of the distribution range of the7organic acids contents from big to small was citric acid, malic acid, quinic acid, oxalic acid, acetic acid, shikimic acid and lactic acid; The variation tendency of the mean value, range and standard deviation of the7kinds of main organic acids in67lines of F1generation fruits were in consistent with the mean value of the parents. In the relations among different organic acids contents in the F1generation, there was a highly significant positive correlation between quinic acid and oxalic acid, shikimic acid; Both of citric acid and acetic acid, shikimic acid and lactic acid showed significant positive correlation;but acetic acid and lactic acid showed significant negative correlation. In the F1generation fruits, among the contents of citric acid, malic acid, quinic acid and oxalic acid showed highly significant difference each other. Moreover, most of the67lines of F1generation were citric acid dominant type.6.The inheritance of the fruit acid trait of mature fruit was analyzed using the F1segregation population (total of67seedlings) derived from the cross’Xinpingli’(Pyrus bretshneider R.)×’94-08’(Pyrus bretshneider R.), markers linked to pear acid gene were screened by amplified fragment length polymorphism (AFLP) technique combined with bulked segregant analysis (BSA) method. One molecular marker which linked to pear acid gene had been identified from64pairs of AFLP primer, named as E-AGG/M-CAG. The genetic distance to pear acid gene were10.6cM. |
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