梨果实发育软化与果胶多糖降解特性的关系

Pectin Polysaccharide Degradation in Relation to the Texture Softening in Pear Fruit

【目的】探讨细胞壁果胶多糖降解特性与梨果实质地软化和贮藏性的关系,进一步阐明果实软化机理,为果实品质的提高及贮藏技术的完善提供理论依据。【方法】以‘鸭梨’和‘京白梨’为试材,根据果实发育和后熟特性,分别在果实发育和后熟软化两个阶段进行定期采样,用质构仪分析比较两品种果实的质构参数变化特性,分别采用生化方法和琼脂糖凝胶色谱柱层析法分析‘鸭梨’和‘京白梨’果实发育软化过程中细胞壁果胶组分含量变化及其分子质量的分布特点,并测定果胶多糖降解相关酶活性的动态变化规律,以探讨贮藏性不同的梨果实果胶多糖降解特性的差异。【结果】发育期,‘鸭梨’果实共价结合果胶(CSP)和离子结合果胶(ISP)含量迅速增加,显著高于‘京白梨’,其水溶性果胶(WSP)含量缓慢增加且低于‘京白梨’,‘鸭梨’果实WSP和CSP均由低分子量组分向高分子量组分转变。贮藏期,‘鸭梨’果实CSP含量高且恒定,WSP含量缓慢增加,但均保持较高的分子质量;而‘京白梨’果实CSP含量迅速降低,WSP和ISP含量快速增加,各果胶显著地由高分子量组分向低分子量组分转变。‘鸭梨’果实WSP、CSP和ISP含量变化仅在发育期与硬度变化显著相关,而‘京白梨’果实果胶与硬度的显著相关性主要表现在贮藏阶段。果胶降解酶活性测定结果表明,两品种的差异主要表现在贮藏期,即在贮藏阶段‘京白梨’果实多聚半乳糖醛酸酶(PG)、果胶甲酯酶(PME)、β-半乳糖苷酶(β-Gal)和α-阿拉伯呋喃糖苷酶(α-Af)的活性和增加速率均显著高于‘鸭梨’,其中β-Gal和α-Af活性在‘京白梨’果实采收后即迅速增加,PG和PME相对滞后,且β-Gal和α-Af活性变化与硬度和各果胶组分含量变化间的相关度均强于PME和PG,此时期‘鸭梨’果实仅α-Af活性与ISP含量变化的相关性显著。综上,‘鸭梨’和‘京白梨’果实的果胶降解特性差异显著,而且在果实软化的不同阶段表现不同,导致两品种果实具有不同的后熟软化和贮藏特性。【结论】耐贮性强的‘鸭梨’果实发育期表现明显的大分子果胶组分积累和随果实后熟软化降解缓慢的特性,不耐贮的‘京白梨’果实发育期积累的大分子量果胶组分随果实软化迅速降解成小分子量组分。其中,难溶性果胶CSP含量的高低及其分子质量的分布是衡量梨果实耐贮性的重要指标。β-Gal和α-Af更促进‘京白梨’果实软化。

【Objective】The aim of this study is to investigate the relationship between the characteristics of cell wall pectinpolysaccharide degradation and textural softening and storability of pear fruit, these results will further demonstrate the mechanism of fruit softening and provide a basis for improving fruit quality and developing better storage technology【.Method】Take the fruits of ‘Yali’ and ‘Jingbaili’ pears as the materials to compare the difference of the changes of pectin polysaccharide content, the molecular weight distribution and the related enzyme activity between two pear cultivars with different storabilities and textural properties during growth and softening.【Result】During development, in ‘Yali’ fruit, the content of both covalent soluble pectin（CSP） and ironic soluble pectin（ISP） increased rapidly, which were significantly higher than that of ‘Jingbaili’ fruit, and the water soluble pectin（WSP） content of ‘Yali’ fruit increased slowly which was lower than that of ‘Jingbaili’ fruit. Moreover, both WSP and CSP components were transferred from the low molecular weight to the high molecular weight in ‘Yali’ fruit. During storage, in ‘Yali’ fruit, the CSP showed a high and constant content with a slow increase in the content of WSP, which all maintained the higher molecular weight. But in ‘Jingbaili’ fruit, the CSP content decreased rapidly and a rapid increase showed in WSP and ISP content, and the pectin components were shifted from higher molecular mass to the lower ones. Moreover, changes of CSP, WSP and ISP contents were obviously related to fruit firmness only during development stage in ‘Yali’ fruit, but in ‘Jingbaili’ fruit, the changes of the contents of the three pectin compositions showed significant correlations with firmness loss mainly at stage of storage. The results indicated that it showed mainly the significant differences of pectin degrading enzymes activity between the two pear cultivars during storage, which the activities of polygalacturonase（PG）, pectinmethylesterase（PME）, β-galactosidase（β-Gal） and α-arabinofuranosidase（α-Af） and the increased rate in ‘Jingbaili’ fruit was significantly higher than that of ‘Yali’ fruit. Among them, β-Gal and α-Af activities increased rapidly just after harvest, while the increase date of PG and PME activities were relatively lagged and the correlations of the changes of β-Gal and α-Af activities with the changes of firmness and pectin components were stronger than that of PME and PG in ‘Jingbaili’ fruit, while in ‘Yali’ fruit only the change of α-Af activity had a significant relationship with the ISP content during storage. In summary, the pectin polysaccharide degradation showed significant differences between ‘Yali’ and ‘Jingbaili’ pear fruits and the related-factors played different roles at different softening stages, thus resulting in significantly different softening and storage characteristics.【Conclusion】The pectin polysaccharide fractions showed obvious accumulation of large molecular component and slowly degradated with fruit ripening in ‘Yali’ fruit, while the larger molecular components accumulated during fruit development degradated rapidly into smaller ones with fruit softening in ‘Jingbaili’ fruit. The insoluble pectin content and its molecular mass distribution may be an important index for measuring storability of pear fruit. Both β-Gal and α-Af played stronger roles in promoting ‘Jingbali’ fruit softening.