细胞壁多糖非酶促调节在桑果采后自溶中的作用机制研究

Mulberries contain abundant anthocyanin and phenolic compounds and are high in antioxidant capacity. They have high nutritional values and health benefits and are important for fresh market as well as for processing. However, mulberries are highly perishable and the fruit quality and market value deteriorate rapidly within 12-24 hours in room temperature. Postharvest research on mulberries in the past focused on finding the techniques for quality maintenance. Little information was available on the mechanisms of softening and breakdown of the flesh. Our preliminary research found that fruit softening of mulberries was related to cell wall hydrolytic enzyme activities and the contents of cellulose and pectic substances. The activities of PME and PG may play important roles in hastening the fruit softening. The non-enzymatic changes of cell wall composition are also closely related to softening of the flesh. However, the mechanisms of how does cell wall compositional change lead to fruit softening is not well understood. This project will investigate the relationship between the hydrolysis of cell wall polysaccharides and fruit softening and breakdown. The mechanisms of non-enzymatic regulation of oxidation and hydrolysis of cell wall polysaccharides will be examined. This research will elucidate the metabolism of oxidative process of cell wall carbohydrates and study the possible mechanisms which cause the deterioration of mulberry fruit. Findings from this project will provide fundamental basis for postharvest techniques for maintaining quality and storage life of mulberries and serve as a guide for promoting the development of other potential berry fruits.

桑椹为高抗氧化性浆果，富含花色苷等，营养价值高，亦是重要的保鲜和加工原料。但桑椹果实采后极易腐烂，常温下12～24h即发生软化和自溶，严重影响品质和商品价值。以往桑果采后研究主要集中在贮藏保鲜技术，对软化产生原因有少数研究，初步发现软化主要与细胞壁水解酶活性、纤维素和果胶含量变化有关，认为酶促降解PME和PG可能是促进软化成因之一。而与软化发生密切相关由非酶促引起的细胞壁组分变化，其导致自溶的机理尚不清楚。本项目围绕非酶促途径介导细胞壁多糖降解与软化自溶的作用机制这一科学问题，拟从非酶促角度，研究桑果采后自溶过程中细胞壁多糖代谢组分变化，探讨活性氧代谢对细胞壁多糖的降解作用，明确介导自溶的主要活性氧种类，阐明细胞壁多糖降解与软化自溶的关系，揭示桑果采后软化自溶发生的可能机制，为调控桑果采后软化自溶，延长贮藏保鲜期提供理论基础和技术支撑，以期指导新潜力的桑椹等浆果产业快速发展。

桑椹为高抗氧化性浆果，但果实采后极易腐烂，常温下12～24h即发生软化和自溶，严重影响品质和商品价值。以往研究认为，酶促降解PME和PG是促进桑果采后软化的主要成因之一。本项目从非酶促角度，探讨活性氧代谢对细胞壁多糖的降解作用，揭示桑果采后软化自溶发生的可能机制，为调控桑果采后软化自溶，延长贮藏保鲜期提供理论基础和技术支撑。主要研究内容和结果如下：. 1. 温度对桑葚贮藏品质、细胞壁降解酶活性、细胞超微结构、细胞壁多糖组分的影响。10℃贮藏条件会加速桑葚果实的软化自溶，0℃低温能够较好的保持桑果品质，抑制果实自溶指数上升，并且能够降低多聚半乳糖醛酸酶、果胶甲酯酶等细胞壁降解酶活性，延缓果实细胞壁降解；随着贮藏时间延长，桑葚果实会发生细胞内容物减少或消失，细胞壁变薄等，0℃贮藏与5℃、10℃相比较，果实细胞形状结构及内容物含量保持最为完好，能够显著抑制果实细胞壁物质以及半纤维素、纤维素含量的下降。. 2. 离体条件下活性氧对桑葚果实细胞壁多糖的降解作用。离体条件下采用O2-.、H2O2和.OH三种活性氧处理桑葚细胞壁，表明.OH在0.5mmol/L浓度条件作用下，会极大加速细胞壁中果胶的水解程度。. 3. 活体条件下.OH对桑葚贮藏品质、活性氧代谢以及抗氧化酶的影响。活性氧.OH是影响果实自溶软化的非酶因素之一， 0.5mmol/L.OH会加速桑果品质下降；加速过氧化氢、羟基自由基、超氧阴离子含量上升，表明外源.OH会加速果实体内自由基的积累，加速细胞的破坏。. 4. 活体条件下.OH对细胞超微结构以及细胞壁多糖组分、含量以及分子量分布的影响。超微结构显示，0.5mmol/L.OH会加速果实细胞结构的破坏、细胞空洞化，刺激囊泡增多并加速细胞的自我溶解，同时.OH能够加速细胞壁的降解、水溶性果胶的上升，以及水溶性果胶中总糖和糖醛酸的降解释放，加速果胶物质的水解，使大分子质量的果胶多糖降解成小分子的单糖或寡糖。