蓝光调控褪绿柑橘果实果皮色泽形成的机理研究

Early varieties of citrus fruit typically do not meet color standards on the tree when the pulp meets maturity standards. In order to meet the requires of the commercial and storage markets, and to avoid winter frost, the fruits must be frequently harvested early, before they are fully colored. The existence of chlorophyll and the less accumulation of carotenoids in the peel, make the fruit less marketable. Application of exogenous ethylene, as a degreening treatment, is a common postharvest citrus fruit practice in order to enhance the color change in early fruit. Ethylene treatment breaks down the green chlorophyll pigment in the exterior part of the peel and allows the yellow or orange carotenoid pigments to be expressed. As compared with fully colored citrus fruits with on-tree maturation, it seems that carotenoids with orange color were less accumulated. Our early research indicated that blue LED light irradiation can increase the a* (The CIE 1976L*a*b* color scale) and the content of carotenoids of ethylene-treated citrus fruits. Effects of blue LED light irradiation on carotenoids with orange color and color formation of ethylene degreening citrus fruit peel were still not clear, and the application of exogenous ethylene makes it more complicated. The objective of this work has been to establish an effective and modified ethylene degreening pattern, based on the application of blue LED light. The analysis of the spectral characteristic and the changes in peel color, the content and composition of the carotenoids as well as the expression of genes that responsible for the successive metabolism of carotenoids, can further clarify the effect of blue LED light irradiation on color formation of ethylene degreening citrus fruit, thereby, might provide new strategies for the improvement of ethylene degreening treatment of citrus fruit.

早熟柑橘果实存在果肉和果皮不同步成熟的问题。但为了满足市场和贮藏对柑橘果肉成熟度的要求，同时避免低温对果实造成的危害，这类柑橘果实通常在果皮充分转色之前采收。此时，由于果皮叶绿素仍然存在，类胡萝卜素积累不足，外观品质明显低于自然着色果实，消费者可接受性差。商业上通常采用乙烯褪绿的方式改善早采果实的着色。但与自然着色果实相比，乙烯褪绿果皮颜色仍然偏浅，常给消费者以不新鲜的感觉。我们前期研究发现，在果实褪绿处理后，对果实进行蓝光处理，可以明显提高果皮色差值a*和类胡萝卜素含量。但乙烯褪绿背景条件下，蓝光处理促进褪绿柑橘果皮着色的机理等信息尚不明确，也更为复杂。该课题期望在建立蓝光改善乙烯褪绿柑橘果实着色效果有效模式的基础上，通过分析果皮呈色物质光谱特性、色素物质含量、代谢关键酶活性和基因表达情况，明确蓝光改善褪绿柑橘果实果皮着色效果的机理，为进一步完善乙烯褪绿技术提供理论参考。

在柑橘众多品种中存在一些果皮果肉成熟时间不同步的早熟品种，其绿皮色泽常降低其消费者接受性和商品价值。因此，商业上常采用乙烯对上述柑橘进行褪绿处理。虽然乙烯褪绿处理能一定程度的加快蜜橘果实转黄，但是与自然成熟果实相比，其褪绿后的色泽偏浅，光泽度和均一性都欠佳，市场接受度低。因此，需要寻求一种能安全、有效的改善乙烯褪绿蜜橘果实色泽的方法。基于此，本项目以重庆本地产早熟蜜橘果实为试材，以外源LED光源(红光LED、蓝光LED)为处理手段，通过测定受试果实色泽和色素(叶绿素和类胡萝卜素)含量的变化，探讨以外源光照处理改善乙烯褪绿蜜橘果实着色效果的可行性及有效处理模式。并通过对有效处理组果实色素物质代谢通路、果皮质体结构变化、乙烯生物合成和信号转导途径相关指标变化的研究，阐明外源LED光源处理改善乙烯褪绿蜜橘果实色泽的机理。同时，项目对该品种蜜橘果实褪绿的温度和褪绿后的主要致病真菌进行了分离鉴定。.研究结果如下：蓝光光照处理(300 lux 10 h)是改善“乙烯褪绿蜜橘”果实着色效果的最有效作用模式。它的作用机制主要是：1)加速“乙烯褪绿蜜橘”果皮质体结构的变化；2)在转录水平上调控叶绿素和类胡萝卜素的代谢；3)通过增加“乙烯褪绿蜜橘”果实乙烯生物合成和信号转导相关基因的表达，增加果实对乙烯的敏感性。以乙烯利对果实进行褪绿处理时，有利于重庆北碚产无核蜜橘褪绿的适宜温度为24℃。胶孢炭疽菌Colletotrichum gloeosporioides，是引起贮藏过程中乙烯褪绿蜜橘果实腐烂的主要致病真菌（>80%），其次为焦腐病菌Lasiodiplodia theobromae（~15%）。综上所示：蓝光对于提高“乙烯褪绿柑橘”果实的外观品质和商业价值具有潜在的应用前景，本项目研究结果也为重庆地区乙烯褪绿蜜橘果实贮藏病害的针对性控制提供了重要的理论参考。