楊桃品種間後熟生理特性之探討

Abstract

楊桃（Averrhoa carambola L.）後熟特性屬於更年或非更年型仍有爭議。本試驗選擇採後乙烯釋放量差異明顯的‘紅龍’與‘金龍’楊桃為材料，觀測兩參試品種於20℃後熟生理變化。楊桃後熟期果實果皮逐漸轉色，但硬度、可滴定酸、總可溶性固型物含量僅些微變化；‘紅龍’ 果實完全轉色時出現乙烯高峰（10.38 μL C2H4•kg-1•h-1），緊隨著果實老化（褐變）與病斑發生；而‘金龍’ 整個採後期間乙烯產生量平穩且低（1.43 μL C2H4•kg-1•h-1）。丙烯1000 µL•L-1處理48小時促進兩參試品種後熟生理變化、乙烯高峰提前、乙烯生合成關鍵酵素ACC合成酶（ACS）及ACC氧化酶（ACO）活性增高；反之，1 μL•L-1乙烯作用抑制劑1-甲基環丙烯（1-MCP）燻蒸24小時則會延緩兩品種後熟。因此根據前述後熟生理特徵並不易判斷其後熟類型。利用RT-PCR策略自楊桃果實組織分離ACS與ACO cDNA選殖系各2個。即時定量PCR分析顯示，AcACS2和AcACO1在兩品種果實發育初期基因穩定表現，進入後熟轉色期逐漸下降；丙烯處理壓抑其mRNA累積，1-MCP處理則促進基因表現，屬於系統1乙烯生合成基因。而AcACS1和AcACO2於後熟期大量表現，受丙烯處理正向誘導、1-MCP負向抑制，研判為系統2乙烯生合成基因。由於‘紅龍’和‘金龍’果實皆具有系統2乙烯生合成系統，本試驗結果支持楊桃為更年型果實。

The ripening behavior, i.e. climacteric or nonclimacteric, of carambola (Averrhoa carambola L.) is still a controversial issue. ‘Hong-Long’ and ‘Jing-Long’, two carambola varieties with discriminating ethylene production after harvest, were used to investigate their physiological changes during ripening at 20℃ in this research. The flesh firmness, titratable acidity, and total soluble solids of these fruits barely changed during ripening. ‘Hong-Long’ showed a surge of ethylene production (10.38 μL C2H4•kg-1•h-1) when fruit was fully ripe, which was tightly followed with flesh senescence (browning) and appearance of pathological lesions. On the other hand, ‘Jing-Long’ displayed a low and stable production of ethylene (1.43 μL C2H4•kg-1•h-1) in postharvest life. However, the physiological changes of ripening, time of ethylene peaks, as well as the enzymatic activities of ACC synthase (ACS) and ACC oxidase (ACO), the two key enzymes in ethylene biosynthetic pathway, were accelerated in these two fruits by exposure of 1000 µL•L-1 for 48 hours. In contrast, fumigation of 1 µL•L-1 1-methylcyclopropene (1-MCP), an ethylene action inhibitor, for 24 hours delayed ripening of the fruits tested. Therefore, it was unable to clearly characterize the ripening pattern(s) based only on the physiological parameters mentioned above. By utilizing RT-PCR strategy, two ACS and two ACO cDNA clones were obtained from fruit tissues of carambola. Real-time quantitative PCR analysis showed that low and stable expressions of AcACS2 and AcACO1 in these two varieties were found in early stage of fruit development; then decreased gradually after onset of ripening. Based on the facts that treatment of propylene suppressed these mRNA accumulations and that 1-MCP treatment up-regulated the expressions, AcACS2 and AcACO1 were considered as System 1 ethylene biosynthesis genes. Conversely, AcACS1 and AcACO2, mainly expressed in ripening stage, were induced by propylene and down-regulated by 1-MCP, which were classified as System 2 ethylene biosynthesis genes. Since System 2 ethylene biosynthesis genes existed in ‘Hong Long’ and ‘Jing Long’, these results suggested that carambola is a climacteric fruit.