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    台灣蓮霧品質優,加上各種研發技術的輔助,使蓮霧成為經濟價值極高的水果,然而,成熟蓮霧果實近果臍處的果肉細胞常會發生木栓化現象(Corky calyx end disorder,CCE),導致外觀與口感不佳,嚴重影響果品價格。台大園藝系教授李國譚及葉德銘帶領學生分析研究,發現果實開始轉色時,靠近果臍部位的導管輸送功能喪失,使鈣濃度偏低而產生木栓化,這個發現不僅具學術價值,亦是蓮霧栽培策略的重要參考,更榮登「FRUITS國際熱帶及亞熱帶園藝期刊」封面故事。

 

    李國譚解釋,木栓化的植物細胞壁會呈現褐化崩塌,水分流失,而木栓化的生理障礙被認為與缺鈣有關,但機制不明。他進一步說明,鈣在果實發育及品質上扮演關鍵角色,諸多影響果實品質的生理障礙,如蘋果苦荳病及梨蜜症,皆與鈣的缺乏有密切相關,然而缺鈣症狀卻又經常發生在土壤鈣元素充足的果園,因此在果實發育過程中,鈣進入果實的效率與在果實內的分布是果樹生理研究上的一個重要課題。

圖一:蓮霧成熟果實果臍端缺鈣所產生的木栓化生理障礙。(圖/陳思如提供)

 

    李國譚及葉德銘和指導的博士班學生陳思如,針對蓮霧分析其果實發育過程中,各部位鈣含量與濃度的變化,並利用導管染色技術,量化果實內部導管的暢通程度。由於鈣在植物體內,主要經由導管內部的水流運移,因此研究團隊假設「容易產生缺鈣部位的導管,在果實發育過程中的特定時機,因物理性或生理性變化而喪失功能,導致該部位鈣濃度偏低,而成為缺鈣生理障礙的好發部位」,並進行實驗驗證。

 

    研究結果指出蓮霧果實的總鈣含量自幼果期起便持續上升,然而平均鈣濃度卻於果實開始著色時快速降低。進一步分析果實內部導管的功能及鈣的分布狀態,發現在果實開始轉色前,各部位的導管功能完整,但果實開始轉色時,靠近果臍部位的導管功能同步喪失,僅少數近果梗處之導管仍具備運輸功能,且成熟果實內部的鈣濃度以果臍端的果肉處最低。李國譚表示,關於導管功能喪失有諸多可能性,也可能數種可能性同時產生,目前學界仍有不同的看法。

圖二:蓮霧成熟果實果臍端的導管在果實轉色期之後失去功能,果肉細胞鈣濃度最低,也是最容易產生木栓化生理障礙的部位。(圖/陳思如提供)

 

    以上的證據顯示,蓮霧果實在發育中期之後,雖然持續吸收並累積鈣元素,但因導管功能喪失,無法將所吸收的鈣運輸至果臍端的果肉部位,所以如果未能在幼果期累積足夠的鈣,成熟時就容易發生缺鈣症狀。

 

    李國譚說,此結果除了具學術價值之外,在產業上也能提供果農田間栽培時,建立良好鈣元素管理策略,例如鈣肥的使用時機等等,亦能作為未來技術開發的重要參考。

 

    本研究成果刊登於FRUITS國際熱帶及亞熱帶園藝期刊(SCI),所提供的蓮霧照片並榮登該期封面故事。

全文連結:https://www.pubhort.org/fruits/74/3/3/index.htm

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Corky calyx end disorder (CCE) often occurs in the pulp cells of mature wax apples near the umbilicus, resulting in poor appearance and taste, which seriously affects the price of the fruit. Professor Kuo-Tan Li and Professor Ming-Te Yeh from Department of Horticulture and Landscape Architecture discovered that when the fruit begins to change color, the delivery function of the vascular tissue near the umbilicus was lost, thereby resulting in a low calcium concentration and corkinzation. This discovery is not only of academic value, but also an important reference for the cultivation strategy of wax apple. Professor Kuo-Tan Li indicated that the cell wall of cork-like plant tissue shows browning, collapse, and water loss. The physiological causes of cork-like tissue formation may be related to calcium deficiency, but the associated mechanism is still unknown. The Figure shows the physiological effects of corkization caused by calcium deficiency in the umbilical end of mature wax apples.

http://www.bioagri.ntu.edu.tw/NTU/upload/point_news/images/%E6%9C%AA%E5%91%BD%E5%90%8D_opt(1).png

Figure. Physiological Effects of Corkization Caused by Calcium Deficiency in the Umbilical End of Mature Wax Apples

Since calcium is mainly transported in the plant body through the water flow inside the vascular tissues, the research team assumed that the tissue that is prone to calcium deficiency will lose its function due to physical or physiological changes at a specific time during the fruit development process. The calcium concentration of the site decreases, and it becomes prone to calcium deficiency physiological disorders, as verified by experiments. The results indicated that the total calcium content of wax apples continuously increases from the young fruit stage, but the average calcium concentration decreases rapidly once the fruit begins to color. Professor Kuo-Tan Li said that there are many possible explanations for this, and there are still different opinions on it in academic circles.

The research results were published in FRUITS on July 24, 2019. See full article at https://www.pubhort.org/fruits/74/3/3/index.htm

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