影響結球萵苣鏽斑病生成之因子及鏽斑病發展過程之解剖觀察

Abstract

結球萵苣（Lactuca sativa L. var. capitata）為菊科萵苣屬萵苣類葉菜中最主要的種類，因為頗耐長途貯運，在全球新鮮農產品市場中佔重要地位。結球萵苣在貯藏及運輸過程中如接觸到乙烯會造成葉柄出現斑點病變，稱為鏽斑病 (Russet spotting)，是結球萵苣採後之重要生理障礙。近十年來結球萵苣產業在台灣的發展迅速，已成為重要的外銷蔬菜作物。本試驗以國內生產之結球萵苣品種為材料，調查影響其鏽斑病發生之多種因子，並以切片染色技術，對鏽斑病發生過程進行解剖學研究。結果顯示，台灣生菜村所生產之‘6號’結球萵苣葉片中肋組織在5℃中，以0.1 μL•L-1乙烯連續處理4天後會出現初期鏽斑病徽狀，但發展緩慢。以1 μL•L-1乙烯處理3天後開始出現徵狀，之後快速發展，於第7天達嚴重程度。鏽斑病的發展在5 °C中最為嚴重，在0°C中發展緩慢，無法用肉眼看到；在10℃及15℃中，鏽斑病之發展比5℃輕微。中肋組織在5℃中以1 μL•L-1乙烯處理1或2天後移至空氣中並不會誘導鏽斑出現；需連續處理3天以上才能誘導鏽斑之生成，鏽斑徵狀出現後，即使移除乙烯，仍會繼續發展至最嚴重程度。個體重量在600 g 以上之結球萵苣，其中肋組織鏽斑病變之發展比發育成熟度者低者為快速。在整球萵苣中以心部葉片的發生最輕微，最外的包葉最嚴重，葉片對乙烯之敏感性由內向外漸增。乙烯處理會誘導中肋組織中苯丙氨酸氨基裂解酶(PAL)活性之增加，以1 μL•L-1乙烯處理8天，PAL活性增加2.67倍。鏽斑徵狀主要發生於組織表層，於肉眼尚無法觀察到的階段，表皮細胞之細胞壁內緣與細胞膜邊境已先開始褐化，並由細胞間絲擴散至鄰近之表皮細胞及葉肉細胞。鏽斑生成中期，表皮細胞出現失水膨壓下降之現象，到了後期，褐變之表皮細胞和近表層的數層葉肉細胞，自表皮細胞開始失水崩解，塌陷堆疊，鏽斑顏色加深，最後形成一顯見之凹陷斑點。另一種形態之鏽斑為內部組織褐變，發生在底層葉肉組織，通常在病變後期出現。結球萵苣之葉肉組織大部分由薄壁細胞組成且含大量水分，本試驗使用Wiesner reagent、Maule reation及Safranin-Aniline blue double stain等木質素染劑，檢視鏽斑病變細胞是否有木質化現象。結果顯示，三種染劑均可使結球萵苣中肋組織之木質部產生顏色變化，但是對鏽斑病變細胞之染色效果不明顯；因此，本試驗並未觀察到鏽斑病細胞有明顯細胞壁木質化之現象。

Iceberg lettuce (Lactuca sativa L. var. capitata) is the most cultivated type within all the vegetable lettuces. Due to its inherited nature for long distance shipment, it occupies a significant position in the world market of fresh produce. Exposed of iceberg lettuce to ethylene during shipping or cold storage will induce the physiological disorder russet spotting (RS). The production of iceberg lettuce in Taiwan expanded rapidly during the past 10 years, and has become an important crop for export market. The object of this research was to study the development of russet spotting in a domestic-grown iceberg lettuce cultivar ‘Number 6’. Tissue segments excised from lettuce midrib tissue were exposed to ethylene to induce russet spotting and the development of the disorder is studied by section staining technology. Exposing to 0.1 μL•L-1 ethylene for 4 days at 5°C will induce RS with slight lesion. Exposing to 1.0 μL•L-1 ethylene for 3 days at 5°C will induce RS, which developed rapidly and became severe on the 7th day. RS development was most severe at 5 ℃, un-noticeable at 0°C, and less severe at 10 and 15℃. At 5 ℃, treating mid-rib sections with 1 μL•L-1 ethylene for 1 or 2 days did not induce the development of russet spotting. It requires at least 3 days to induce the RS, and the symptom will continue to development even after the removal of ethylene. Lettuce heads weighing more than 600 g developed RS more rapidly and severely than less mature heads. Within a single head, the out wrapper leaves were more sensitivity to ethylene than the inner heart leaves. Ethylene induced a 2.67 fold increase in the phenylalanine ammonia lyase (PAL) activity in the mid-rib tissue. RS appeared mostly on the surface of tissue. In the early stages these are light yellow; later they become deep red-brown and the predominant color is russet. In advanced stages, the epidermis and mesophyll cells near the surface collapsed and pitting appeared. Most of the russet spots occurred as depression of surface cells, but some occurred below the surface. The latter appear diffuse and dark below the surface layers of normal white cells in the midribs. In order to verify whether cell wall lignifications took place in the disordered cell, the section of lettuce midribs were histologically stained with three lignin-specific stains: the Wiesner reagent test, the Maule reation test and the Safranin-Aniline blue double stain test. The results showed that the lignified xylem was stained by all three tests, but the color at the RS lesion did not change significantly. This result indicated that little or no lignifications in the RS affected cells.