結球萵苣為臺灣第二大外銷蔬菜，2013年起出口量均超過1 萬公噸且逐年成長。惟現行品種在溫度偏高環境易出現頂燒症，徵狀為葉緣焦枯或葉球內部呈現乾腐，導致喪失商品價值。故本研究藉由引入新品種，於不同季節試作，先篩選出無頂燒症及抽苔者，再評估於低溫期間不同施肥量其影響葉球形狀之程度。結果顯示'45-08 RA'、'EX-8380'、'合歡新9號'、'包心萵苣3號' 及'長生50號'在104年秋期作均未出現頂燒症及抽苔。105年冬期作以合理化施肥栽培，該5品種葉球形狀均呈圓球形，而加倍施肥量處理組之' 長生50號'及對照品種'常利6號'均出現高圓球形葉球。本研究證實在外銷產季初期秋作及後期春作之高溫環境，適合篩選低頂燒率品種，另在低溫期以加倍施肥量處理，可篩選葉球球形穩定之品種。
Cultivar Evaluation of Iceberg Lettuce for Low Tipburn Rate and Stable Heading Shape
Hsieh, M. H. and C. W. Lin
Iceberg lettuce is the second largest export vegetable with an average amount over 10,000 tons per year and is increasing year by year in Taiwan since 2013. However, tipburn is a widespread physiological disorder of the current cultivars, with the symptom of necrosis on margins and inner leaves of the head, causing the loss of commercial value. To overcome this problem, we evaluated heading characteristics and tipburn rates in different cultivars in fall under high temperatures. Then, we investigated the cultivars with no tipburn or bolting on the effects of two different amounts of fertilizer on heading shape and other characteristics in winter at low temperatures. Results showed that tipburn and bolting did not occur in '45-08 RA', 'EX-8380', 'Ho-Huan New No.9', 'Sonpo No.3', and 'Acegreen No. 50' in the fall trial in 2015. In 2016, although the heading shape of the five cultivars were all round in the winter trial with rational application of fertilizer, the heads of 'Acegreen No.50' and the comparison cultivar ('Ho-Huan No.6') were both long and round with double amount of fertilizer. This research has proved that the tipburn tolerant cultivars are suitable for export selection in the fall and late spring under high temperatures and the stable heading shape cultivars are suitable for selection of cultivars with more fertilizer in winter under to low temperatures.
What is already known on this subject?
Previous research has proved that rational fertilizing is an effective approach to reduce the tipburn rate in iceberg lettuce. However, the tipburn still occurred in the fall and late spring for the current cultivars for export.
What are the new findings?
The cultivars in the trials were cultivated in the fall and late spring under high temperatures. Five cultivars without tipburn or bolting were selected. Then, the selected cultivates were cultivated in the winter under low temperatures with two different amounts of fertilizer. Four cultivars were found with stable heat shapes.
What is the expected impact on this field?
By cultivating in different seasons in high or low temperatures and treating with different amounts of fertilizer, some cultivars without tipburn or head deformity were successfully selected. In addition, most of their weights were similar to the comparison cultivar. Therefore, the cultivars will be recommended to iceberg lettuce farmers for export production.
Key words: Iceberg lettuce, Heat tolerance,
Rational application of fertilizer, Physiological disorder, Tipburn
Accepted for publication: September 6, 2016