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Identification of good storability in short-day onion and its mechanism of resistance to Aspergillus niger
onion (Allium cepa L.)
total soluble solids
volatile flavor compounds
|摘要:||在熱帶和亞熱帶地區，腐敗造成短日型洋蔥鱗莖的貯藏損失很嚴重，延長洋蔥的貯藏壽命將可達到週年供應。本研究針對品種之差異性、化學組成份和病理性等影響貯藏力的因子做一系列的探討。十二個洋蔥品種在亞蔬中心進行三年的貯藏試驗，經過貯藏三個月後，其平均貯藏損失介於21.1%至98.8%。貯藏損失受到基因型(G)、年度(Y)和G×Y交感作用之影響。1997年在洋蔥成熟期間豐沛的雨量，導致採收時田間罹病率增加，也影響了貯藏期間洋蔥罹病率增加。貯藏損失與乾物質含量和可溶性固形物呈負相關，但卻與罹細菌性軟腐和黑黴菌病呈正相關。黑黴病係由黑黴菌所引起，是洋蔥在高溫高濕貯藏下之主要病害。檢測不同黑黴菌之菌株，結果指出An4-2和An39-9的致病力最強。黑黴菌在鱗片之感染面積和產孢量隨接種濃度增加而增加；在低濃度(1×104 conidia per ml)或溫度低於20 oC時，感染面積小且產孢量少。鱗片接種之方法如下：鱗片組織塊用針剌傷，再接種40 ml的孢子懸浮液(1 × 105 conidia per ml)，放置在 25oC，12小時的生長箱中四天後調查感染之病斑和產孢情形。進行42個品種抗病篩選，結果有一些耐貯藏品種如‘Red Pinoy’、 ‘Serrana’、‘Dehydrator No.3’和‘Moonlight’比較抗黑黴菌之感染。為探討洋蔥抗黑黴病之機制，有進一步的測定洋蔥的可溶性醣和揮發性香氣成份抑菌試驗。可溶性醣是洋蔥主要的貯藏性碳水化合物，在十三個測試品種中其含量佔總乾物質之48%至65%。耐貯藏品種如‘Red Pinoy’ 和‘Dehydrator No.3’具有較多的可溶性固形物和較多的全可溶性醣 (分別含622和635 mg/g dwt，其中果聚醣佔全可溶性醣的85%)。反之，不耐貯藏品種’CAL606’有較多的單糖和雙糖。將黑黴孢子培養在不同洋蔥品種的可溶性醣溶液中，結果指出黑黴孢子在耐貯藏品種之醣液中萌芽率和發芽管生長較不耐貯藏者來得差。相關性分析結果指出貯藏期間洋蔥罹黑黴病和黑黴孢子發芽率與單糖和雙糖含量呈正相關；卻與果聚糖呈負相關。倒置培養法試驗結果指出洋蔥的揮發性香氣成份可抑制黑黴菌之生長，此抑制效果依品種和濃度而異。所有洋蔥的精油原液和耐貯藏品種經稀釋5倍之處理組，對黑黴菌有殺菌效果，不耐貯藏品種則只有靜菌之效果。然而所有的參試品種經稀釋10倍之後，各別的抑菌效果會降低。同樣的，抑菌效果隨處理的時間增加而減少。GC-MS分析結果指出耐貯藏的品種含豐富之propenyl methyl disulfide、dimethyl trisulfide和methyl propyl trisulfide，較不耐貯藏品種高5至13倍。|
Deterioration of short-day onion bulbs in storage is a serious problem in the tropics and subtropics. Extending the storage life of onions can meet its year-round demand. The present research explored the genetic variability, chemical factors, and the pathological aspects responsible for onion storability. Storage trials were conducted at the Asian Vegetable Research and Development Center (AVRDC). After three months of storage, mean storage loss of 12 onion genotypes varied from 21.1% to 98.9% over three years. Results indicated that storage loss was influenced by genotype (G), year (Y), and G×Y interaction. In 1997, heavy rainfall during bulb maturity increased disease incidences in the field and during storage. Storage loss was negatively correlated with dry matter and total soluble solids, but was positively correlated with bacterial rot and black mold incidence. Onion black mold, caused by Aspergillus niger is a serious postharvest disease. Effective methods to screen for black mold resistance is unavailable at present. Effects of pathogen isolate, inoculum density, incubation temperature, and inoculation methods on severity of black mold were investigated. Isolates An4-2 and An39-3 were identified to be highly virulent to onion tissue. Disease severity increased with the increase in inoculum densities and incubation temperatures. Small lesion and no sporulation were observed when onion scales were inoculated with 1×104 conidia per ml or incubated at temperature lower than 20 oC. Based on the result the following screening protocol has been developed: onion slice was wound inoculated with 40 ml inoculum suspension of 1×105 conidia per ml, and incubated at 25oC, 12 h photoperiod; lesion diameter and sporulation was recorded at 4 days after inoculation. A total of 42 onion cultivars were screened for resistance to A. niger. Cultivars with better storability like ‘Red Pinoy', ‘Serrana', ‘Dehydrator No.3', and ‘Moonlight' were found to be tolerant to A. niger. On the other hand, cultivars with poor storability like ‘Texas Early Grano 502', ‘Granex 429', and ‘Explorer' were found to be susceptible to A. niger. Lesion diameter incited by scale inoculation method correlated significantly with bulb storage loss, dry matter content, and total soluble solids. To study the mechanism of resistance to A. niger, soluble sugar and volatile compounds of onions were used in microbial testing. Soluble sugars are the major storage carbohydrates in onions. Total soluble sugar varied from 48% to 65% of dry weight among 13 tested cultivars. Good storability cultivars such as ‘Red Pinoy' and ‘Dehydrator No.3' had more total soluble solids (TSS), and more total soluble sugar (622 and 635 mg/g dwt, of which more than 85% of total sugar was fructans). On the other hand, cultivar ‘CAL606' had more mono- and di-saccharide. In the culture of the conidia of A. niger in sugar solution of onions, it was found that conidia of A. niger germinated less in the soluble sugar of good storability cultivars and had shorter germ tube than cultivars which had poor storability. The correlation study showed that black mold incidence and conidia germination were positively correlated with mono- and di-saccharide, but negatively correlated with fructans. Inverted lid Petri dish experiment suggested that volatile compounds of onions could inhibit the growth of A. niger. The inhibitory effect of essential oil on the growth of A. niger was cultivar and concentration dependent. All original essential oils of tested cultivars and essential oil of good storability cultivars, after diluting 5X, had fungicidal effect against A. niger. However, diluting the essential oils of all tested cultivars by more than 10 times could reduce its inhibitory effect. Likewise, the inhibitory effect of the essential oils decreased gradually over prolonged incubation period. Good storability cultivars were rich in propenyl methyl disulfide, dimethyl trisulfide, and methyl propyl trisulfide (5-13 times higher) compared to those cultivars with poor storability.
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