番石榴果實寒害指標評估及採後處理技術之研究

Abstract

番石榴（Psidium guajava L.），為台灣重要的熱帶果樹，果實採後容易因失水或病原菌感染而造成品質損失。雖然低溫貯藏可以有效延長產品的櫥架壽命，但番石榴為低溫敏感性作物，在臨界溫度下會出現寒害，造成品質損失。因此，如何評估果實採後寒害發生與嚴重程度及增加組織低溫耐受性是番石榴採後處理重要的課題。 ‘珍珠拔’屬於非更年型番石榴，為台灣主要的番石榴栽培品種。本論文第一個研究目的在觀察‘珍珠拔’番石榴果實寒害發生的臨界溫度及徵狀，並比較不同寒害檢測技術何者可迅速且可靠的作為番石榴寒害評估指標。‘珍珠拔’番石榴果實寒害徵狀包括果皮褐化變色、表面凹陷斑、增加腐爛率，且嚴重時會出現果心水浸狀。‘珍珠拔’番石榴果實在0℃貯藏4天或5℃貯藏22天後會出現寒害徵狀，且於20℃回溫3天後會加速寒害的嚴重程度。推測‘珍珠拔’番石榴果實的寒害臨界溫度約在5℃。寒害會誘導番石榴果實在回溫後呼吸率及逆境乙烯生成率上升，但個體之間差異大，無法區別果實寒害的嚴重程度並不適合作為番石榴寒害評估的測定。相反地，寒害會降低‘珍珠拔’番石榴果實光系統II最大光化學量子效率（Fv/ Fm），並增加離子滲漏百分比和a*值，且和寒害指標具有很高的相關性。當‘珍珠拔’番石榴果實在0℃貯藏4天會出現輕微的寒害徵狀，Fv/ Fm即顯著地下降，由0.77下降至0.71，但離子滲漏百分比卻無顯著的變化。因此離子滲漏百分比作為番石榴果實寒害發生初期的寒害指標靈敏度較差。另外，在0℃貯藏試驗中，寒害高敏感型番石榴品種‘梨仔拔’果實的Fv/ Fm比值下降速度較‘珍珠拔’迅速，可見高靈敏度的葉綠素螢光分析技術也可明確的鑑別番石榴物種內不同品種的耐寒力。Fv/ Fm不僅在本試驗中為測量寒害靈敏度最佳的方法，且可在番石榴果實寒害徵狀出現前預測寒害的發生。‘珍珠拔’與‘梨仔拔’在0℃環境，Fv/ Fm若分別降至0.7及0.75以下，則於20℃回溫3天後會出現寒害徵狀。這比值代表這兩個品種番石榴果實寒害由初級反應進入次級反應的轉折點葉綠素螢光參數 。果皮a*值在果皮出現寒害徵狀（褐化變色）後才會出現顯著差異，因此無法用來預測番石榴果實寒害的發生。 本研究中所使用的貯前熱處理或水楊酸處理條件皆無法有效降低‘珍珠拔’番石榴果實在0℃環境的寒害徵狀。但是，100 mg/ L茉莉酸甲酯（MJ）燻蒸處理16小時則可有效減緩在0℃環境貯藏14天回溫後的寒害徵狀，且不會影響‘珍珠拔’果實貯藏後硬度、酸度及維生素C含量的變化。經此一MJ處理的‘珍珠拔’果實Fv/ Fm、離子滲漏百分比及a*值分別為0.71、20.79％和-10.3，較對照組果實0.65、31.15％和-2.95變化波動較小。但MJ處理若以浸泡及減壓方式，則都無法減輕‘珍珠拔’番石榴果實於0℃貯藏的寒害表現。

Guava（Psidium guajava L.）, an important tropical fruit crop in Taiwan, is a highly perishable horticultural produce due to its rapid water loss and very susceptible to pathogen infection after harvest. Although cold storage is the most effective tool for extending the shelf life of fresh commodity, low temperature may cause chilling injury (CI) and, therefore, quality loss when guava fruit, a chilling-sensitive produce, is held below critical temperature. Therefore, how to assess CI occurrence and how to enhance chilling tolerance are pivot issues for postharvest and handling process of guava fruits. The first objective of this thesis is to define CI symptoms and threshold temperature of ‘Jen-Ju Bar’ guava fruits, the nonclimateric-type, dominate cultivar in Taiwan, as well as to investigate a quick and reliable CI assessment for the product by comparing different CI measurement methods. The CI symptoms, including grayish scald-like peel discoloration, surface pitting, increased susceptibility to decay, and, in severe cases, water-soaking core, of ‘Jen-Ju Bar’ guava fruits were noticeable after exposure to 0℃ for 4 days or 5℃ for 22 days. The lesions become more obviously upon transfer to 20℃ condition for 3 days. The CI threshold temperature for ‘Jen-Ju Bar’ guava fruit is estimated around 5℃. Because of large variations among samples and lack of quantitative correlation, transient increases of both respiration and stress ethylene release rates detected after CI fruit transferred to 20℃ were not evaluated as reliable CI measurements for guava. In contrast, CI rendered decrease of maximum quantum efficiency of Photosystem II photochemistry (Fv/ Fm) and increase of electrolyte leakage percentage and Hunter a* value of exocarp of ‘Jen-Ju Bar’ guava, and all these measurements showed a good quantitative correlation with the CI severity graded by vision assessment (CI index). After treatment at 0℃ for 4 days, a condition for trace CI induction in ‘Jen-Ju Bar’ guava, Fv/ Fm ratio significantly decreased from 0.77 to 0.71, but no difference was observed in electrolyte leakage measurement. Therefore, electrolyte leakage percentage was not a sensitive index for the very early stage of CI occurrence in guava fruits. In addition, the fact that Fv/ Fm ratio decreased more rapidly in ‘Li-Tzy Bar’ guava, a very chilling-sensitive cultivar, than that of ‘Jen-Ju Bar’ during 0℃ storage, chlorophyll inflorescences technique was sufficiently sensitive to discriminate clearly between two cultivars within a species. Fv/ Fm ratio was not only the most sensitive parameter among the techniques tested in this research for CI severity, but also could be used as a reliable parameter for CI occurrence prediction in guava fruits before any CI symptom appearance. When Fv/ Fm ratio dropped to 0.7 and 0.75 under 0℃ environment for ‘Jen-Ju Bar’ and ‘Li-Tzy Bar’, respectively, CI symptoms appeared after transfer at 20℃ for 3 days. These ratios implied the turning point between primary and secondary events of CI occurrence in the two guava cultivars. Hunter a* value of guava exocarp, instead, could not be used for prediction of CI occurrence in guava, since this parameter showed significant change only when visible symptom (brown discoloration) became obvious. Neither heat treatment nor salicylic acid/ methyl salicylate treatment used in this study affected CI severity of ‘Jen-Ju Bar’ guava fruits stored in 0℃ condition. However, ‘Jen-Ju Bar’ guava fruits with 100 mg/ L methyl jasmonate (MJ) fumigation treatment for 16 hours did effectively alleviate CI symptoms without affecting fruit quality, such as firmness, titratable acid, and ascorbic acid, at 0℃ storage for 14 days. The Fv/ Fm ratio, electrolyte leakage percentage, and Hunter a* value of the MJ treated ‘Jen-Ju Bar’guava were less fluctuated, namely 0.71, 20.79 and -10.3, respectively, when compared with 0.65, 31.15, and -2.95 in untreated control samples. Unlike fumigation treatment, MJ application by immersion or infiltration did not reduce CI severity in ‘Jen-Ju Bar’ guava fruits at 0℃.