摘" " 要:【目的】探究遮光對獼猴桃發(fā)育期間果實香氣的影響,為獼猴桃精細栽培管理提供參考?!痉椒ā恳垣J猴桃紅陽為試驗材料,從花后20~140 d,以20 d為一個時段將其分為6個時段(時段Ⅰ~時段Ⅵ),每個時段為一組使用套袋進行遮光處理(T1~T6組),T7為全時段遮光,對照(CK)為全時段未遮光。采用頂空固相萃?。℉S-SPME)和氣相色譜-質譜聯(lián)用(GC-MS)測定香氣,并對T7和對照果實進行轉錄組測序分析。【結果】在不同遮光處理下,共檢測出38種香氣物質,包括15種酯類、11種醛類、8種萜烯類和3種醇類。各組香氣種類介于19~37種之間,其中T6種類最多,T2和T5最少??傁銡夂縏4最高,其次為T2和T5。除T4處理外,其他試驗組果實酯類含量均顯著降低;T4、T5和T6處理組醛類含量與對照無顯著差異,其他處理組顯著低于對照;T1和T2處理組萜烯類含量與對照無顯著差異,T4處理組無累積,其他處理組顯著高于對照。醇類物質含量在T6處理果實中顯著高于對照,T2和T5處理組無累積,其他處理組低于對照。聚類分析顯示,T4處理與對照成分組成最相似。主成分分析顯示,T6處理組綜合得分最高,其次為T4、T5和T2。轉錄組分析共發(fā)現(xiàn)1248個差異表達基因,T7較對照下調基因788個,上調基因460個。篩選出了15個與香氣合成相關的差異表達基因,其中遮光處理降低了T7果實中乙醇脫氫酶(ADH)、丙酮酸轉氨酶(AGXT)、天冬氨酸轉氨酶(COT1)和糖醇-4-磷酸胞苷酰轉移酶(IspD)基因的表達量,升高了焦磷酸脫羧酶(MDV)、醛脫氫酶(ALDH)和苯丙氨酸解氨酶(PAL)基因的表達量?!窘Y論】紅陽獼猴桃的各類香氣成分具有不同的合成時期,其中時段Ⅵ為酯類合成重要時期;時段Ⅰ-時段Ⅲ對醛類物質合成積累有重要意義;時段Ⅳ為萜烯類物質合成關鍵時期。遮光抑制了ADH、AGXT和COT1基因的表達,同時上調了MVD和ALDH基因的表達量,是導致遮光果實中萜烯類香氣物質含量升高和酯類、醛類、醇類含量下降的可能因素。綜合分析表明,T2、T4和T5處理的獼猴桃果實香氣含量較高,受遮光影響較小,即時段Ⅱ(40~60DAA)、時段Ⅳ(80~100DAA)和時段Ⅴ(100~120DAA)為適宜遮光時段。結合前人研究,建議從時段Ⅱ到時段Ⅴ(40~120DAA)進行果實遮光,時段Ⅵ去除套袋使果實正常接收光照。研究結果可以為獼猴桃精細管理提供參考。
關鍵詞:獼猴桃;紅陽;香氣;遮光;果實發(fā)育
中圖分類號:S663.4 文獻標志碼:A 文章編號:1009-9980(2024)11-2285-15
Effects of shading on the aroma component of fruits at different developmental periods in Hongyang kiwifruit
LIU Ruonan1, LIU Cuixia2#, YE Kaiyu2, GAO Jianyou2, LI Jiewei2, GONG Hongjuan2, XIA Liming2, ZHU Rongxiang2, WANG Faming2*, ZHA Manrong1*
(1College of Biology and Environmental Science, Jishou University, Jishou 416000, Hunan, China; 2Guangxi Key Laboratory of Functional Phytochemicals Research and Utilization/Guangxi Institute of Botany, Chinese Academy of Sciences, Guilin 541006, Guangxi, China)
Abstract: 【Objective】 Aroma is an important trait of fruit quality, constituted by a variety of compounds including esters, aldehydes, ketones, alcohols, terpenes and volatile phenols, as well as a range of other aromatics. The compound and content of aroma undergo consistent changes throughout fruit development, and can be affected by various environmental factors. It has been demonstrated that sunlight, as a crucial factor influencing fruit development, has a significant effect on the content and composition of aroma. Bagging is widely practiced in fruit cultivation. The fruit with bagging tends to exhibit a superior external appearance and neater shapes compared to non-bagged, which makes them more popular by consumers. However, the practice of bagging limits the fruit’s exposure to sunlight, which in turn affects the formation of fruit aroma. In this study, the effects of shading on fruit aroma types and content at different developmental periods were determined to explore the significance of each period in aroma compounds synthesis, and identify the optimal timing for bagging. Furthermore, a transcriptome analysis was employed to explore the molecular mechanisms underlying the impact of shading on aroma synthesis. The findings of this study will serve as a valuable reference for the refined cultivation of kiwifruit. 【Methods】 The experimental design spanned from 20 days after anthesis (20 DAA) to 140 DAA, dividing the maturation process into six periods (periods Ⅰ to Ⅵ) with 20-day intervals. Each period underwent light exclusion using double-layer black bags (Groups T1 to T6), with T7 representing total shading and CK representing no shading. The fruits were harvested when the soluble solids content reached 7% and allowed to ripen at room temperature until the soluble solids content reached 20%. Aroma compounds were then analyzed by headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS). Data were processed and analyzed using Excel 2016 and SPSS 20.0, and graphs were created using Origin 2021. Samples from T7 and CK fruits were sent for transcriptome sequencing and KEGG and GO enrichment analysis of DEGs were conducted using the company's platform. 【Results】 A total of 38 aroma compounds were detected in Hongyang kiwifruits with different treatments, including 15 esters, 11 aldehydes, 8 terpenes, 3 alcohols and a single other compound. The number of aroma compounds in each group ranged from 19 to 37, with T6 exhibiting the highest diversity at 37 types. In contrast, T2 and T5 exhibited the lowest diversity, with both groups containing 19 compounds. The highest total aroma content was observed in T4, followed by T2 and T5 among the groups with shading. The proportions of the major aroma constituents in the Hongyang kiwifruit remained largely unchanged by the exclusion of sunlight. Ester compounds consistently accounted for over 90% of the total content in each treatment group, with aldehydes comprising 1% to 7% and terpenes and alcohols each accounting for less than 2%. The ester content was found to be significantly reduced under shading in all groups with the exception of T4, which exhibited the highest ester content, followed by T2 and T5. The aldehyde content of T4, T5 and T6 exhibited no significant difference compared to CK, whereas the other groups displayed a significantly lower content than CK. The terpene levels in T1 and T2 were not significantly different from CK, which exhibited no accumulation in the fruit of T4. In contrast, the other treatment groups displayed significantly higher terpene levels than CK. Furthermore, all groups except T4, which was bagged, showed an increase in terpene compound diversity, with the highest variety found in T7, followed by T6. The only treatment group in that the alcohol content was significantly higher than CK was T6. No accumulation of alcohol was observed in T2 and T5, while all other treatment groups exhibited lower alcohol levels compared to CK. The results of the cluster analysis indicated that the aroma composition of T4 was most similar to that of CK. T2 and T5 were grouped together, while T1 and T3 were placed in a separate cluster. T6 and T7 were classified into their own distinct categories. The principal component analysis demonstrated that T6 exhibited the highest overall score, followed by T4, T2 and T5. Transcriptome analysis identified 1, 248 differentially expressed genes, with 788 downregulated and 460 upregulated ones in T7 compared to CK. 15 genes related to aroma synthesis were selected, with shading resulting in reduced expression of ethanol dehydrogenases (ADH), alanine aminotransferase (AGXT), and aspartate aminotransferase (GOT) genes in T7 fruit, while increasing the expression of pyrophosphate decarboxylase (MDV), and aldehyde dehydrogenases (ALDH). 【Conclusion】 The impact of shading in different developmental periods resulted in varying degrees of aroma content and compound constitution, indicating that the synthesis of various aroma components in Hongyang kiwifruit occurred at disparate optimal periods. The results demonstrated that period Ⅵ was a critical period for ester synthesis; periods Ⅰ to Ⅲ were significant for the accumulation of aldehydes; and period Ⅳ was crucial for terpene synthesis. Comprehensive analysis showed that kiwifruits from T4, T5 and T2 had higher aroma content and demonstrated reduced sensitivity to shading, showing superior aroma quality. Shading suppressed the expression of ADH, AGXT and COT1 genes while increased the expression levels of MVD and ALDH genes, which may account for the increased content of terpene aroma compounds and the decreased levels of ester, aldehyde and alcohol compounds in shading fruit. Thus, period Ⅱ (40-60 DAA) and periods Ⅳ to Ⅴ (80-120 DAA) were identified as suitable periods for bagging. Combined with previous research, it was recommended to begin bagging kiwifruits in the late portion of period Ⅰ and continue from period Ⅱ to period Ⅴ (40-120 DAA). The bagging process should be discontinued upon the completion of period Ⅴ to permit the fruits to receive sunlight in a natural manner. These results can serve as a valuable reference for the precise management of kiwifruit cultivation.
Key words: Kiwifruit; Hongyang; Aroma; Shading; Fruit development
獼猴桃作為中國重要的水果,在全國多地均有種植。多年來,眾多學者針對獼猴桃的種植條件、抗病蟲害能力和營養(yǎng)成分等方面進行了研究,旨在促進獼猴桃產(chǎn)業(yè)發(fā)展[1-3]。紅陽獼猴桃(Actinidia chinensis ‘Hongyang’)為中華獼猴桃變種,由四川蒼溪縣野生中華獼猴桃中心培育,果心帶有紅色放射狀條紋,果實大且果形整齊,口感豐富,富含糖分和維生素C等營養(yǎng)物質,深受消費者喜愛,并且它的抗逆性強、果實耐貯藏,是中國重要的經(jīng)濟樹種[1,4]。
果實香氣是品質的重要指標,由酯、醛、酮、醇、萜烯類等多種揮發(fā)性香氣物質綜合構成。這些香氣物質的組成以及含量會隨著果實的成熟而改變,使得果實在不同階段釋放出不同的氣味,因此,香氣可側面反映水果成熟度,用作評價果實適宜采收期的標準之一[5]。此外,水果的特征香氣還可以刺激消費者的嗅覺,引起消費者食欲和購買欲,對水果產(chǎn)業(yè)發(fā)展具有重要意義[6]。
套袋在水果栽培中被廣泛應用,能通過物理防護有效避免果實因雨水、鳥類、昆蟲以及陽光照射不均等外界因素導致的落果、水果畸形以及食用口感差等問題,從而提高商品果率[7]。但套袋也會阻礙果實發(fā)育過程中接受光照,對果實的內(nèi)在品質產(chǎn)生影響,這一影響因品種而異。對番石榴套袋,提高了果實質量以及抗壞血酸含量[7];對枇杷套袋能夠顯著提高果實質量和可溶性糖含量,降低可滴定酸含量[8];但對西南冷涼高地紅富士蘋果套袋則會降低果實品質[9]。就獼猴桃而言,有研究表明套袋顯著降低海沃德獼猴桃果實中蔗糖以及葉綠素含量,但單果質量、可溶性固形物含量及干物質含量均無明顯變化[10];而用適宜顏色的果袋對紅陽獼猴桃套袋卻能提高果實質量、干物質含量、可溶性糖含量、ASA含量和固酸比,降低可滴定酸含量[11]。
遮光對果實香氣也具有顯著影響。在果實的各個發(fā)育時期進行去光處理均會顯著抑制玫瑰香葡萄中萜類物質的合成,但在果實成熟階段進行遮光處理則會促進京香玉葡萄中醛、醇和酮的積累[12-13]。在硬核期前對白鳳水蜜桃進行套袋處理能顯著提高果皮中的香氣含量,對果肉中的香氣含量無影響[14]。利用橙色袋在果實緩慢增長期對玉露水蜜桃進行套袋遮光能顯著提升果實中的醛類含量[15]。對紅富士蘋果進行套袋則減少了果實中香氣物質種類和含量[16]。目前探究遮光對獼猴桃香氣的影響文獻較少,僅集中在探討采后貯藏方式對香氣的影響方面。0.5 μL·L-1的1-甲基環(huán)丙烯(1-MCP)被證明是改善獼猴桃布魯諾采后品質并保持在貯藏期間香氣發(fā)展的最適濃度[17];低溫貯藏后使用脫落酸(ABA)處理增加了采后獼猴桃香氣成分,并提高了相關合成酶醇酰基轉移酶(AAT)、支鏈氨基酸轉氨酶(BCAT)和過氧化氫裂解酶(HPL)活性[18];Han等[17]、Günther等[18]和Huan等[19]對華特獼猴桃進行冷藏后乙烯處理,發(fā)現(xiàn)相比未處理獼猴桃具有更濃郁的果香。此外,關于獼猴桃香氣合成關鍵基因已有研究,Zhang等[20]通過轉錄組分析鑒定出3個酯類合成相關基因AdFAD1、AdALDH2和AdAT17;張琳等[21]研究表明,乙烯抑制劑和外源乙烯處理下獼猴桃果實中的基因AcAT1、AcAT2、AcAT17表達量變化與AAT活性變化呈正相關;董婧等[22]對比6種獼猴桃品種發(fā)現(xiàn)基因AcAT16表達量與丁酸乙酯含量呈正相關。關于遮光影響獼猴桃香氣合成的機制仍然未知?;诖?,筆者以紅陽獼猴桃為材料,將其從幼果到果實成熟分為六個時段利用套袋分別進行遮光處理,使用頂空-固相微萃取/氣相色譜-質譜聯(lián)用技術探究不同時段去光對紅陽獼猴桃果實香氣的影響,并對全時段遮光和全時段未遮光果實開展轉錄組測序分析,以期篩選出受光調控的香氣形成關鍵基因,為探究遮光影響獼猴桃果實香氣的分子機制奠定基礎,同時找到獼猴桃套袋的適宜時段,為進一步優(yōu)化獼猴桃栽培措施、生產(chǎn)高品質獼猴桃提供理論依據(jù)。
1 材料和方法
1.1 材料
筆者以紅陽獼猴桃為材料,采自于廣西植物研究所獼猴桃種質資源圃(東經(jīng)110°17,北緯25°4)。試驗樹為5~6年生中華砧紅陽獼猴桃,株行距為3 m×3 m,雌雄比例為8∶1,棚架栽培[23]。
1.2 試驗處理
對獼猴桃果實使用黑色雙層果袋進行遮光處理,遮光度95%。用花后天數(shù)(days after anthesis,簡稱DAA)標記處理的時間,選取6株生長狀態(tài)一致的果樹,每組處理時分別在6株果樹上各選取20顆位置、大小和發(fā)育相對一致的果實進行套袋,每2株作為一個生物學重復。在整個發(fā)育過程暴露于陽光下的果實為對照(CK),從花后20 d開始到果實成熟,每20 d遮光為一個處理組,具體處理見表1。
果實可溶性固形物含量達到7%后采摘。采摘下來的果實在室溫放置直到后熟完成,檢測可溶性固形物含量達到20%,即進行香氣成分的提取和檢測[24]。
1.3 香氣測定
香氣測定方法參考劉翠霞等[25]。采用頂空固相萃?。℉S-SPME)和氣相色譜-質譜聯(lián)用(GC-MS)檢測果實香氣成分。每個處理隨機取5~10個果實,去皮后液氮速凍,加入10%的氯化鈣,用磨樣機預冷后打磨成粉末狀,每次稱取5 g,3次重復,加入10 μL 41 mg·L-1的3-辛醇溶液作為內(nèi)標,于20 mL頂空瓶中進行萃取。萃取結束后,將萃取頭插入氣相色譜的進樣口進行解析。萃取頭采用30 μm PDMS/DVB SPME萃取頭(Supelco公司,美國)。采用氣質聯(lián)用儀對芳香物質進行分析鑒定。氣相色譜型號為7890A,質譜儀型號為 5975C(安捷倫公司,美國)。定性分析通過 AMDS 解卷積軟件。解卷積后檢索標準化合物的質譜圖(NIST08)、廣西落葉果樹團隊已經(jīng)分析過的標準品的保留時間以及與相關文獻報道進行保留指數(shù)等信息比對。對揮發(fā)物的定量分析采用3-辛醇作為內(nèi)標物進行半定量分析。
1.4 轉錄組分析
測序樣品為全時段遮光(T7)和全時段未遮光(CK)果實。取用于香氣測定的T7和對照果實樣品液氮速凍后使用磨樣機快速研磨成粉末,裝入離心管放至-80 ℃冰箱保存。委托武漢邁維代謝生物科技股份有限公司完成樣品RNA提取和轉錄組測序,3次重復?;诒磉_定量結果使用DESeq2軟件進行樣品組間的差異表達分析,篩選閾值為:|log2Fold Change| ≥ 1、FDR<0.05,將篩選出的基因在基因本體GO(Gene Ontology,GO)以及京都基因與基因組百科全書(Kyoto encyclopedia of genes and genomes,KEGG)數(shù)據(jù)庫進行比對,獲得不同樣品差異表達基因的功能及相關代謝通路信息。根據(jù)GO和KEGG注釋結果,對差異基因進行功能分類與富集分析。
1.5 數(shù)據(jù)處理
采用Excel 2016對數(shù)據(jù)進行處理,采用SPSS 20.0進行顯著性分析以及主成分分析并計算綜合得分,計算方法參考張文霖[26]的報道,使用Duncan法對數(shù)據(jù)進行多重比較,采用Origin2021作圖。
2 結果與分析
2.1 不同時段遮光處理下香氣種類及含量分析
在紅陽獼猴桃的處理組和對照組中,共檢測到38種香氣物質,包括15種酯類、11種醛類、8種萜烯類、3種醇類、1種其他類。
紅陽獼猴桃果實在不同處理下的香氣種類介于19~37種(圖1-A),其中T6和T7果實香氣種類較多,分別為37和32種;T2和T5果實香氣種類最少,均為19種;T1、T3和T4的果實香氣種類與對照相近,分別為24種、25種和22種。酯類物質種類在每個處理組中較穩(wěn)定,均在14~15種。醛類物質種類在各處理組中的變化較大,介于3~11種之間,其中T6紅陽獼猴桃果實中醛類物質種類最多,為11種,T7其次,為8種,T5果實中醛類物質種類最少,為3種。萜烯類物質種類在各處理組中變化也較大,介于0~8種之間,T7紅陽獼猴桃果實中萜烯類物質種類最多,為8種,T6其次,為7種,T4果實中無萜烯類物質,T2、T5和對照,僅為1種。各處理組中醇類物質種類較少,在0~3種之間。
在各時段遮光均抑制了紅陽獼猴桃果實總揮發(fā)性香氣的積累。其中,T4果實中香氣含量最高,T2和T5其次。除T4外,其他處理組香氣物質總含量均顯著低于對照,T2和T5約為對照香氣物質總含量的76%和66%,而全遮光處理的T7只有對照的13%(圖1-B)。酯類物質為各處理組中紅陽獼猴桃果實中的主要香氣成分,在各處理組中含量均占各組香氣物質總含量的90%以上;醛類物質次之,介于1%~7%之間;萜烯類和醇類物質含量在各處理組總含量中的占比均在2%以下(圖1-C~D)。
各個時期遮光對各類香氣物質的影響程度具有差別。對不同處理組中的酯類、醛類、萜烯類和醇類含量進行比較發(fā)現(xiàn):T4紅陽獼猴桃果實中酯類含量與對照無顯著差異,T1、T2、T3、T5、T6和T7果實中的酯類含量顯著低于對照,分別約占對照果實中酯類物質含量的24%、77%、41%、66%、30%和13%(圖2-A);T4和T6果實中醛類物質含量與對照無顯著差異,T1、T2、T3和T7中紅陽獼猴桃醛類含量顯著低于對照,分別約占對照果實中醛類物質含量的53%、64%、50%和49%(圖2-B);T1和T2中萜烯類含量與對照無顯著差異,T3、T5、T6和T7中萜烯類含量顯著高于對照,相比對照果實中萜烯類物質含量提高59%、63%、147%和202%,此外,T4果實中無萜烯類物質累積(圖2-C);僅T6紅陽獼猴桃果實的醇類物質含量顯著高于對照,相比于對照果實中的醇類物質含量提高136%,T2和T5果實中無醇類物質累積,其他處理組果實中醇類物質含量顯著低于對照(圖2-D)。
2.2 不同時段遮光處理下紅陽獼猴桃果實香氣組分分析
在檢測到的香氣成分中,各處理組中紅陽獼猴桃果實都含有的成分有15種,分別是丁酸乙酯、丁酸甲酯、苯甲酸甲酯、丁酸丁酯、己酸乙酯、己酸甲酯、乙酸乙酯、苯甲酸乙酯、反式巴豆酸甲酯、甲酸丙酯、辛酸乙酯、戊酸乙酯、(E)-2-己烯醛、己醛和戊醛(表2)。
酯類物質作為獼猴桃果實的主要芳香物質,種類最多,含量也最豐富。由表2可以看出丁酸乙酯、丁酸甲酯、丁酸丁酯、己酸乙酯、乙酸乙酯、苯甲酸甲酯、苯甲酸乙酯是紅陽獼猴桃果實中酯類香氣物質的主要部分,其中丁酸乙酯、丁酸甲酯、己酸乙酯可以產(chǎn)生典型的獼猴桃香氣,主要呈現(xiàn)果香和甜香,苯甲酸甲酯、苯甲酸乙酯可呈現(xiàn)花香[27]。(Z)-2-己烯醛、己醛、戊醛是醛類香氣物質的主要部分,多呈現(xiàn)青草香[28]。鄰傘花烴是主要萜烯類物質,僅在T4中無積累。此外,萜烯類中的D-檸檬烯、桉葉油醇、萜品烯-4-醇、(Z)-β-羅勒烯、α-蒎烯、β-蒎烯、α-松油醇可分別在除T4外的處理組中檢測到。
對不同處理組中紅陽獼猴桃果實香氣成分進行聚類分析發(fā)現(xiàn),不同時段遮光處理對果實的影響可分為4類,其中T1、T2、T3和T5的果實為一類,T4和對照果實為一類,T6和T7分別為一類。說明T4中紅陽獼猴桃的香氣物質的組成和含量與對照更為接近(圖3)。此外,酯類香氣成分乙酸乙酯、己酸乙酯、苯甲酸乙酯、苯甲酸甲酯、丁酸甲酯、丁酸乙酯在T2、T5和T4中含量較高,說明時段Ⅱ、時段Ⅳ和時段Ⅴ遮光果實果香較時段Ⅰ、時段Ⅲ、時段Ⅵ以及全時段遮光果實濃郁。α-蒎烯、桉葉油醇、D-檸檬烯、α-松油醇、(Z)-β-羅勒烯、萜品烯-4-醇、庚醛、(Z)-2-丁烯-1-醇、1-己醇在T2、T4、T5和對照中無累積,但在T6果實中均有發(fā)現(xiàn),說明時段Ⅵ遮光促進了上述成分的積累(圖3)。
2.3 不同時段遮光處理下紅陽獼猴桃果實香氣的主成分分析
利用SPSS軟件對紅陽獼猴桃果實香氣物質進行主成分分析,以特征值大于1提取主成分。結果顯示提取了5個主成分,第1主成分和第2主成分累積解釋了總方差的90.147%,說明這兩個主成分可以很好地反映樣品中的大部分數(shù)據(jù)(表3)。利用主成分1和主成分2的特征值和貢獻率分別計算各組的綜合得分,并進行排名。結果顯示除對照外,T6綜合得分最高,可能是因為T6中醛類和醇類含量較其他組高。其次為T4、T5和T2。結合香氣總含量綜合分析,T4、T2和T5的果實香氣要優(yōu)于其他處理(圖4)。
2.4 全時段遮光(T7)和全時段未遮光(CK)果實揮發(fā)性香氣物質相關基因的表達
2.4.1 轉錄組數(shù)據(jù)整體概況 對T7和對照果實進行轉錄組學分析,各樣品Clean Data均達到6.74 GB以上,Q20堿基含量最小值為97.58%,Q30堿基含量最小值為93.35%,GC含量最小值為49.63%,符合質量要求。PCA分析顯示,主成分1和主成分2共解釋了樣品的52.41%,各組的樣品都較高程度地聚集在一起,說明他們具有較好的重復性,組間差異顯著(圖5)。
DEGs分析結果顯示,T7和對照之間共有1248個差異表達基因,其中,T7相對對照下調基因788個,上調基因460個(圖6)。
2.4.2 差異表達基因GO和KEGG富集分析 基于GO數(shù)據(jù)庫,分析差異表達基因在生物過程(biological process)、細胞組成(cellular component)和分子功能(molecular function)的富集情況。這些差異基因注釋在40個GO條目中,在生物過程主要注釋到了細胞脂質代謝過程(cellular process)、代謝過程(metabolic process)、對刺激的響應(response to stimulus)和生物調控(biological regulation)等;在分子功能上,主要富集的條目是結合(binding)、催化活性(catalytic activity)、轉錄調控活性(transcription regulator activity)和轉運載體活性(transporter activity)等;在細胞組成中富集的基因條目為細胞整體結構(cellular anatomical entity)和蛋白復合體(protein-containing complex)(圖7)。
基于KEGG數(shù)據(jù)庫將T7和對照之間的DEGs進行KEGG富集分析,差異基因注釋在130個KEGG條目中,柱狀圖顯示前50個富集最顯著的條目。結果表明,大部分的差異基因富集在代謝中,其中,差異基因主要富集的條目是代謝途徑(metabolic pathways)、次級代謝物的生物合成(biosynthesis of secondary metabolites)、內(nèi)質網(wǎng)中的蛋白質加工(protein processing in endoplasmic)和氨基酸的生物合成(biosynthesis of amino acid)(圖8)。以上結果說明遮光引發(fā)了紅陽獼猴桃果實顯著的生理變化。
2.4.3 受光照影響的香氣合成基因分析 果實的揮發(fā)性香氣物質合成途徑主要為脂肪酸代謝途徑、氨基酸代謝途徑和萜類代謝途徑[29]。對這3個代謝途徑上的DEGs進行分析,共篩選出15個與香氣合成相關的基因(圖9)。其中包括2個乙醇脫氫酶(ADH),1個脂氧合酶(LOX),1個羥基肉桂?;D移酶(HCT),3個苯丙氨酸解氨酶(PAL),1個天冬氨酸轉氨酶(GOT),1個丙酮酸轉氨酶(AGXT),1個焦磷酸脫羧酶(MVD),1個2-C-甲基-d-赤蘚糖醇-4-磷酸胞苷酰轉移酶 (IspD),1個萜烯合酶(TPS),3個醛脫氫酶(ALDH)。遮光處理降低了T7果實中ADH1、AGXT、GOT1和IspD基因的表達量,升高了MVD、ALDH和2個PAL基因的表達量,這可能是T7果實香氣含量顯著低于對照的原因。
3 討 論
酯類、醛類和醇類在獼猴桃果實香氣中多有研究。其中酯類是獼猴桃特征香氣的重要組分,為獼猴桃提供濃郁的果香和甜香,醛類和醇類則為獼猴桃富集青草香氣[23-24]。本試驗對對照中的紅陽果實進行檢測發(fā)現(xiàn),其酯類含量占比超過90%,丁酸乙酯、丁酸甲酯在紅陽果實酯類成分中的占比最大;醛類含量位居第二,(E)-2-己烯醛和己醛在醛類成分中占比最高,與前人的研究結果一致[25,30]。趙玉[27]等檢測了翠香、徐香、秦美和華優(yōu)獼猴桃的香氣成分,將己醇鑒定為獼猴桃關鍵香氣組分之一,但在紅陽中己醇含量較低,未達到閾值,可能是因為品種差異。
將遮光處理組的果實香氣與對照進行對比,結果顯示除時段Ⅳ外,其他各時段的遮光處理均會導致果實酯類含量下降,其中T2和T5果實中酯類含量較其他組高,分別是對照的77%和66%,T1、T3和T6組中含量分別為對照的24%、41%和30%,表明時段Ⅰ、時段Ⅲ和時段Ⅵ為酯類合成的重要時期。在T4、T5和T6中,果實的醛類物質與對照無顯著差異,而在T1、T2和T3中,醛類含量顯著低于對照,說明時段Ⅰ~Ⅲ是醛類合成關鍵時期,已有研究也證明了醛類香氣物質在紅陽獼猴桃果實發(fā)育前期含量最高,而后逐漸降低[31]。上述結果說明了紅陽獼猴桃中酯類和醛類的合成時期不完全相同,與前人在金艷獼猴桃中得出的結論類似[32]。此外,持續(xù)遮光顯著降低了己醛的含量,但對(E)-2-己烯醛的含量無明顯影響,與Liu等[33]、王繼源等[34]的結果一致。
萜烯類多呈現(xiàn)花果香[35]。對照中只有鄰傘花烴一種萜類,而王逍遙[29]研究發(fā)現(xiàn)桉葉油醇是紅陽獼猴桃的主要萜類物質,可能是地域不同導致的。萜烯類的合成途徑有甲羥戊酸途徑(MVA)和2-C-甲基-D-赤藻糖醇-4-磷酸途徑(MEP),其中,單萜通過MEP途徑產(chǎn)生[36]。在除對照和T4外的其他組中分別檢測到包含鄰傘花烴在內(nèi)的多種萜類物質,且均為單萜,說明紅陽獼猴桃的萜烯類化合物合成途徑為MEP途徑,并且各類物質在果實發(fā)育的不同時段產(chǎn)生,依賴避光環(huán)境進行積累。T4果實中萜烯類物質的累積被完全抑制,而在其他時段遮光對果實內(nèi)萜烯類物質含量無影響或顯著提高,說明時段Ⅳ是萜烯類合成的關鍵時期。
在脂肪酸代謝途徑中,前體物質經(jīng)脂氧合酶(LOX)和乙醇脫氫酶(ADH)等酶的催化,轉化為醛類和醇類,再轉化成酯[37]。張曾等[31]認為AcLox3、AcLox4和AcLox6是紅陽獼猴桃成熟過程中調控醛類合成的相關基因,AcLox1和AcLox5是調控酯類香氣成分合成的相關基因。本研究轉錄組分析僅發(fā)現(xiàn)LOX1_5基因(novel.3064)在遮光處理下表達量升高,認為其受遮光誘導,在獼猴桃酯類香氣形成中起負調控作用。ALDH能夠將醛類催化為酸,從而進入倍半萜合成途徑[38]。在T7中2個ADH基因(Achv4p10g015598,Achv4p11g016521)表達被遮光抑制,而3個ALDH基因(Achv4p11g0116573,Achv4p06g008766,Achv4p04g006368)高表達,可能是T7果實醇類、醛類和酯類香氣含量降低的原因。
在氨基酸代謝途徑中,氨基酸可經(jīng)過轉氨酶催化形成酮酸,再由ADH催化形成各類香氣成分,也可在PAL催化下形成醛,再向醇和酯轉化[37]。在轉錄組分析結果中,AGXT基因(Achv4p28g044216)和COT1基因(Achv4p03g003648),以及PAL基因(Achv4p24g037630)在T7中被抑制表達,而PAL基因(Achv4p28g04395,Achv4p26g040956)在T7中表達量上調。推測PAL基因(Achv4p28g043957,Achv4p26g040956)對香氣合成起負調控作用,同時遮光降低AGXT(Achv4p28g044216)、COT1(Achv4p03g003648)和PAL基因(Achv4p24g037630)表達量,抑制T7果實醇類、醛類和酯類香氣合成。
紅陽獼猴桃萜烯類合成途徑主要是MEP途徑,IspD、MVD和TPS是該過程中的關鍵酶[39-40]。轉錄組分析結果顯示,相比于對照,MVD和TPS基因在T7中表達上調,IspD基因表達下調。而T7果實中萜烯類含量顯著高于對照,推測MVD基因(Achv4p22g034538)和TPS基因(Achv4p15g023672)是萜烯類合成的關鍵基因。有研究表明,TPSs基因表達受光照影響,如擬南芥中的TPS合成基因AtTPS03、AtTPS06和AtTPS29受光誘導[41]。但在本試驗中,TPS基因表達在遮光條件下上調,可能是由物種不同所致。
前人研究發(fā)現(xiàn)紅陽獼猴桃套袋適宜開始在落花后30~40 d,過早套袋,容易傷害獼猴桃幼果,影響果實正常發(fā)育;而過晚套袋,獼猴桃果皮容易變得粗糙,達不到套袋的目的[42]。根據(jù)試驗結果綜合分析,對果實香氣影響最小的遮光時段為時段Ⅳ,其次是時段Ⅴ和時段Ⅱ以及時段Ⅲ。結合已有研究,建議從時段Ⅰ后期(30~40 DAA)開始對獼猴桃套袋,時期Ⅴ結束(120 DAA左右)時取袋。試驗結果顯示,時段Ⅵ(120~140 DAA)遮光顯著降低果實中酯類物質和總香氣含量,已有研究也表明獼猴桃海沃德和紅陽中酯類物質會在果實成熟后期大量累積[21,31],因此建議保持該時期果實充分的光照條件。
4 結 論
紅陽獼猴桃果實香氣含量被遮光處理顯著影響,其中,T4、T5、T2果實香氣受遮光影響最小,說明在40~60 DAA(時段Ⅱ)和80~120 DAA(時段Ⅳ和時段Ⅴ)為適宜遮光時段。遮光抑制了ADH、AGXT和COT1基因的表達,提高了MVD和ALDH基因的表達量,是遮光果實中萜烯類香氣物質含量升高和酯類、醛類、醇類含量下降的可能原因。此外,從試驗結果來看,時段Ⅰ、Ⅲ、Ⅵ和時段Ⅳ分別是酯類和萜烯類香氣物質合成的重要階段,時段Ⅰ至時段Ⅲ為醛類物質合成的重要階段。此結果可為獼猴桃精細化管理提供有益參考。
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