‘香拔’與‘珍珠拔’番石榴果實發育與後熟特性之研究

Abstract

‘香拔’為非更年性‘珍珠拔’芽條變異的番石榴(Psidium guajava L.)品種，其果實生長發育體、積重量變化與母本幾乎相同，皆呈現雙S曲線生長模式。不同於‘珍珠拔’，‘香拔’從盛花後108天控制果實後熟的系統Ⅱ乙烯合成基因PgACS1開始大量表現，轉譯生成1-aminocyclopropane-1-carboxylic acid合成酶(ACC synthase, ACS)，且在盛花後122天達到ACS活性高峰為0.21 ± 0.04 μmole ACC g-1 protein h-1，也是乙烯生成高峰前一週，因此果肉組織內ACC開始累積，完熟時達0.3 ± 0.04 nmole ACC g-1 FW，並在盛花後108天開始顯著提高乙烯合成，生成速率2.73 ± 1.00 μL C2H4 kg-1 h-1乙烯高峰及57.86 ± 6.18 mL CO2 kg-1 h-1呼吸率高峰，導致果肉軟化、果皮葉綠素降解及香氣生成等更年性番石榴後熟特徵。‘珍珠拔’具有ACC氧化酶(ACC oxidase，ACO)的活性與基因表達，酵素活性甚至高於‘香拔’，然而PgACS1在‘香拔’中的表現量為‘珍珠拔’的5-48倍，證明芽條變異在‘香拔’造成PgACS1的表現與母本‘珍珠拔’相異，使得‘香拔’恢復為更年性果實。將‘香拔’、‘珍珠拔’與更年性‘梨仔拔’綠熟果採收後貯藏於20℃，觀察比較三者間後熟行為，‘香拔’PgACS1表現量為‘珍珠拔’的2.5-30倍，但僅有‘梨仔拔’的0.04-0.47倍，導致‘香拔’乙烯生成量、ACC含量及ACS活性分別僅有2.43 ± 0.33 μL C2H4 kg-1 h-1、0.50 ± 0.27 nmol g-1 FW及0.173 ± 0.073 mole ACC g-1 protein h-1，皆略高於‘珍珠拔’但遠低於‘梨仔拔’。本試驗結果顯示‘香拔’恢復部分的ACS活性，推測因此能有少量的ACC及乙烯生合成。綜合以上試驗結果，芽條變異在‘香拔’可能透過反轉錄跳躍子(retrotransposon)造成PgACS1部分表現，因此合成部分具有活性的ACS酵素，並使ACC及乙烯生成促進果實後熟。番石榴果實發育過程的澱粉含量皆低於2%，顯示番石榴非澱粉貯藏型果實，果實可溶性醣類顯示掛樹完熟及採後後熟的果實主要可溶性糖分別為蔗糖及果糖，顯示蔗糖為番石榴貯藏性醣類，採收後的果實將蔗糖降解為果糖及葡萄糖，並消耗葡萄糖以進行三羧酸循環及磷酸戊醣途徑代謝維持基本代謝。

‘Shiang Bar’(SB) is a bud sport mutant cultivar of guava (Psidium guajava L.) derived from ‘Jen-Ju Bar’(JJB), a non-climacteric variety. Both of SB and JJB fruit developmental curves, based on fruit size and weight, displayed a double-sigmoid pattern. However, in contrast to JJB, expression level of PgACS1, a system Ⅱ 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) gene, in SB began to increase dramatically on 108 days after blooming (DAB). It showed the highest ACS activity 0.21 ± 0.04 μmole ACC g-1 protein h-1 on 122 DAB; therefore, ACC began to accumulate in SB fruit and finally achieved 0.3 ± 0.04 nmole ACC g-1 fresh weight (FW) in SB fruit at the full-ripe stage. Likewise, SB significantly increased ethylene emission and respiration on 108 DAB and achieved peak values of 2.73 ± 1.00 μL C2H4 kg-1 h-1 and 57.86 ± 6.18 mL CO2 kg-1 h-1, respectively. This led SB to behaved a climacteric ripening characteristic, including peel degreening, flesh softening, and volatile emissions. Although ACC oxidase (ACO) activity and PgACO gene expression in JJB were higher than those of SB, the expression level of PgACS1 in SB was 5 to 48 folds of JJB’s. These results implied that the bud sport mutation occurred in SB partially resumed the expression of PgACS1 and massive ethylene production during ripening stage; SB consequently exhibited a climacteric ripening manner. Moreover, comparisons of ripening characteristics of detached mature-green SB, JJB, and ‘Li-Tzy Bar’(LTB), a climacteric variety, were conducted at 20℃. PgACS1 expression level in SB was 2.5 to 30 times higher than JJB, but only 0.04 to 0.47 fold of LTB’s; resulting the ethylene production (2.43 ± 0.33 μL C2H4 kg-1 h-1), ACC content (0.50 ± 0.27 nmol g-1 FW), and ACS activity (0.173 ± 0.073 mole ACC g-1 protein h-1) of SB were higher than those of JJB but lower than LTB’s. Again, the data supported the hypothesis just mentioned that SB gained the ability of partially synthesis of system Ⅱ ACS, ACC, and ethylene. Taken together, the bud sport mutation occurred in SB might lead to partial restorations of PgACS1 expression via retrotransposon removal in the promoter region and ACS activity during fruit ripening. The starch contents in SB and JJB flesh were less than 2% over the fruit development; therefore, a guava was not considered as a starch-storing fruit. The main soluble sugars of tree-ripe and detached ripe fruit were sucrose and fructose, respectively. It indicated sucrose is the major storage carbohydrate in guava fruit.