探討兔眼藍莓果實無子或少子之機制

Abstract

兔眼藍莓(Vaccinium virgatum Aiton)具有高度自交不親合性，多數品種需異交授粉才可著果。‘NTU025’是由兔眼藍莓‘Blueshower’開放授粉所得之實生苗選育出的品系，有反季節(off-season)開花特性，在花期無授粉樹的條件下仍結實，且著果率高，成熟果實內部無子或少子。本研究目的為探討兔眼藍莓‘NTU025’果實無子或少子之原因，試驗假說為本品系可能是五倍體、具有花粉發育不正常、花粉無活力、自交親和、單為結果或略精結實等特性，而造成種子不發育。 以流式細胞技術進行染色體倍體數分析後，顯示‘NTU025’及對照品種‘Blueshower’與‘Climax’均為六倍體，且DNA含量相近。‘NTU025’小孢子母細胞減數分裂正常，形成四分體複合型花粉，平均直徑為46.9 ± 2.3 μm、極軸(polar axis)平均長度25.8 ± 1.0 μm，赤道軸(equatorial diameter)平均長度36.7 ± 1.5 μm，極軸與赤道軸比值0.7，單粒花粉形狀呈現扁圓形(oblate)，小於母本‘Blueshower’，但與‘Climax’無顯著差異。‘NTU025’外壁紋飾在極面中間較平滑(psilate)，邊緣為皺紋狀(rugulate)，與對照品種‘Blueshower’之花粉形態差異不大。‘NTU025’花粉萌發率為69.5%，與‘Climax’ 71.8%無顯著差異，但高於‘Blueshower’ 58.8%之花粉萌發率，結果顯示‘NTU025’花粉形態與活力皆正常。 ‘NTU025’、‘Blueshower’與‘Climax’自交72 h後，花粉管皆可生長至子房內，無典型之配子體型自交不親合現象。‘NTU025’自交72 h後，觀察到精細胞與卵細胞質融合現象。自交2週後，接合子停滯發育，4週後胚珠敗育，但果實仍持續發育，花後4週之果實平均重量為0.40 ± 0.02 g，與異交處理0.42 ± 0.02 g在重量上無顯著差異。對照之‘Climax’自交後皆無觀察到受精現象，4週後果實平均重量為0.13 ± 0.01 g，與同時期異交果實平均重量0.45 ± 0.01 g有顯著差異。‘NTU025’、‘Blueshower’與‘Climax’在開放授粉條件下，著果率無顯著差異，‘NTU025’果實直徑、重量與總種子數較對照品種低。三品種以除雄處理，只有‘NTU025’結果。三者以自交處理，‘NTU025’著果率較對照品種高且果實發育較佳。試驗結果證實，‘NTU025’為六倍體兔眼藍莓，可產生正常有活力之花粉，在無外來花粉源的條件下，具有單為結果之特性，而自交後，可觀察到受精完成，惟合子初期敗育，果實則持續發育，因此亦具有略精結實之特性。

Rabbiteye blueberry (Vaccinium virgatum Aiton) cultivars are highly self-incompatible and require cross pollination for commercial berry set. ‘NTU025’ is a breeding line selected from an open pollinated population using ‘Blueshower’ as the female parent. ‘NTU025’ possesses characters of off-season blooming, high berry set, and seedless. This study aimed to investigate the mechanism of self-fruitfulness and seedlessness of ‘NTU025’. It was hypothesized that ‘NTU025’ is either a pentaploid or self-compatible line, or producing abnormal pollen grains but capable of bearing parthenocarpic or stenospermocarpic berry. Flow cytometry analysis showed that ‘NTU025’, ‘Blueshower’, and ‘Climax’ were all hexaploid with similar DNA contents. Microspore mother cells of ‘NTU025’ were normal during meiosis and produced pollen tetrads with an average diameter of 46.9 ± 2.3 μm, an average polar axis length of 25.8 ± 1.0 μm, and an equatorial axis length of 36.7 ± 1.5 μm. The ratio of polar axis to equatorial axis was 0.7, and the shape of single pollen grain was oblate. ‘NTU025’ pollen was smaller than ‘Blueshower’ but was not significantly different from ‘Climax’. Apocolpial exine sculpture of ‘NTU025’ pollen were psilate in the middle of the polar view, and the edges were rugulate, which was not significantly different from ‘Blueshower’. The pollen tetrad germination rate of ‘NTU025’ was 69.5%, and 71.8% of ‘Climax’, both were higher than the 58.8% germination rate of ‘Blueshower’. The results showed that ‘NTU025’ pollen morphology and viability were both normal. After 72 h of self-pollination, the pollen tubes of ‘NTU025’, ‘Blueshower’, and ‘Climax’ entered the ovary, indicating no typical gametophytic incompatibility. The fusion of sperm cells and egg cells was observed 72 h after self-pollination in ‘NTU025’. However, the development of zygotes ceased 2 weeks, and the embryo aborted 4 weeks after self-pollination. However, the berry continued to develop. The average weight of the young berry was 0.40 ± 0.02 g 4 weeks after pollination treatments, similar to the berry after cross-pollination (0.42 ± 0.02 g). These results indicated that ‘NTU025’ produced seedless berry through stenospermocarpy. On the other hand, no fertilization was observed in ‘Climax’ after self-pollination and the average fruit weight 4 weeks after self-pollination was 0.13 ± 0.01 g compared to 0.45 ± 0.01 g of cross-pollinated berries. There was no significant difference in the percentage of berry set among ‘NTU025’, ‘Blueshower’, and ‘Climax’ subjected to open pollination in the field condition. The berry diameter, berry weight, and number of seeds of ‘NTU025’ were lower than ‘Blueshower’ and ‘Climax’. After emasculating treatments, only ‘NTU025’ set berry. In the ‘NTU025’, ‘Blueshower’, and ‘Climax’ self-pollination trial, the percentage of berry set in ‘NTU025’ was higher than the other two commercial cultivars. These results indicated that ‘NTU025’ is a hexaploidy rabbiteye blueberry producing normal and functional pollen grains and capable of parthenocarpic berry set in the absence of exotic pollen donors. When self-pollinated, zygotes formed but aborted after 4 weeks while berry growth continued. Therefore, ‘NTU025’ is also capable of stenspermocarpic berry setting.