蓖麻種子結(jié)構(gòu)的解剖和顯微觀察

郭學(xué)民 趙曉曼 徐 珂 王芯蕊 張辰瑜 東方陽

蓖麻種子結(jié)構(gòu)的解剖和顯微觀察

郭學(xué)民*趙曉曼 徐 珂 王芯蕊 張辰瑜 東方陽

河北科技師范學(xué)院農(nóng)學(xué)與生物科技學(xué)院, 河北昌黎 066600

本文通過萌發(fā)法、分離法、石蠟切片法和顯微觀察技術(shù), 分別以番紅固綠和希夫試劑染色, 系統(tǒng)觀察了蓖麻種皮、胚乳和胚的結(jié)構(gòu)及其維管束分布。(1)蓖麻種子背側(cè)基部的種孔并未被種阜所覆蓋。(2)種皮包括外種皮、內(nèi)種皮和種阜3部分, 其中, 外種皮由外至內(nèi)分別為長柱狀表皮層、海綿組織層和柵欄組織層, 而內(nèi)種皮則依次為馬氏層、海綿組織層和內(nèi)珠被內(nèi)層, 在種阜端由內(nèi)種皮內(nèi)層和外層共同圍成氣室, 類似于雞蛋的氣室。(3)在外種皮和內(nèi)種皮中均有維管束分布, 其中外種皮的大型維管束僅分布在種子腹側(cè)種脊的海綿組織層內(nèi), 它從種臍延伸至種阜相對一端; 而內(nèi)種皮維管束也分布在海綿組織層中, 通過維管束索, 逐級分枝, 止于種阜端氣室邊緣; 內(nèi)、外種皮的維管束通過合點(diǎn)端的連接點(diǎn)相連續(xù), 共同構(gòu)成種皮維管系統(tǒng)。(4)種阜由表皮和薄壁組織構(gòu)成, 其中薄壁組織由外側(cè)小型薄壁細(xì)胞和內(nèi)側(cè)大型薄壁細(xì)胞構(gòu)成。種阜維管束僅2束, 分布在種阜中部腹側(cè), 獨(dú)立構(gòu)成種阜維管系統(tǒng); 在種阜中, 還有種阜管道, 其內(nèi)端開口于內(nèi)種皮的氣室, 外端為盲端。(5)胚乳由含大量糊粉粒的薄壁細(xì)胞構(gòu)成, 其中未見維管束分布。在胚中, 維管束為外韌維管束, 它們分化于胚軸, 從胚軸頂端兩側(cè)分別進(jìn)入2片子葉主脈, 在主脈中由4個(gè)逐步減少為1個(gè), 在子葉中分枝變細(xì), 構(gòu)成胚維管系統(tǒng); 子葉表皮和葉肉細(xì)胞中均含糊粉粒, 但在胚芽、胚軸和胚根中未見糊粉粒; 2片子葉近軸面分離, 遠(yuǎn)軸面則通過粘液層與胚乳相連接; 胚軸和胚根在空間上與胚乳分離。這些結(jié)果為全面掌握植物蓖麻種子的結(jié)構(gòu)和研究種子發(fā)育和萌發(fā)過程及其物質(zhì)運(yùn)輸提供了解剖學(xué)依據(jù)。

蓖麻; 種子; 維管束; 解剖結(jié)構(gòu)

大戟科(Euphorbiaceae)的蓖麻(L.)是重要的經(jīng)濟(jì)作物。蓖麻種子是一種植物學(xué)模式材料, 蓖麻油是全球特種化學(xué)工業(yè)的重要原料[1-5],它是羥基化脂肪酸(hydroxylated fatty acid)的唯一商業(yè)來源。蓖麻種子是生物燃料的重要資源[6], 其提取物具有重要的抗病活性[7-8], 尤其是它所含蓖麻毒蛋白是最強(qiáng)的植物源毒素之一, 主要應(yīng)用于生物農(nóng)藥領(lǐng)域和醫(yī)藥領(lǐng)域[9-12]。我國是蓖麻油的主要生產(chǎn)國和消費(fèi)國, 加強(qiáng)蓖麻的基礎(chǔ)研究, 對蓖麻資源的綜合開發(fā)利用、提升國民經(jīng)濟(jì)水平具有十分重要的意義。

國外許多學(xué)者觀察了授粉后50 d的蓖麻種皮的結(jié)構(gòu)[13]、種阜的結(jié)構(gòu)[14]、萌發(fā)5 d的蓖麻種子子葉和胚乳的結(jié)構(gòu)[15]、種子萌發(fā)后6~7 d胚乳從種皮側(cè)到子葉側(cè)超微結(jié)構(gòu)的變化[16]以及在子葉衰老過程中葉綠體等細(xì)胞器的變化[17]。國內(nèi)不少《植物學(xué)》教材中都有關(guān)于蓖麻種子的形態(tài)和胚乳細(xì)胞糊粉粒的扼要描述[18-19]。這些研究關(guān)注的是種子發(fā)育或萌發(fā)過程中某一時(shí)期某一部分的結(jié)構(gòu)及其變化, 目前, 尚未見到完整成熟種子結(jié)構(gòu)和維管束分布的報(bào)道。

本文利用石蠟切片法和顯微照相技術(shù), 較為系統(tǒng)地觀察了成熟蓖麻種子的解剖學(xué)結(jié)構(gòu)和維管束分布, 以期為從整體上認(rèn)識(shí)種子的結(jié)構(gòu), 以及研究種子發(fā)育和萌發(fā)過程中物質(zhì)運(yùn)輸?shù)纳?、生化和分子生物學(xué)提供基本資料。

1 材料與方法

1.1 材料

試驗(yàn)材料為從市面上購買的健康、飽滿蓖麻種子。

1.2 方法

1.2.1 種子形態(tài)觀察 選擇30粒左右成熟飽滿、結(jié)構(gòu)完整的蓖麻種子,用蒸餾水浸泡12 h,分別游離其中15粒種子各組成部分, 以觀察其形態(tài)結(jié)構(gòu);其余15粒在潮濕的培養(yǎng)皿中室溫下進(jìn)行萌發(fā)試驗(yàn),以觀察種孔的位置,先后分別在OLYMPUS SZX16體視顯微鏡下觀察、照相。

1.2.2 種子解剖結(jié)構(gòu)觀察 取完整飽滿成熟蓖麻種子10粒, 以蒸餾水浸泡12 h,分離出外種皮、內(nèi)種皮、種阜、胚乳和胚5個(gè)部分。

以FAA固定液分別固定種子背側(cè)、側(cè)面和腹側(cè)外種皮, 24 h后, 以軟化劑(過氧化氫∶冰醋酸=1∶2)軟化2個(gè)月; 隨后, 乙醇梯度脫水、二甲苯透明、浸蠟、換蠟、包埋、修塊, 用ESM-100L大型滑走式切片機(jī)切片, 厚度14 μm, 經(jīng)展片、烘片、脫蠟、染色、封片等, 制成永久切片。

同樣, 以FAA固定內(nèi)種皮、種阜、胚乳和胚, 經(jīng)過上述步驟處理, 用KD-2258輪轉(zhuǎn)切片機(jī)連續(xù)切片, 切片厚度8~12 μm, 制作切片。

分別用番紅固綠對外種皮和內(nèi)種皮染色, 以番紅固綠與希夫試劑分別對種阜、胚乳、子葉和胚染色, 旨在顯示不可溶性多糖(希夫試劑)和木質(zhì)化、木栓化和角質(zhì)化的細(xì)胞壁(番紅固綠雙重染色)。

在光學(xué)顯微鏡Olympus BX51下觀察切片, 用DP72相機(jī)照相。選擇代表性照片, 用Photoshop CS 6.0軟件制作圖版。

1.2.3 內(nèi)種皮維管束分布的觀察 選擇成熟完整蓖麻種子, 去除外種皮, 將其余部分在蒸餾水中浸泡12 h, 從種阜端向上輕輕剝下相對完整的內(nèi)種皮。用番紅染色, 酸乙醇分色, 甘油封片, 在顯微鏡Olympus BX51下觀察并照相記錄。

2 結(jié)果與分析

2.1 種子形態(tài)

蓖麻種子呈扁卵形或扁橢圓形, 較平坦的一面為腹側(cè), 隆起一面為背側(cè), 它由種皮、胚乳和胚3部分構(gòu)成(圖1-A~E)。種子萌發(fā)時(shí), 胚根從種阜基部、靠近種子背面的種孔突破種皮(圖1-F), 種阜依然完整, 這表明種孔并未被種阜所覆蓋。

圖1 蓖麻種子的形態(tài)

A: 種子腹面觀, 示外種皮的紋飾和種脊(箭頭); B: 去外種皮的種子, 示內(nèi)種皮維管束(箭頭)的分布; C: 去種皮的種子, 示乳白色的胚乳; D: 與子葉平面垂直的正中縱切面, 示胚在種子中的位置; E: 游離的胚, 示子葉、胚軸和胚根; F: 去部分種皮、胚根突破種孔的萌發(fā)種子, 示種孔在種阜背側(cè), 并未被種阜覆蓋。HI: 種臍; CA: 種阜; EN: 胚乳; TE: 種皮; EM: 胚; CO: 子葉; HY: 胚軸; RA: 胚根。Bar = 2 mm。

A: ventral view of a seed, showing the ornamentation and raphe (arrow) of a seed; B: a seed without episperm, showing the distribution of vascular bundles (arrow) in endopleura; C: a seed without testa, showing the milky white endosperm; D: median longitudinal section perpendicular to the plane of cotyledon, showing the position of an embryo in a seed; E: a free embryo, showing cotyledon, hypocotyle and radicle; F: a germinating seed with partial testa and radicle breaking through the micropyle, showing the micropyle on the dorsal side of the caruncle and not covered by the caruncle. HI: Hilum; CA: Caruncle; EN: Endosperm; TE: Testa; EM: Embryo; CO: Cotyledon; HY: Hypocotyle; RA: Radicle. Bar = 2 mm.

2.2 種皮的解剖結(jié)構(gòu)

蓖麻種皮包括外種皮(圖2-A~E)、內(nèi)種皮(圖2- F~I;圖3)和種阜(圖1-A; 圖4-A~C)。

2.2.1 外種皮解剖結(jié)構(gòu) 外種皮表面光滑, 有黑色和黃褐色的紋飾;種脊位于種子腹側(cè)中央,與種子長軸平行,并未向外突出,它是倒生胚珠的珠柄與珠被愈合處留于外種皮上的痕跡(圖1-A)。

種子各部分外種皮的結(jié)構(gòu)基本相同,由外至內(nèi)分別為長柱狀表皮層、海綿組織層和柵欄組織層(圖2-A~E)。外種皮腹側(cè)種脊海綿組織層貫穿有大型維管束(圖2-D, E),始于種臍,止于種阜相對一端(相當(dāng)于胚珠的合點(diǎn)端)的外種皮內(nèi)側(cè)與內(nèi)種皮維管束索的連接點(diǎn)(圖2-G),而在外種皮背側(cè)和側(cè)面,均未發(fā)現(xiàn)維管束分布(圖2-A, C)。

2.2.2 內(nèi)種皮解剖結(jié)構(gòu) 內(nèi)種皮包裹著胚乳, 是半透明膜質(zhì)結(jié)構(gòu)。在種阜端由內(nèi)種皮內(nèi)層和內(nèi)種皮外層圍成1個(gè)氣室, 類似于雞蛋的卵殼膜在一端所形成的氣室(圖2-F, G; 圖4-C; 圖7-A, B)。內(nèi)種皮由5~6層細(xì)胞組成, 由外到內(nèi)依次為馬氏層(Malpighian layer, ML)、海綿組織層和內(nèi)珠被內(nèi)層, 其中馬氏層為1層細(xì)胞, 海綿組織層由3~4扁平的薄壁細(xì)胞組成, 其中有維管束穿過, 而內(nèi)珠被內(nèi)層為1層薄壁細(xì)胞(圖2-H, I)。

內(nèi)種皮維管束分布很有規(guī)律。在未被染色時(shí), 維管束呈黃褐色, 主要分布在合點(diǎn)端絕大部分面積的內(nèi)種皮中, 而在種阜端內(nèi)皮層中, 未見維管束分布(圖1-B)。被番紅染色后, 內(nèi)種皮維管束木質(zhì)部呈紅色(圖3-A), 其中與外種皮腹側(cè)種脊大型維管束相連的維管束索, 分枝為3個(gè)較大的維管束(圖3-B), 然后, 從合點(diǎn)端開始不斷分枝變細(xì), 終止于內(nèi)種皮內(nèi)層和外層結(jié)合處附近(圖3-B~E), 這也進(jìn)一步驗(yàn)證了上述在種阜端的內(nèi)種皮內(nèi)層和外層部分無維管束分布的結(jié)果(圖3-A, E)。外種皮種脊中的大型維管束和內(nèi)種皮的各級維管束, 共同構(gòu)成種皮維管系統(tǒng)。

(圖2)

A: 種子背側(cè)外種皮垂直于種子長軸橫切面, 示長柱狀表皮層、海綿組織層和柵欄組織層; B: 外種皮表皮細(xì)胞橫切面, 示表皮細(xì)胞橢圓形形態(tài); C: 種子側(cè)面外種皮垂直于種子長軸橫切面, 示長柱狀表皮層、海綿組織層和柵欄組織層; D: 種子腹側(cè)外種皮沿種脊延伸方向的橫切面, 示柱狀表皮層、海綿組織層和柵欄組織層以及位于海綿組織層的大型維管束; E: 種子腹側(cè)外種皮垂直于種脊延伸方向的橫切面, 示大型維管束; F: 內(nèi)種皮分布示意圖, 示在種阜端內(nèi)種皮內(nèi)層和外層圍成的氣室; G: 外種皮腹側(cè)內(nèi)側(cè)體視顯微鏡圖像, 示種阜端外種皮和內(nèi)種皮外層不易分離, 以及種脊大型維管束在種阜相對一端與內(nèi)種皮維管束索的連接點(diǎn)(箭頭); H: 內(nèi)種皮脈間區(qū)橫切面, 示馬氏層、扁平的海綿組織層和內(nèi)珠被內(nèi)層; I: 內(nèi)種皮沿維管束延伸方向的橫切面, 示維管束位于海綿組織層中。EP: 表皮層; SP: 海綿組織層; PP: 柵欄組織層; LB: 大型維管束; IE: 內(nèi)種皮內(nèi)層; OE: 內(nèi)種皮外層; ML: 馬氏層; IL: 內(nèi)珠被內(nèi)層; EB: 內(nèi)種皮維管束。

A: transection of episperm on the back of a seed perpendicular to the long axis, showing columnar epidermis, spongy parenchyma and palisade parenchyma; B: transection of epidermal cells from the episperm, showing epidermal cells in oval shape; C: transection of lateral episperm of a seed perpendicular to the long axis, showing columnar epidermis, spongy parenchyma and palisade parenchyma; D: transection of episperm at the ventral surface of a seed along the direction of raphe extension, showing columnar epidermis, spongy parenchyma, palisade parenchyma, and the large bundle in spongy parenchyma; E: transection of episperm at the ventral surface of a seed perpendicular to the direction of ridge extension, showing large vascular bundle; F: a schematic diagram of the distribution of endopleura, showing an air chamber surrounded by inner and outer layers of endopleura at caruncle end; G: stereomicroscopic image from the inner side of the ventral episperm, showing the nonseparable pattern of episperm and endopleura, and the junction point (arrow) between the large bundle of raphe and the bundle band of endopleura at the opposite end of the caruncle; H: transection of endopleura from the vein-islet, showing malpighian layer, flat spongy parenchyma and inner layer of inner integument; I: transection of endopleura along the direction of bundle extension, showing the vascular bundle located in spongy parenchyma. EP: epidermis; SP: spongy parenchyma; PP: palisade parenchyma; LB: large bundle; IE: inner layer of endopleura; OE: outer layer of endopleura; ML: malpighian layer; IL: inner layer of inner integument; EB: endopleura vascular bundle.

圖3 蓖麻內(nèi)種皮維管束分布

A: 部分游離的內(nèi)種皮, 示被染成紅色的、代表維管束的木質(zhì)部和維管束的分枝格局; B: 圖A中方框a的放大圖, 示與外種皮腹側(cè)種脊大型維管束相連的維管束索, 分枝為3個(gè)較大的維管束; C: 圖A中方框b的放大圖, 示管胞末端側(cè)壁相連(箭頭); D: 圖A中方框c的放大圖, 示維管束分枝的格局; E: 圖A中方框d的放大圖, 示內(nèi)種皮維管束末端的螺紋管胞(箭頭)。BB: 維管束索。

A: partial free endopleura, showing the reddish xylems representing vascular bundles and branching pattern of vascular bundles; B: detailed view of box a in figure A, showing that the vascular bundle band, connecting with large vascular bundle in the raphe from episperm at the ventral surface of a seed, branched into three larger vascular bundles; C: detailed view of box b in figure A, showing terminal side wall junction (arrow) of the tracheids; D: detailed view of box c in figure A, showing the pattern of branching of vascular bundles; E: detailed view of box d in figure A, showing the helical tracheid (arrow) at the end of vascular bundle of endopleura. BB: bundle band.

2.2.3 種阜的解剖結(jié)構(gòu) 種阜是外種皮在種子一端延伸形成膨大的淺棕色海綿狀隆起物(圖1-A; 圖4-A~C; 圖7-A, B)。從種阜上部看, 種阜是由2個(gè)淺棕色半球狀的結(jié)構(gòu)相互連接構(gòu)成的(圖4-A, B)。種阜由薄壁細(xì)胞構(gòu)成, 外側(cè)3層細(xì)胞比內(nèi)部細(xì)胞小, 細(xì)胞壁上有發(fā)達(dá)的紋孔, 紋孔來源于增厚的果膠質(zhì)(圖4-D~G)。在種阜中部橫切面腹側(cè), 有2個(gè)維管束斜向分布, 構(gòu)成種阜維管系統(tǒng), 其中管胞也被希夫試劑染成紅色(圖4-H, J); 稍靠種阜背側(cè)中部有一個(gè)種阜管道, 呈“丫”字形(圖4-H, I)。在種阜縱切面上, 種阜管道外端為盲端, 內(nèi)端開口于內(nèi)種皮的氣室(圖4-C, K, L; 圖7-A), 該管道壁細(xì)胞較小, 沿管道呈徑向排列(圖4-H, I)。從染色情況看, 種阜細(xì)胞壁由不可溶性糖(纖維素和半纖維素, 具親水性)和果膠構(gòu)成(圖4-D~L)。

圖4 蓖麻種子種阜的形態(tài)與結(jié)構(gòu)

A~C: 體視顯微鏡圖片, 示種阜的形態(tài); D~J: 橫切面; K和L: 縱切面; D和H~J: 希夫試劑染色, 示細(xì)胞壁由糖類物質(zhì)構(gòu)成; E~G、K、L: 番紅和固綠雙重染色, 示細(xì)胞壁不角質(zhì)層和含木質(zhì)素。A: 種阜背面觀, 示種阜中部的淺溝將種阜上部分為2個(gè)球形突起; B: 種阜腹面觀, 示種阜和外種皮相連接; C: 種阜底面觀, 示種阜管道在內(nèi)種皮內(nèi)層和外層之間的開口(箭頭), 該開口靠近種子背側(cè), 以及白色的內(nèi)種皮外層; D: 種阜上部橫切面, 示2個(gè)球形突起; E: 種阜淺溝基部橫切, 示未出現(xiàn)種阜管道; F: 種阜中部外側(cè)橫切面, 示外側(cè)3層小型的細(xì)胞和內(nèi)部大型的細(xì)胞; G: 種阜中部橫切放大圖, 示發(fā)達(dá)的紋孔(箭頭); H: 種阜中部橫切面, 示出現(xiàn)種阜管道; I: 圖H中方框a放大圖, 示Y字形種阜管道; J: 圖H中方框b放大圖, 示種阜中的維管束(箭頭); K: 距種阜管道中軸一點(diǎn)距離的種阜縱切面, 示種阜管道(箭頭)上端并未和淺溝基部聯(lián)通; L: 沿種阜管道中軸的縱切面, 示種阜管道基部的開口(箭頭)。EM: 外種皮; CC: 種阜管道。

A–C: stereomicrograph, showing the shape of the caruncle; D–J: transection; K and L: longitudinal section; D and H–J: images stained with Schiff’s reagent, showing cell walls composed of carbohydrates; E–G, K, L: images with double staining of saffron and fixed green, showing cell walls without lignin and cuticle. A: view on the back of the caruncle, showing the shallow ditch in the middle part of the caruncle, which divided its upper part into two spherical protrusions; B: ventral view of the caruncle, showing the tight combination of caruncle and episperm; C: view on the bottom of the caruncle, showing an opening (arrow) of caruncle channel between the inner and outer layers of the endopleura and near the dorsal surface of the seed, and the outer layer of the white endopleura; D: transection of the upper part of the caruncle, showing two spherical protrusions; E: transection of shallow ditch base of the caruncle, showing no caruncle channel; F: transection of the middle lateral side of the caruncle, showing three layers of small and cuticle-free cells on the outside and large parenchyma cells inside; G: detailed view of transection of the middle part of the caruncle, showing developed pits (arrow); H: transection of middle part of the caruncle, showing the caruncle channel; I: detailed view of box a in figure H, showing Y-shaped caruncle channel; J: detailed view of box b in figure H, showing vascular bundle in the caruncle; K: longitudinal section of the caruncle a short distance from the central axis of the caruncle channel, showing the upper end of the caruncle channel (arrow) which did not connected with the base of the shallow ditch; L: longitudinal section along the central axis of the caruncle channel, showing the opening (arrow) at the base of the caruncle channel. EM: episperm; CC: caruncle channel.

2.3 胚乳和胚的解剖結(jié)構(gòu)

胚乳呈白色, 光滑飽滿, 占種子體積的絕大部分, 以不同厚度包裹著胚(圖1-C; 圖7-A, B)。胚由子葉、胚芽、胚軸和胚根組成(圖1-D; 圖7-A, B)。子葉2片, 緊貼胚乳, 薄而大, 主脈紋明顯, 側(cè)脈分布不對稱(圖1-E; 圖7-B)。2片子葉基部之間是胚芽(圖5-I; 圖6-C, F)。胚根呈光滑圓錐形, 胚軸連接胚芽與胚根(圖1-E; 圖6-C; 圖7-A, B)。

胚乳由表皮和胚乳組織構(gòu)成, 其中未發(fā)現(xiàn)維管束。胚乳細(xì)胞都含有大量的含有擬晶體和磷酸鹽球形體的糊粉粒(圖5-A~D), 具有貯藏營養(yǎng)物質(zhì)的功能。胚乳細(xì)胞的細(xì)胞壁由果膠、纖維素和半纖維素等親水性物質(zhì)構(gòu)成(圖5; 圖6-A, B)。

從胚、胚乳及其各部分的連接情況看, 胚軸和胚根在空間上與胚乳分離(圖5-E, G, I~K), 而2片子葉近軸面相互分離, 但其遠(yuǎn)軸面分別通過非細(xì)胞結(jié)構(gòu)的粘液層與胚乳連接在一起(圖5-A, D, 圖6-A)。

從胚維管束看, 胚根維管束尚未分化(圖5-E, F; 圖6-C, D), 而胚軸已分化出8個(gè)外韌維管束(圖5-G, H), 它們通過2片子葉主脈基部的8個(gè)維管束(圖5-I~K; 圖6-C, F; 圖7-A), 與子葉各級側(cè)脈維管束(圖1-E; 圖5-A; 圖7-B)相連接, 構(gòu)成胚維管系統(tǒng)。其中, 每片子葉主脈基部的4個(gè)維管束在延伸過程中由粗變細(xì)(圖5-K, L), 數(shù)量由4個(gè)減少到1個(gè)。

從胚細(xì)胞所含糊粉?？? 子葉細(xì)胞中亦含有大量糊粉粒, 但胚根(圖5-E, F)、胚軸(圖6-C, E)和胚芽(圖6-I, J)細(xì)胞中很鮮見。

從細(xì)胞壁組成看, 胚細(xì)胞的細(xì)胞壁也由果膠、纖維素和半纖維素等親水性物質(zhì)構(gòu)成(圖5; 圖6)。

3 討論

本研究發(fā)現(xiàn), 蓖麻種子的種孔并未被種阜覆蓋, 種阜端的內(nèi)種皮具一氣室, 種阜管道內(nèi)端開口于該氣室; 2片子葉近軸面分離, 遠(yuǎn)軸面則通過粘液層與胚乳相連接; 胚乳與子葉的細(xì)胞均含糊粉粒, 而在胚芽、胚軸和胚根細(xì)胞中未發(fā)現(xiàn); 胚軸和胚根與胚乳相分離; 種皮維管系統(tǒng)、種阜維管系統(tǒng)和胚維管系統(tǒng)在空間上相對獨(dú)立、功能上相互聯(lián)系。

3.1 種皮

種孔是由胚珠頂端的珠孔發(fā)育而成的孔道, 為種子萌發(fā)時(shí)吸收水分和胚根伸出的部位。有的《植物學(xué)》教材[18-19]認(rèn)為種孔被種阜所覆蓋, 只有剝?nèi)シN阜才能看到, 這與本研究的結(jié)果不一致。事實(shí)上, 如果種阜覆蓋住種孔, 意味著種阜將成為胚根伸出種皮的阻礙, 那么, 只有種阜被破壞或胚根經(jīng)過彎曲才能突破種皮, 這樣, 將削弱種阜在種子萌發(fā)中的吸水功能, 或影響胚根迅速扎入土壤, 這不符合植物結(jié)構(gòu)與功能相適應(yīng)的觀點(diǎn)。因此, 蓖麻種子的種孔不可能被種阜所覆蓋。

本研究中種皮結(jié)構(gòu)與Bianchini和Pacini[13]結(jié)果相比較, 不一致之處是, (1)外種皮表皮細(xì)胞明顯徑向伸長, 因此推測授粉50 d后可能是表皮細(xì)胞徑向生長的重要時(shí)期; (2)外種皮表皮角質(zhì)層和種脊大型維管束木質(zhì)部并未被番紅染為紅色, 該現(xiàn)象也在合歡種皮解剖結(jié)構(gòu)觀察中發(fā)現(xiàn)過[20], 可能是因?yàn)榉N皮角質(zhì)層成分特殊和管狀分子未木質(zhì)化的緣故, 這還有待于進(jìn)一步驗(yàn)證; (3)內(nèi)種皮細(xì)胞不是2層, 而是5~6層。另外, 馬煒梁[19]將蓖麻種子種皮分為外種皮、中種皮和內(nèi)種皮, 與本實(shí)驗(yàn)結(jié)果、Bianchini和Pacini[13]的結(jié)果以及國內(nèi)其他《植物學(xué)》教材的描述均不一致。一方面, 我們?yōu)榇苏J(rèn)真研究了馬煒梁蓖麻種皮圖像的標(biāo)注, 認(rèn)為這可能是因?yàn)? 他誤將外種皮的海綿組織層和柵欄組織層當(dāng)作“中種皮”的緣故; 另一方面, 我們認(rèn)為蓖麻種皮的發(fā)育還有待于進(jìn)一步深入研究。特別是, 本研究發(fā)現(xiàn), 內(nèi)種皮維管組織發(fā)達(dá), 在種阜端形成氣室, 氣室與種阜管道內(nèi)口相連通, 這為蓖麻種子的水分運(yùn)輸提供了重要解剖學(xué)基礎(chǔ)。

3.2 種阜

“elaiosome”是Sernander[21]提出的一個(gè)生態(tài)術(shù)語, 用來表示螞蟻散布的所有肉質(zhì)和可食用的植物附屬物(傳播體)。Bhojwani和Bhantnagar[22]在研究大戟科植物“elaiosome”的吸水功能時(shí), 將外珠被增生形成的、具有吸收作用的一類“elaiosome”稱為“carubcle”, 即種阜。所以, 《植物生物學(xué)詞典》[23]認(rèn)為, 種阜是大戟科和無患子科植物受精后, 在珠柄處經(jīng)細(xì)胞分裂與增大而形成的一種墊狀結(jié)構(gòu)。

圖5 希夫試劑染色的蓖麻胚的解剖結(jié)構(gòu)

A: 子葉中部橫切面, 示2片子葉被胚乳包圍的格局; B: 圖A的局部放大圖, 示胚乳細(xì)胞中大量含有擬晶體和磷酸鹽球形體的糊粉粒; C: 圖A中胚乳邊緣的局部放大圖, 示外側(cè)9~11層小型胚乳細(xì)胞和內(nèi)側(cè)大型的胚乳細(xì)胞, 且均含有糊粉粒; D: 圖(A)中子葉中段橫切面, 示含有糊粉粒的小型的子葉細(xì)胞、大型的胚乳細(xì)胞以及子葉和胚乳之間的粘液層(箭頭); E: 胚根橫切面, 示基本分生組織和原形成層; F: 圖E的局部放大圖, 示基本分生組織和原形成層細(xì)胞缺少糊粉粒; G: 胚軸橫切面, 示皮層、原形成層和維管柱; H: 圖G的局部放大圖, 示皮層、原形成層和維管柱細(xì)胞缺少糊粉粒; I: 胚芽橫切面, 示2個(gè)胚芽突起被包圍在2片子葉主脈之間; J: 圖I的局部放大圖, 示胚芽細(xì)胞缺少糊粉粒; K: 距圖I所在平面上面一點(diǎn)距離的子葉基部橫切面, 示2片子葉主脈結(jié)構(gòu); L: 圖K的局部放大圖, 示主脈細(xì)胞缺少糊粉粒。圖中圓圈代表外韌維管束。GM: 基本分生組織; PR: 原形成層; CX: 皮層; PL: 胚芽; VC: 維管柱。

A: transection of middle cotyledon, showing the pattern of two cotyledons surrounded by endosperm; B: local enlargement of figure A, showing plenty of aleurone grains containing crystalloids and phosphate spheres in endosperm cells; C: local enlargement of endosperm margin in figure A, showing the outer 9–11 layers of small endosperm cells and the inner large ones containing aleurone grains; D: transection of the middle cotyledon in Figure A, showing small cotyledon cells and large endosperm ones, which contained aleurone grains, and the mucilaginous layer between the cotyledon and endosperm (arrow). E: transection of the radicle, showing ground meristem and procambia; F: local enlargement of figure E, showing ground meristem and procambia cells lacking aleurone grains; G: transection of the hypocotyle, showing cortex, procambia and vascular cylomnder; H: local enlargement of figure G, showing cortex, procambia and vascular cylomnder cells lacking aleurone grains; I: transection of plumule, showing two plumule protuberances surrounded by two main veins of cotyledons; J: local enlargement of figure I, showing plumule cells lacking aleurone grains; K: transection a short distance above the level in figure I in cotyledon base, showing anatomical structure of main veins of two cotyledons; L: cells in main veins lacking aleurone grains. The circles in the figure represented collateral vascular bundles. GM: ground meristem; PR: procambia; CX: cortex; PL: plumule; VC: vascular cylinder.

圖6 番紅固綠雙重染色的蓖麻胚的解剖結(jié)構(gòu)

A和B: 種子中部的橫切面; C~F: 垂直于子葉所在平面的縱切面。A: 子葉中段橫切, 示2片子葉分離、子葉與胚乳通過粘液層(箭頭)相互連接的格局, 以及大型胚乳細(xì)胞和小型子葉細(xì)胞均含糊粉粒; B: 胚乳橫切面, 示含大量糊粉粒的胚乳細(xì)胞; C: 去大部分子葉的胚的縱切面, 示部分子葉、胚芽、胚軸和胚根; D: 圖C中胚根縱切面, 示原表皮、基本分生組織和原形成層的分化格局; E: 圖C中胚軸的縱切面, 示表皮、皮層和維管柱的分化格局; F: 胚芽和部分子葉的縱切面, 示胚芽和聯(lián)系胚軸與子葉的維管束(箭頭)。

A and B: the transverse section of the middle part of the seed; C–F: longitudinal section perpendicular to the plane of the cotyledon. A: transection of the middle cotyledon, showing the patterns of cotyledon separation, cotyledon-endosperm interconnection through the mucilaginous layer (arrows), large endosperm cells and small cotyledon ones with aleurone grains; B: transection of the endosperm, showing endosperm cells containing large amounts of aleurone grains; C: longitudinal section of embryos with most cotyledons removed, showing partial cotyledon, plumule, hypocotyle and radicle; D: longitudinal section of the radicle in figure C, showing differentiation pattern of primary epidermis, ground meristem and procambium; E: longitudinal section of the hypocotyle in figure C, showing differentiation pattern of epidermis, cortex and vascular cylomnder; F: longitudinal section of plumule and partial cotyledon, showing the plumule and vascular bundles (arrows) connecting the hypocotyle and cotyledon.

種阜的主要功能有兩方面: (1)生態(tài)功能: 蟻媒傳播(myrmecochory), 許多螞蟻被種阜中的脂類、蛋白質(zhì)、淀粉和維生素所吸引, 將種子攜帶到其巢穴, 使種子離開母體[3]; (2)生理功能: ①誘導(dǎo)休眠, 有的植物種子在去除種阜后才能迅速萌發(fā), 但蓖麻種阜不具此功能[24]; ②促進(jìn)種子脫水, 授粉40 d后, 種阜可將胚乳和胚細(xì)胞的水分轉(zhuǎn)運(yùn)到外界, 有利于種子壽命延長; ③促進(jìn)種子吸水, 在種子萌發(fā)期間, 種阜從土壤吸收水分, 可暫時(shí)貯存, 或?qū)⑵滢D(zhuǎn)移到種子的其余部分[13]。

本研究結(jié)果與Lisci等[14]報(bào)道基本一致, 但種阜維管系統(tǒng)和種阜管道是首次報(bào)道(圖4-F, G, K, L)。蓖麻種阜適于種子成熟時(shí)干燥脫水以及發(fā)育與萌發(fā)過程中吸水的結(jié)構(gòu)可能包括: (1)細(xì)胞壁組分, 種阜主要由細(xì)胞壁果膠增厚的薄壁細(xì)胞構(gòu)成, 細(xì)胞壁組分是果膠、纖維素和半纖維素, 均屬于親水性物質(zhì); (2)紋孔, 種阜細(xì)胞壁紋孔發(fā)達(dá), 有利于胞間水分流動(dòng); (3)維管束, 種阜維管束有利于水分在種阜內(nèi)的長距離輸導(dǎo); (4)種阜管道, 它可能是種子萌發(fā)時(shí), 水分和空氣通過種阜管道的毛細(xì)作用和擴(kuò)散作用進(jìn)出種子的通道。

3.3 維管系統(tǒng)

在種子發(fā)育期間, 水分、無機(jī)鹽和有機(jī)物等物質(zhì)通過種皮維管系統(tǒng), 從果實(shí)胎座被輸送到胚乳和胚, 供胚乳和胚發(fā)育和貯藏; 種子萌發(fā)時(shí), 水分亦可沿此路徑進(jìn)入種子, 使其吸脹而萌發(fā)(圖7-A~C)。在蓖麻種子成熟過程中脫水和萌發(fā)過程中吸水時(shí), 可通過種阜維管系統(tǒng)進(jìn)行較長距離的水分輸導(dǎo)(圖7-D)。

在種子萌發(fā)、形成幼苗的過程中, 子葉作為中介, 也可將胚乳貯藏的營養(yǎng)物質(zhì)通過子葉各級分枝維管束、主脈維管束和胚軸維管束運(yùn)輸?shù)脚叩母鱾€(gè)部分來利用; 子葉變綠能進(jìn)行光合作用、制造有機(jī)物后, 也通過此途徑將營養(yǎng)物質(zhì)輸導(dǎo)至胚根、胚軸和幼苗的其他部分來利用。當(dāng)種子萌發(fā)、胚根具有吸水能力后, 水分和無機(jī)鹽則沿著相反的路徑運(yùn)動(dòng)(圖7-A~C)。

圖7 蓖麻種子維管束分布示意圖

A: 與子葉面垂直的正中縱切面, 示內(nèi)、外種皮維管束的關(guān)系及來源于胚軸的子葉主脈維管束; B: 與子葉面平行的正中縱切面, 示內(nèi)種皮維管束和子葉維管束空間上的分離; C: 與種子長軸垂直的正中橫切面, 示外種皮、內(nèi)種皮和子葉維管束的空間關(guān)系; D: 種阜中部局部橫切面, 示種阜維管束。VC: 子葉主脈維管束; HB: 胚軸維管束; AC: 氣室; BV: 子葉分枝維管束; BC: 種阜維管束。

A: median longitudinal section perpendicular to the plane of cotyledon, showing the relationship between vascular bundles of endosperm and episperm and main vascular bundles of cotyledons originating from hypocotyle; B: median longitudinal section parallel to the plane of cotyledon, showing spatial separation of vascular bundles in endopleura and cotyledon; C: median transverse section perpendicular to the long axis of seed, showing spatial relationship of vascular bundles in episperm, endopleura and cotyledon; D: local transverse section of middle part of caruncle, showing bundles in caruncle. VC: the vascular bundles in the main vein of cotyledons; HB: hypocotyle bundles; AC: air chamber; BV: branched vascular bundles in cotyledons; BC: bundles in the caruncle.

3.4 胚乳與胚

被子植物胚乳是指胚囊中2個(gè)極核受精后發(fā)育形成的三倍體特殊營養(yǎng)結(jié)構(gòu), 貯藏有豐富的營養(yǎng)物質(zhì), 這些營養(yǎng)物質(zhì)在種子萌發(fā)中由胚長成幼苗時(shí)被吸收利用, 或在胚的發(fā)生中立即被吸收消耗[22]。蓖麻成熟種子中, 胚乳細(xì)胞均含豐富的糊粉粒(圖5-A, B; 圖6-B)和圓球體(脂質(zhì)體)[16]。子葉是一種側(cè)生在胚軸上的、片狀或線形的、特殊的吸收器官, 其功能是吸收和轉(zhuǎn)移異養(yǎng)幼苗所需的大量物質(zhì)。本研究中子葉的觀察結(jié)果(圖1-E; 圖5-A; 圖7-A~C)與Kriedemann和Beevers[15]的發(fā)現(xiàn)一致。

從結(jié)構(gòu)上看, 首先, 蓖麻胚乳和子葉與胚芽、胚軸和胚根細(xì)胞中糊粉粒分布具有顯著差異(圖5;圖6), 這一方面反映其功能的不同, 另一方面反映其功能在時(shí)空上是分離的, 即胚乳和子葉在種子發(fā)育和萌發(fā)時(shí)分別承擔(dān)營養(yǎng)貯藏、轉(zhuǎn)化和轉(zhuǎn)運(yùn)功能, 而胚芽、胚軸和胚根則在種子萌發(fā)時(shí)進(jìn)行生長和發(fā)育。其次, 2片子葉遠(yuǎn)軸面分別通過非細(xì)胞結(jié)構(gòu)的粘液層與胚乳大面積相連接(圖5-A, D; 圖6-A), 這擴(kuò)大了子葉和胚乳之間物質(zhì)轉(zhuǎn)運(yùn)的面積, 有助于種子發(fā)育和萌發(fā)時(shí)物質(zhì)的快速轉(zhuǎn)運(yùn)和貯藏。

從功能上看, 蓖麻胚乳與胚密切相關(guān)[15,25]。蔗糖在胚乳中合成, 通過自由空間(粘液層), 與質(zhì)子協(xié)同轉(zhuǎn)運(yùn)(Cotransport of sucrose and protons)至子葉, 被子葉吸收, 成為子葉、胚軸、胚根和嫩枝生長和發(fā)育的碳源, 直到綠化的子葉可以進(jìn)行同化物的生產(chǎn)和輸送[26]。通常從胚乳進(jìn)入子葉的糖分有80%以上被傳遞到胚軸[15]?？梢? 在蓖麻種子萌發(fā)過程中, 糖的異生作用、轉(zhuǎn)運(yùn)和利用在空間上是明顯分離的, 而且是單向運(yùn)輸。

蓖麻種子胚軸和胚根在空間上與胚乳分離(圖5-E, K, I), 這種格局可消除種子萌發(fā)過程中, 胚軸向上伸長和胚根向下扎入土壤時(shí), 胚乳對二者的生長發(fā)育可能造成的阻力, 有利于種子在合適的條件下迅速萌發(fā)。

4 結(jié)論

采用石蠟切片和顯微照相技術(shù)全方位觀察了成熟蓖麻種子種皮(包括外種皮、內(nèi)種皮和種阜)、胚乳和胚的結(jié)構(gòu)特征和維管系統(tǒng)的構(gòu)成, 為全面掌握蓖麻成熟種子的結(jié)構(gòu)和研究種子發(fā)育和萌發(fā)過程中的物質(zhì)運(yùn)輸提供了結(jié)構(gòu)證據(jù)。

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Anatomy and microscopic observation ofseed structure

GUO Xue-Min*, ZHAO Xiao-Man, XU Ke, WANG Xin-Rui, ZHANG Chen-Yu, and DONG-FANG Yang

College of Agronomy and Biotechnology, Heibei Normal University of Science & Technology, Changli 066600, Hebei, China

We systematically observed the structure and distribution of vascular bundles of castor () testa, endosperm and embryo with germination, separation, paraffin sectioning technique, and microscopic observation techniques, stained with safranine and fast green, along with schiff’s reagent, respectively. (1) Micropyle at the dorsal base of the seed was not covered by the caruncle. (2) The testa consisted of three parts: episperm, endopleura and the caruncle, in which the episperm was composed of the long columnar epidermal layer, sponge parenchma and palisade parenchma from outside to inside, while the endopleura was composed of Malpighian layer, sponge parenchma and the inner integument layer, with an air chamber surrounded by the inner layer and outer layer of endopleura at the end of the caruncle, similar to that of eggs. (3) Both the episperm and endopleura were distributed with vascular bundles, and the large ones of the episperm only distributed in the spongy parenchma of the ventral raphe of the seed, which extended from the hilum to the opposite end of the caruncle; while those of endopleura were also distributed in the spongy parenchtma, and they branched step by step through the bundle band, ending at the edge of the air chamber at the end of the caruncle; those of both the episperm and endopleura were continuous through the junction point of the chalazal end to form the vascular system of the testa. (4) The caruncle was composed of epidermis and parenchyma, in which parenchyma consisted of small parenchyma cells on the outside and large parenchyma cells on the inside. In the middle and ventral part of the caruncle, there were only two vascular bundles, which constituted the vascular system of the caruncle independently. In the caruncle, caruncle channel was also found, with the inner end opening in the air chamber of the endopleura and the outer end being blind. (5) No vascular bundles were found in the endosperm, which consisted of parenchymal cells containing a large number of aleurone grains. In the embryo, the vascular bundles were collateral, which differentiated in the hypocotyle and entered the main vein of two cotyledons from both sides of the top of the hypocotyle respectively, gradually reduced from 4 to 1 in the main veins, and the branches became thinner in the cotyledon, forming the vascular system of embryo. The cotyledon epidermis and mesophyll cells contained aleurone grains, but no aleurone grains were found in the plumule, hypocotyle and radicle. Two cotyledons separated from each other on the adaxial surface, while their abaxial surfaces were connected with the endosperm through the mucilaginous layers. The hypocotyle and radicle were spatially separated from the endosperm. These results provide an anatomical basis for comprehensively understanding the structure of the seeds and studying the seed development and germination and their material transportation.

; seed; vascular bundle; anatomical structure

10.3724/SP.J.1006.2020.94141

本研究由河北科技師范學(xué)院博士基金(2013YB021)資助。

This study was supported by the Doctor Foundation of Hebei Normal University of Science & Technology (2013YB021).

郭學(xué)民, E-mail: xueminguo@126.com, Tel: 0335-2037496

2019-09-23;

2020-01-15;

2020-02-17.

URL: http://kns.cnki.net/kcms/detail/11.1809.S.20200214.1746.008.html