Embryology of a Himalayan Glasshouse Plant, Rheum Nobile
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ふか直陥附同成且刈涜恥山か平トいτrvE誌 1yJ JBnra 司 MqJ咽・ーοAUnu崎、】、‘ I EF 一 Embryology of a Himalayan Glasshouse Plant ， Rheum nobile Hook.f. & Thoms. (Polygonaceae) a Y 吋i OMORl and Hideaki OHBA b aYokosuka aYokosuka City Museum ，Fukadadai 95 ，Yokosuka ， 238-0016 JAPAN; bDep 釘 tment of Botany ，University Museum ，University of Tokyo ， Hongo Hongo 7-3-1 ，Bunkyo ，Tokyo ， 113-0033 JAPAN (Received (Received on January 11 ， 2003) The embryologic a1 process of a large perenni a1 Himalayan “glasshouse plan t" Rheum nobile was anatomically an a1 yzed. It s megag 卸 netophyte development was classi- fied fied as PO かgonum type ， embryogenic type as Polygonad type and endosperm formation as as the Nuclear type. These embryological features of R. nobile were almost the same as those those of R. palmatum and some species of Polygonum. The embryological evidences sug- gest gest that this species steadily carries out normal amphimixis and produces a great number of of seeds at the final ye 紅 of its life under severe environmental conditions such as low temperature temperature with much rain ，strong window and intense UV radiation. Key words: alpine plant ，embryology ，glasshouse plant ，Polygonaceae ，Rheum nobile. Rheum nobile Hook .f. & Thoms. is the et al. 1999 ，Omori et al. 2000). l紅 gest perennial herb reaching 1- 1. 5 m in The reproductive morphology or ecology height ，which grows at subtropical alpine of the plants adapted to cool and wet envi- zone of the East Himalayas ， at 4000 -4 500 m ronment as in the East Himalayas has not above the sea leve l. It is called as been performed. Regardless of low tempera- 'glasshouse' 'glasshouse' plant (Ohba 1988) because of ture during growing season and poor insect having having pale yellow translucent bracts cover- fauna as pollinators in the Himalayan alpine ing ing the 1紅 ge compound raceme. In this 紅 ea ， regions ， R. nobile produced many matured we cannot find any large plant except R. fruits. This suggests autogamy and apomixis nobile ， as short growing season ，low tem- might occur in this species. Embryological perature ， strong wind and UV radiation make data are not enough in the alpine plants. But shrubs shrubs and herbs dwarfish. Under such se- alpine species in general have higher ploidy vere vere conditions ， R. nobile grows in 1訂 ge ro- levels than lowland species (Bliss 1971 in sette sette for seven to eight years and produces k りmer 1999) ，which are much correlated to many in fruits large infructescence at the autogamy and apomixis. For making sure of final final year (Masuzawa et al. 1993). the reproduction of this species ，we exam- In In a series of the ecological and morpho- ined the embryology next to the pollen de- logical studies studies logical about R. nobile ，its main habi- velopment (Omori and Ohba 1996). tats ， the photosynthetic rate of the rosulate The embryological studies conceming of leaves ， the anatomical features of the bracts ， Polygonaceae have not done sufficiently and so on have been analyzed (Terashima (Sou とges 1948 ，Comer 1976). Wang et al. et et al. 1993 ，Omori and Ohba 1996 ，Ki kuchi (1992) investigated developmentally only the -145- 146 146 植物研究雑誌第78 巻第3号 平成15 年6月 ovule ovule and macrosporangium of R. palmatum. cut in 7-10 凹n by rot 訂 y microtome. For For clarifying the role of the translucent Counterstaining was by Heidenhain's Hema- bracts bracts of R. nobile and a series of studies we toxylin ，Safranin and Fast green FCF. have have here investigated microclimatic condi- tions tions (Ki kuchi et al. 1999) ， the bract mor 田 Results phology phology (Omori et al. 2000 ，Omori 2002) of We examined 60 ovules of six individuals this this species. As ap 紅 t of these investigations at various stages from egg cell to he 紅 t- we present here the results conceming the shaped embryo. We did not find any abnor- development development of ovule ，embryo and seed coa t. mal embryo sac and embryo in all samples from the anatomical standpoin t. Materials Materials and Methods Megagametophyte and nucellus - The The in f1 0rescence and infructescence of ovule of Rheum nobile is attached to basal Rheum nobile Hook.f. & Thoms. were col- placenta and orthotropous ， and bitegmic lected lected near Banduke ， at 4300 m above sea (Fig. 1: U ， o i). The outer in 旬gument is two- level ， in Ja りale Himal in the east Nepal or three-cells thick and the inner one-two (Ohba 1992) and were stocked in FAA (five cells thic k. The micropyle is formed only by p紅 ts stock form a1 in; five p紅白 glacial acetic the inner integument and the nucellus is acid; acid; 90 parts of 50 % ethanol) solution. crassinucellate (Fig. 3). After After development a1 stages from f1 0ral bud Megaspore mother cell (Fig. 3.: mc) di- to to young fruit were dehydrated by n-butanol- vides transversely at the first meiotic divi- ethanol ethanol series ， they were embedded in sion ， forming a dyad. The embryo sac (Fig. Palaplast Palaplast or Histosec (56-58 OC) and were 4) finally consists of one egg cell (ec) ，two Figs. Figs. 1-2. Gynoecium (L S: longitudinal section) and fruit (LS) of Rheum nobile. nobile. Fig. 1. Gynoecium: ovary wall (ow) ， outer integument (oi) ， inner integument integument (i i) ，nucellus (nu). Scale bar= 100μm. Fig. 2. Fruit ，pericarp (p e) ， seed coat (sc) ，epidermis of nucellus (ep). Scale bar=50μm. June June 2003 Journal of Japanese Botany Vo l. 78 No. 3 147 synergids ，two pol 紅 nuclei (p n) and three thin inner layer. The endotesta withers and antipodal antipodal cells (α c) ，therefore the type of degenerates as the seed ripens. megagametophyte development of R. nobile is is classified as Polygonum type (Willemse Discussion and van Went 1984). Rheum nobile has an orthotropous ， Early Early embryogenesis and endosperm- bitegmic and crassinucellate ovule on the The zygote is located at the micropylar end basal placenta ，and the megaspore mother of of the embryo sac (Fig. 5: zy). The first divi- cell divides transversely at first division. sion sion is transverse ，dividing the zygote into an These morphological features 紅 e almost in apical apical cell ca and a basal cell cb (Figs. 6，16- accordance with the previous description of 1). 1). The apical cell ca divides longitudinally P. palm αtum (Wang et al. 1992) and two times into the quadrant cells q， whereas Polygonaceae (Corner 1976). Corner (1 976) the the basal cell divides transversely ， forming did not decide whether the seed coat type of the the middle cell m and the proximal cell ci. this family was truly exotestal seed or not ， The proximal cell ci divides transversely into because the seed coat formation of the two superposed cells n and n' ， and the cell Polygonaceae had not been studied suffi- n' n' becomes 0 and p cells by transverse divi- ciently. The seed coat type of R. nobile is sion sion (Figs. 7 ，16-2). The quadrant cells q di- supposed to be exotestal seed coat because vide vide transversely into two cell layers 1 and the outer epidermis of the outer integument l' l' and the middle cell m divides longitudi- remains as mechanical layer. The seed coat nally nally (Fig. 16-3). The celllayers 1 and l' di- of R. nobile is morphologically similar to vide vide periclinally and 紅 e formed protoderms that of R. palm αtum (Marek 1958) in having (Figs. (Figs. 16 四 3-5). firm testa with tannin. In the case of The middle cell layer m further divides Polygonum pensylvanicum ， the outer layer of transversely transversely into two to three cell layers nucellus and two integuments s勘ink to be 四 (Figs. (Figs. 16-5-6: m) ，its proximal layer contri- come thin cover of the seed (Neubauer butes butes to the formation of the initials of the 1971). root root cortex (Figs. 16-7-8: iec). The celllayer Rheum nobile agrees with Polygonum n divides transversely into two or three cell persicaria (Sou とges 1948) and P. layers layers (Figs. 16-5-7: n) to form the root cap pensyl να nicum (Neubauer 1971) in its basic (Figs. (Figs. 14 ， 15: ra). Based on the differentia- embryological characteristics and in the de- tion tion of the initials of the root cortex iec ， the viation of the initial cell of root cortex iec. embryogenic embryogenic type of R. nobile is classified as These common embryological characters Polygonad Polygonad type (Yamazaki 1982). The draw a conclusion that R. nobile is also in- procambium is recognized by subsequent cluded in Polygonum variation of Asterad cell cell divisions (Figs. 14 ， 15: pr). type by Johansen (1 950) and Polygonad type After After fertilization ， the primary endosperm by Yamazaki (1 982). nucleus nucleus becomes directly free nuclear (Fig. The fruits of R. nobile 紅 e fully ripened in 8:fn). 8:fn). As the embryo develops ， the cell walls spite of so harsh condition of alpine zone of 紅 e formed from the periphery. Therefore the the Himalayas that it was expected at first endosperm formation of R. nobile is the that the agamospermy rather than normal Nuclear Nuclear type (Vijayaraghavan and Prabhakar sexual reproduction occurred. However ， the 1984). 1984). megaspore formation and embryogeny Seed coat- The seed coat consists of two (embryogenesis) examined here 紅 e normal layers layers of exotesta; a sclerenchymatous and and co 町 espond to the previous reports in the tanniferous tanniferous outer layer (Fig. 2: sc) and very genus (Soueges 1948 ，Wang et al. 1992). 148 148 植物研究雑誌第78 巻第3号 平成15 年6月 June June 2003 Journal of Japanese Botany Vo l.