Bryqniaefolius(Cucurbitaceae)
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The JapaneseSocietyJapanese Society forforPlant Plant Systematics ISSNOOOI-6799 Acta Phytotax. Geobot. 50(1): 59-73(1999) Floral Morphology and Viztsculature of Schizopepon bryQniaefolius(Cucurbitaceae) " 2 TATSUNDO FUKUHARAi and JUNICHI AKIMOTO iGraduate schoot ofHuman and Environmental Studies, K),oto University, K},oto 606-8501 2Centerfor j<),oto University, Kbeoto 606-8502 EcologicalResearch, AbstracL FIoral characters of an annual andredioecious vine Schizopepon bryoniaefoJius are with special reference (Cucurbitaceae-Cucurbitoideae-Schizopeponeae) described ,to the arrangement of stamens and ovary locules, vasculature, nectaries, trichomes and phenology. Both hermaphrodite and ma]e flowers bear two dithecal and one monothecal stamens, as in many taxa of Cucurbitoideae, However, S, bryoniaojbtius differs from them in that each stamen is recognized as a single mound in an early stage of the development and receives a single vascular bundle. The anther trichomes are snowman-ljke and are different in cell shape from those of the other previously examined taxa, There is no er small time-lag between anther dehiscence and stigma maturation in hermaphrodite fiowers, In both of the sexual morphs, there is a nectary at the bottom of the receptacular cup. Key words: androdioecy, Cucurbitaceae, fioral anatomy, Schizopqpon Received February 5, J999, acceptedt"prit IO, 1999 The present study describes floral characters of a cucurbitaceous annual vine Schizopepon bryoniaojbtius Maxim. (Cucurbitaceae-Cucurbitoideae- Schizopeponeae) with special reference to the arrangement of floral components, vasculature, nectaries, stomata and trichomes. Two aims are intended by these observations. The first is to support or refute the current taxonomic position of the genus Schizopepon by floral characters. Recent systems treat Schizopepon either as the only genus of Cucurbitoideae-Schizopeponeae (Jeffrey, 1980, 1990; Lu and Zhang, 1985) or under Cucurbitoideae-Melothrieae-Sicydiinae (Lu and Zhang, 1986). Jeffi:ey (1964) established Schizopeponeae based on the observation that the pollen grain surface of Schizopepon is reticuloid (retipilate, in the sense of Punt et al. [1994] and Hoen [1998]), a unique feature in the family. However, judging from the data of Lu and Zhang (1985), it is not reticuloid but reticulate with spinulose muri and thus the isolated position of the genus is not supported "Present address: c/o Mr. Satake, Ichijoji-Saikata-cho 7, Sakyo-ku, Kyote 606-8157. e-mail: [email protected] Labels for the fioral vascular bundles. DA: dithecal stamen bundle; GD: carpel dorsal bundle (=stigma bundle) ; GV: carpel ventral bundle (=ovary septum bunclle) ; MA: monothecal stamen bundle; PC: central bundle of the corolla lobe; PL: lateral bundle of the corolla lobe; S: ca]yx lobe bund]e, The bund]es that diverge into more than one bundle in the upper level of the flower are designated by linear formulas of the "S+2PV'). labe]ings (e,g,, NII-Electronic Library Service The JapaneseSocietyJapanese Society forforPlant Plant Systematics 60 Acta PhytQtax. Geobot. Vol, 50 by explicit synapomorphies but by the combination of characters (Lu and Zhang, 1985). Schizopqpon and Sicyoeae (=Melothrieae-Sicydiinae) share a solitary pendulous ovule per locule, whereas other floral characters of Schizopepon are not shared by Sicyoeae but by the rest of Cucurbitoideae, namely, the androecial architecture composed of two dithecal stamens and a monothecal ene arranged "2+2+1 in a whorl (referred to as type"), tricolporate pollen grains with reticulate surface and 3-locular ovaries (Lu and Zhang, 1985, 1986). A close study on floral characters might provide infOrmation for the issue. The second aim concems the functional aspects of fiowers in relation to the androdioecious sexual system of Schizopepon bryoniaqfblius. A recent examination revealed that some of the populations of the species are androdioecious (Akimoto et al., 1999; earlier suggested by Makino [1940], Ohwi [1953], Kitamura et al. I1957], and Lu and Zhang [1985]). This is a very rare condition among angiosperms, and there are no more than five suflliciently documented examples (see Ishida and Hiura [1998] and Akimoto et al. I1999] for review). We therefbre examine the aspects of floral characters that may be functionally related to the reproductive ecology, and attempt to compare the characters with those of the few other androdioecious species. Materials and methods Flower sarnples were collected from two localities, Oguchi-mura, Ishikawa pref. and Kiyomi-mura, Gifu pref., both in central Japan. The populations were androdioecious in the former locality but hermaphrodite in the latter. The samples were fixed in FAA (water: ethanol: fbrmalin: acetic aci,d=9: 9: 1: 1 in volume). Paraffin sectioning and the preparation for SEM observation fbllowed conventional procedures. Moral vasculature was described based on the serial cross sections (at 10-15 pm in thickness) of 10 hermaphrodite flowers and five male flowers. Because there was no significant difference between the two localities in the observed characters of the hermaphrodite flowers, the localities are not mentioned in each part of the results. The text fo11ows the tribal and generic definition of Jethey (1990). Results Injlorescence - Hermaphrodite plants At each fertile (flower-bearing) node, the leaf subtends a solitary fiower, a dwarf flowering shoot and a tendri1 (Fig. IA, B). The dwarf shoots have a solitary flower at each node, short intemodes and small leaves, and appear simi1ar to racemes. The solitary flower usually blooms when the dwarf shoot at the node is not yet extended (Fig. IA). - Male plants An ebracteate raceme, a raceme-bearing shoot and a tendril are subtended at each of the fertile nodes (Fig. IC). The raceme-bearing shoots spirally bear ebracteate racemes that are not subtended by a bract but occasionally some nodes in the upper part show branchings with three to five branches and a small subtending fbliage leaf. Thus they may be described as NII-Electronic Library Service The JapaneseSocietyJapanese Society for Plant Systematics August 1999 FUKUHARA & AKIMOTO: Floral anatomy of Schizopepon 61 ole ri[ (primordiuaL} FiG. 1. Semidiagrams illustrating flower-bearing nodes of Schizopepon br[yoniaojiolius. A, B, herrnaphrodite plant; C, Male plant. Arrows indicate the courses of the vines, double-racemes alternatively. Flower - Hermaphrodite flowers The receptacles are green and cup-shaped (Fig. 2B) and the upper part forms a low (O.2-O.5 mm) hypanthium (Fig. 2A), that continues to the calyx and white corolla. Both corolla and calyx have a short (less than O.5 mm) tube and five lobes. Corolla lobes are quincuncially arranged rllhree befbre opening (Fig. 5A). stamens, two dithecal ones and a monothecal one, are inserted at the base of the hypanthium (Fig. 2B). Filaments have round to elliptic cross sections (Fig. 5A, B) and are continuous to the slightly thickened anther connective, which contains a cluster of cytoplasm-rich cells (Fig. 2B). Anther thecae are alternate to the corolla lobes (Fig. 2B) and are attached to the abaxial sides of the connectives (the anthers are extrorse). Each theca has two locules and dehisces by opening an abaxial longitudinal slit (Fig. 2C). The anthers and the stigmas are located close to each other. An inferior 3-locular ovary is present below the receptacle (Fig. 2A, B), and a style protmdes from the center of the receptacular bottom and continues to three stigmas, each of which is further bilobed by an apical cleavage (Fig. 2C, 7A). The base of the style has a narrow (ca. 100 um in width) nectary zone with densely distributed stomata (Fig. 7A, B). In nectary zone, each stoma occupies the apex of mound-like convexities en the surface (Fig. 7B) and the underlying mesophyll cells contain dense cytoplasm befbre the corolla opens (Fig. 2B, arrowhead). Ovaries have triangular cross sections (Fig. 5E). Each of the three sides has a central longitudinal rib or two longitudinal ribs, depending on the number of the vascular bundles (Fig. 5E). The position of floral parts in relation to the axis could not be determined due to the complex architecture of the nodes (Fig. 1A, B). When the flower buds are less than 250 pm in diameter (Fig. 4A-I), calyx lobes (I-V in Fig. 4A-I) enclose the corolla lobes (1-5 in Fig. 4A-I). The future ovary locules are recognized as a trilobed furrow (s, t and u in Fig. 4A-I) and the androecium is composed of three mound-like (when the buds are less than 200 stm) to clavate (200-250 pm, Fig. 4A-I) stamen primordia (a, b and c in Fig. 4A- I), that are not significantly diflierent from each other in size and shape. In these stages, the stamen primordia are placed opposite a calyx lobe (a--I in Fig. 4A-I, respectively) and two corolla lobes (b--3, and c--4 in Fig. 4A-I), respectively. NII-Electronic Library Service The JapaneseSocletyJapanese Society forforPlantSystematlcs Plant Systematics 62 Acta Phytotax. Geobet. vol. 5e 2 ig ,g FiG. 2. Hermaphrodite flowers of Schilopepon besonianjioltus. A. Bucl shoray before opening. The portion of the cerolla surface (arrow) is enlarged in Fig. 7D. B, Longitudinal section of a bud. C. Floral center when bath stamens ancl stigmas are functional, D, Stigma sudece at maturity. E. Close-up of Fig. 2D, showing conical cell surface and germinating pollen grains. Scales = 1 rnm (A, B), 250 "m (C) ; 100 pm (D) ; 1Opm e). NII-Electronic Library Service The JapaneseSocietyJapanese Society forforPlant Plant Systematics / August 1999 FUKUHARA & AklMOIOI"'Flbfaa"tifititomy bf Schizqpepon 63 The future ovary locules alternate with the stamen primordia. wnen the buds are ca. 300 pm in diameter (at microspore mother cells stage), the anthers and the connectives are differentiated from each other. Of 30 opened hermaphrodite flowers, five have stigmas that are not receptive and anthers that are not dehiscent. In these fiowers, the stigmatic lobes (Fig. 5A, B) do not extend yet and have colliculose surfaces on which there is no pollen grain stuck to it.