FLORAL INITIATION AND DEVELOPMENT OF TRICYRTIS HYBRIDA
Seiichi FUKAI
Abstract
Morphological development of inflorescence and floral organs in Tricyrtis hybrida were determined using scanning electron microscopy （SEM）. Inflorescence of Tricyrtis was a heteromorphous compound inflorescence; the main axis was indeterminate and secondary axes were determinate. The apex of the main axis produced primary bracts continuously, but the apex of each axil of the primary bracts produced a secondary bract and terminated in the first flower. The secondary bract subtended an axil that produced a bract and terminated in the second flower. Tertiary and higher order bracts and flowers appeared similarly, leading to the formation of a cincinnus. The flowers consisted of three outer and inner perianths, six stamens, and a pistil, as in other Liliaceae plants. The basal part of outer perianths swelled and developed to spurs in later development.
Key words：cincinnus, floral initiation, Tricyrtis
Introduction
The genus Tricyrtis （Liliaceae） comprises about 20 species distributed throughout East Asia, including 10 species native to Japan. Tricyrtis （toad lilies） are useful as ornamental plants because they display widely various colors and morphological variations in their inflorescence. To date, only a few genotypes of Tricyrtis have been used as cut flowers and pot plants, al-though wild herb fanciers have bred many hybrids in Tricyrtis. They are also useful as materials for a shade garden because most Tricyrtis species are tolerant of moderate shade.
Understanding of floral development in each plant is im-portant basic knowledge of floriculture. Both genetic and en-vironmental control of inflorescence architecture is essential for high-quality cut flower production. Floral initiation and development in Tricyrtis have not been described to date. This study was undertaken to elucidate the morphological development of floral organs of Tricyrtis using scanning elec-tron microscopy （SEM）.
Materials and Methods
Rhizomes of T. hybrida were planted in plastic pots （1.2 L） containing a mixture of sandy soil and manure （3:1） in March. They were grown under shade （45%） in a field. For SEM observations, shoot apices were collected during April-October for three years （2005 2007）. Samples were fixed
immediately with FAA （formalin: acetic acid: 70% ethanol – 5:5:90）. The specimens were dissected under a binocular microscope, dehydrated in an ethanol-acetone series, dried at the critical point, coated with Pt, and observed using SEM （S-2150; Hitachi, Ltd.）.
The sequence of events during floral initiation and develop-ment was defined according to those of Lilium longiflorum（1）
with slight modifications: I, vegetative; II, dome formation; III, floret primordium formation; IV, outer perianth primor-dium formation; V, inner perianth primorprimor-dium formation; VI, stamen primordium formation; VII, pistil primordium forma-tion; VIII, pollen tetrad formaforma-tion; and IX, flowering.
Results and Discussion
Inflorescence initiation and development
Time-course of floral initiation and development of T.
hybrida were identical in both 2006 and 2007 （Fig. 1）. New
shoots that had developed from rhizomes were vegetative until mid-July. The vegetative shoot apex was flat with trian-gular leaf primordia that surrounded the apex with alternate phyllotaxy （Fig. 2A）. The first sign of inflorescence initia-tion, dome formation （Fig. 2B）, occurred in mid-July. Inflo-rescence initiation occurred earlier in thicker stems. Then an axil swell （Fig. 2C） developed into a secondary inflorescence, leading to the formation of a mixed inflorescence （Figs. 2E, 2F）. Once flower initiation occurred, the phyllotaxy changed
Tech. Bull. Fac. Agr. Kagawa Univ., Vol. 61, 2009
to spiral. The main stem apex continued to produce some leaf primordia （primary bracts）. The apex of the axil produced a secondary bract and terminated in the first flower （Fig. 2D）. The secondary bract subtended an axil that produced a bract and terminated in the second flower. Tertiary and higher order bracts and flowers appeared similarly, leading to formation of a cincinnus （Fig. 2）. The cincinnus （scorpioid cyme） shows a cylindrical inflorescence with axes on different planes, re-sulting in branching alternately to one side and the other. After 2-4 secondary inflorescences were produced, the apex of the main stem stopped to produce leaves.
Those results demonstrate that T. hybrida has a hetero-morphous compound inflorescence. The inflorescence is des-ignated as thyrse: the main axis is indeterminate （raceme） and the secondary axis is determinate （cyme）, according to Hickey and King（2）. Inflorescence with the same structure is
visible in Zingiberaceae species; Curcuma（3）
, Hedyciu（4）
and
Scaphochlamys（5）
. In Liliaceae, various types of inflorescence are observed. Lilium longiflorum（6）
and a branching type of tu-lip（7）
produce a raceme （in wide definition） in which each axil produces a bract and terminates in a flower. A typical raceme is observed in Gloriosa（8）
; bostryx in Hemerocallis（9）
, and in
some modified umbels in Allium（10）
.
In early October, blooming started. The upper 2–3 cincinni developed faster and became larger, and cincinni at the lower position of the main stem developed to become somewhat smaller. Some upper cincinni appeared at the opposite site of the main bract （leaf） on the main stem （Fig. 3）. In this case, a lower part of the cincinnus stem fuses to the main stem, resulting in non-axillary branching, as shown in some Lilium species.
Flower development
The swollen apex of the axil became round and the sec-ondary bract appeared （Fig. 4A）. The apex sequentially produced primordia of three outer perianth （Fig. 4B）, three inner perianth （Fig. 4C）, and six stamens （Fig. 4D）. The outer perianth became broad, whereas inner perianths had a clear midrib on the abaxial side （Figs. 4E, 4F）. This struc-ture closely resembles that of Lilium longiflorum（6）
and
Glori-osa superba（8）
but not tulip（7）
. A primodium of pistil appeared at the center of the flower when both outer and inner perianth primordia grew upward and covered other floral organs （Fig. 4F）. The pistil primordium developed into a triangle shape
I II III IV V VI VII VIII IX
30.Jun ●●●●● ○○○○○ 10.Jul ●●●●● ○○○○○ 20.Jul ● ○ ●● ○○○○ ●● 30.Jul ● ●● ○○○○ ● ○ ● 10.Aug ● ○ ○○○ ● ● ○ ●●● ○ 20.Aug ○ ● ○○ ●●●●●○○○ 30.Aug ●●●● ●● 10.Sep ●●●●● ○○○○○ 20.Sep ●●●●● ○○○○○ 10.Oct ●●●●● ○○○○○ Fig. 1 Floral development of Tricyrtis hybrida.
I, vegetative; II, dome formation; III, floret primordium formation, IV, outer perianth primordium formation; V, inner perianth primordium formation; VI, stamen primordium formation ; VII, pistil primordium formation, VIII, pollen tetrad formation, IX, flowering.
●;2006,○;2007. 22
Fig. 2 Inflorescence initiation and development of T. hybrida. A, Vegetative shoot apex. Bar = 100 µm.
B, Dome formation. Bar = 100 µm.
C, Arrow indicates swollen axillary apex. Bar = 200 µm.
D, Each axillary apex produced a secondary bract and terminated in the flower. Bar = 100 µm, Arrows indicate secondary bracts.
E, Each axil developed to a secondary inflorescence. 1, 2 and 3 respectively signify the first, second and third secondary inflorescence. Bar = 100 µm.
F, Developed inflorescence. Perianths became hairy. Bar = 1 mm.
of T. hirta x T. formosana. Because the plants show intermedi-ate morphological characteristics in shoot and inflorescence, the plant is completely sterile. Artificial breeding between the two species is known to produce sterile plants（11）.
Two distinctly different types of inflorescence structure are known in the genus Trycyrtis: upright or pendulous flowers. It is assumed that the pendulous flower types are derived from （Fig. 4G） and grew upward. Finally, the center space of the
pistil primordium closed and the top of it developed into a tri-fid stigma. Each stigma branch became bitri-fid at the distal end. Tricyrtis had a calcarate corolla （Fig. 4H）. The basal parts of three outer perianths swelled and developed into short spurs. Tricyrtis hybrida used for this study is circulated as T. hirta in the commercial market: the plant is assumed to be a hybrid
Tech. Bull. Fac. Agr. Kagawa Univ., Vol. 61, 2009
upright types.（12）
The specific inflorescence structure described here is not clear in pendulous flower type species because the species with pendulous flowers have only one or two flowers in each axil,.
Acknowledgments
The authors thank Dr. Masaru Nakano of Niigata University for providing some plant materials.
References
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(Received October 31, 2008)
Fig. 3 Top part of T. hybrida inflorescence. Arrows indicate fused parts of the stem. 24
Fig. 4 Development of flower of T. hybrida. A, Swollen apex of the axil. Bar = 100 µm. B, Outer perianth primordia appeared. Bar = 100 µm. C, Inner perianth primordia appeared. Bar = 100 µm.
D, Six stamen primordia appeared inside perianth primordia. Bar = 100 µm. E, Outer and inner perianth primordia developed into different shapes. Bar = 100 µm. F, Pistil primordia appeared as triangle shapes at the flower center. Bar = 100 µm. G, Pistil primordia grew upward. Bar = 200 µm.
H, Blooming flower. Bar = 1 cm.
Tech. Bull. Fac. Agr. Kagawa Univ., Vol. 61, 2009 ホトトギス（Trycyrtis hybrida）の花芽分化・発達過程を走査型電子顕微鏡で明らかにした．ホトトギスの花序は，主 軸は無限成長型で各腋芽は有限花序型に発達する異型複合花序であった．花序の形成は，苞葉の腋芽の肥大に始まり， その後も主軸の茎頂は苞葉を分化し続けた．花序主軸の各腋芽の茎頂は，一枚の二次苞葉を形成して頂部が花になっ た．その二次苞葉の腋芽は再び一枚の苞葉を形成して頂部が花になり，その後も同様の分化方式をつづけて，３−５個 の花からなるサソリ型花序となった．各花は３つの外花被，内花被，６つの雄ずいと１つの雌ずいからなり，花被の下 部は短い距を発達させた．
