植物研究雑誌 J. J. Jpn. Bo t. 79: 79: 326-333(2004)

Possible Possible Role of the Nectar-Guide-like Mark in Explosion in in Desmodium paniculatum (L.) DC. (Leguminosae)

Hiroshi Hiroshi TAKAHASHI

Departrilent Departrilent of Biology ，Faculty of Education ，Gifu University ，Gifu ， 501-1193 JAPAN (Received (Received on January 21 ， 2004)

Desmodium paniculatum ， which is from North America and is naturalized widely in Japan ，exhibits characteristics typical to explosive f1 owers ，i. e. ， has no retum to original position position in the wings and keel ，spreads a cloud at flower explosion ，rarely has re- visits visits by ， and is nectarless. The explosion is induced by bee-proboscis inser- tion into into tion the opening between the standard- and the wing-base ， with no force other needed. needed. The f1 0wer possesses marked spots in the basal part of the standard ，that 訂 e very similar similar to the nect 紅 guides common in the nectariferous f1 0wers of Papilionoideae.

However ，they 紅 e not guide marks to introduce bees to reward objects ， because it does

not not have any reward in the basal p紅t. They appe 征 to function as a guide mark to help bees bees make the f1 0wer explode. Bees can obtain the pollen reward only after insertion of their their proboscis into the opening under the mark.

Key words: Desmodium pαniculatum ，explosive f1 ower ，Leguminosae ，nectar guide ，pol- len len guide.

Flowers that provide their pollinators nec- was referred to as a tongue-guide by tar tar as a reward ，especially those with hidden Westerkamp (1997). During this process the

nectaries nectaries inside ， usually possess nectar anthers and the stigma in the wing 同 keel com- guides guides (Sprengel 1793). Pollinators can eas- plex come out to touch the bee abdomen. ily ily approach the nectaries by their help. In The pollen grains dusting the abdomen may Papilionoideae Papilionoideae of the family Leguminosae ， be a secondary reward for the pollinators. most nectariferous flowers have nectar Legume species with typical explosive guides guides in the basal part of the standard. Their flowers present a pollination mechanism a nectaηis at the base of the ovary and little different from most species with non- can take nectar through holes in the base of explosive flowers. The stamens and pistil in the the filaments with their proboscis (Faegri the flowers of the genera such as Medicago ， and van der P討1 1971). The nect 的 is invisi- Genista and Cytisus are rapidl y released ble ble because it is covered with the basal p紅白 from the wing-keel complex by a visit of of of . Bees ， the most common pollina- their pollinators ，and never come back into tors ， land on the wing-keel complex and in- the kee l. After explosion the flowers are sel- sert sert their proboseis into the opening under dom visited again by the pollinators (Kugler the the nectar guide. The opening ，which is be- 1955 ， Faegri and van der P討1 1971 ， Proctor tween the standard- and the wing-base ，intro- and Yeo 1973 ，Lopez et al. 1999). Pollen is duces duces the proboscis of nectar-feeding insects almost the only reward for the pollinators in into into the basal holes of the filament-tube ，and these flowers (Kugler 1955 ， Faegri and van

-326- October October 2004 Journal of Japanese Vo l. 79 No. 5 327 der der P討1 1971) ， although Mdeicago spp. se- Macro EF 100 mm lens and a Kenko UV crete crete much nect 訂 (Free 1993 ， Perez-Banon 360 filter) under artificial UV ligh t. The et et al. 2003) and Galloni and Cristofolini color photographs were changed to mono- (2003) (2003) detected glucose at the base of the chrome with a computer using Adobe standard standard in Cystisophyllum and Spartium and Photoshop 6.0. These monochrome photo- at at the filament-tube in Genista. Westerkamp graphs are quite similar to ones made from

(1 997) pointed out that the tongue-guide ex 田 monochrome-film (Takahashi unpublished). isted isted even in the papilionate f1 0wers that did Glucose reaction was examined in various not not provide nect 訂， and that bees were con- parts of the f1 0wer with glucose-test paper strained strained into a position suitable for effecting (UROPIECE S made by Toyo Roshi Kaisha pollination. pollination. However ， there are few studies Ltd.) to check nectar exudation. The test addressing addressing how the pollinators know where paper was cut into small stripes (about 1 mm the the anthers ，which 訂 e hidden in the keel- in width) if needed. The paper was softly wing complex ，訂e located and how they ob- pressed against each testable part of the tain tain pollen in the f1 0wers providing pollen as f1 0wer to avoid injuring the tissues. the the main reward. Voucher specimens have been deposited Many species of the genus Desmodium in the herbarium of Gifu University (GIFU). have have explosive f1 0wers (Faegri and van der P討1 1971 ， Ohashi et al. 1981) ， and their ex- Results plosion plosion is .caused by some force additional Flower morphology and blooming move- exerted exerted from outside by the ment (Faegri (Faegri and van der P詰1 1971). In The calyx is short; the tube is about 1.5 Desmodium ，however ， there are no reports mm long ， the lowermost lobe about 2 mm on what is the most important force to cause long ， the other lobes about 1 mm long. The explosion. explosion. The f1 0wers of Desmodium standard extends a short distance forward at paniculatum paniculatum (L.) DC ，a species from North the base and then curves upw 紅 d at a right America and naturalized widely in Japan ，訂e angle or greater (Fig. 1). The f1 ag part is 6- exploded exploded by a bee- visi t. They have marked 7 mm long and 6-8 mm wide. Two marked spots spots like. a nectar guide in the basal area of green-spots 紅 e found at the standard base the the standard ， although no nectar appe 征 s to (Fig. 2). The wing-keel complex is 6-8 mm

be produced. 1 will show the elegant explo 四 long in the non-explored f1 ower. The sion sion of a D. paniculatum f1 0wer by insertion androecium is diadelphous ， but the filaments of of bee proboscis into the opening ， and dis- make a column without any opening between cuss cuss the possible role of the standard spots in the free stamen and the nine fusing stamens the the explosion. in the base (Figs. 5 and 6). The in pistil the column exposes only the distal p紅 t of the Materials Materials and Methods style ，which curves weakly upw 紅 d (Fig. 5). Field Field investigations were made in the There are no nectaries in the f1 ower. plant plant populations of Yanagido ，Sano and Desmodium paniculatum blooms from late Hinakura Hinakura in Gifu City. August to early October. The f1 0wers open Anthers Anthers were counted in the f1 0wers that non-simultaneously in the moming (7-10 had just had a bee visitation and in ones that am). Intact f1 0wers that have not been visited were artificially explored with a needle. by insects extend the wing-keel complex at a UV absorption of the corolla was exam- right angle or greater to the standard (Fig. 3). ined ined through photographs taken by a digital The anthers and stigma 紅 e situated within camera (Canon EOS 10D with a Canon the kee l. After explosion ， the wings and keel 328 328 植物研究雑誌第79 巻第5号 平成16 年 10 月

Figs. Figs. 1 -6. The flower of Desmodium paniculatum. 1: front view of the pre-explored flower. 2: the spots in the lower lower p訂 t of the standard. 3: side view of the pre-explored flowe r. 4: side view of the explored flowe r. 5: side side view of the reproductive column comprising the diadelphous androecium and the pisti l. 6: upside view of of the basal part of the column ，which shows no opening between one free stamen and nine fusing stamens. Bar=2 mm. October October 2004 Journal of Japanese Botany Vo l. 79 No. 5 329 curve curve down at a horizontal angle to the stan- Pollinators and their behavior dard ， but the column ，consisting of the Honeybees (Apis mellifera L.) and solitary stamens stamens and pistil ， stays in the same place ， bees (Nomia punctulata Dalla Torre and resulting resulting in exposure of the whole column Megachile spissula Cockerell) visited the (Fig. (Fig. 4). The down ward curving movement flowers and were effective pollinators. The of of the wings and keel appe 紅 to be due to honeybees approached the flower frontally to recurving recurving in the basal p訂 t of the keel petals. touch the spots in the standard base with the Most pollen grains in the anthers ，which antennae and landed on the wing-keel com- have have already dehisced ， 征 e spread during plex (Fig. 7). Body weight did not cause their their enforced appe 訂 ance through the adher- flower explosion ， probably because the ing ing upper-edge of the keel ， and some anthers peduncles and also the branches with flowers may be pulled off from the filaments (Fig . were thin and flexible (Fig. 8). Explosion oc- 4). 4). The stigmas were apparently receptive curred just after the honeybees pushed their when they appeared. The flowers ， even if not proboscis into the opening between the stan- explored ， began wilting around 3 pm. dard and the wings (Fig. 9). Flower explo-

Figs. Figs. 7-10 . The bees of Apis mell 俳ra visiting the flowers of Desmodium paniculatum. 7: the bee is landing on the wing-keel complex. 8: the bee inserts its proboscis into the opening under the spots in the standard. 9. 9. the flower explored and the anthers and stigma touch the underside of the abdomen . 10: the bee is glooming glooming while grasping the filament 加 be. Bar = 1 cm. 330 植物研究雑誌第79 巻第5号 平成16 年10 月 sion sion causes a forced appearance of the contact and a few ones pull off by the keel in anthers anthers from the keel inside to sprinkle the the explosion. In the artificially explored dry dry pollen grains onto the underside of bee flowers ， 8.9 of the anthers were pulled off on abdomen ， which touches the emergent an- average (N = 35 ，SD = 1.3 ， range 5-10). thers thers and stigma at the same time. Next ， the bees bees usually grasped the stamens and some- UV absorption in the flower times times also the keel and wings and groomed The spots on the standard base ， each of the the body with the mid- and hind-legs to send which is composed of an upper light-green pollen pollen onto the pollen baskets. This process part and an under grayish-white p紅 t，com- was completed in a short time ， and the bees pletely absorbed UV ligh t. The other area of left left the flower usually within a few seconds the standard and wings ， which are pink or after after landing on it. Honeybees rarely landed bluish pink ，also strongly absorbed (about 80 on the explored flowers even though they did %) UV light (Figs. 13-16). These results approach approach them. show that the spots in the flower should be The solitary bees the visited flowers in al- conspicuous for bees. most most the same way as the honeybees did ， but they they sometimes visited the explored flowers ， Glucose reaction in the flower although although they soon left them without insert- No glucose reactions were detected in ing ing proboscis into the flowers. The bees usu- every testable intact-surface of the flower; ally ally groomed in places other than on the the calyx ，corolla filaments and pistil ， while flowers. flowers. Syrphid flies sometimes visited the glucose was clearly detected in almost all ar-

explored explored flowers to lick the anthers ， if pre 目 tificially. i吋ured parts in the concentration of sented ， and the stigmas. Although the flies about 1 mg/m l. It was impossible to intactly sometimes sometimes the visited non-explored flowers examine the ovary base because the filament and and appe 紅 ed to look for something in the tube tightly covered it. spotted spotted area ， they failed to obtain pollen. Discussion Discussion Artificial Artificial explosion of the flower Explosion in the Desmodium paniculatum Explosion Explosion of the flowers was artificially flower is elegan t. In the papilionate flowers caused caused by insertion of a needle into the open- of Leguminosae ，some outside fource by pol- ing ing between the standard and the wings linators is needed to expose the stamens or

(Figs. (Figs. 11 ， 12). The insertion appeared to pollen and the stigma ，which 紅 e concealed bring bring about the explosion by the following in the keel (Faegri and van der Pり1 1971). process: process: the needle tip presses the column The most important force is the downward downwards in the inside of the opening and press from the legs of pollinators ， which is the the downward movement of the column principally the same in the explosive flowers presses presses the basal part of the keel petals ， (Faegri and van der 町1 1971 ，Westercamp which which are tightly connected to the column. 1997 ， Galloni and Cristofolini 2003) al- This This pressure causes a strong recurvature of though the stamens and style do not come the the keel base. back into the kee l. In D. paniculatum ，how- ever ，a needle-insertion into the opening be- Anther number in the explored flowers tween the standard- and the wing-base easily Anthers Anthers that remained in the flowers just causes explosion; showing that bee- after after a bee-visit that led to an explosion ，av- proboscis insertion is enough to explode the eraged eraged only 1.2 in number (N = 33 ，SD = flower without any other force. The explo- 1.8 ， range 0-7). Most anthers fell off by bee sion occurs almost simultaneously with in- October October 2004 Journal of Japanese Botany Vo l. 79 No. 5 331

Figs. Figs. 11-16. Artificial explosion with a needle in the flower of Desmodium paniculatum (Figs. 11 ，12) ， and UV absorption absorption of the flower (Figs. 13-16). 11: a needle is inserted into the opening between the standard- and the the wing- base. 12: the flower is explored and a cloud ofpollen is spread. 13 and 14: photographs under vis- ualligh t. 15 and 16: photographs under UV ligh t. Bar = 0.5 mm.

sertion. sertion. tion in the wings and keel ， spread of pollen DeSniodium paniculatum possesses many cloud at flower explosion ， rare re-visit of of of the features that have been described in pollinators ， and lack of nectar. The anthers explosive explosive flowers; no retum to original posi- fall out easily ， and only a few anthers were 332 植物研究雑誌第79 巻第5号 平成16 年10 月 left left on the filaments after one bee- visi t. guide and do not directly indicate pollen- Pollen Pollen seems to be the only reward for polli- existing places. They may be called an “ex- nators. nators. Although it had been generally be- plosion guide." It is hypothesized that the ex- lieved lieved that explosive f1 0wers produced no or plosion guide is homologous with a “true" little little nect 紅 (Kugler 1955 ，Faegri and van der nectar guide ， and that D. paniculatum was 町1 1971) ，Medicago sativa (Free 1970) and derived from nectariferous species because it M. citrine (Perez-Banon et al. 2003) secrete has diadelphous stamens ， which are usually much nectar. Galloni and Crisatofolini found in nectariferous species of Papilio- (2003) (2003) showed that Spartium junceum ， noideae. Nonetheless ， the pollinators behave Genista Genista cilentina ， and G. radiata produced as if they know well how to obtain the pol- nectar nectar at the base of the standard or on the len. The pollen-collecting bees may learn reproductive reproductive column. They observed some how to pollen obtain first by the experience nectar-feeding nectar-feeding insects ， such as long-tongued of inserting their proboscis into the tube to Anthophora ， and male solitary bees on their seek nectar. f1 owers. Insertion of bee-proboscis into the opening opening between the standard and the wings 1 would like to greatly thank D r. H. in in the Desmodium paniculatum f1 0wer is ap- Ohashi for his kind suggestions about leg- parently parently similar to the nectar-seeking behav- ume f1 owers. ior ior in many legume species. The bees ， however ， did not appe 紅 to target nectar ，be- References cause cause they quickly pulled their proboscis out B 訂 th F. G. 1985. Insects and Fl owers; the Biology of and and left the f1 0wers in a short time ， which a Partnership. Princeton University Press ， was enough for taking pollen. Nevertheless ， Princeton. Faegri Faegri K. and van der P訪1 L. 1971. The Principle of the the bees ， which tend to f1 y out from Pollination Pollination Ecology ，2nd rev. ed. Pergamon Press ， rewardless rewardless populations ， continued to visit Oxford. many f1 0wers in the same population of D. Free J. B. 1970. Pollination of Crops. Academic paniculatum. paniculatum. Press ， London. The marked spots in the standard of the D. Galloni M. and Cristofolini G. 2003. Fl oral rewards paniculatum paniculatum f1 0wer are very similar to the and pollination in Cytiseae (Fabaceae). Plant Sys t. Evo l. 238: 127-137. nectar nectar guides that are common in nectari 目 Kevan P. K. 1983. Floral colors through the insect eye: ferous ferous f1 0wers of Papilionoideae ， although what they are and what they mean. In: Jones C. E. many of them are lines that converge toward and Little R. J. (eds.) ，Handbook of Experimental the the entrance to the f1 0wer (Kevan 1983). In Pollination Biology. pp. J-30. Scientific and Academic Editions ，New Y ork. D. D. panuculatum ，however ， the spots appe ぽ to to function as a guide mark leading to f1 0wer Kugler H. 1955. Einfeurug in die Bl utenokologie (Japanese (Japanese version translated by H. Nakano). explosion. explosion. The bees always inserted their Hirokawa-shoten ， Tokyo. proboscis proboscis into the opening under the spots on Lopez J.， Rodriguez-Riano T. ，Ortega-Olivencia A.， the the nectarless f1 owers. Barth (1985) showed Devesa J. A. and Ruiz T. 1999. Pollination mecha- some examples of pollen guides that intro- nisms and pollen-ovule ratios in some Genisteae duce duce visitors to the pollen location in anther- (Fabaceae) from Southwerstern Europe. Plan t. Sys t. Evo l. 216: 23 -4 7. hidden hidden f1 owers. Most of them are imitations Ohashi H. ，Polhill ， R. M. and Schubert ， B. G. 198 1. of of anthers or pollen ，contrasting to nectar Tribe 9. Desmoideae (Benth.) Hutch. In: Polhill ， R. guides guides in which their designs appear to be M. and Raven ， P. H. (eds.) ，Legume Systematics ， unrelated unrelated to any food source. The spots of 292-300. Royal Botanic Gardens ，Kew. Desmodium pα niculatum are quite unique Perez-Banon c.， Juan A.， Petanidou T.， Marcos-Garcia because because they are very similar to a nectar M. A. and Crespo M. B. 2003. The reproductive October October 2004 Journal of Japanese Botany Vo l. 79 No. 5 333

ecology ecology of Medicago citrine (Font Quer) Greuter in the structure and fertilization of flowers (English (Leguminosae): (Leguminosae): a bee-pollinated plant in Medi- version translated by P. Haase). In: Ll oyd D. G. teηanean islands where bees are absen t. Plant Sys t. and Barrett S. C. (eds よFl oral Biology ， pp. 3-4 3. Evo l. 241: 29 -4 6. Chapman and Hall ，New Y or k. Proctor Proctor M. and Yeo P. 1973. The Pollination of Westerkamp C. 1997. Keel blossoms: Bee flowers with Flowers. Flowers. Collins ， London. adaptations against bees. Flora 192: 125- l3 2. Sprengel Sprengel C. K. 1793. Discovery of the secret of nature

高橋 弘:アレチヌスビトハギの爆裂花における 蜜標類似マークの役割 アレチヌスピトハギは北アメリカ原産で，日本 とは言えない.訪れたハナパチは，吸蜜を目的と の広い地域に帰化している.この花は典型的な してこの標識の下の隙聞に口吻を挿入しているよ “爆裂花"で，一度爆裂すると翼弁と舟弁は下方 うには見えない.ハナパチ，特にミツバチは，花 へ降りたまま元に戻らず，爆裂時に花粉を雲状に 蜜を求めて訪花する場合 報酬物が少ない集団で まき散らし，爆裂後は送粉者であるハナバチはほ いつまでも探査を続けることはない.ハチは，口 とんど訪れず，また花蜜を出さない.爆裂はハナ 吻を挿入して花が爆裂すると，直ちに口吻を引き バチによる翼弁-舟弁複合体の押し下げによるの 抜いて花粉収集に熱中する.すなわち，標識の下 ではなく，口吻を旗弁と翼弁の基部の隙間に挿入 の隙聞に口吻を挿入すると花が爆裂して花粉が得 することにより引き起こされる.他の力を要しな られることを知っているかのように振る舞うので いことは，針の挿入だけで爆裂させ得ることによ ある.しかし，アレチヌスピトハギは 2 体雄蕊を り明らかである.旗弁の基部にはマメ亜科の多く 持つことから，花蜜を生産する群から進化した可 の種に見られる蜜標に似た斑紋がある. しかし， 能性が高く，爆裂誘導指標は蜜標から由来したも アレチヌスピトハギは蜜を分泌しないので「蜜標j のと考えられる(岐阜大学教育学部)