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ISSN OOOI-6799 Acta Phytotax. Geobot. 46 (1): 55-65 (1995)
The Enantiostyly and the Pollination Biology of
Monochoria korsakowii (Pontederiaceae)
GUANGXI WANG, REllCHI MIURA and TOKUICHI KUSANAGI
fuculty ofAgriculture, KYoto Uletiversity, Kyoto 606-Ol
Abstract. The flower morphology and pollination biology of Monochoria korsakowii were studied in natural and transplantcd populations. The plant has somatic enantiostyly, i.e., each plant bears two rnorphs ef flowers, left- and right-handed fiowers, with the style dcflcc- tion to the left and right, rcspectively. The enantiostyly is accompanied by a stamen
dimorphism; cach llowcr has ilve srnall and one largc stamens, the large anther and the stig- ma are symmetrieal with respect to the median plane of the flower. Thc major pollinaters observed were Apis cerana iaponica, Kylocopa circumvolans and Bombus spp. The large anthcr ef a fiower morph touches the same position of bee's abdornen as the style of the other morph, thereby apparently facilitating hitermorph pollination. However, since the enantiestyly is somatic, this seems to lead to cross-pollination only when flowers of only onc
morph, by chance, are open on an individual. 1[he fiower was found to be sclf-compatible and capable of setting seed even in thc absence of pollinators, It was the small stamcn that contributed the autogamy.
Key words: aquatic annual, buzz-pollination, enantiosty]y, geitonogarny, Moreochoria korsakeveii
Received February J, 1995y accepted March 2S, 1995
Heterostyly is a flower polymorphism in which the heights of the stig- ma and the anther differ reciprocally among flower morphs. This condition is regarded as a type of reciprocal herkogamy (Webb and Lloyd, 1986; Lloyd and Webb, 1992). Reciprocal herkogamy can be realized by an alternative way: narnely enantiostyly, the left- and right-hand polymor- phism (Webb and Lloyd, 1986). For example, in the left-handed flower of Solanum rostratum, the style is declined to the left, and one of the anthers, much larger than the rest, is declined to the right; vice versa in the right- handed flower (Bowers, 1975). Although much rarer than heterostyly, enantiostyly has long been known and reported from at least 14 genera belonging to 9 families (Mul- ler, 1883; Ptiry Shaw, 1973; Bowers, 1975; Vogel, 1978; Ornduff and Dulberger, 1978; Dulberger and Ornduff, 1980; Dulberger, 1981; Webb and Lloyd, 1986). They include Cassia (Caesalpiniaceae), Cochliostema (Commelinaceae), Exacum (Gentianaceae), Sainrpaulia (Gesneriaceae), Barberetta, Dilatris and Wachendoi:ICia (Haemodoraceae), Anthericum, Chlorqphytum and Clyanella (Liliaceae), Allionia (Nyctaginaceae), Heteran- thera and Monochoria (Pontederiaceae), and Solanum (Solanaceae). One
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interesting feature of enantiostyly is that it is usually a somatic polymor- i.e., phism, each individual produces flowers of both morphs (an exception is Wachendoijia (Ornduff and Dulberger, 1978)). So far, only several attempts have been made to elucidate the pollination biology of the enan- tiostylous flowers. In the genus Monochoria, M. korsakowii Regel et Maack, and some other congeners, are known to be enantiostylous (Iyengar, 1932; Ecken- walder and Barrett, 1986) but little is known about their pollination biolo- gy. M. korsakowii is an annual, emergent aquatic plant occurring in pools, ditches, canals and rice-fields of eastern Asia (Cook, 1989; Kadono, 1994). Ip Japan, it is now regarded as one of the endangered species because con- tmuous usc of herbicides and exploitation of wetlands eradicated this spe- cies from many of its habitats (Japan Society of Plant Taxonomists, 1993; Kadono, 1994; Wang and Kusanagi, 1994). In this paper we report the re- sults of some observations and experiments on the floral biology of M. konsakowii.
Materials and Methods
One of the authors (Wang) made a preliminary study on flower mor- phology in natural populations in China (Heilongjiang, Jilin and Liaoning) from 1990 to 1991. Additional observations were made in natural and transplanted populations in Japan from 1992 to 1994. The natural popula- tion situated in a shallow-standing water of an abandoned rice field in Mikata, Fukui Prefecture. The site was surrounded by marshes and ponds, and was rich in aquatic flora. The transplanted population, about 2 × 10 m, was established frorn 1992 to 1994 at the Experimental Farm of Kyoto University, Kyoto Prefecture, by sowing seeds every spring. The distributions of Ieft- and right-handed flowers within inflores- cences and individuals were recorded for the Mikata population. Opening date of flower buds and withered flowers could be judged from their appearance within the range of two days before or after the opening; one- day census thus covered five days of flowering. Flower visitors were col- lected at both Kyoto and Mikata populations. The flower is basically zygo- morphic and has two different types of stamens at upper (adaxial) and low- er (abaxial) positions. To investigate the function of this structure, insect behavior was observed in manipulated flowers as fo11ows: 1) One of the two types of stamens was removed, 2) The whole infiorescence was held upside down to keep flower parts at reverse positions. To determine the pollen fertility and self-compatibility, inflorescences of the transplanted population were bagged, emasculated and hand-polli- nated. Cross- and self-pollinations were performed using each of the two types of stamens separately. Fruit set and number of seeds per fruit were examined. In addition, pollen germination on stigma and subsequent pol- len tube growth were observed following the method of Scribailo and Bar- rett (1991). Pistils were collected at 20-minute intervals after hand-pollina- tion and fixed in FAA. After clearing of the tissue and staining with ani- line blue, the pistil was observed under fluorescence microscopy.
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To determine the mode of self-pollination, bagging and emasculation experiments were conducted using the transplanted population: 1) the large stamen was removed before flower opening; 2) the srnall stamens were removed before flower opening; 3) All of the stamens were removed before flower openjng; 4) All of the stameRs were removed just before flower closing; 5) The flower was bagged but not emasculated (control). To avoid mechanical influence of bagging, each inflorescence was held in a box made of sulfated paper instead of a paper envelope, so that the wall of the box did not touch the flower parts.
Results
Phenology and flower moilphotogy Monochoria korsakowii behaved as a.summer annual both in the natu- ral (Mikata) and the transplanted (Kyoto) populations. The seedling emerged in April and fiowered from late August to mid-October with a peak around mid September. When observed in Kyoto on September 14, flower opening began at 7:30 and was completed at 8:OO. The flower began to wither at 16:30 and it completely closed at 17:15. Each flower opened only one day. No noticeable differences in fiower morphology were found between plants of Monochoria korsakoveii of China and Japan. Description below is based on the Japanese materials. The flower is 2.0-2.8 cm in diameter, bowl-shaped, weakly zygomor- phic, and horizontally-oriented at opening (Fig. 1), The purplish blue perianth consists of six segments, each 1.4-2.2 cm long. Among the seg- ments, the three exterior are 0.5-O.7 cm wide and thc three interior are broader, 0.8-1.2 cm wide. Each flower has two types of stamens (heteran- thery): an upper (adaxial) bunch of five small stamens with yellow anthers, and a lower (abaxial) large one with a purplish-blue anther (Figs. 1 and 2), The filament of the small and large stamens are 3-6 mm and 4-6.5 mm long, respectively. The anther length of the small and the large stamens are 2.0-3.5 rnm and 4-5 mm, respectively. The filaments of the small sta- mens are simple, while that of the large one has a trigger-like appendage emerging parallel with the filaments of the small stamens (Fig. 2). Anthers of both types have two longitudinal slits, but have an poricidal appearance because the slits open widely only at the apex of the anther. The flower is hypogynous and the pistil is approximately as long as the large stamen. The flower is odorless and does not produce nectar, The flower is enantiostylous: each individual produces two morphs of flowers with the stylc curving either to down left or down right (as viewed from the plant; Figs. 1-3). These morphs are referred to as the left-hand- ed (L-) and the right-handed (R-) flowers*, respectively. The large anther and the stigma are symmetrical with respect to the median plane of the flower, The distance bctween the stigma and the tip of the large anther is
' '1'his definitien is corrcsponding to Bowers (1975) but opposite to that of Ornduff and Dulbci- ger (1978) and Dulberger (1981). In the latter, the left and right were defincd from the obscrver'sviewpoint.
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rrhe F[Gs. 1--3. tluwer of Monochoria korsakowii 1: A left-handed flower. 2: A sideview of a lcft- handed flower (pcrianth removed). P: pistil; SS/ small stamcns; LS: large stamen; A: appendage on the fiiament of thc large stamen. 3: A worker of Apis cerana 1'aponica visiting the same ftewer us in Fig. 1, Another f]ower beLow is right-handed.
O.8-1.0 cm. The small anthers are at the central upper position in the flower. The terminal inflorescence, bearing 10-30 fiowers, is probably a raceme of seyeral cymes. The lower cymes are longer and bear 2-4 flow- ers. L- and R-flowers alternate along each cyme by turns. Only one flower (either L or R) at most opens on each cyme per day (Fig. 4), Table 1 shows the distributions of L- and R-form flowers in the natural population in Mikata in late September, 1992. The total numbers of L- and R-flowers assigned were 119 and 121, respectively: very close to the ratio of 1:1.
lnsect visitons and their hehaviors Eight and five insect species were collected on the flowers of M. kor- sakowii in the Mikata natural population and the Kyoto transplanted population, respectively (Table 2). The most frequent visitor was the work- er of Apis cerana joponica in both populations (Table 2). In Kyoto on September 14, 1994, the bee visited the flower from 8:OO to 12:OO with a peak around 8:45, The bee foraged only pollen from this nectarless flower. When the mass visiting ceased, most of the flower anthers became empty. When foragin g pollen, the bee landed on the flower with its head upward,
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TABi.E 1. Distribution of L- and R-ilowers within infiorescences and individuals at the natural popu]a- tion in Mikata,
Number of LtR-fiowers pcr inflorcsccnce Individuals ancl inflorescences") Scpt, 24 25 26 27 28 Totalb)
1 111 ell312etl 1/32XO2/12!22/53112111121113/O311110111110310Oll1/11/1214211213211312411321104f21/Oo/o4fOO14O13210O134/1O12Oll 3f6 2 9ld 3 Ot3 315 4 311O/3Oll 51310f9 5 4XO113OX21f2Otl3fO4tl212110II2 6 719 7 6/7 8 616 9 112O!2Oll 316 10 8/5 11 314 12a 2LLIXIlfl 315111213 6X6 12b 2/3 111 617 13a 4/4 13b 1/1OflOX2O12IAlflOf22112Xl25130110 5/1 14a Otl 1111/O 114 14b 213 14c ltO 213 15 2fl 112 6/8 16 312 17 110 210 11e3/3O1311718 6/6 18 715 19Total 2fl16/17 lfl26f24 818119/121
a) Letters following the same numbcr indicate different infiorescences of the individual.
b) lncluding only flowcrs that open during the five days, and may bc less than the total flowers on each infloresccncc.
"milked" grasped the small stamens and then the anther with its fore- and mid-legs (Fig. 3). During this, the bee abdomen is between the style and large stamen. Although the abdomen is narrower than the distance be- tween the stigma and the anther, it occasionally touched them as the bee moved. The Apis workers visited the flower of which the large stamen had been removed, but not the fiower of which all the small stamens had been removed (23 and no visits, respectively, from 9:OO to 9:30 on September 10, 1992). When the intact flower was held upside-down, the /tpis workers visited it and collected pollen from the small yellow anthers, but never touched the stigma nor the large blue anther positioning above the small stamens. Larger bees, JY]ylocqpa circumvolans, BombiLs ignitus and B. diversus diversus also foraged pollen from the flower. These bees approached the flower with their heads upward but the flower was immediately forced to bend down due to their body weight. The bee hung on the flower, curled
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TABLH 2. Insects collected on the flower of Monochoria korsakowii at the natural population in Mika- ta and the transplanted population in Kyote.
NumbeT of individuals collected
Insect species Mikata (50min.) Kyoto (30 min.)
Apidae Apis cerana 1'crponica 1515 12 Bombus diversus diversus B. ignitus Halictidne Lasioglossum sp. 1 Anthophoridae lb,tocopa circumvolans 1 5 Colletidae flylaeus nippon 6 Sphecidae Cerceris rybiensts iaponica 1 Calliphoridae Stomorhina ebsoleta 2 Syrphidae Mesembriusflariceps 31 2 dyiatropaflorea
its body over the small stamens, clasped the yellow anthers with the fore- and mid-legs, and emitted a sound characteristic of buzz-pollination (Buchmam), 1974). During this, the bee abdemen touched the stigma by one side and the large anther by the other side, at the same time. H]ylaeus nippon and other small insects (Table 2) foraged pollen from the small anthers, without touching the stigma or the large anther.
Efilects of bagging, emasculation and handipollination Results of the hand-pollination experiments are shown in Tables 3 and 4. All four pollination modes, i.e., combinations of crosslself pollinations and the large!small stamens, gave 100 % fruit set. However, the number of seeds per fruit was significantly affected by the stamen type (two-way
TABLE 3. Effects of crosslself hand-pollinations and the types of stamens on the number of viable
seeds in a fruit.
Number of viable seeds in a fruit Crosstself Numbcr of Fruitage pollinationsLarge/small stamens samples (%) RangeMeun S,D.
CrossCrossSelfSelfLaTgeSmallLargeSmall13131013 10010010010056-19020-17681-21041-145137.46114.15162.0097.2344.7750,5439.6035,23
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16
S
FIG. 4. A diagrarnrnatic Tepresentation of an infiorescencc of Monochoria korsakowii. 1[he date of flowering in SeptembeT, 1994 is shown at the shoulder of each circle. The plant was grown in Kyoto by sowing seed collected at the Mikata populat{on.
analysis of variance, P > O.OOI; Table 4): hand-pollinations with large and small stamens gave 148 and 106 seeds per fruit in average, respectively. The seed numbers of cross- and self-pollinated fruits were not significantly different (P > O.5; Table 4). Table 5 summarizes the results of bagging and emasculation experi- ments carried out at the transplanted population. Overall, bagging and emasculation treatments affected both fruit set (Chi-square test, P < O.Ol)
TABLE 4. A two-way analysis of variance for the effects of crosslself hand-pollinations and the types of stamens on the number of viable sccds in a fruit.
Source df M.S. F-ratio Probability
Crosstseli' pollinations 111an 83.324232.U5683.3 o.o"12.864 > O.5< Large/smal1 stamcns O.oo1>
Interaction 3.017 O.05 Residual 1883.7
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TABLE 5. Effccts of bagging and emasculation on fruit and seed sets of Monochoria korsakowii.
Number of seeds
Number Numberof Fruit Perfiower Per fruit '' - of fiewers set samples fruited (%) Mean S.D, MeanS,D.
Bagged and emasculated: Large stamen was removed before opening 302S2018492920 18 60 45.0 47,6 75.lb2) 3s.5 Smal[ stamens were removed before opening o o o.o o.o - AII stamcns wcrc removed before opening 1 5661006910U O.2 O,7 3.o3) All stamcns werc removed befure closing 12 2S,1 31,2 40,lu 30.1 Bagged and hand-pollinated 49 125.6 48,1 125.6c 48.1
Bagged, not hand-pollinated 20 54.6 56,8 79.2b 52.1 Open p61linatedi) 20x2 155,2 15,9 155.2d 15.9 =, 122.4 ANOVA: F = 22.401 (p < O.Ol) (p < O.UOI)
i)Flowers were visited predeminantly by Apis cerana iaponica. 2)Means within a column followed by the same letter are not significantly different at the 5 % Level according to Duncan's multiple range test. 3}Excluded from the analysis of variance.
and seed number per fruit (one-way analysis of variance, P < O.OOI). With visits by Apis, almost all of the open-pollinated flowers fruited. Each imit contained 155 seeds in average. Bagging of fiowers reduced fi;uit set to about 70 % and seed number per fruit to about half. Emasculation of the bagged flowers before flowering almost completely inhibited seed set. Emasculation of only the small stamens, leaving the large stamen intact, also resulted in zero seed set. However, when only the large stamen had been removed, 60 % of the flowers set fruits and the seed number was not significantly different from that of the non-emasculated flowers. Emascula- tion performed just before flower closing did not inhibit fruiting but re- duced seed set to half.
Pollen germination and pollen tube growth The pollens of both stamen types are yellow, about 40/ttm long and 38 ym in diameter; the size was not significantly different between stamen types (t-test, P > O.05). Pollen grains germinated on the stigma 25-35 minutes after contact. The pollen tube grew down the center of the style and reached the ovules about 3.5 hours after germination. Pollens of the two stamen types did not apparently differ in viability and pollen tube growth rate, although quantitative data were not obtained.
Discussion
The flowers of M. korsakowii were visited most consistently by Apis cerana japonica. However, the fit between fiower parts and insect body suggests that the larger bees, Bomb"s and .81ylocopa, arc more effective pollinators. When the bee fbrages on a L-flower, the large anther touches
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the left side of bee's hairy abdomen. Then, if the bee next visits an R- flower, the pollen will be deposited on the stigma that touches the left side of the abdomen. It is expected that random visits to L- and R-flowers would lead to higher frequency of intermorph than intramorph pollination. Pollen of the small stamen is the only reward to the bee, since the flower does not produce nectar. The bee visited fiowers whose large sta- men had been removed, but never those flowers whose small stamens had been removed. This indicates that the small yellow stamens play a key role "feeding" in attracting the bee. The yellow color of the anther may be a deceit provoking visits for a longer time than pollen is available (Vogel, 1978). On the other hand, the bee did not forage pollen of the large sta- men. The purplish blue color of the large anther, almost the same as the perianth, seems to camouflage itself effectively. The trigger-like appendage on the filament appears to help rubbing movement of the large anther against the bee abdomen, being pulled by bee legs unintentionally. These features consists a syndrome shared by many of the enantiostylous taxa (Dulberger, 1981). In particular, the overall appearance of the flower is re- markably similar to that reported of CYanella lutea (Dulberger and Ornduff, 1980). It was once assumed that in Monochoria hastata and M. vaginalis, the five small stamens were sterile and the large one fertile (Iyengar, 1932). Our experiment with M. korsakowii demonstrated that both stamens pro- duce fertile pollen