REGULATION OF THE TIMING OF POLLEN GERMINATION BY THE PISTIL IN TALINUM MENGESII (PORTULACACEAE)

In angiosperms selection among male gametophytes may take place between the time a flower is pollinated and the time its ovules are fertilized. With insect pollination, a surplus of male gametophytes in the form of pollen grains may be deposited on the stigma of a pistil and this provides the potential for gametophytic competition and selection. In all plants examined from two Talinum mengesii populations, pollen germination was delayed for up to two hours after pollination. In plants of two other populations pollen germination was not delayed or the time of germination varied. Reciprocal pollinations between plants with and without the delay trait and pollinations at different times during anthesis revealed that the timing of pollen germination was regulated by the maternal parent. Theoretically, the effect of the delay trait would be to promote the accumulation of pollen on the stigma and induce simultaneous germination. This would intensify intermale competition and provide a mechanism for the maternal parent to influence the paternal parentage of its offspring.     

A multi-year study of factors affecting fruit production in Aristolochia paucinervis (Aristolochiaceae)

Pollen limitation, resource limitation, fruit abortion, and predation have all been proposed as factors explaining low fruit set in hermaphroditic plants. We conducted a 5-year study combining field observations and pollination experiments to determine the causes of the low fruit set in Aristolochia paucinervis, a Mediterranean species with a specialized pollination system in two populations in SW Spain. Fruit initiation was markedly low, and between 28.6 and 75.0% of the flowering stems did not initiate any fruit. In most flowers, the number of germinated pollen grains was less than the number of ovules, and supplemental pollination significantly increased fruiting, indicating deficient pollination. In A. paucinervis, autonomous self-pollination seems to be a decisive factor in fruit production because the number of germinated pollen and the fruit set from flowers bagged before anthesis were similar to those in free-pollinated flowers. Only in 2005 did flowers that were successfully pollinated outnumber ripened fruits, suggesting that other factors limit fruiting. We found a significant positive correlation between tuber mass and fruit set. Deficient pollination and lack of resources could explain the low fruit set, but the relative consequences seem to vary spatially and temporarily.     

Self‐pollination and parthenocarpic ability in developing ovaries of self‐incompatible Clementine mandarins (Citrus clementina)

This study aimed to determine if self‐pollination is needed to trigger facultative parthenocarpy in self‐incompatible Clementine mandarins (Citrus clementina Hort. ex Tan.). ‘Marisol’ and ‘Clemenules’ mandarins were selected, and self‐pollinated and un‐pollinated flowers from both cultivars were used for comparison. These mandarins are always seedless after self‐pollination and show high and low ability to develop substantial parthenocarpic fruits, respectively. The time‐course for pollen grain germination, tube growth and ovule abortion was analyzed as well as that for carbohydrates, active gibberellins (GA₁ and GA₄), auxin (IAA) and abscisic acid (ABA) content in the ovary. ‘Clemenules’ showed higher pollen grain germination, but pollen tube development was arrested in the upper style 9 days after pollination in both cultivars. Self‐pollination did not stimulate parthenocarpy, whereas both un‐pollinated and self‐pollinated ovaries set fruit regardless of the cultivar. On the other hand, ‘Marisol’ un‐pollinated flowers showed greater parthenocarpic ovary growth than ‘Clemenules’ un‐pollinated flowers, i.e. higher ovule abortion rate (+21%), higher fruit set (+44%) and higher fruit weight (+50%). Further, the greater parthenocarpic ability of ‘Marisol’ paralleled higher levels of GA₁ in the ovary (+34% at anthesis). ‘Marisol’ ovary also showed higher hexoses and starch mobilization, but lower ABA levels (?64% at anthesis). Self‐pollination did not modify carbohydrates or GA content in the ovary compared to un‐pollination. Results indicate that parthenocarpy in the Clementine mandarin is pollination‐independent with its ability to set depending on the ovary hormone levels. These findings suggest that parthenocarpy in fertile self‐incompatible mandarins is constitutively regulated.     

Fluorescence and scanning electron microscopic study on self-incompatibility in distylous Averrhoa carambola L.

Various combinations of intermorph, selfing and intramorph pollinations were carried out in Averrhoa carambola and the pollinated pistils were observed under fluorescence and scanning electron microscopes in time-course experiments. In both compatible and incompatible pollinations, similar behavior of pollen germination and penetration was observed in the first 4 h after pollination. In compatible intermorph pollination, pollen tubes were found at the base of the transmitting tract of the style at 8 h and 24 h after pollination in both the pin and thrum morphs. With thrum flowers, selfing resulted in pollen tubes being uniformly arrested at the junction between the stigmatic and stylar tissues. Penetration of pollen tubes into the upper portion of style was observed in thrum intramorph pollination and, when the second member was treated as the gynoecial tissue under the same pollination, penetration of tubes was further enhanced. Pin flowers, on selfing, resulted in pollen tube penetration farther down the style than was the case with thrum selfing. Intramorph pollination of pin morph behaved in a similar manner to selfing and was not affected by genotypes. Beside the stamen-style dimorphism, the receptive surface of the cob stigma was larger in pin than thrum flowers. While pin pollen was round, thrum pollen was oblong in shape with pin to thrum ratio on the polar axis being 1.2 and on the equatorial axis 0.8. The stigma of pin morph belonged to the dry type, while that of the thrum resembled the wet type.     

Stigma development and receptivity in almond (Prunus dulcis)

BACKGROUND AND AIMS: Fertilization is essential in almond production, and pollination can be limiting in production areas. This study investigated stigma receptivity under defined developmental stages to clarify the relationship between stigma morphology, pollen germination, tube growth and fruit set. METHODS: Light and scanning electron microscopy were employed to examine stigma development at seven stages of flower development ranging from buds that were swollen to flowers in which petals were abscising. Flowers at different stages were hand pollinated and pollen germination and tube growth assessed. Artificial pollinations in the field were conducted to determine the effect of flower age on fruit set. KEY RESULTS: Later stages of flower development exhibited greater stigma receptivity, i.e. higher percentages of pollen germination and more extensive tube growth occurred in older (those opened to the flat petal stage or exhibiting petal fall) than younger flowers. Enhanced stigma receptivity was associated with elongation of stigmatic papillae and increased amounts of stigmatic exudate that inundated papillae at later developmental stages. Field pollinations indicated that the stigma was still receptive and nut set was maintained in older flowers. CONCLUSIONS: Stigma receptivity in almond does not become optimal until flowers are past the fully open stage. The stigma is still receptive and fruit set is maintained in flowers even at the stage when petals are abscising. Strategies to enhance pollination and crop yield, including the timing and placement of honey bees, should consider the effectiveness of developmentally advanced flowers.     

Studies in Garcinia, dioecious tropical forest trees: the phenology, pollination biology and fertilization of G. hombroniana Pierre

RICHARDS, A. J., 1990. Studies in Garcinia , dioecious tropical forest trees: the phenology, pollination biology and fertilization of G. hombroniana Pierre . Garcinia hombroniana is a facultative agamosperm which is pollinated by Trigona bees. Nectar is restricted to the large discoid stigma (or pistillode in male flowers), which also captures and hydrates pollen. The 'wet' stigma and binucleate pollen suggest that Garcinia arose from hermaphrodite plants with a gametophytic self-incompatibility system.
On stigmas, nectar is secreted early on three or four successive days. On male pistillodes, nectar is secreted when anthers dehisce, on the second morning after anthesis. Pollen is most viable when freshly collected, but some viability remains four days after collection. Pollen germinates within 24 h of hydration. Similar results to pollinations are obtained by germinating pollen in 1 % sucrose.
Garcinia hombroniana flowers principally from January to June. Cultivated females are considered as 'big bang' strategists. Male flowers are considered as 'steady state' strategists.     

ON THE DARWINIAN HYPOTHESIS OF THE ADAPTIVE SIGNIFICANCE OF TRISTYLY

Darwin proposed that the function of the stamen-style polymorphism in heterostylous plants is to increase the probability of legitimate (compatible) pollinations among the floral morphs. Conspicuous pollen trimorphism in tristylous Pontederia cordata enables a test of the hypothesis. Comparison of the composition of pollen loads in naturally pollinated stigmas of intact and emasculated flowers were made at a population in Paugh Lake, Ontario, which was visited primarily by bumblebees. The magnitude of legitimate pollination was analyzed by ANOVA. In intact flowers, significant legitimate pollination was detected in the long-styled morph only. Following emasculation legitimate pollination was evident in the long- and short-styled morphs, with the mid-styled morph just short of displaying significant legitimate pollination. Similar results were obtained by chi-square analysis. It has been suggested that heterostyly may reduce mutual interference between maternal and paternal reproductive function. Two aspects of pollen-stigma interference were investigated in P. cordata. The potential importance of stigmatic or stylar clogging by incompatible pollen was examined by controlled field pollinations and measurements of seed set. The results indicate that prior application of large amounts of incompatible pollen has no significant effect on the seed set of open-pollinated inflorescences. Comparison of legitimate pollen capture in intact and emasculated flowers provided no evidence that the presence of stamens within flowers of the floral morphs interferes with the receipt of legitimate pollen. Pollen-stigma interference remains to be demonstrated in heterostylous plants.     

Influence of number of pollinations and pollen load size on maternal fitness costs in Collinsia heterophylla: implications for existence of a sexual conflict over timing of stigma receptivity

Costs related to pollen competition have rarely been considered, but are expected in the case of sexual conflict where male and female sexual functions have opposing evolutionary interests. In Collinsia heterophylla, delayed stigma receptivity is beneficial as it enhances pollen competition. A sexual conflict over timing of stigma receptivity has been proposed in this species as early pollination, following one-time pollinations, is advantageous to pollen donors at a cost of reduced maternal seed set (measured as seed number). In this study, we explored whether the maternal cost was still present following an additional pollination. We hypothesized that the cost is caused either by harm related to early pollen presence or by factors unrelated to harm. We performed pollinations at different stages of floral development, either one or two pollinations (24-h time lag), and varied the size of the first pollen load in the latter category. Early pollination reduced seed biomass also after two-time pollinations, suggesting a persistent maternal cost of early pollen presence. Further, pollen load size modified seed production, possibly indicating that dose-dependent harm influences the maternal cost of early fertilization. Our results strongly suggest negative effects of pollen competition on maternal fitness following early pollination, which is consistent with the existence of a sexual conflict over timing of stigma receptivity. In conclusion, we propose that much could be gained if more plant studies considered the potential for fitness costs in relation to sexual conflict, particularly those investigating pollen-pistil interactions.     

Exploring the boundary between pollination syndromes: bats and hummingbirds as pollinators of<Emphasis Type="Italic"> Burmeistera cyclostigmata</Emphasis> and<Emphasis Type="Italic"> B. tenuiflora</Emphasis> (Campanulaceae)

In this study I documented the degree of specialization in the pollination systems of Burmeistera cyclostigmata and B. tenuiflora (Campanulaceae) to explore the potential role of floral isolation in the diversification of the genus. I asked which floral characteristics are important in specializing on either bat or hummingbird pollination, and whether overlap between these floral syndromes can exist. I examined nocturnal and diurnal pollen deposition, pollinator visitation rates, and single visit effectiveness and related them to intra- and interspecific variation in Burmeistera floral characteristics at Monteverde, Costa Rica. Bats and hummingbirds visited both Burmeistera species, and bats pollinated both species. Owing to differences in floral morphology, however, hummingbirds effectively pollinated only B. tenuiflora. The generalized pollination system of B. tenuiflora demonstrates that there can be overlap in the boundary between ornithophily and chiropterophily, and that nectar production and timing of anthesis do not serve as barriers between these syndromes. The high intraspecific variation in floral color from green to red or purple did not correlate with either nocturnal or diurnal pollen deposition. Degree of flower accessibility did affect pollination; nocturnal pollen deposition significantly decreased as flowers become more obstructed. In Burmeistera, floral morphology and accessibility appear to be the most important floral characteristics for specialization at the boundary between ornithophily and chiropterophily.     

The effects of stigma age on receptivity in Silene alba (Caryophyllaceae)

Silene alba, a perennial, dioecious plant, produces flowers that open in the evening and can remain open and receptive to pollination for up to 5 d, though in hot and dry conditions the flowers will wilt during the day only to reopen night after night. In the field, it is visited by two different kinds of pollinators with differential success: moths visit the flowers at night, and their movements result in broad pollen dispersal and large seed production, whereas bees, wasps, and flies visit the flowers in the mornings and have decreased pollination effectiveness. However, this differential success may be due to a decrease in stigmatic receptivity soon after the flowers open. We performed controlled pollinations to determine the effect of stigma age on pollen germination and seed set. We pollinated flowers at 12-h intervals up to 120 h and divided these into two sets: from one set, we removed stigmas 24 h after pollination to examine percentage of pollen germination. The second set of flowers was allowed to produce fruits, and the seeds were counted and weighed. Pollen germination declined significantly with stigma age, but there was no significant effect of stigma age at pollination on the number or mass of resulting seeds. Thus, the decreased pollination success of bees is not due to a decrease in stigmatic receptivity but is most likely a result of pollinator inefficiency.     

The pollination ecology of Paraboea rufescens (Gesneriaceae): a buzz-pollinated tropical herb with mirror-image flowers

BACKGROUND AND AIMS: Gesneriaceae is a pantropical plant family with over 3000 species. A great variety of pollination mechanisms have been reported for the neotropical members of the family, but the details of buzz-pollination and enantiostyly for the family have not been described. We investigated the floral biology and pollination ecology of Paraboea rufescens in Xishuangbanna, south-west China, considering three aspects: (1) the type of enantiostyly exhibited; (2) whether the species is self-compatible; and (3) whether pollinator behaviour could enhance the precision of pollen transfer between flowers of contrasting stylar orientation. METHODS: Flowering phenology was monitored once a month during vegetative growth, and once a week during flowering both in the field and under cultivation. Pollination manipulations and pollinator observation in the field were conducted. KEY RESULTS: Anthesis occurred early during the morning, and flowers remained open for 1-5 d, depending on weather conditions. Controlled pollinations revealed that P. rufescens is self-compatible, and exhibited inbreeding depression in seed set. Plants were pollinator limited in natural populations. The similar stylar deflection among flowers within a plant limits autonomous self-pollination as well as pollination between flowers. Two species of bumble bees (Bombus spp.), Amegila malaccensis and Nomia sp. effectively pollinated P. rufescens. These pollinators visited flowers in search of pollen with almost the same frequency. None of the pollinators appeared to discriminate between left- or right-handed flowers. CONCLUSIONS: Paraboea rufescens exhibits monomorphic enantiostylous flowers and a buzz-pollination syndrome. Floral morphology in P. rufescens and pollinator foraging behaviour seems likely to reduce self-pollination and pollinations between flowers of the same stylar deflection.     

聚果榕传粉榕小蜂传粉与产卵之间的权衡

【目的】榕树(Ficus)依赖专性榕小蜂(Agaonidae)传粉,同时为传粉榕小蜂提供繁衍后代的场所,两者形成动植物间经典的协同进化关系。在雌花期果内,榕小蜂需在有限的存活时间内完成传粉和产卵,而传粉榕小蜂如何在传粉与产卵之间进行权衡仍然是悬而未解的问题。本研究旨在明确传粉榕小蜂——一种栉颚榕小蜂Ceratosolen sp.在雌雄同株的聚果榕Ficus racemosa雌花期果内的行为活动及繁殖模式。【方法】借助测微尺测量聚果榕榕果雌花花柱长度与传粉榕小蜂(Ceratosolen sp.)产卵器长度,通过显微视频记录传粉榕小蜂在雌花期果内搜索、传粉及产卵行为;结合单果控制性引蜂试验,测定不同阶段榕小蜂个体大小、孕卵量、携粉量,以及雄花期最终繁殖的榕小蜂后代和榕果种子数量。【结果】聚果榕雌花花柱长度存在树间变异,榕小蜂产卵器长度比绝大多数的雌花花柱长,说明该小蜂可以产卵于大部分的雌花子房里。通常个体大的榕小蜂孕卵量更多,但个体大小与携粉量之间相关性不显著。观察发现,榕小蜂进入雌花期榕果内,前6 h集中产卵,可产下孕卵量的95%,平均搜索用时27 s,产卵用时46 s,此期间传粉行为少见,花粉筐中携带花粉量亦无明显变化;榕小蜂进果后6-24 h,主要执行传粉,其行为主动,连贯高效,单次传粉用时平均为2 s,最终可传完携粉量的80%。控制引蜂试验也证实榕小蜂进入榕果内前6 h主要执行产卵繁殖后代,之后6-24 h主要执行传粉以繁殖榕树种子。【结论】在雌雄同株的聚果榕雌花期榕果内,榕小蜂先产卵、后传粉。本研究首次展示了传粉榕小蜂在聚果榕雌花期榕果内的产卵和传粉行为,并获得与行为相匹配的产卵量和传粉繁殖量,反映了具主动传粉行为的榕小蜂在传粉和产卵之间存在时间和数量上的权衡。     

Reproductive timetable for the tropical Vireya rhododendron,R. Macgregoriae

Summary The reproductive development of Rhododendron macgregoriae F.v.M., Section Vireya (Ericaceae) has been followed from 10 days before anthesis to the production of mature germinable seeds about 145 days after anthesis. The species is self-compatible but shows both protandry and physical separation of pollen from the receptive stigma. Pollen is mature and viable from shortly before anthesis until the corolla and attached anthers abscise some 9–12 days after the flowers open. Spontaneous dehiscence, however, occurs mostly in the first few days after opening. The stigma becomes receptive 6–7 days after anthesis, and nectar is produced at this time. Female gametophytes are not mature until about 10 days after anthesis. Fertilization occurs about 6–7 days after pollination, and although the endosperm commences development immediately, development of the embryo proper does not begin until some 40–45 days later. Pollinations made with fresh pollen between 5 and 14 days after anthesis were successful, but those made on dry stigmas in the first 4 days after anthesis, or on senescing pistils 21 days after anthesis, gave little or no seed set. In nature, autogamy is probably uncommon, the majority of pollinations are likely to be geitonogamous, but there is considerable potential for outcrossing.     

Floral reward presentation favored the expression of male function in the pollen‐only flower Melastoma malabathricum

Abstract Animal‐pollinated plant species modulate the presentation of pollinator rewards to maximize reproductive success. In plants providing pollen as the only reward for pollinators, it is usually difficult to unravel the dual roles of reward presentation and the realization of male and female functions (pollen removal and deposition). Exploiting the two types of anther in the androecia of Melastoma malabathricum L., we examined whether the removal of pollen for reward is regulated primarily to favor male function or female function. Pollen removal by carpenter bees from the feeding and pollination anthers, as well as pollen deposition on the stigmas, were quantified during anthesis of M. malabathricum. There was no significant difference in pollen removal rates from the feeding and pollination anthers of M. malabathricum between the onset of anthesis and flower wilting. The stigmatic pollen loads exceeded the ovule number after three sonication bouts, and female function was satisfied earlier than male function. The results support the hypothesis that the presentation of pollination reward in this species is regulated primarily to favor the expression of male function, rather than female function, in agreement with the pollen‐donation hypothesis. A cooperative relationship between the feeding and pollination anthers was demonstrated in heterantherous flowers, which optimizes the balance in investments between pollinator rewards and “functional pollen” for gene transfer.     

A new technique for monitoring pollen flow in orchids

Summary The orchid Prasophyllum fimbria is pollinated by nectar-feeding native bees and wasps. The pollinia are patially separated from the viscidium by a stipe so that pollinia can be labelled with coloured histochemical stains without interfering with pollinarium removal. Pollen flow was monitored by following the movement of the coloured pollen in several populations of P. fimbria in Western Australia. Statistical analysis confirmed that pollen labelling did not interfere with pollinarium removal or subsequent pollination of the labelled flower. Fifty eight labelled pollinaria were removed by vectors from 16 test spikes, with a total of 125 flowers on 47 spikes receiving labelled pollen. An average of 2 flowers received pollen for every pollinium removed but up to 6 flowers received pollen from a single collinium. No significant differences between mean vector flights and pollen flow distances were detected. On average, geitonogamous transfers only accounted for 22% of all pollinations. This is a simple and inexpensive technique for the direct labelling of pollen with minimal disruption to the pollination system and may have applications in other plant families.     

Effects of Inhibitors of Protein Serine/Threonine Phosphatases on Pollination in Brassica

We have examined the effect of the protein phosphatase inhibitors okadaic acid and microcystin on pollen-pistil interactions in Brassica. Inhibitor-treated flowers or floral buds were pollinated with untreated pollen and examined for pollen tube growth by fluorescence microscopy. Our results show that type 1 or type 2A serine/threonine phosphatases play a crucial role in the pollination responses of Brassica. We observed two distinct effects of protein phosphatase inhibitors on pollination: (a) the inhibition of pollen tube growth during cross-pollination in flowers, and (b) the break-down of self-incompatibility or promotion of pollen tube growth during self-pollination in flower buds just prior to anthesis. Thus, treatment of flower pistils with protein phosphatase inhibitors resulted in the inhibition of pollen tube growth at the surface of the papillar cells of the stigma in crosses between different self-incompatible Brassica oleracea strains, in an interspecific cross between B. oleracea and Brassica campestris, and in self-pollinations of a self-fertile Brassica napus cultivar. With four different self-incompatibility genotypes, treatment of mature flowers with protein phosphatase inhibitors had no effect on self-pollination response. In contrast, treatment of flower buds just prior to the anthesis stage allowed self-pollen tube invasion of papillar cells. However, the magnitude of this effect was genotype dependent, being most pronounced in the S22 genotype. The data support the conclusion that pollinations in Brassica are controlled in part by the presence of phosphorylated proteins in the papillar cells of the stigma, and that the quantity of these proteins or their levels of phosphorylation changes during stigma development.     

Pollen limitation and fruiting failure related to canopy closure in Calypso bulbosa (Orchidaceae), a northern food‐deceptive orchid with a single flower

Natural fruit set is constrained by pollen limitation and fruiting failure, and pollen limitation is expected to be especially severe in deceptive orchids. We performed hand cross‐pollinations in ten populations of a food‐deceptive orchid, Calypso bulbosa, under sparse and dense canopies in three non‐consecutive years. We explored the relationships between natural fruit set, pollen limitation and fruiting failure. Mean natural fruit set over the years was 60%, which is exceptionally high for a deceptive orchid. On average, hand cross‐pollination increased fruit set by 23%. Among open‐pollinated plants that did not set a fruit, 55.5% were estimated to be pollen limited and 44.5% to be limited by fruiting failure, i.e. inability to set a fruit after pollination. In species with high natural fruit set, hand cross‐pollination experiments may not always detect statistically significant pollen limitation. In our case, pollen limitation tended to become significant when the natural fruit set dropped below 60%. Canopy cover had a significant effect on fruiting failure, which was more severe under a dense canopy. Although our results demonstrate pollen limitation in many cases, they also highlight the fact that food deception can be a very effective pollination strategy. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013 , 171 , 744–750.     

Genetic consequences of specialization: yucca moth behavior and self-pollination in yuccas

Reciprocal specialization in interspecific interactions, such as plant-pollinator mutualisms, increases the probability that either party can have detrimental effects on the other without the interaction being dissolved. This should be particularly apparent in obligate mutualisms, such as those that exist between yucca and yucca moths. Female moths collect pollen from yucca flowers, oviposit into floral ovaries, and then pollinate those flowers. Yucca moths, which are the sole pollinators of yuccas, impose a cost in the form of seed consumption by the moth larvae. Here we ask whether there also is a genetic cost through selfish moth behavior that may lead to high levels of self fertilization in the yuccas. Historically, it has been assumed that females leave a plant immediately after collecting pollen, but few data are available. Observations of a member of the Tegeticula yuccasella complex on Yucca filamentosa revealed that females remained on the plant and oviposited in 66% of all instances after observed pollen collections, and 51% of all moths were observed to pollinate the same plant as well. Manual cross and self pollinations showed equal development and retention of fruits. Subsequent trials to assess inbreeding depression by measuring seed weight, germination date, growth rate, and plant mass at 5 months revealed significant negative effects on seed weight and germination frequency in selfed progeny arrays. Cumulative inbreeding depression was 0.475, i.e., fitness of selfed seeds was expected to be less than half that of outcrossed seeds. Single and multilocus estimates of outcrossing rates based on allozyme analyses of open-pollinated progeny arrays did not differ from 1.0. The discrepancy between high levels of behavioral self-pollination by the moths and nearly complete outcrossing in mature seeds can be explained through selective foreign pollen use by the females, or, more likely, pollen competition or selective abortion of self-pollinated flowers during early stages of fruit development. Thus, whenever the proportion of pollinated flowers exceeds the proportion that can be matured to ripe fruit based on resource availability, the potential detrimental genetic effects imposed through geitonogamous pollinations can be avoided in the plants. Because self-pollinated flowers have a lower probability of retention, selection should act on female moths to move among plants whenever moth density is high enough to trigger abortion. Received: 18 March 1996 \Accepted: 30 July 1996     

Functional analysis of synchronous dichogamy in flowering rush, Butomus umbellatus (Butomaceae)

Dichogamy is one of the most widespread floral mechanisms in flowering plants and is thought to have evolved to reduce interference between pollen import and export within flowers, especially self-pollination. Self-pollination between flowers may also be reduced if dichogamy is synchronous among flowers on an inflorescence. The analysis of dichogamy at both levels requires that the sexual phases of individual flowers be defined functionally in terms of pollen deposition and removal. We conducted morphological and functional analyses to investigate the degree of dichogamy within flowers and the synchronicity of dichogamy between flowers within inflorescences in an emergent, aquatic monocot, flowering rush (Butomus umbellatus). Based on daily observations of the development of marked flowers, data on the schedule of anther dehiscence within flowers, and repeat surveys of floral sex ratios in three populations, individual flowers appear to be strictly protandrous. On average, each flower spends ～1 d in each of male and female phases with an intervening 1-d neuter phase during which there is no available pollen in anthers and stigmas are not yet exposed to receive pollen. Morphological criteria used to delimit the beginning and end of each of these three sex phases were validated by quantifying the temporal schedule of pollen removal from anthers and pollen deposition on stigmas. Experimental pollinations showed that the morphological changes marking the end of female phase are hastened by pollen deposition. At the umbel level, synchronous development within sequential cohorts of flowers reduced overlap of male and female sexual phases between flowers. On average (±1 SE), 72 ± 3% of flowers completed their female phase while no other flowers on the same umbel were in male phase. Computer simulations of umbel development showed that this value is significantly higher than expected if the timing of flower development within umbels was random (30 ± 1%). Surveys of floral sex ratios in three populations revealed that 87% of umbels were either unisexual male or female at any given time. Pollinators usually visited more than one flower in sequence when foraging on umbels, suggesting that synchronous dichogamy may be an adaptation to avoid geitonogamy. The adaptiveness of both flower- and umbel-level dichogamy is also suggested because both traits are expressed to a lesser extent in obligately clonal, triploid populations, where flowers do not make seeds and hence floral adaptations are not maintained by natural selection.