Beyond pollen:ovule ratios: Evolutionary consequences of pollinator dependence and pollination efficiency for pollen and ovule production in angiosperms

Premise

The relative per-flower production of ovules and pollen varies broadly with angiosperm mating systems, with outcrossing types commonly producing more pollen grains per ovule than selfing types. The evolutionary causes of this variation are contentious, especially the relevance of pollination risk. Resolution of this debate may have been hampered by its focus on pollen:ovule (P:O) ratios rather than on the evolution of pollen and ovule numbers per se.

Methods

Using published mean ovule and pollen counts, we analyzed associations with the proportion of removed pollen that reaches stigmas (pollen-transfer efficiency) and differences between pollinator-dependent and autogamous forms within and among species. Analyses involved Bayesian methods that simultaneously considered variation in pollen and ovule numbers and accounted for phylogenetic relatedness. We also assessed the utility of P:O ratios as mating-system proxies and their association with female outcrossing rates.

Results

Median pollen number declined consistently with pollen-transfer efficiency among species, whereas median ovule number did not. Similarly, in both intraspecific and interspecific analyses, pollinator-dependent plants produced more pollen than autogamous plants, whereas ovule production did not differ statistically. Distributions of P:O ratios overlapped extensively for self-incompatible and self-compatible species and for different mating-system classes, and P:O ratios correlated weakly with outcrossing rate.

Conclusions

Our findings demonstrate that pollinator dependence and pollination efficiency commonly influence the evolution of pollen number per flower but have more limited effects on ovule number. P:O ratios provide ambiguous, possibly misleading, information about mating systems, especially when compared among clades.

     

Multi‐year stigmatic pollen‐load sampling reveals temporal stability in interspecific pollination of flowers in a subalpine meadow

Co‐flowering plants may commonly experience interspecific pollination. It remains unknown, however, whether interspecific pollination is a largely stochastic process or consistent enough over years to exert selection for traits that can reduce interspecific pollination or ameliorate its deleterious effects on reproduction. To assess the likelihood of this precondition being met, stigmatic pollen loads on 17–34 insect‐pollinated plant species over three consecutive years were scored in a subalpine meadow in southwestern China. Plant species varied significantly in the amount and proportion of heterospecific pollen (HP) on stigmas. Both the number of HP species and the proportion of the pollen load that was HP for each recipient species correlated positively between years (reflected in pairwise correlations for all year‐by‐year combinations). Although inter‐annual variation was smaller for conspecific pollen (CP) than for HP loads, species tended to experience either consistently high or consistently low HP proportions across years. We found that species with higher stigmatic HP proportions generally experienced lower proportional variation in stigmatic HP, an unexpected result if high HP loads are the result of rare stochastic events. The novel finding of between‐year consistency in stigmatic loads of heterospecific pollen suggests that adaption to stigmatic loads of HP is possible, and two divergent strategies may have evolved: HP avoidance and HP tolerance. The observation of temporally consistent differences among species in levels of HP supports the idea that natural selection may be operating either to increase tolerance or to minimize arrival of heterospecific pollen on stigmas in co‐flowering plants. Such adaptations may be important for the maintenance of high levels of local plant diversity in biodiversity hotspots such as our study area.     

Evaluating the effects of pollinator‐mediated interactions using pollen transfer networks: evidence of widespread facilitation in south Andean plant communities

Information about the relative importance of competitive or facilitative pollinator‐mediated interactions in a multi‐species context is limited. We studied interspecific pollen transfer (IPT) networks to evaluate quantity and quality effects of pollinator sharing among plant species on three high‐Andean communities at 1600, 1800 and 2000 m a.s.l. To estimate the sign of the effects (positive, neutral or negative), the relation between conspecific and heterospecific pollen deposited on stigmas was analysed with GLMMs. Network analyses showed that communities were characterised by the presence of pollen hub‐donors and receptors. We inferred that facilitative and neutral pollinator‐mediated interactions among plants prevailed over competition. Thus, the benefits from pollinator sharing seem to outweigh the costs (i.e. heterospecific deposition and conspecific pollen loss). The largest proportion of facilitated species was found at the highest elevation community, suggesting that under unfavourable conditions for the pollination service and at lower plant densities facilitation can be more common.     

Patterns of pollen removal and deposition in tristylous Pontederia cordata L. (Pontederiaceae)

In tristylous Pontederia cordata (Pontederiaceae), conspicuous differences in the size of pollen grains and discrete variation in the length of reproductive organs provide a suitable experimental system for the study of fine-scale pollination events. At a population of P. cordata at Pothole Lake, Ontario, the majority of flowers are visited by bumble bees which remove on average 45% of the pollen during single visits to previously unvisited flowers. The amount and proportion of pollen removed are significantly different among floral morphs and stamen levels. Deposition of the three pollen types on the bodies of Bombus spp., Apis mellifera and Melissodes apicata is non-random: large- and medium-size pollen tends to remain in greatest concentrations where it is initially deposited, whereas small-size pollen is displaced from the proboscis to more posterior body parts, probably as a result of grooming activities. Stigmatic pollen loads of individual flowers following single bumble bee visits indicate that the mid-styled morph captures the largest total pollen load, and the short-styled morph the smallest. The largest proportion of compatible pollen grains is deposited on stigmas of the long-styled morph. Pollen load data from "single visit" flowers is in general agreement with previously published population surveys involving multiply-visited flowers.     

Pollen transfer by hummingbirds and bumblebees, and the divergence of pollination modes in Penstemon

We compared pollen removal and deposition by hummingbirds and bumblebees visiting bird-syndrome Penstemon barbatus and bee-syndrome P. strictus flowers. One model for evolutionary shifts from bee pollination to bird pollination has assumed that, mostly due to grooming, pollen on bee bodies quickly becomes unavailable for transfer to stigmas, whereas pollen on hummingbirds has greater carryover. Comparing bumblebees and hummingbirds seeking nectar in P. strictus, we confirmed that bees had a steeper pollen carryover curve than birds but, surprisingly, bees and birds removed similar amounts of pollen and had similar per-visit pollen transfer efficiencies. Comparing P. barbatus and P. strictus visited by hummingbirds, the bird-syndrome flowers had more pollen removed, more pollen deposited, and a higher transfer efficiency than the bee-syndrome flowers. In addition, P. barbatus flowers have evolved such that their anthers and stigmas would not easily come into contact with bumblebees if they were to forage on them. We discuss the role that differences in pollination efficiency between bees and hummingbirds may have played in the repeated evolution of hummingbird pollination in Penstemon.     

Timing of invasive pollen deposition influences pollen tube growth and seed set in a native plant

Invasive plants may threaten the reproductive success of native sympatric plants by modifying the pollination process. One potential mechanism takes place through the deposition of invasive pollen onto native stigmas when pollinators are shared among species. We explore how pollen from the invasive plant Brassica nigra influences pre- and post-fertilization stages in the native plant Phacelia parryi, through a series of hand pollination experiments. These two species share pollinators to a high degree. P. parryi flowers were hand-pollinated with either pure conspecific pollen (the control) or with B. nigra pollen applied prior to, simultaneously with, or following conspecific pollen. Application of B. nigra pollen lowered seed set, with the simultaneous application resulting in the highest reduction. Pollen tube growth was also influenced by the presence of invasive pollen, with fewer conspecific pollen tubes reaching the base of P. parryi styles in treatments where B. nigra pollen was applied prior to or simultaneously with conspecific pollen. The deleterious effects of invasive pollen on native seed set in this study are likely not due to loss of stigmatic receptivity since seed set was less affected when heterospecific pollen was applied prior to conspecific pollen, but may instead involve interactions between interspecific pollen grains on the stigma or within the style. Our study highlights the importance of timing of foreign pollen deposition on native stigmas and suggests that interspecific pollen transfer between native and exotic plants may be an important mechanism of competition for pollination in invaded plant communities.     

The potential significance of nectar‐feeding bats as pollinators in mangrove habitats of Peninsular Malaysia

We tested the hypothesis that bats are effective pollinators of mangroves in Malaysia. Bats (Eonycteris spelaea) visited flowers of two Sonneratia species frequently, and deposited large quantities of conspecific pollen grains on stigmas. The bats are likely to be important pollinators of the two mangrove species.     

Foraging behavior affects pollen removal and deposition in Impatiens capensis (Balsaminaceae)

Flowers of most plant species are visited by a variety of animals. Some of these visitors are effective pollinators while others remove resources without transferring pollen. Studies comparing the effectiveness of different visitors as pollinators often compare taxa without considering variation in behavior within a taxon. Wilson and Thomson (Ecology 72: 1503-1507, 1991) documented the effects of honey bees and bumble bees on the pollination dynamics of Impatiens capensis. They found that pollen-collecting honey bees removed large numbers of pollen grains from anthers but deposited little of it on stigmas; bumble bees, which sought nectar, removed less pollen but deposited more of it on stigmas. It is unclear whether the low pollen transfer efficiencies of honey bees are explained by their morphology or by their pollen-collecting behavior. We repeated the work of Wilson and Thomson at a site where honey bees were foraging for nectar, not pollen. We measured the quantity of pollen remaining in anthers, the number of pollen grains deposited on stigmas, and seed production after single visits by honey bees and bumble bees. The differences between the taxa disappeared when they were foraging in a similar manner. Our results clearly demonstrate the importance of foraging behavior on the pollination effectiveness of floral visitors.     

Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass‐flowering species

Pollinator and/or mate scarcity affects pollen transfer, with important ecological and evolutionary consequences for plant reproduction. However, the way in which the pollen loads transported by pollinators and deposited on stigmas are affected by pollination context has been little studied. We investigated the impacts of plant mate and visiting insect availabilities on pollen transport and receipt in a mass‐flowering and facultative autogamous shrub (Rhododendron ferrugineum). First, we recorded insect visits to R. ferrugineum in plant patches of diverse densities and sizes. Second, we analyzed the pollen loads transported by R. ferrugineum pollinators and deposited on stigmas of emasculated and intact flowers, in the same patches. Overall, pollinators (bumblebees) transported much larger pollen loads than the ones found on stigmas, and the pollen deposited on stigmas included a high proportion of conspecific pollen. However, comparing pollen loads of emasculated and intact flowers indicated that pollinators contributed only half the conspecific pollen present on the stigma. At low plant density, we found the highest visitation rate and the lowest proportion of conspecific pollen transported and deposited by pollinators. By contrast, at higher plant density and lower visitation rate, pollinators deposited larger proportion of conspecific pollen, although still far from sufficient to ensure that all the ovules were fertilized. Finally, self‐pollen completely buffered the detrimental effects on pollination of patch fragmentation and pollinator failure. Our results indicate that pollen loads from pollinators and emasculated flowers should be quantified for an accurate understanding of the relative impacts of pollinator and mate limitation on pollen transfer in facultative autogamous species.     

Incomplete partitioning of pollinators by Linum suffruticosum and its coflowering congeners

Premise

Linum suffruticosum shows variations in pollinator fit, pollen pickup, and local pollinators that predict pollen deposition rates. The species often coflowers with other Linum species using the same pollinators. We investigated whether L. suffruticosum trait variation could be explained by local patterns of pollinator sharing and associated evolution to reduce interspecific pollen transfer.

Methods

Pollinator observations were made in different localities (single species, coflowering with other congeners). Floral traits were measured to detect differences across populations and from coflowering species. Reproductive costs were quantified using interspecific hand pollinations and measures of pollen-tube formation, combined with observations of pollen arrival on stigmas and pollen-tube formation after natural pollination in allopatric and sympatric localities.

Results

The size and identity of the most important pollinator of L. suffruticosum and whether there was pollinator sharing with coflowering species appeared to explain floral trait variation related to pollinator fit. The morphological overlap of the flowers of L. suffruticosum with those of coflowering species varied, depending on coflowering species identity. A post-pollination incompatibility system maintains reproductive isolation, but conspecific pollen-tube formation was lower after heterospecific pollination. Under natural pollination at sites of coflowering with congeners, conspecific pollen-tube formation was lower than at single-species localities.

Conclusions

Trait variation in L. suffruticosum appears to respond to the most important local pollinator. Locally, incomplete pollinator partitioning might cause interspecific pollination, imposing reproductive costs. These reproductive costs may generate selection on floral traits for reduced morphological overlap with coflowering congeners, leading to the evolution of pollination ecotypes.     

Relationship between herkogamy,incompatibility and reciprocity with pollen–ovule ratios in Melochia (Malvaceae)

Heterostyly is a floral polymorphism that increases pollination efficiency by promoting cross-pollination and reducing pollen wastage. Efficiency in pollination has been related to plant investment in gamete production and to the pollen to ovule ratio (P/O), which has been proposed as an indication of the likelihood of enough pollen grains reaching the stigmas to result in maximum reproductive success. In heterostylous species, cross-pollination is promoted by the reciprocal position of sexual organs between morphs and a heteromorphic incompatibility system, which precludes selfing and fertilizations among plants of the same morph. Morphological features like reciprocity (between morphs) and herkogamy (within morph) together with the breeding system are thought to influence pollination quality. Therefore, a close relationship between the pollination efficiency, morphological characteristics, and incompatibility would be expected. Pollination treatments and morphological measurements were carried out to describe the breeding system, herkogamy, and reciprocity of six Melochia species. Afterward, the relation between the P/O (as a surrogate of the efficiency in pollination), and reciprocity, herkogamy and incompatibility was evaluated. Monomorphic M. nodiflora and distylous M. pyramidata are self-compatible species, whereas the rest of the species are self- and morph-incompatible. There was a positive relationship between the P/O value and the degree of herkogamy and incompatibility. However, P/O values appear to increase when higher reciprocity is found in the populations. As expected, the lower values of P/O are associated with lower levels of herkogamy and compatibility in the Melochia species studied. The relationship between the factors is discussed under different scenarios of the pollinators’ predictability.     

Pollen flow in a population of Primula vulgaris Huds.

A British population of Primula vulgaris was visited four times between 1971 and 1976 for study of the nature and functioning of distyly. The stainability and production of pollen from pins exceeded those of thrums. About 20% of pin pollen and 48% of thrum pollen was removed from anthers by insect visitors. Pins and thrums were present in a 1:1 ratio in 1971 but pins outnumbered thrums in 1976. Thrums produced more seeds per flower than pins though the number of ovules produced by each was similar. Two methods for collecting stigmas from open flowers and analyzing them for pollen loans produced somewhat different results. For pin stigmas, the pollen load consisted of 2–23% thrum pollen; for thrum stigmas, the pollen load consisted of 0–71% pin pollen with most stigmas having less than 50% pin pollen. In general, intermorph pollen flow is less than would be expected if pollen flow were random. It is probable that most intramorph pollen on stigmas is a result of self- or geitonogamous pollination. The extensive literature concerning the natural pollinators of the primrose is reviewed. Although Darwin's hypothesis concerning the functional significance of distyly in promoting intermorph pollination was never quantified, the pollen flow patterns observed in P. vulgaris are unexpectedly deviant and are similar to those patterns observed in several unrelated heterostylous species in other plant families.     

Interspecific pollen transfer between two coflowering species was minimized by bumblebee fidelity and differential pollen placement on the bumblebee body

Aims When sympatric flowering plant species in a natural community share pollinators, study of plant–plant interactions via interspecific pollen transfer (IPT) is essential for understanding species coexistence. However, little is known about the extent of IPT between interactive species and its causes.Methods To explore how sympatric flowering plants sharing pollinators minimize deleterious effects of IPT, we investigated the pollination ecology of two endemic species, Salvia przewalskii and Delphinium yuanum, in an alpine meadow in the Hengduan Mountains, southwest China. We quantified conspecific and interspecific visits by shared bumblebee pollinators, amounts of pollen placed on different body sites of the pollinators and stigmatic pollen loads on open-pollinated flowers. To examine whether IPT affects female fitness, we measured pollen germination and seed production in the two species in an artificial pollination experiment.Important findings One bumblebee species, Bombus trifasciatus, was found to be the sole effective pollinator for the two coflowering species. Pollination experiments indicated that deposition of heterospecific pollen could significantly decrease seed set in both species. Experiments showed that S. przewalskii pollen could germinate well on stigmas of D. yuanum, inhibiting conspecific pollen germination in D. yuanum. However, seed set was not lower under open pollination than under cross-pollination within species, suggesting that no female fitness loss was caused by IPT. In foraging bouts with pollinator switches, switches from S. przewalskii to D. yuanum were relatively more frequent (8.27%) than the converse (1.72%). However, IPT from S. przewalskii to D. yuanum accounted for only 1.82% of total stigmatic pollen loads while the reverse IPT to S. przewalskii was 8.70%, indicating that more switches of bumblebees to D. yuanum did not result in higher IPT. By contrast, selection for reduced IPT to S. przewalskii would limit pollinator switches from D. yuanum. We found that a bumblebee generally carried pollen grains from both species but the two species differed in the position of pollen placement on the bumblebee's body; S. przewalskii ' s pollen was concentrated on the dorsal thorax while D. yuanum ' s pollen was concentrated ventrally on the head. This differential pollen placement along with pollinator fidelity largely reduced IPT between the two species with a shared pollinator.     

野菊与菊花杂交中花粉活力和柱头可授性及胚胎发育研究

应用石蜡制片、活体压片、光学显微镜及扫描电子显微镜观察等方法,研究了四倍体河南云台山野菊(Dendranthema indicum)与栽培菊花'钟山金山'(D.grandiflorum 'Zhongshanjinshan')种间杂交中父本花粉活力、花粉在柱头萌发、花粉管生长及胚胎发育情况等.结果发现,父本云台山野菊的花粉活力在授粉时为12%左右.人工授粉后的不同时间,在柱头上都观察到正常萌发的花粉粒,且花粉管都能进入柱头,其中,在授粉后0.5 h时,平均每柱头有5.9粒花粉萌发;12 h时,为59.9粒;而24和48 h时,则分别降为47.1和35.7粒.此外,在授粉后8、10、12和15 d时,分别在49.1%、40.8%、39.7%和38.5%子房内观察到正常发育的胚胎,最终杂交结实率为44.8%,而母本自然开放结实率为52.3%.研究表明,授粉前其多数母本雌蕊发育良好、授粉后多数花粉能在柱头正常萌发和花粉管正常生长,在受精后大部分胚胎发育正常是野菊与栽培菊种间杂交较高结实率的重要保证,而授粉前父本较低的花粉活力对杂交结实率影响不大.     

Shortened anther-stigma distance reduces compatible pollination in two distylous Primula species

Aims Distyly has been regarded as an adaptation to improve compatible pollination between two floral morphs with reciprocal herkogamy. The hypothesis that the different positions of anthers and stigmas within flowers as well as their reciprocal position between morphs, reduce the probability of self pollination raised by Darwin has been rarely tested. In this study, we measured stigmatic pollen loads in response to reduced reciprocal herkogamy in two Primula species.Methods To see whether reciprocal herkogamy can increase compatible and/or reduce incompatible pollen deposition, thus promoting compatible pollination, we shortened the distance between anthers and stigmas within the flowers by changing the position of the corolla tube, to which the anthers were fused, i.e. reduced herkogamy in natural populations of Primula secundiflora and P. poissonii and quantified stigmatic pollen loads in the field over 2 years.Important findings In both species, stigmatic pollen loads were significantly higher in the long-styled (L-morph) than in the short-styled morph (S-morph) in both control and manipulated flowers, but percentage of compatible pollen in S-morph were higher. Flowers manipulated to halve the anther–stigma distance showed a similar pattern for 2 years: total pollen grain counts on stigmas did not differ significantly but compatible pollen grains in L- and S-morphs were significantly decreased in both species. The percentage of compatible pollen loads was decreased by 68.7% in P. secundiflora and 65.3% in P. poissonii in L-morphs, while it decreased by 30.6% and 2.9% in S-morphs, respectively. Our manipulation of the relative position of anthers and stigmas in the two distylous species indicated that a lower degree of herkogamy reduced compatible but incompatible pollen transfer was likely to increase. The higher proportion of compatible pollen in the S-morph than in the L-morph in the two Primula species could be attributed to the accessibility of two-level sexual organs, floral orientations and pollinator behaviors. This is a first attempt to manipulate intraflower herkogamy for understanding adaptation of heterostyly, shedding insights into how the reciprocal herkogamy promotes compatible pollination.     

The plot thickens: does low density affect visitation and reproductive success in a perennial herb, and are these effects altered in the presence of a co-flowering species?

Plants may experience reduced reproductive success at low densities, due to lower numbers of pollinator visits or reduced visit quality. Co-occurring plant species that share pollinators have the potential to facilitate pollination by either increasing numbers of pollinator visits or increasing the quality of visits, but also have the potential to reduce plant reproductive success through competition for pollination. I used a field experiment with a common distylous perennial (Piriqueta caroliniana) in the presence and absence of a co-flowering species (Coreopsis leavenworthii) in plots with one of four different distances between conspecific plants. I found strong negative effects of increasing interplant distance (related to conspecific density) on several components of P. caroliniana reproductive success: pollinator visits to plants per plot visit, visits received by individual plants, conspecific pollen grains on stigmas, outcross pollen grains on stigmas, and probability of fruit production. Although P. caroliniana and C. leavenworthii share pollinators, the co-flowering species did not affect visitation, pollen receipt or reproductive effort in P. caroliniana. Pollinators moved very infrequently between species in this experiment, so floral constancy might explain the lack of effect of the co-flowering species on P. caroliniana reproductive success at low densities. In co-occurring self-incompatible plants with floral rewards, reproductive success at low density may depend more on conspecific densities than on the presence of other species.     

Variation of pollination and resource limitation in a low seed-set tree, Liriodendron chinense (Magnoliaceae)

Pollen limitation and resource limitation have been documented as the major factors responsible for plants commonly producing more ovules than seeds, but few studies have examined pollen deposition directly in natural populations at different sites and times. We investigated the causes of low seed set in four populations of Liriodendron chinense (Magnoliaceae), an insect‐pollinated endangered tree endemic to southern China, over 2–3 years. One pistil potentially produces two ovules. The number of pistils per flower varies among populations, but in three of the four populations the variation in a given population was not significantly different among years. Overall, populations with higher pistil numbers tend to set more seeds per flower, but a positive correlation between pistil numbers and seed production per flower was observed in only one of the four populations. The numbers of pollen grains deposited per stigma varied from 0 to 60. The proportion of pollinated stigmas per flower ranged from 44% to 88% among populations and years. The numbers of pollen grains deposited per stigma and the percentages of pollinated stigmas were significantly different between populations, and two populations showed significant differences between years. A positive correlation between stigmatic pollen load and seed set was sought in ten population‐by‐year combinations but, in a given population, high stigmatic pollen loads did not always result in high seed set. Examination of pollen deposition, pistil and seed production over several sites and years showed that in addition to pollination, other factors such as resource or genetic loads were likely to limit the (lower than 10%) seed set in L. chinense. It appears that small, isolated populations experience severe pollination limitation; one population studied had seed/ovule ratios of 0.84% and 1.88% in 1995 and 1996. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 140 , 31–38.     

Pollination Rates and Pollen Tube Growth in a Vulnerable Plant,Liriodendron chinense (Hemsl.) Sarg. (Magnoliaceae)

The limited seed production of insect-depended plant, Liriodendron chinense was once considered to be pollen-limited and insufficient cross pollination. In this study, we counted pollen grains deposited on stigmas in three populations in Guizhou, Hunan and Zhejiang provinces of China respectively. Over 61.9% stigmas were pollinated. From 1994 to 1996, the mean number of pollen grains on each stigma ranged from 4.4 to 42.6, much more than ovules(2) in each pistil. Based on observations of three years, both the pollination rate and pollen quantity on stigmas significantly affected seed set. When flowers opened without stamens dehiscencing at the early stage of anthesis, stigmas received considerable quantity of pollen grains. Pollen grains from different sources were able to germinate and pollen tube growth rates were not greatly variable. It is very likely that pollen grains arrived first would fertilize eggs. Since only several pollen tubes went through the stylar canal, the potential pollen competition may exist. In this case, there would be strong selection on floral syndrome which benefit early insect visits. Pollen grains from the early visits would have a greater chance to fertilize ovules than those from later visits, which implies that cross pollination is the predominant breeding system of this plant. The conclusion was also confirmed by following four artificial experiments. Three treatments, including flowers bagged, netted or with the perianth removed, all reduced seed set clearly, but flowers with the stamens removed (emasculation)had a higher seed production by open pollination. As the rates of deposited stigmas in three populations were 6～8 times more than full seed set, we considerthat lower seed production in this plant may not mainly be due to pollen limitation.     

茨藻目植物柱头特征与传粉系统的演化

茨藻目包含有风媒、水表水媒和水下传粉的多种传粉方式。在前人工作的基础上,补充了4科9 种植物的柱头特征和传粉机制的观察。该目花粉和柱头特征可分为两类,传粉系统表现出两种不同的柱头捕获花粉的机制。花粉为球形的种与其宽大的柱头相配;而那些花粉演化为丝状或花粉在水中具先期萌发能力的种,其花粉易被其丝状的柱头所捕获。此外,还推测了导致这个类群传粉系统分化的可能原因。     

Evolution of pollination system and characters of stigmas in Najadales

Diverse ways of pollination, including aerial, epihydrophilous and hypohydrophilous ones occur in the Najadales. Although the knowledge of pollination biology in this order has been largely accumulated in recent years, most researchers ignored the role of stigmas in the pollination process. Based on the previous works, we supplemented observations on stigmatic characters and pollination mechanism in nine species from four families. The flowers of Aponogeton lakhonensis ( Aponogetonaceae ), Triglochin palustre ( Juncaginaceae), Potamogeton crispus, P. distinctus, P. gramineus, P. pectinatus, P. perfoliatus, P. pusillus (Potamogetonaceae), are all found to open above or on water surface. Stigmas in these species have a large surface area. Correspondingly, the pollen grains are all found to be globular. Of the eight species, except for P. pectinatus and P. pusillus, the remaining six obviously expose their stigmas out of the perianth. Another species , Najas marina (Najadaceae), has filiform stigmas and ellipsoidal pollen grains which can germinate in water before being transferred onto stigmas in natural conditions. Pollination takes place in this species strictly under water. The characters of pollen and stigmas could be divided into two types, and the pollination system is of two kinds of mechanisms of stigmas capturing pollen in the Najadales. The species with globular pollen grains have wide, large stigmas while those producing filiform or precocious pollen grains, which are likely to be captured by stigmas during the pollen dispersal by water currents, normally have filiform stigmas. It is inferred that various water stresses might have resulted in the diversification of pollinationsystem in the Najadales.