Nectar Sugar Production across Floral Phases in the Gynodioecious Protandrous Plant Geranium sylvaticum

Many zoophilous plants attract their pollinators by offering nectar as a reward. In gynodioecious plants (i.e. populations are composed of female and hermaphrodite individuals) nectar production has been repeatedly reported to be larger in hermaphrodite compared to female flowers even though nectar production across the different floral phases in dichogamous plants (i.e. plants with time separation of pollen dispersal and stigma receptivity) has rarely been examined. In this study, sugar production in nectar standing crop and secretion rate were investigated in Geranium sylvaticum, a gynodioecious plant species with protandry (i.e. with hermaphrodite flowers releasing their pollen before the stigma is receptive). We found that flowers from hermaphrodites produced more nectar than female flowers in terms of total nectar sugar content. In addition, differences in nectar production among floral phases were found in hermaphrodite flowers but not in female flowers. In hermaphrodite flowers, maximum sugar content coincided with pollen presentation and declined slightly towards the female phase, indicating nectar reabsorption, whereas in female flowers sugar content did not differ between the floral phases. These differences in floral reward are discussed in relation to visitation patterns by pollinators and seed production in this species.     

The effects of floral display on pollinator visitation vary among populations ofPhacelia linearis (Hydrophyllaceae)

Summary Individual plants in gynodioecious populations ofPhacelia linearis (Hydrophyllaceae) vary in flower gender, flower size, and flower number. This paper reports the effects of variation in floral display on the visitation behaviour of this species' pollinators (mainly pollen-collecting solitary bees) in several natural and three experimental plant populations, and discusses the results in terms of the consequences for plant fitness. The working hypotheses were: (1) that because female plants do not produce pollen, pollen-collecting insects would visit hermaphrodite plants at a higher rate than female plants and would visit more flowers per hermaphrodite than per female; and (2) that pollinator arrival rate would increase with flower size and flower number, the two main components of visual display. These hypotheses were generally supported, but the effects of floral display on pollinator visitation varied substantially among plant populations. Hermaphrodites received significantly higher rates of pollinator arrivals and significantly higher rates of visits to flowers than did females in all experimental populations. Flower size affected arrival rate and flower visit rate positively in natural populations and in two of the three experimental populations. The flower size effect was significant only among female plants in one experimental population, and only among hermaphrodites in another. The effect of flower number on arrival rate was positive and highly significant in natural populations and in all experimental populations. In two out of three experimental populations, insects visited significantly more flowers per hermaphrodite than per female and visited more flowers on many-flowered plants than on few-flowered plants, but neither effect was detected in the third experimental population. Because seed production is not pollen-limited in this species, variation in pollinator visitation behaviour should mainly affect the male reproductive success of hermaphrodite plants. These findings suggest that pollinator-mediated natural selection for floral display inP. linearis varies in space and time.     

Sexual differences in reproductive characters and pollinator attractiveness in gynodioecious Glechoma longituba (Lamiaceae)

We quantified the differences in floral characters and attractiveness to flower visitors under natural conditions between the sexual types in the gynodioecious plant Glechoma longituba. We also manipulated flowers by altering corolla size or nectar volume, or by removing anthers, to examine the effect of these primary and secondary attractants (i.e. rewards and advertisements) on attractiveness. A change in corolla size and shape reduced visiting frequency and pollen load. Removal of anthers did not affect visiting rates, but significantly reduced pollination rates and stigmatic pollen load. A decrease in the nectar volume of a flower was associated with a reduction in handling time and pollen loads on stigmas. These results show that corolla size is an important advertisement to pollinators (particularly at greater distance), which associate hermaphrodite flowers with a larger corolla and a larger volume of nectar than female flowers. We found that artificial changes in population structure affected the behavior of pollinators as well as the pollination rates of flowers. We suggest that the pattern of distribution of hermaphrodite and female clones in a population may serve to avoid pollen limitation in a female clone or patch. This effect may ensure female reproductive success and allow for the maintenance of female individuals in natural populations of this gynodioecious plant.     

Pollinator visits to floral colour phases of Fuchsia excorticata

Abstract

Fuchsia excorticata is a gynodioecious tree (endemic to New Zealand) which is pollinated by honeyeater birds. Red, tubular flowers are common among bird-pollinated plants, and the tubular flowers of F. excorticata change colour from green to red. The purpose of the present study was to describe the timing of the colour change, dropping of the floral tube, and nectar production of F. excorticata and to determine how bellbirds (Anthornis melanura) and two introduced species of nectar robbers (Zosterops lateralis and Bombus sp.) respond to the different colour phases.

Floral tubes fell off about 11 days after anthesis in both sexes, with colour change occurring on about Day-4 for female trees and on about Day-5 for hermaphrodite trees. Green-phase hermaphrodite flowers produced significantly more nectar/day than did green-phase female flowers, while red-phase flowers did not produce nectar in either sex. All three floral visitors studied preferentially visited green-phase flowers and virtually ignored the nectarless flowers in the red phase. These results contrast with the general association between red, bird-pollinated flowers and the presence of a nectar reward. We suggest that the non-migratory habit of the New Zealand honeyeaters and the lack of native insect visitors to this species may account for this anomalous green-to-red colour change.     

Male‐biased hermaphrodites in a gynodioecious shrub,Daphne jezoensis

Gynodioecy, a state where female and hermaphrodite plants coexist in populations, has been widely proposed an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy. In the gynodioecy–dioecy pathway, hermaphrodites may gain most of their fitness through male function once females invade populations. To test this prediction, comprehensive studies on sex ratio variation across populations and reproductive characteristics of hermaphrodite and female phenotypes are necessary. This study examined the variation in sex ratio, sex expression, flower and fruit production and sexual dimorphism of morphological traits in a gynodioecious shrub, Daphne jezoensis, over multiple populations and years. Population sex ratio (hermaphrodite:female) was close to 1:1 or slightly hermaphrodite‐biased. Sex type of individual plants was largely fixed, but 15% of plants changed their sex during a 6‐year census. Hermaphrodite plants produced larger flowers and invested 2.5 times more resources in flower production than female plants, but they exhibited remarkably low fruit set (proportion of flowers setting fruits). Female plants produced six times more fruits than hermaphrodite plants. Low fruiting ability of hermaphrodite plants was retained even when hand‐pollination was performed. Fruit production of female plants was restricted by pollen limitation under natural conditions, irrespective of high potential fecundity, and this minimised the difference in resources allocated to reproduction between the sexes. Negative effects of previous flower and fruit production on current reproduction were not apparent in both sexes. This study suggests that gynodioecy in this species is functionally close to a dioecious mating system: smaller flower production with larger fruiting ability in female plants, and larger flower production with little fruiting ability in hermaphrodite plants.     

Nectar robbers influence the trait–fitness relationship of Primula secundiflora

• The trait–fitness relationship influences the strength and direction of floral evolution. To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations.
• We experimentally quantified phenotypic selection on floral traits through female fitness and estimated the casual effects of nectar robbing with different nectar robbing intensities on trait–fitness relationships in both the L‐ (long‐style and short‐anther phenotype) and S‐morph (short‐style and long‐anther phenotype) flowers among Primula secundiflora populations.
• A larger number of flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in the P. secundiflora populations. The indirect effects of these two traits on female fitness were mediated by nectar robbers. The indirect effect of the number of flowers on female fitness increased with increasing nectar robbing intensity. In most populations, the direct and/or indirect effects of floral traits on female fitness were stronger in the S‐morph flowers than in the L‐morph flowers. In addition, nectar robbers had a direct positive effect on female fitness, but this effect varied between the L‐ and S‐morph flowers.
• These results show the potential role of nectar robbers in influencing the trait–fitness relationships in this primrose species.

     

Sex Ratio and Sex‐specific Latitudinal Variation in Floral Characteristics of Gynodioecious Kallstroemia grandiflora (Zygophyllaceae) in Mexico

In species with broad distribution, geographic variation could be associated to particular environmental factors that may affect plant reproduction. In dimorphic species, environmental factors may influence evolutionary processes governing sex ratio variation and floral traits evolution. Kallstroemia grandiflora is an annual insect‐pollinated gynodioecious herb with broad distribution from the Sonoran desert to the semiarid west coast of Mexico. In this study, we explored if female frequency increases in harsh environments and the associated changes in floral morphology in female and hermaphrodite plants. We selected 23 populations along the pacific coast of Mexico where we estimated female frequency and in eight of them we also measured flower size in hermaphrodite and female plants. The average decrease in precipitation was correlated with the increase in female frequency, and a reduction in the flower size of hermaphrodite plants. Our results support the idea that harsh environments may influence female frequency, but also flower size and possibly breeding system evolution.     

Selective nectar robbing in a gynodioecious plant (Glechoma longituba) enhances female advantage

Nectar robbing not only affects the reproductive fitness of the plant but it may also potentially affect the pollination dynamics of the associated coflowering individuals. In this study, we established that the nectar robber Xylocopa sinensis robs nectar only from the hermaphrodite ramets of the gynodioecious plant Glechoma longituba but not from the female ramets. In populations with high rates of nectar robbing, this results in hermaphrodite ramets having reduced seed set whereas the female ramets have a slightly increased seed set. We hypothesize that selective nectar robbing confers an advantage to female individuals and thus ensures their maintenance in gynodioecious populations. Results of controlled experiments indicated that the reduction in the amounts of nectar available occasioned by nectar robbing resulted in some legitimate pollinators switching to visiting flowers on female rather than hermaphrodite ramets. This resulted in lower pollination rates and seed set for hermaphrodites and higher pollination rates and seed set for females. This study presents a previously unreported mechanism causing female advantage in gynodioecious plants.     

Gynodioecy in Cirsium chikushiense Koidz. (Compositae)

A study of the morphology and function of flowers in Cirsiumchikushiense revealed that the species was distinctly gynodioecious.Self-incompatible hermaphrodite florets produced both seedsand pollen grains, while female ones produced seeds but no pollengrains at all. The degenerated stamens of females were not onlysmaller but also sometimes occurred at a lower position insidethe floral tube than in hermaphrodites. The stigmata of femalesoften developed more fully than those of hermaphrodites. Thefrequencies of female plants in natural populations varied from15·5 to 50%. Almost all the pollinators stayed on bothfemale and hermaphrodite heads only to collect nectar. The femaleplants of this species may be more specialized in their genderby saving the cost of not only pollen grains but also stamens,and may be maintained by sufficient pollinators in natural populations.This gynodioecy may provide an example of nuclear-cytoplasmicmale sterility.Copyright 1994, 1999 Academic Press Sex expression, gynodioecy, Cirsium chikushiense, Compositae, male sterility, degenerated stamen, female frequency     

Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation

Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.     

Sex‐biased seed predation in gynodioecious Dianthus superbus var. longicalycinus (Capryophyllaceae) and differential influence of two seed predator species on the floral traits

Gynodioecy, the co‐occurrence of hermaphrodite and female individuals within a species, is maintained by differential reproductive success between sexes. Recently, researchers recognized that not only pollinators but also herbivores are important agents in the evolution and maintenance of gynodioecy, when herbivory is hermaphrodite biased. In this study, we investigated whether there is hermaphrodite‐biased herbivory in a gynodioecious plant, Dianthus superbus var. longicalycinus, and if so, what floral traits influenced hermaphrodite‐biased herbivory. We measured flower morphology (flower diameter, calyx tube length, corolla height and petal width) and phenology of flowers of female individuals, hermaphrodites and gynomonoecious individuals in a natural population. We also investigated seed predation and predator species. At the study site, Sibinia weevils (Curculionidae; Coleoptera) and Coleophora moths (Coleophoridae; Lepidoptera) were common pre‐dispersal seed predators in this species. The weevil appeared early in the flowering season, and weevil predation correlated with flower phenology. Because female individuals did not flower early in the season, weevil predation was less frequent in female individuals. Moth predation correlated with calyx length. The calyx length of flowers of female individuals was smaller than those of hermaphrodites, but a direct comparison of moth predation rates failed to find a significant difference among sex morphs. We found that the two seed predators had different effects on floral traits in D. superbus var. longicalycinus. We suggest that weevil predation contributes to the maintenance of gynodioecy because female individuals successfully escaped weevil predation by flowering late. It remains unclear why flower phenology is different among sex morphs.     

Maytenus obtusifolia Mart. (Celastraceae): a tropical woody species in a transitional evolutionary stage of the gynodioecy–dioecy pathway

‘Gynodioecy–dioecy’ is one of the pathways by which dioecy can arise from hermaphroditism. Studies on sex determination and development of gynodioecious systems have focused on temperate and/or annual species. Little is known about the evolutionary dynamics of gynodioecy and dioecy in perennial tropical species, where these systems have more frequently evolved. Maytenus obtusifolia is an abundant species in restingas in southeastern Brazil. The sexual system of M. obtusifolia was investigated by studying the floral structure and reproductive biology. We considered the sexual system as an intermediate step in the pathway gynodioecy-dioecy. The characterization of the hermaphrodite morph was complex, because of a gradient of variation in floral morphology and reproductive characteristics (sizes of the style and stigmatic surface, pollen viability, embryo sacs containing hypertrophied synergids, and fruit set). This variation leads to different proportions of functional male and female flowers among hermaphrodite plants and is responsible for the different levels of reproductive success. Female reproductive success and pollen viability were negatively correlated with the hermaphrodite morph (r = ?0.67). The higher fruiting intensity and fruit/flower ratio of females (41 %) compared to hermaphrodites (2 %) and the fact that female plants produce more and better-quality seeds support the female compensation. We suggest that female sterility may be linked to the set of changes in the carpels. The differences in the quality and quantity of pollen grains of hermaphrodite plants, and the similar individual rates of pollen viability observed for three consecutive flowering events, may indicate a relationship with nuclear cytoplasmic sex determination.     

Sex‐specific floral attraction traits in a sequentially hermaphroditic species

• ● Many angiosperms are hermaphroditic and produce bisexual flowers in which male (pollen export) and female (stigma receptivity) functions are separated temporally. This sequential hermaphroditism may be associated with variation in flower size, color, or pattern, all of which may influence pollinator attraction. In this study, we describe variation in these traits across discrete functional sex stages within and between 225 greenhouse‐grown individuals of Clarkia unguiculata (Onagraceae). In addition, to identify the effects of floral phenotype on pollinator attraction in this species, we examine the effects of these floral traits on pollen receipt in ~180 individuals in an experimental field array.
• ● Petal area, ultraviolet (UV)‐absorbing nectar guide area, and blue and green mean petal reflectance differ significantly across the functional sex stages of C. unguiculata. Male‐ and female‐phase flowers display significantly different pollinator attraction traits. Petal and UV nectar guide area increase as flowers progress from male phase to female phase, while blue reflectance and green reflectance peak during anther maturation.
• ● In field arrays of C. unguiculata, female‐phase flowers with large UV nectar guides receive more pollen than those with small nectar guides, and female‐phase flowers with high mean blue reflectance values are more likely to receive pollen than those with low blue reflectance. Female‐phase flowers with green mean reflectance values that differ most from background foliage also receive more pollen than those that are more similar to foliage. These findings indicate that components of flower color and pattern influence pollen receipt, independent of other plant attributes that may covary with floral traits. We discuss these results in the context of hypotheses that have been proposed to explain sex‐specific floral attraction traits, and we suggest future research that could improve our understanding of sexual dimorphism in sequentially hermaphroditic species and the evolution of features that promote outcrossing.

     

Synchronized dichogamy and dioecy in neotropicalLauraceae

In hermaphrodite neotropicalLauraceae a highly evolved dichogamous system is present which represents a kind of temporal dioecy. This system involves the existence of two flower morphs which are characterized by reciprocal phases of receptivity of the stigmas and pollen release. In some genera (Persea, Cinnamomum), nectar is produced as a reward for the flower visitors, while in other genera (Aniba, Clinostemon, Licaria), nectar is absent and pollen seems to be the only reward. This implies that in this case the flowers in the female stage must be deceptive flowers. In dioecious species of the generaOcotea andNectandra, both the male and female flowers attract the visitors with nectar. The pollen-ovule ratio of theLauraceae is comparatively low. — The type of reproductive system that characterizes theLauraceae, comprising functional dioecy, small, inconspicuously coloured flowers, pollination by small bees, and large, one-seeded fruits dispersed by birds, is quite prominent among trees of various families in the tropical lowland forest. The relationship between the different modes of flowering within theLauraceae and the causes for the correlation of their reproductive traits are discussed.     

Pollinators shift to nectar robbers when florivory occurs,with effects on reproductive success in Iris bulleyana (Iridaceae)

• Studies have indicated that florivory and nectar robbing may reduce reproductive success of host plants. However, whether and how these effects might interact when plants are simultaneously attacked by both florivores and nectar robbers still needs further investigation.
• We used Iris bulleyana to detect the interactions among florivory, nectar robbing and pollination, and moreover, their effects on plant reproductive success. Field investigations and hand‐pollination treatments were conducted on two experimental plots from a natural population, in which Experimental plot was protected from florivores and Control plot was not manipulated.
• The flower calyx was bitten by sawflies to consume the nectary, and three bumblebee species were pollinators. In addition, the short‐tongued pollinator, Bombus friseanus, was the only robber when there was a hole made by a sawfly. The bumblebee had significantly shortened flower handling time when robbing, as compared to legitimate visits. Pollinator visitation and seed production decreased significantly in damaged flowers. However, seed production per flower after supplementary hand‐pollination did not differ significantly between damaged and undamaged flowers. Compared to the Experimental plot, bumblebees visited fewer flowers per plant in a foraging bout in the Control plot.
• The flowers damaged by florivory allowed B. friseanus to shift to a nectar robber. Florivory and nectar robbing collectively decreased plant reproductive success by consuming nectar resources, which may reduce attractiveness to pollinators of the damaged flowers. However, the changes in pollinator behaviour might be beneficial to the plant by reducing the risk of geitonogamous mating.

     

Sex‐biased oviposition by a nursery pollinator on a gynodioecious host plant: Implications for breeding system evolution and evolution of mutualism

Dioecy, a breeding system where individual plants are exclusively male or female, has evolved repeatedly. Extensive theory describes when dioecy should arise from hermaphroditism, frequently through gynodioecy, where females and hermaphrodites coexist, and when gynodioecy should be stable. Both pollinators and herbivores often prefer the pollen‐bearing sex, with sex‐specific fitness effects that can affect breeding system evolution. Nursery pollination, where adult insects pollinate flowers but their larvae feed on plant reproductive tissues, is a model for understanding mutualism evolution but could also yield insights into plant breeding system evolution. We studied a recently established nursery pollination interaction between native Hadena ectypa moths and introduced gynodioecious Silene vulgaris plants in North America to assess whether oviposition was biased toward females or hermaphrodites, which traits were associated with oviposition, and the effect of oviposition on host plant fitness. Oviposition was hermaphrodite‐biased and associated with deeper flowers and more stems. Sexual dimorphism in flower depth, a trait also associated with oviposition on the native host plant (Silene stellata), explained the hermaphrodite bias. Egg‐receiving plants experienced more fruit predation than plants that received no eggs, but relatively few fruits were lost, and egg receipt did not significantly alter total fruit production at the plant level. Oviposition did not enhance pollination; egg‐receiving flowers usually failed to expand and produce seeds. Together, our results suggest that H. ectypa oviposition does not exert a large fitness cost on host plants, sex‐biased interactions can emerge from preferences developed on a hermaphroditic host species, and new nursery pollination interactions can arise as negative or neutral rather than as mutualistic for the plant.     

Pollination and breeding system of the enigmatic South African parasitic plant Mystropetalon thomii (Mystropetalaceae): rodents welcome,but not needed

• Unrelated plants adapted to particular pollinator types tend to exhibit convergent evolution in floral traits. However, inferences about likely pollinators from ‘pollination syndromes’ can be problematic due to trait overlap among some syndromes and unusual floral architecture in some lineages. An example is the rare South African parasitic plant Mystropetalon thomii (Mystropetalaceae), which has highly unusual brush‐like inflorescences that exhibit features of both bird and rodent pollination syndromes.
• We used camera traps to record flower visitors, quantified floral spectral reflectance and nectar and scent production, experimentally determined self‐compatibility and breeding system, and studied pollen dispersal using fluorescent dyes.
• The dark‐red inflorescences are usually monoecious, with female flowers maturing before male flowers, but some inflorescences are purely female (gynoecious). Inflorescences were visited intensively by several rodent species that carried large pollen loads, while visits by birds were extremely rare. Rodents prefer male‐ over female‐phase inflorescences, likely because of the male flowers’ higher nectar and scent production. The floral scent contains several compounds known to attract rodents. Despite the obvious pollen transfer by rodents, we found that flowers on both monoecious and gynoecious inflorescences readily set seed in the absence of rodents and even when all flower visitors are excluded.
• Our findings suggest that seed production occurs at least partially through apomixis and that M. thomii is not ecologically dependent on its rodent pollinators. Our study adds another species and family to the growing list of rodent‐pollinated plants, thus contributing to our understanding of the floral traits associated with pollination by non‐flying mammals.

     

Population Size,Pollination and Breeding System of Silene stockenii Chater (Caryophyllaceae), an Annual Gynodioecious Species of Southern Spain

The distribution area, phenology, sex polymorphism, floral characteristics and breeding system of Silene stockenii (Caryophyllaceae), a narrowly endemic annual species of southern Spain, were studied. Only five populations were found in a total area of 2 ha. Silene stockenii is a gynodioecious species with fully female, fully hermaphrodite and intermediate plants bearing hermaphrodite, male-sterile, and partially male-sterile flowers. Male-sterile flowers are typically smaller than hermaphrodites. Nectar production was significantly higher in hermaphrodite plants and during the female phase of hermaphrodite flowers. The red flowers appear during the spring (March-May) and are pollinated by long-tongued Bombyliidae. Hand pollinations revealed that the species is self-compatible; however, natural self-pollination is rare due to marked protandry. Hand pollination significantly increased the number of seeds per fruit and seed set, indicating limited pollination in the field. In controlled pollinations female plants of S. stockenii produced higher seed set than hermaphrodite plants, but in freely pollinated plants fruit set and seed production was similar in both morphs, indicating that pollinators do not discriminate in favour of hermaphrodite plants.     

Does competition with wind‐pollinated species alter Echium plantagineum's attractiveness to a common pollinator Bombus terrestris?

1. In insect‐pollinated plants, pollinator attraction is influenced by flowers (e.g. number, size) and their associated rewards (e.g. pollen, nectar). These traits can depend on plant interactions. Indeed, below‐ground competition between plants can lead to a decrease in flower or reward production in insect‐pollinated species. 2. Wind‐pollinated plants, in particular, which are almost never studied in plant–pollinator networks, can alter insect‐pollinated plants' attractiveness through competition for nutrients. The response of pollinators to such changes has never been investigated. 3. A pot experiment was carried out in which an insect‐pollinated species, Echium plantagineum, was grown in binary mixture with three wind‐pollinated species selected to exert a panel of competitive interactions. Below‐ground competition was controlled using dividers limiting interspecific root competition. Floral traits of E. plantagineum (i.e. flower production, floral display size, flower size and nectar production) were measured. For each species mixture, the visits (i.e. first visit, number of visits, 10‐min sequences) of Bombus terrestris individuals released in a flight cage containing two pots were followed, one with and one without below‐ground competition. 4. Below‐ground competition significantly affected nectar's sucrose concentration but did not influence flower and nectar production. Likewise, pollinator visits were not influenced by below‐ground competition. Competitor identity significantly influenced flower and reward production of E. plantagineum, with a decrease in the presence of the most competitive wind‐pollinated species. A tendency for faster flower visitation events was also detected in the presence of the least competitive competitor. This study raises new questions regarding the influence of wind‐pollinated plants on plant–pollinator interactions.     

Reproductive isolation is mediated by pollen incompatibility in sympatric populations of two Arnebia species

To explore uncertain aspects of the processes that maintain species boundaries, we evaluated contributions of pre‐ and postpollination reproductive isolation mechanisms in sympatric populations of Arnebia guttata and A. szechenyi. For this, we investigated their phylogenetic relationships, traits, microenvironments, pollinator visits, action of natural selection on floral traits, and the outcome of hand pollination between the two species. Phylogenetic analysis indicates that A. szechenyi is a derived species that could be closely related to A. guttata, and both could be diploid species. Arnebia guttata flowers have larger parts than A. szechenyi flowers, but smaller nectar guides. Soil supporting A. szechenyi had higher water contents than soil supporting neighboring populations of A. guttata (in accordance with their geographical distributions). The pollinators shared by the two species preferred A. szechenyi flowers, but interspecific visitations were frequent. We found evidence of conflicting selection pressures on floral tube length, flower diameter and nectar guide size mediated via male fitness, and on flower diameter and floral tube diameter via female fitness. Hand‐pollination experiments indicate complete pollen incompatibility between the two species. Our results suggest that postpollination prezygotic mechanisms are largely responsible for reproductive isolation of sympatric populations of the two Arnebia species.