Pollination system of Holcoglossum rupestre (Orchidaceae): A special and unstable system

The pollination system of Holcoglossum rupestre was studied in northwestern Yunnan. The species is self-compatible but pollinator-dependent and offers nectar as a reward to visitors. A species of beetle, Hybovalgus bioculatus Kolbe, is identified as the pollinator among all visitors observed. The beetles carried the pollinia in two different positions, i.e. on the buttock and the foreleg, and pollinated the flowers by two different mechanisms. The analysis suggests that Holcoglossum rupestre and its pollinator may be in an unstable pollination relationship. The natural fruit set is low, which is attributed to the low visiting frequency of pollinator and the effective mechanism to prevent self-pollination.     

The effect of petal-size manipulation on pollen removal,seed set,and insect-visitor behavior in Campanula americana

We investigated the effect of petal-size variation in tall bellflower, Campanula americana, on pollen removal rates, seed set, and pollinator behavior. Pollen removal, seed set, and the behavior of two bee visitors (Bombus and Halictus) were assessed in flowers that had 0%, 50%, or 100% of their petal lobes removed. Pollen removal rates did not differ significantly among the three treatment groups, probably due to the presence of an ineffective (ugly) pollinator (Halictus), which quickly removed pollen from completely reduced flowers. Seed set was significantly positively correlated with petal size. Bumblebees discriminated against reduced flowers, thereby lowering pollen movement to reduced flowers. Halictid bees rarely contacted stigmatic surfaces and thus pollen delivery to reduced flowers was limited. The presence of an ugly pollinator probably obscured the effect of petal reduction on pollen receipt, thus masking the impact of attractive structures on male fitness. Given the positive relationship between seed set and petal size, we caution against ascribing expenditure on attractive structures exclusively to male function. In plants with many olules, expenditure on attractive structures may affect seed set.     

What We Think We Know vs. What We Need to Know About Orchid Pollination and Conservation: <Emphasis Type="Italic">Cypripedium</Emphasis> L. as a Model Lineage

While Darwin (1862, 1877) showed that reproductive success in orchid populations depended on adaptive floral morphology coupled with pollinator visitation a more recent review of the literature (Tremblay et al., 2005) confirmed that many out-breeding species are pollinator-limited because most orchid species showing low fecundity also lack rewards. The absence of rewards depresses both pollinator fidelity and the frequency of pollinator visits to an orchid population even though orchid flowers that lack rewards retain the same interlocking floral structures for precise pollinia removal and deposition found in related species that offer rewards. Using the genus, Cypripedium, as a model lineage of non-rewarding flowers this study also shows that the correlation between low fruit set in a Cypripedium sp. and its specific pollinator(s) is insufficient to predict specific frequencies of low fecundity. Annual rates of fruit set often vary broadly between populations of the same species and within the same population over several seasons. We speculate that fruit-set rates also decline when orchid demography and additional biotic and abiotic factors interrupt rates of pollinator activity (pre-zygotic) and fertilization/fruit maturation (post-zygotic). We suggest that that traditional field studies on pollination ecology and breeding systems be combined with data sets recording genetic variation and orchid flower demography in relation to seasonal variation in climate. We also propose that the same information be collected in regard to genetic variation, demography and phenology of populations of known orchid pollinators and co-blooming angiosperm species native to orchid habitats.     

Diverse visitors with various pollinator importance and temporal change in the important pollinators of <Emphasis Type="Italic">Geranium thunbergii</Emphasis> (Geraniaceae)

Assessing pollinator importance of each floral visitor to a plant species is a key to understanding plant–pollinator interaction. The present study examined visitation frequency, pollination efficiency, and pollinator importance of the full range of floral visitors to Geranium thunbergii natural population, by measuring seed-set. During 2 years of observations, the flowers were visited by at least 45 insect species belonging to four orders. Among the main 22 visitor species, 11 species belonging to three orders (Hymenoptera, Diptera, and Lepidoptera) acted as the efficient pollinators. In both years, Hymenoptera, especially bees, was the most important pollinator to G.thunbergii. Thus, the flowers could be considered as bee-pollinated. However, the most important species were not constant between years. The study also documented that the efficient pollinators have larger body sizes. The dish-shaped floral morphology, taxonomically diverse pollinators, and temporal change in the most important pollinators indicate that G.thunbergii–pollinator interaction is a rather generalized system. The results suggest that casual observations of visitation, or even precise measurement of pollinator importance in a single season is insufficient to identify important pollinators.     

Pollination biology of Eulophia alta (Orchidaceae) in Amazonia: effects of pollinator composition on reproductive success in different populations

Background and Aims

Spatial variation in pollinator composition and abundance is a well-recognized phenomenon. However, a weakness of many studies claiming specificity of plant–pollinator interactions is that they are often restricted to a single locality. The aim of the present study was to investigate pollinator effectiveness of the different flower visitors to the terrestrial orchid Eulophia alta at three different localities and to analyse whether differences in pollinator abundance and composition effect this plant''s reproductive success.

Methods

Natural pollination was observed in vivo, and manipulative experiments were used to study the pollination biology and breeding system of E. alta at three sites near Manaus, Brazil. To gain a better understanding of the underlying mechanisms of pollinator attraction, nectar composition and secretion patterns were also studied, floral scent composition was analysed and a bioassay was conducted.

Key Results

Flower visitors, pollinator composition, pollinia transfer efficiency of particular pollinator species and natural fruit set differed among the investigated populations of E. alta. Flowers were self-compatible, partially autogamous and effectively pollinated by five bee species (four Centris species and Xylocopa muscaria). Visiting insects appeared to imbibe small amounts of hexose-rich nectar. Nectar sugar content was highest on the third day after flower opening. Floral fragrance analyses revealed 42 compounds, of which monoterpenes and benzenoids predominated. A bioassay using floral parts revealed that only floral tissue from the labellum chamber and labellum tip was attractive to flower visitors.

Conclusions

The data suggest that observed differences in reproductive success in the three populations cannot be explained by absolute abundance of pollinators alone. Due to behavioural patterns such as disturbance of effective pollinators on flowers by male Centris varia bees defending territory, pollinia transfer efficiencies of particular pollinator species also vary between study sites and result in differing reproductive success.     

Flexibility of Pollinator-Flower Relationship in Isodon umbrosus and I. effusus (Lamiaceae) and its Relation to Fruit-set and Seed-set

Isodon umbrosus was pollinated by two bumblebee pollinators, Bombus honshuensis and B. diversus. B. diversus was a secondary pollinator when both pollinators were present, but in the absence of B. honshuensis, B. diversus pollinated I. umbrosus, and the resulting seed production was comparable to when visited by the two bumblebee species together. Not only the high visitation frequency of B. diversus but also the low ratio of seed to ovules (= average seed set × fruit set) of I. umbrosus compensated for the low pollination efficiency of B. diversus. Since, the low pollination efficiency of B. diversus, based on the finding that the low average seed set of I. umbrosus did not require a definite morphological match between flowers and pollinators, and did not require very frequent visitations to each flower. In other words, Isodon umbrosus could briefly maintain constant seed production under the condition of the low ratio of seeds to ovules (the low seed set and the low fruit set) and a large number of flowers without B. honshuensis as legitimate pollinator. On the other hand, I. effusus was exclusively pollinated by B. diversus and produced almost the same volume of seeds as I. umbrosus. Isodon effusus had fewer flowers than I. umbrosus, but this smaller number of flowers was compensated for by the high ratio of seeds to ovules in I. effusus, which was maintained by frequent visitations of the morphologically matched pollinator, B. diversus. Received 17 September 1999/ Accepted in revised form 10 February 2000     

A nocturnal Provespa wasp species as the probable pollinator of epiphytic orchid Coelogyne fimbriata

Many vespid wasps visit flowers to forage nectar. These hymenopterans sometimes contribute to flower pollination. However, none of the nocturnal wasp species is a known pollinator. We collected individuals of light‐attracted Provespa nocturna workers in a montane rainforest on Peninsular Malaysia: some wasps collected bore orchid pollinia on their thoraxes. Among 114 trapped individuals, four bore pollinaria and nine bore only viscidia, suggesting that pollinia had been successfully transported. Molecular barcoding of the pollinia (based on their ITS sequences) assigned the orchid to a species in Coelogyne fimbriata complex. These findings and our other analyses suggest that this nocturnal wasp contributes to pollination of an epiphytic nectarless orchid that probably releases olfactory attractants. This discovery sheds light on the importance of mutualistic relationships between the nocturnal social wasps and epiphytic orchids in Southeast Asian tropical rainforest canopies.     

HasIxianthes (Scrophulariaceae) lost its special bee?

Ixianthes retzioides is a rare, moderately sized, shrub with large yellow flowers that are lined with glandular trichomes. Preliminary analysis indicates that the trichomes secrete a complex mixture of lipids. Floral morphology suggests thatIxianthes should be pollinated by a large oil-collecting bee, yet only one possible candidate,Rediviva gig, is known and it occurs outside of the distributional range ofIxianthes. Despite observations ofIxianthes at different sites in different years, no oil-collecting bee has been observed visiting the flowers. This together with low fruit and seed set at the main study site suggests thatIxianthes has lost, at least locally, its specialized pollinator. Plants are self-compatible and set a limited amount of seed as a result of visits by pollen-collecting bees.     

Breeding systems of the Japanese-endemic carnivorous sundew species Drosera makinoi and Drosera toyoakensis (Droseraceae)

In response to unfavorable environmental conditions, some Drosera species in the northern hemisphere do not open their flowers but form bud-like flowers. The bud-like flowers exhibit pseudo-cleistogamous behavior and produce similar numbers of seeds as open flowers. However, it is not clear how common this type of reproduction is in the genus Drosera. In this study, we examined the effects of flowering (opening flowers/forming bud-like flowers) and pollinator visitation on seed production in Drosera makinoi and D. toyoakensis, two endemic species in Japan. D. makinoi and D. toyoakensis plants did not open 20%–40% chasmogamous flowers to form bud-like flowers. The numbers of mature seeds produced by bud-like flowers were significantly lower than those produced by open flowers (0%–2% of open flowers). In D. makinoi, the percentage of bud-like flowers correlated negatively with the seed production per plant, suggesting that the production of bud-like flowers decreased the lifetime reproductive success. In both D. makinoi and D. toyoakensis, pollinator-excluded flowers produced similar numbers of seeds as control flowers. Our results suggest that pseudo-cleistogamy does not occur in D. makinoi and D. toyoakensis, and they produce seeds without pollinator visits once flowers open.     

Can short‐billed nectar thieving sunbirds replace long‐billed sunbird pollinators in transformed landscapes?

• Pollinator specialisation through exploitation barriers (such as long floral tubes) does not necessarily mean a lack of pollination when the favoured pollinator is rare or absent. Theory predicts that suboptimal visitors will contribute to plant reproduction in the absence of the most effective pollinator. Here I address these questions with Chasmanthe floribunda a long‐tubed plant species in the Cape Floristic Region, which is reliant on one species of pollinator, the long‐billed Malachite Sunbird. In contrast to short‐billed sunbirds, the Malachite Sunbird occurs in lower abundance or is absent in transformed landscapes. Short‐billed sunbirds rob and thieve nectar from long‐tubed flowers, but their potential contribution towards pollination is unknown.
• Experiments assessing seed set after single flower visits were performed to determine whether thieving short‐billed sunbirds can act as substitute pollinators. To determine whether short‐billed sunbirds reduce pollen limitation in transformed areas, pollen supplementation was done by hand and compared to natural fruit set.
• Short billed sunbirds are unable to act as substitute pollinators, and seed set is significantly lower in the flowers that they visited, compared to flowers visited by long‐billed sunbirds. This is substantiated on a landscape scale, where fruit production in Chasmanthe floribunda could artificially be increased by 35% in transformed landscapes, but not so in natural areas.
• These findings have important consequences for the management and conservation of long‐tubed bird‐pollinated plant species that exist in recently transformed landscapes. The potential vulnerability of specialised plant species in transformed landscapes is highlighted.

     

Plant interactions for pollinator visits: a test of the magnet species effect

Summary From 1985–1987, patterns of fruit and seed set were studied in a population of mayapple (Podophyllum peltatum), a clonal, self-incompatible herb found in deciduous woods in eastern North America. Mayapple flowers do not produce nectar, but depend on infrequent visits by nectar-seeking queen bumble bees for pollination. In all years female reproductive success in mayapple colonies was influenced by colony size (number of flowers), by the distance to neighbouring colonies and by proximity to lousewort plants (Pedicularis canadensis), a prolific nectar producer heavily visited by bumble bees. In all years fruit and seed set were greater in mayapple colonies <25 m from lousewort flowers than in matched colonies which were >50 m from lousewort. In 1985 and 1987 the frequency of queen bumble bee visits to flowers in colonies close to lousewort was about four times greater than to distant colonies. In 1986 I removed about 80% of lousewort flowers to test whether the enhanced fruit and seed set in mayapples close to lousewort was pollinator mediated. Mayapple colonies close to flowerless lousewort patches did not differ in fruit or seed set from matched colonies >50 m from lousewort. In contrast, mayapples close to flowering lousewort patches had greater fruit and seed set compared with distant colonies. Over all years, a larger proportion of mayapples close to flowering lousewort patches had enhanced fruit and seed set compared with colonies close to louseworts without flowers. Though rarely documented, this type of facilitative interaction between plants that are highly attractive to pollinators (magnet species), and co-flowering species that are rarely visited by pollinators, may be widespread in plant communities.     

Do pollinators influence the assembly of flower colours within plant communities?

The co-occurrence of plant species within a community is influenced by local deterministic or neutral processes as well as historical regional processes. Floral trait distributions of co-flowering species that share pollinators may reflect the impact of pollinator preference and constancy on their assembly within local communities. While pollinator sharing may lead to increased visitation rates for species with similar flowers, the receipt of foreign pollen via interspecific pollinator movements can decrease seed set. We investigated the pattern of community flower colour assembly as perceived by native honeybee pollinators within 24 local assemblages of co-flowering Oxalis species within the Greater Cape Floristic Region, South Africa. To explore the influence of pollinators on trait assembly, we assessed the impact of colour similarity on pollinator choices and the cost of heterospecific pollen receipt. We show that flower colour is significantly clustered within Oxalis communities and that this is not due to historical constraint, as flower colour is evolutionarily labile within Oxalis and communities are randomly structured with respect to phylogeny. Pollinator observations reveal that the likelihood of pollinators switching between co-flowering species is low and increases with flower colour similarity. Interspecific hand pollination significantly reduced seed set in the four Oxalis species we investigated, and all were dependant on pollinators for reproduction. Together these results imply that flower colour similarity carries a potential fitness cost. However, pollinators were highly flower constant, and remained so despite the extreme similarity of flower colour as perceived by honeybees. This suggests that other floral traits facilitate discrimination between similarly coloured species, thereby likely resulting in a low incidence of interspecific pollen transfer (IPT). If colour similarity promotes pollinator attraction at the community level, the observed clustering of flower colour within communities might result from indirect facilitative interactions.     

Variation of pollinator assemblages and pollen limitation in a locally specialized system: the oil-producing Nierembergia linariifolia (Solanaceae)

Background and Aims

Few studies have examined the dynamics of specialist plant–pollinator interactions at a geographical scale. This knowledge is crucial for a more general evolutionary and ecological understanding of specialized plant–pollinator systems. In the present study, variations in pollinator activity, assemblage composition and pollen limitation were explored in the oil-producing species Nierembergia linariifolia (Solanaceae).

Methods

Pollen limitation in fruit and seed production was analysed by supplementary hand pollination in five wild populations. Pollinator activity and identity were recorded while carrying out supplementary pollination to assess the effect of pollinators on the degree of pollen limitation. In two populations, pollen limitation was discriminated into quantitative and qualitative components by comparing supplementation and hand cross-pollination in fruit set and seed set. The effect of flower number per plant on the number of flowers pollinated per visitor per visit to a plant was examined in one of these populations as a possible cause of low-quality pollination by increasing geitonogamy.

Results and Conclusions

Although pollen limitation was evident along time and space, differences in magnitude were detected among populations and years that were greatly explained by pollinator activity, which was significantly different across populations. Floral display size had a significant effect on the visitation rate per flower. Limitation by quality clearly affected one population presumably due to a high proportion of geitonogamous pollen. The great inter-population variation in plant–pollinator interaction (both in pollinator assemblages composition and pollinator activity) and fitness consequences, suggests that this system should be viewed as a mosaic of locally selective processes and locally specialized interactions.Key words: Nierembergia linariifolia, Centris, Chalepogenus, pollen limitation, pollen quality, oil-producing flowers, specialized pollination, floral display, assemblage composition, geographic variation, Solanaceae, tests of equivalence     

Breeding system and factors limiting fruit production in the nectarless orchid Broughtonia lindenii

Low fruit set values in most orchids (especially epiphytic and tropical species) are normally thought to be the consequence of pollination constraints and limited resources. In particular, pollination constraints are modulated by pollinator visitation rates, pollinator visitation behaviour (promoting crossing or selfing), the type and number of pollinia deposited on stigmas (in the case of orchids with subequal pollinia) and the amount of pollen loaded per inflorescence. In order to assess to what extent these factors can affect fruit set in specific orchid-pollinator systems, the repercussions of some of these aspects on reproduction of Broughtonia lindenii were examined in a coastal population in western Cuba. The study focused on plant breeding system, importance of pollen load and type of pollinia on subsequent fruit and seed, limiting factors of seed production and interaction with pollinators. This species presents long-lasting flowers that senesce after all forms of effective visit. Pollinator dependence for fruit production was demonstrated, while hand-pollination experiments revealed self-compatibility and inbreeding depression at seed level. More pollinia on stigmas enhance the proportion of well-developed seeds. In contrast, the pollinia type used in pollination is not important for seed quality of fruits, suggesting that small pollinia are not rudimentary. Natural fruit set in two consecutive years was substantially affected by pollinator activity, and also by systematic depredatory activity of ants and a caterpillar. Considering that this orchid completely lacks nectar and that the local assemblage of pollinators and predators influenced its reproduction, a minor importance of resource constraints in this epiphyte (with long-lasting reserve structures) is confirmed at least for a short time.     

Pollinators and crossability as reproductive isolation barriers in two sympatric oil-rewarding <Emphasis Type="Italic">Calceolaria</Emphasis> (Calceolariaceae) species

Pollinator species are widely accepted as an important factor in plant reproductive isolation. Although mostly investigated in plants visited by different groups of pollinators (e.g., hummingbirds vs bees), few studies have examined the role of pollinators belonging to the same taxonomic group (e.g., only bees) on plant reproductive isolation. In this study, we investigate this question by evaluating pre- and post-zygotic mechanisms putatively involved in the reproductive isolation of two oil-rewarding sympatric Calceolaria species (i.e., Calceolaria filicaulis and C. arachnoidea) in an Andean ecosystem of Chile. We estimated reproductive isolation values using a combination of field (pollinator visitation rates) and experimental (intra and interspecific manual cross-pollination and seed germination of parents and hybrids) evidence. The two Calceolaria species were preferentially visited by different oil-collecting bee species. Results from hand cross-pollination experiments indicate that intraspecific crossings produced significantly more seeds than interspecific ones. Notwithstanding, seed germination essays did not reveal differences between parental and hybrids. Taken together, these results suggest that pollinator species are responsible for most of the reproductive isolation in the two Calceolaria species studied here. This study is the first assessment of pollinator-mediated reproductive isolation in Calceolaria species and the first to document reproductive barriers in oil-rewarding plants.     

Honey bees are the dominant diurnal pollinator of native milkweed in a large urban park

In eastern North America, the field milkweed, Asclepias syriaca L. (Asclepiadaceae), is used in planting schemes to promote biodiversity conservation for numerous insects including the endangered monarch butterfly, Danaus plexippus (Linnaeus) (Nymphalidae). Less is known about its pollinators, and especially in urban habitats where it is planted often despite being under increasing pressure from invasive plant species, such as the related milkweed, the dog‐strangling vine (DSV), Vincetoxicum rossicum (Kleopow) Barbar. (Asclepiadaceae). During the A. syriaca flowering period in July 2016, we surveyed bees in open habitats along a DSV invasion gradient and inspected 433 individuals of 25 bee species in 12 genera for pollinia: these were affixed to bees that visited A. syriaca for nectar and contain pollen packets that are vectored (e.g., transferred) between flowers. Of all bees sampled, pollinia were found only on the nonindigenous honeybee, Apis mellifera (43% of all bees identified), as well as one individual bumblebee, Bombus impatiens Cresson. Pollinia were recorded from 45.2% of all honeybees collected. We found no relationship between biomass of DSV and biomass of A. syriaca per site. There was a significant positive correlation between A. syriaca biomass and the number of pollinia, and the proportion vectored. No relationship with DSV biomass was detected for the number of pollinia collected by bees but the proportion of vectored pollinia declined with increasing DSV biomass. Although we find no evidence of DSV flowers attracting potential pollinators away from A. syriaca and other flowering plants, the impacts on native plant–pollinator mutualisms relate to its ability to outcompete native plants. As wild bees do not appear to visit DSV flowers, it could be altering the landscape to one which honeybees are more tolerant than native wild bees.     

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.

     

Floral biology of Echinopsis chiloensis ssp. chiloensis (Cactaceae): Evidence for a mixed pollination syndrome

In this study the flower biology of Echinopsis chiloensis ssp. chiloensis, a columnar cactus occurring in Central Chile, is investigated, in particular its pollination syndrome, its visitors, their frequencies and behaviors and their pollination efficiencies. As statements on floral anthesis of this species are contradictory, this study also intended to elucidate both its beginning and duration. A pollinator exclusion study of a total of 162 flowers from 12 plants was conducted at one of the two study sites. Fruit and seed production as well as seed viability were documented to evaluate pollinator efficiencies.     

Seed set in Phormium: interactive effects of pollinator behaviour,pollen carryover and pollen source

Summary Fruit and seed set was studied in New Zealand flax (Phormium tenax Phormiaceae) a large monocot that preferentially sets outcrossed seeds. Fruit set was low and in particular situations could result from insufficient pollinator visitation. Observations of pollinator (Meliphagidae) movements showed that birds preferentially visited male phase flowers and predominantly moved pollen within inflorescences of the same plant. More dominant resident birds moved more between plants that subordinate non-resident birds. Combination of results of fluorescent dye carryover with known bird movements allowed predictions of fruit set and seed size that closely approximated observed levels. Resident birds account for almost half the observed foraging bouts but are predicted to be responsible for the vast majority of the viable seeds.     

Pollination biology of the endangered orchid Cypripedium japonicum in a fragmented forest of Japan

Pollination biology studies of the endangered orchid Cypripedium japonicum were conducted in its natural habitat using pollinator observation and hand‐pollination experiments. The observed fruit set was as follows: artificial outcross‐pollinated, 100%; artificial self‐pollinated, 100%; pollinator‐excluded, 0%; and emasculated flowers, 0%. These results show that this species, although self‐compatible, is neither autogamous nor agamospermous. The fruit set for open‐pollinated flowers was 14.9%, which suggests that the study population was subject to pollinator limitation. The nectarless flowers of C. japonicum were exclusively visited and pollinated by the queens of two bumblebee species (Bombus ardens and B. diversus diversus). It is probable that the nectarless flowers of C. japonicum attract pollinators through a generalized food deceptive system.