Pollen limitation in invasive populations of Solanum rostratum and its relationship to population size

Aims small plant populations may be more likely to suffer more severe pollen limitation due to the lower number of potential mates or suitable pollinators.For invasive species,this phenomenon may be more common when an invading population colonizes a new habitat.Here,we investigated whether pollen limitation occurs in invasive populations of Solanum rostratum during its invasion from North america to China and evaluated the patterns between pollen limitation and population size.Methods Pollen addition experiments were performed on six invasive populations of S.rostratum.by comparing fruit set and seed production with open pollination treatment,we calculated the index of pollen limitation and regressed it to population size and density.Important Findingsamong the six sampled invasive populations of S.rostratum,the fruit set and seed production per fruit were 0.346±0.014 and 52.38±9.29,respectively,with open pollination treatment and 0.572±0.022 and 56.28±10.79,respectively,with pollen addition treatment.Compared with open pollination,pollen addition significantly increased fruit set and seed production by 65.3 and 7.4%,respectively.The standardized index of pollen limitation ranged from 0.022 to 0.125,with an average of 0.065,suggesting that invasive populations of S.rostratum do suffer from pollen limitation.The index of pollen limitation was negatively correlated with population size,which is consistent with the pattern that smaller populations suffer from more severe pollen limitation.

Reproduction of Hedysarum scoparium (Fabaceae) in patched habitat is pollen limited,but not just pollinator limited

Pollen limitation of plant reproduction occurs across Angiosperms, particularly those in patched habitats We investigated the, relationship between pollen limitation and patch variables （patch size, visitation frequency） in the desert plant Hedysarum scoparium （Fabaceae）, which is an important xerophyte in the arid and semi-arid regions of Northwest China and can grow well as a pioneer plant in shifting sand dunes. We observed insect visitation to H. scoparium over two flowering seasons and estimated pollen limitation using fruit set and seed production. Our results indicate that fruit set and seed production increased significantly with pollen supplementation compared with open pollination. Hedysarum scoparium was pollinated by over 8 species of bees, with 88.4% of visits made by introduced honeybees （Apis mellifera）. Bee visitation varied significantly among the patches of habitats, but not associated with patch size of habitat. In general, pollen limitation occurred more strongly during fruit set than during seed production. The patches that received higher rates of pollinator visits were less pollen limited for fruit set. Pollen limitation for seed production, however, was not associated with pollinator visitation frequency. We conclude that pollen limitation in H. scoparium was caused by more than one reason, not just pollinator visits.

Does pollinator dependence decrease along elevational gradients?

Plants have long been thought to be less dependent on pollinators for seed production at higher elevations due to adverse pollination environments.However,recent research has yet to consistently support the generality of this expectation.In this study,we asked whether pollinator dependence decreases along an elevational gradient and how it varies with various reproductive traits.To answer these questions,we quantified pollinator-plant associations and various reproductive traits for 112 flowering plants spanning a large elevational gradient(990-4260 m a.s.l.)in the Qinghai-Tibet Plateau.We found that flowering plants in the Qinghai-Tibet Plateau region are highly dependent on pollinators for seed production(76.2%of seed production was contributed by animal pollinators and 44.6%of plants would produce no seed without pollinator visitation).Contrary to our expectation,there was no significant elevational gradient in pollinator dependence index.Although the pollinator dependence index was not significantly correlated with pollen limitation,flower size,floral longevity,or reward type,it was correlated with compatibility status and flowering time.These findings indicate that pollinator dependence does not decrease along an elevational gradient in the Qinghai-Tibet Plateau.Our study also highlights the severe vulnerability of flowering plant seed production to pollinator declines under global change in the Qinghai-Tibet Plateau region,particularly for early-flowering or self-incompatible plants growing at higher elevations(e.g.,subnival belt).

Floral traits and pollination system of Zygophyllum xanthoxylum in the managed and wild populations in an arid region of Northwest China

Zygophyllum xanthoxylum, which belongs to Sarcozygium of Zygophyllaceae, is one of the ecologically important species in Northwest China. In order to understand the pollination system of Z. xanthoxylum, we investigated the following characteristics of this species in the Urat Desert-grassland Research Station in western Inner Mongolia of China： flowering dynamics, pollen viability, pollen limitation, floral visitors and breeding system. The results showed that the flowering period and flowering peak were different between the wild and managed popula- tions, being longer in the managed population. Z. xanthoxylum was pollen-limited, and pollen limitation was more intense in the wild population than in the managed population. Chalicodoma deserticola （Hymenoptera） was found to be the most frequent pollinator in the wild population, while Anthophora fulvitarsis （Hymenoptera） was the most frequent and effective visitor in the managed population. Out-crossing was dominant in the breeding system and self-pollination just played an assistant role to assure the reproduction of Z. xanthoxylum.

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.

Pollinator diversity benefits natural and agricultural ecosystems,environmental health,and human welfare

Biodiversity loss during the Anthropocene is a serious ecological challenge.Pollinators are important vectors that provide multiple essential ecosystem services but are declining rapidly in this changing world.However,several studies have argued that a high abundance of managed bee pollinators,such as honeybees(Apis mellifera),may be sufficient to provide pollination services for crop productivity,and sociological studies indicate that the majority of farmers worldwide do not recognize the contribution of wild pollinator diversity to agricultural yield.Here,we review the importance of pollinator diversity in natural and agricultural ecosystems that may be thwarted by the increase in abundance of managed pollinators such as honeybees.We also emphasize the additional roles diverse pollinator communities play in environmental safety,culture,and aesthetics.Research indicates that in natural ecosystems,pollinator diversity enhances pollination during environmental and climatic perturbations,thus alleviating pollen limitation.In agricultural ecosystems,pollinator diversity increases the quality and quantity of crop yield.Furthermore,studies indicate that many pollinator groups are useful in monitoring environmental pollution,aid in pest and disease control,and provide cultural and aesthetic value.During the uncertainties that may accompany rapid environmental changes in the Anthropocene,the conservation of pollinator diversity must expand beyond bee conservation.Similarly,the value of pollinator diversity maintenance extends beyond the provision of pollination services.Accordingly,conservation of pollinator diversity requires an interdisciplinary approach with contributions from environmentalists,taxonomists,and social scientists,including artists,who can shape opinions and behavior.

Natural selection on floral traits of Caltha scaposa (Ranunculaceae), an alpine perennial with generalized pollination system from Northwest Yunnan

Floral traits, including those invisible to humans but visible to pollinators, that increase pollination efficiency may be selected by pollinators in plant species with pollen limitation of seed production, but the importance of pollinators as selective agents on different floral traits needs to be further quantified experimentally. In the present study, we examined selective strength on flower diameter, flower height,UV bulls-eye size, sepal size and UV proportion via female fitness in Caltha scaposa, based on openpollinated and hand-pollinated flowers, through which pollinator-mediated selection was calculated for each of floral traits. Our results suggest that seed production of C. scaposa is pollen limited in natural conditions. There was directional selection(△β_(pollinator)=-0.12) for larger flowers in open-pollinated flowers, while no significant selection was found in flower height, UV bulls-eye size, sepal size or UV proportion. Statistically significant selection was found in UV bulls-eye size, sepal size and UV proportion in hand-pollinated flowers, but interactions with pollinators contributed only to flower diameter. We conclude that in C. scaposa, floral traits that are subjected to selection might be driven by multiple selective agents, and suggest the importance of investigating floral traits that are invisible to human but visible to pollinators in measuring pollinator-mediated selection via male fitness.

Pollen and resource limitations to lifetime seed production in a wild population of the endangered plant Disanthus cercidifolius var.longipes H.T.Chang(Hamamelidaceae)

Disanthus cercidifolius Maxim.var.longipes H.T.Chang,a plant species that only occurs in a few counties in Hunan,Jiangxi and Zhejiang Provinces and with a relatively small number of individuals,is ranked as a second Class endangered species for conservation in China.We have studied the effect of pollen and resources available to female reproduction,and the reproductive mechanism of“excess flowers with low fruit set”in Disanthus cercidifolius Maxim.var.longipes H.T.Chang was discussed.Results are as follows:Pollen from different sources has significant effects on fruit set and seed set of Disanthus cercidifolius Maxim.var.longipes H.T.Chang.The pollen source rather than pollen numbers significantly affected reproduction of this species.In wild populations,producing one fruit needs about 54.8 flow-ers,and one satiation seed needs about 6.60 flowers or 83.19 ovules.After fertilizing,which was propitious to flower develop-ment,the abortion rate of flower buds was decreasing,but the flowering rate was increasing.The fruit set and seed set was also significantly increasing,while abortion rate of fruit was significantly decreasing.With the increasing percentages of cutting leaves,the fruit set decreased,but the abortion rate of fruit shows no significant differentiation among treatments.After cutting branches that were puny,broken and insect-infested branches,the flower number seemed to be decreas-ing,but the fruit set and seed set all increased significantly.After removing some flowers,the fruit set was calculated with respect to the number of flowers remaining after the treatment increased with increasing of percentages of flower removal,whereas fruit set calculated with respect to the initial number of flowers remained constant,and the mean weights of per fruit and per seed all decreased significantly.Sufficient spatial or temporal heterogeneities in nutrient levels might allow limitation of seed set by resources and pollen in a natural population,while supplying resources may indirectly affect pollination by increasing attraction of the flowers to pollinators.There were very low fruit and seed sets in natural populations of Disanthus cercidifolius Maxim.var.longipes H.T.Chang.Different factors may have interacted to effect a low fruit set.A joint adoption of the“selection abortion hypothesis”,“ovary reserve hypothesis”and“male function hypothesis”seems to be the most likely explanation for the reproductive strategy of“excess flowers with few fruit sets”in Disanthus cercidifolius Maxim.var.longipes H.T.Chang.

Variation in Floral Sex Allocation and Reproductive Success in Sequentially Flowering Inflorescence of Corydalis remota var. lineariloba(Fumariaceae)

In hermaphroditic plants, female reproductive success often varies among different positions within an inflorescence. However, few studies have evaluated the relative importance of underlying causes such as pollen limitation, resource limitation or architectural effect, and few have compared male allocation. During a 2-year investigation, we found that female reproductive success of an acropetally flowering species, Corydalis remota Fisch. ex Maxim. var. lineariloba Maxim. was significantly lower in the upper late developing flowers when compared with the lower early flowers. Supplementation with outcross pollen did not improve female reproductive success of the upper flowers, while removal of the lower developing fruits significantly increased female reproductive success of the upper flowers in both years, evidencing resource limitation of the upper flowers. Female production in upper flowers was greatly improved by simultaneous pollen supplementation of the upper flowers and removal of the lower fruits, suggesting that, when resources are abundant, pollen may limit the female reproductive success of the upper flowers. The less seed mass in the upper flowers didn＇t increase in all treatments due to architecture. In the upper flowers, ovule production was significantly lower and the pollen ： ovule ratio was significantly higher. These results suggest that male-biased sex allocation in the upper flowers may lead to increased male reproductive success, whereas the lower flowers have higher female reproductive success.

Mast fruiting in a hawkmoth- pollinated orchid Habenaria glaucifolia: an 8-year survey

Aims The hypothesis of predator satiation has been proposed to explain mast fruiting in various flowering plants.it considers that the simultaneous production of large numbers of seeds by a plant population reduces the risk of seed predation for each individual.orchids produce huge numbers of seeds per fruit and rarely experience seed predation.it remains unclear which factors may affect fluctuating fruit production in orchids,which generally suffer a widespread pol-len limitation.To explore the temporal pattern of fruiting and poten-tial factors related to fluctuation in fruit production,we investigated reproductive success of a long-spurred orchid(Habenaria glaucifo-lia)in an alpine meadow with thousands of individuals over 8 years.Methods To estimate reproductive success,pollinator observation was con-ducted by day and at night,and pollinia removal and receipt were recorded in the field population for 8 years.To examine whether fruit set and seed set are pollen limited,we conducted supple-mentary pollination experiments and compared fruit set,seed set and pollinia movement of open-pollinated flowers from 2011 to 2013.We measured lengths of spurs and pollinator probos-cises,and nectar volume and concentration,to identify potential pollinators.Important Findings Hawkmoths were seen to be effective pollinators for H.glauci-folia in 3 years,whereas in the remaining 5 years no pollinators were observed,and consequently pollinia were rarely transferred.Numerous pollinia movements were observed in 2012,2013 and 2014(pollinia removal:48,59 and 85%;pollinia receipt 51,70 and 80%),and correspondingly fruit set was significantly higher in 2012 and 2013(59 and 46%)than in 2011(25%).it was fruit set,rather than seed set,that was pollen limited in this orchid in the 3 years,in that supplementary pollination increased fruit set but did not increase seed set per fruit compared to natural.Three species of hawkmoths had proboscis lengths that matched the spur length of H.glaucifolia.Fruit set in this long spurred orchid depends on the activity of long-tongued hawkmoths,resulting in significant tempo-ral variation in fruit production.mast fruiting in this alpine orchid could be attributed to a‘sit and wait’strategy,awaiting an abun-dance of effective pollinators.

Potential Autonomous Selfing in Gesneria citrina (Gesneriaceae),a Specialized Hummingbird Pollinated Species with Variable Expression of Herkogamy

Species with mixed mating systems often demonstrate variable expression of breeding system characteristics and thus represent the opportunity to understand the factors and mechanisms that promote both outcrossed and selfed seed production. Here, we investigate variation in levels of herkogamy （variation in stigma-anther separation distance） in a Puerto Rican population of hummingbird pollinated Gesneria citrina Urban. There is significant variation in herkogamy levels among individuals of this species and stigma-anther separation is negatively associated with the ability to set fruits and seeds in the absence of pollinators. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of hummingbird pollinators. We also describe a novel breeding system in G. citrina, where stamens elongate over time to reach stigma height, but stamen elongation is accelerated by pollination. These results suggest that once the flowers are pollinated, stamen elongation may favor increased pollen removal and siring success, while the reduction in stigma-anther distance no longer imposes the risk of interference between male and female functions. We discuss our findings of breeding system variation in the context of pollination system evolution in an island setting （Antillean islands）.

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa, a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites,the production of ovules,fruits and seeds commonly declines from basal(early-opening)to distal(late-opening)flowers,while pol-len production remains constant or only changes slightly,with the result that distal(late-opening)flowers become functionally male.However,few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired,a more direct measure of male fitness.in pseudogamous apomicts,fitness depends on the number of seeds produced;thus,a contrasting pattern of variation in the pollen-to-ovule(P/o)ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa,a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally.We quan-tified variations in pollen number,ovule number,the P/o ratio and fruit and seed production,from 2007 to 2011,among flowers within racemes of four populations of H.ventricosa in southwest China.Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition,flower removal and sup-plemental pollination.Important Findings Pollen number,ovule number and the P/o ratio declined from basal-to-distal positions in all sampled populations(years).Fruit and seed production decreased distally in most populations(years).low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided.The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common.The low P/o ratio of distal flowers in H.ventricosa might be favored because they were unable to obtain fitness by donat-ing pollen and siring seeds on other plants.our study may help to understand the adaptive significance of sex allocation among flow-ers within inflorescences of sexually reproducing hermaphrodites.

Floral biology and pollination mechanisms of four Mexico-endemic Fuchsia species with contrasting reproductive systems

Aims The evolution of the outstanding variation of reproductive systems in angiosperms has been considered an important driver of line-age diversification.Closely related hermaphroditic and dioecious species with biotic pollination provide the opportunity to study and compare traits related to pollinator attraction and their con-sequences on reproductive components.A higher predictability of pollination syndromes is expected in dioecious species,which are dependent on pollinators,than in self-compatible hermaphroditic taxa.Dioecious species may suffer pollen limitation depending on the quality of floral rewards and the kind and abundance of pollina-tors,whereas no pollen limitation is expected in hermaphroditic spe-cies with autonomous self-pollination.Additionally,in the absence of pollen limitation,more or better seeds are expected in dioecious species,according to the sexual specialization hypothesis.Methods In natural populations of the hermaphroditic Fuchsia fulgens and Fuchsia arborescens and dioecious Fuchsia parviflora and Fuchsia obconica,all endemic to Mexico,we first described flower phenol-ogy,flower production and longevity and nectar volume and concen-tration.Then,we evaluated the correspondence between floral visitors and pollination syndromes.In hermaphrodite plants,we determined the level of herkogamy and the potential for autonomous self-pollina-tion.Finally,we evaluated the effect of pollen limitation on fruit set and seed number and assessed seed germination for all species.Important Findings In contrast to our prediction,dioecious species did not show a higher correspondence between pollination syndromes and floral visitors than did hermaphrodites;however,male flowers exhib-ited a higher correspondence than female flowers.No pollen limitation was detected in dioecious species,for which visitation rate did not differ between male and female flowers.The her-maphroditic F.fulgens showed pollen limitation for seed number,despite the presence of autonomous selfing.Fruit set from auton-omous pollination was higher in F.arborescens,which showed a lower level of herkogamy compared with F.fulgens.Finally,dioecious species produced fewer but heavier seeds compared with hermaphrodite species.Although Fuchsia is classified as an outcrossing genus,both hermaphroditic species showed autono-mous self-pollination.The heavier but lower number of seeds per fruit in dioecious species may be related to the more efficient resource allocation expected from sexual specialization.This could play an important role in the evolution of dioecy;however,a comparative phylogenetic approach is required to confirm this hypothesis.

Floral resources, pollinators and fruiting in a threatened tropical deciduous tree

Nothapodytes nimmoniana(Family Icacinaceae)is a deciduous tree species distributed in Asia facing severe population decline.Wood chips from the tree are a source of camptothecin,a globally soughtafter alkaloid with cancer-treating properties,and are harvested unsustainably in natural forests.We studied the pollination ecology of the species and asked if there are constraints in pollination and fruiting success in its natural populations.We also discuss the potential effects of wood extraction on pollinators and reproductive success in the population.Methods The study was carried out during the flowering season in two populations with varying exploitation levels,both located in the Protected Area Network in the Western Ghats of Karnataka State.We assessed floral resource availability and measured pollinator diversity and activity in the canopy from the perspective of the biology of the species.We quantified pollinator abundance and percent fruit set as a function of population density.Important Findings Flowers belong to the fly pollination syndrome and are open to visits by generalists such as Apis dorsata and Trigona iridipennis.Fruiting success did not vary between exploited and unexploited sites,but there was a significant difference in pollinator assemblage.In a lean male flowering year,fruit set was low,suggesting pollen limitation in the population.No fruiting was recorded in the second year at the site where trees were felled soon after the flowering period.We argue that when male floral resources are altered in exploited populations,pollinators of generalist nature may show a shift in foraging pattern.Also,generalist plants may be as susceptible to pollination loss as are specialist plants.The need for outcrossing in the species would probably exacerbate this vulnerability.

Altitudinal variation in the reproductive performance of the Mediterranean shrub Rhamnus lycioides L.

Aims Rhamnus lycioides L.subsp.Oleoides(Rhamnaceae)is a perennial shrub native to the Mediterranean Basin distributed along an altitudinal gradient,from sea level up to 1000 m a.s.l.The specific goals of our study were(i)to compare plant morphology between two contrasting populations,(ii)to determine the reproductive system of R.lycioides,quantifying the relative importance of insects and wind as pollen vectors,(iii)to test if pollen limitation differs between populations,(iv)to study the main factors influencing fruit set and(v)to compare plant reproductive performance(mass allocation to flowers,fruits and seeds)between the two habitats.Methods In the present study,we examined plant morphology and the reproductive performance of R.lycioides L.in contrasting environments in two populations located at the extremes of its altitudinal range in the island of Mallorca(Balearic Islands,Western Mediterranean Basin)along a 3-year period.Plant morphology,the relative importance of insects and wind as pollen vectors,the pollen limitation to seed production and the plant reproductive performance(mass allocation to flowers,fruits and seeds)were determined.Important Findings Rhamnus lycioides individuals showed a higher plant surface/plant height ratio at the mountain than at the coast.This species appeared to be ambophilous despite its inconspicuous flowers,although the relative importance of wind as a pollination vector was higher at the mountain than at the coastal site.Fruit set was much higher at the mountain,where pollen appeared to be a limiting factor.By contrast,fruit set was not limited by pollen availability at the coastal population,where resource(water and nutrients)limitation seemed to be more determining.Flower size was greater at the coast,in contrast to fresh fruit weight that was higher at the mountain.Despite the relatively few differences between sites in precipitation patterns along the study period,water availability appeared to be the key factor explaining not only fruit set but also the reproductive performance of this species in the study populations.