Floral display and pollination success in Senecio jacobaea (Asteraceae): Interactive effects of head and corymb size

The self-sterile Senecio jacobaea (Asteraceae) presents its rayed heads in large, compound inflorescences (corymbs). I examined the role of head and corymb size in pollinator attraction, and whether the positive effect of intact rays (if any) depends on the size of the corymb. Using female fertility as a measure of pollination success, I assessed the performance of stems representing four experimentally produced character combinations: (1) few heads without rays, (2) many heads without rays, (3) few heads with rays, and (4) many heads with rays. The proportion of flowers setting fruit was higher for intact stems (treatments 2, 4) than for stems on which the majority of the heads had been removed (treatments 1, 3), suggesting selection for maximum inflorescence production. By contrast, experimental removal of all rays had a relatively weak negative effect on fruit set, with few-headed stems (treatment 1) experiencing a greater reduction than stems with many heads (treatment 2). These results suggest that clusters of heads produce a synergistic effect on pollinator attraction, allowing plants to maintain high visitation rates even if there are drastic reductions in the basic attraction units. Hence, the number of heads and the attractiveness of the individual heads interacted in their effect on pollination success. Fruit set per flower differed greatly between sites and was positively correlated with plant density.     

Floral display and pollination success in Achillea ptarmica (Asteraceae)

In a natural population of the self-incompatible Achillea ptarmica , many-headed inflorescences attracted most pollinators (flies), and each pollinator visited more heads on large inflorescences than on smaller ones. The visitation rate did not increase disproportionately with inflorescence size, as would be expected if clusters of heads had a synergetic effect on pollinator attraction. Both the calculated visitation rate per head and the percentage seed set increased slightly as a function of head number. The removal of rays from all heads in the inflorescence reduced the approach rate by 51%, but had only a small negative effect on seed set (12%). Hence, ray removal probably affected male fertility (pollen donation) to a greater extent than female fertility. The decline in pollination success was similar for all inflorescence sizes, suggesting that ray and inflorescence size have independent effects on the total display, and thus represent separate targets for pollinator-mediated selection. Residual variation in seed set was strongly correlated with patch identity, indicating clonal or environmentally induced differences in female reproductive success.     

Sequential variation in the components of reproductive success in the distylousJasminum fruticans (Oleaceae)

A 2-year study of three natural populations of the distylousJasminum fruticans showed that mean fruit and seed production were significantly greater in shortstyled plants (thrums) than in long-styled plants (pins). In this study, we investigated the role of four sequential factors which may differentially influence fruit and seed set in the two floral morphs: (1) differences in flowering phenology, (2) a limitation of pollen transfer towards pins, (3) a differential capacity of the two morphs to act through famale and male function and (4) differential fruit abortion in the two morphs. Fruit set was significantly influenced by differences in flowering phenology although there were no differences in flowering time between the two morphs. supplementary pollinations in a natural population significantly increased fruit set and reduced the difference in fruit set between the two morphs in relation to controls, indicating a limitation on pollen transfer which was most severe towards pin stigmas. In reciprocal crosses, seed set was significantly dependent on the paternal and maternal identity of the pin parent. There was no significant variation among thrums for their performance as male or female parent. Furthermore, individual pin plants with relatively high percentage seed set as female parents gave poor seed set as male parents and vice-versa. Whereas fruit removal had no effect on seed number in thrum plants, a greater proportion of viable seeds were produced on pin plants which were left to naturally mature their fruits than on pins which had fruits artificially removed, suggesting the occurrence of selective fruit abortion in pins but not in thrums. The initially greater maternal fitness of thrums due to their greater success as pollen recipients may thus be opposed by increased viable seed set in the pins due to factors acting after the pollination stage. The relative reproductive success of floral morphs in the distylousJ. fruticans is thus differentially influenced by ecological factors occurring at different stages of the reproductive process.     

Fruit removal efficiency and success: influence of crop size in a neotropical treelet

The reproductive success of animal-dispersed plants is closely linked to the number of seeds that they are able to disperse. The fruit crop size hypothesis states that a plant with large fruit crop size will attract more dispersers than a plant with a smaller fruit crop, which may result in more seeds being dispersed from the foremost. In this study, we experimentally examined the effect of crop size and other factors on primary seed dispersal in a neotropical shrub/tree, Casearia corymbosa (Flacourtiaceae). We used two predictive variables of reproductive success, which produce an accurate picture of seed dispersal ratio: fruit removal efficiency (proportion of a fruit crop removed by frugivores) and fruit removal success (relative contribution of each individual tree to the number of fruits removed in the population). We established two levels of fruit crop size at the C. corymbosa individuals, using plants with large (150 fruits) and small crops (50 fruits). We found that individual plants with larger crops had significantly higher values of fruit removal efficiency (92.6%) and success (5%) than plants with smaller crops (69.3% and 1.3%, respectively). Fruit removal efficiency was related to vegetation type, plant height and fruit width, but the variance explained by these variables was low ( < 8%). Fruit removal success was significantly related to crop size ( > 90% of the variance explained). These results suggest that fruit removal efficiency and success are strongly related to fruit crop size of C. corymbosa plants.     

Variation in fruit set within and among natural populations of the self-incompatible herb Centaurea scabiosa (Asteraceae)

Various factors known to affect fruit set in self-incompatible plants were studied in 11 populations of Centaurea scabiosa . The importance of genetic variation was estimated using isozymes. A low overall genetic differentiation among populations suggested high levels of either current or historical gene flow. Large variation in fruit set was found among populations, among plants within populations, and among flower heads within plants. Most of this variation was explained by differences among flower heads within plants. Within the range in population size studied, there was no indication that plants growing in the smaller populations had lower fruit set than plants in larger ones. A significant positive correlation between genetic variation and population reproductive success was found. Pollinator visitation rate was very high and dominated by bumblebees. It was rendered unlikely that reproduction was limited by pollinator availability. It was even suggested that a very high visitation rate might have negative effects on fruit set in some populations due to an increased proportion of geitonogamous pollination resulting in stigma clogging with non-compatible pollen. The number of flower heads produced by individual plants did not affect fruit set in individual flower heads.     

Pollination efficiency and reproductive patterns in relation to local plant density,population size,and floral display in the rewarding Listera ovata (Orchidaceae)

Pollination efficiency and reproductive success vary strongly among populations of most animal‐pollinated plant species, depending on their size and local density, whereas individual plants within populations experience varying levels of reproductive output as a result of differences in floral display. Although most orchid species have been shown to be severely pollination limited, few studies have investigated the impact of the above‐mentioned factors on pollination success and reproduction, especially in rewarding species. In this study, the impact of population size, local density of flowering plants, and floral display on the rates of pollinia export and fruit production was investigated in 13 natural populations of the rewarding terrestrial orchid Listera ovata. In addition, an emasculation experiment was set up to examine how floral display and local density of flowering plants affected the relative importance of cross‐ vs. geitonogamous pollination in determining fruit set. In the studied populations, pollination efficiency, pollen removal, and fruit set increased with increasing population size until a threshold value of 30–40 flowering plants was reached, above which pollination efficiency and reproductive output decreased again. On average, plants with large floral displays showed higher proportional pollinia removal and fruit set compared with smaller plants. Fruit production was also significantly and positively related to local plant density, whereas emasculation did not affect the relationship between local plant density and fruit set, suggesting that geitonogamous pollination did not affect the outcome of female function. The results of this study are discussed in the light of the flowering mechanism of the species and its generalized pollination system. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 713–721.     

Fitness consequences of floral variation in Senecio jacobaea (Asteraceae): evidence from a segregating hybrid population and a resource manipulation experiment

The present study examines some of the ecological factors that might exert selection on floral morphology in Senecio jacobaea , a self-sterile composite which exhibits geographic variation in the frequency of rayed and discoid individuals. Regression analyses of phenotypic data from a large, segregating hybrid population, established in a semi-natural (garden) environment and studied over a 2-year period, revealed a negative relationship between the size of the rays and the average germination rate of the maternal seed crop, a pattern that can be attributed to the reduced germination speed of achenes from ray florets. There was no effect of ray size on the amount of cross-pollination achieved, the proportion of heads infested by larvae of seed flies ( Pegohylemyia ) and the amount of resources retained for the next flowering season. The lack of resource costs was also apparent in a manipulation experiment with greenhouse-grown plants of the rayed phenotype: artificial removal of all rays at the early bud or flowering stage had no detectable effect on subsequent flower and fruit development, regardless of whether the plants experienced high or low water stress. Given these and other observations, I hypothesize that plant-animal interactions and resource costs sometimes play a minor role in exerting selection on flower morphology and that spatially varying selection on germination behaviour accounts for some of the morph frequency variation in S. jacobaea.     

Temporal and spatial variation in flower and fruit production in a food-deceptive orchid: a five-year study

Deceptive orchids are generally characterized by low levels of fruit set; however, there may be substantial variations in fruit set between sites and years. Within a single population, individual plants may also differ greatly in their reproductive output as a result of differences in inflorescence size or local density. In this study, we determined flower and fruit production over 5 years in two populations of the food-deceptive orchid, Orchis purpurea . All plants were monitored annually for survival and flowering at each site to determine whether flowering and fruiting induced costs. The number of flowers per inflorescence varied considerably from year to year (min: 36.6, max: 49.5). Average fruit set was low (7%) and varied considerably among years and populations. A considerable proportion of plants also failed to set any fruit. However, the probability of producing at least one fruit was not affected by inflorescence size or local density. The number of fruits was significantly related to inflorescence size, but proportional fruit set was not. Local density also did not affect the number of fruits, nor proportional fruit set. There was also no evidence that plants with large inflorescence size or high fruiting success had a larger probability of remaining vegetative the year after flowering than plants with small inflorescence size or low fruiting success. Our results suggest that pollinator-mediated selective forces on inflorescence size through female reproductive success alone are weak, most likely because of the low overall level of visitation and the resulting uncertainty of pollination at the individual level. Our results further demonstrate that investigation of patterns of fruit set over several years is needed to better understand the variability in female reproductive success that is typical of most plant–pollinator interactions.     

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 pollen limitation. To explore the temporal pattern of fruiting and potential factors related to fluctuation in fruit production, we investigated reproductive success of a long-spurred orchid (Habenaria glaucifolia) in an alpine meadow with thousands of individuals over 8 years.Methods To estimate reproductive success, pollinator observation was conducted 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 supplementary 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 proboscises, and nectar volume and concentration, to identify potential pollinators.Important findings Hawkmoths were seen to be effective pollinators for H. glaucifolia 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 temporal variation in fruit production. Mast fruiting in this alpine orchid could be attributed to a 'sit and wait' strategy, awaiting an abundance of effective pollinators.     

Pollinator and nonpollinator selection on ray morphology in Leucanthemum vulgare (oxeye daisy, Asteraceae)

Despite evidence that both pollinators and nonpollinator agents of selection can shape the evolution of floral characters, there have been few attempts to compare the strengths and directions of selection from pollinators and other agents in the same study system. In this investigation of Leucanthemum vulgare, a self-incompatible composite known for its conspicuous white rays, I obtained data from a ray removal experiment in the field and from a segregating F(2) population in an experimental garden to assess the role of pollinator and nonpollinator selection as stabilizing factors on floral evolution in this species. Removal of all rays reduced the pollination success of heads by 31-35%, but did not significantly affect the level of infestation by larvae of the fly Tephritis neesii. Data from F(2) plants indicated a potential for indirect selection on ray morphology, mediated through links between ray morphology and measures of vegetative size and plant vigor. The results of this study show that individuals of the normal, rayed phenotype have a clear selective advantage, in terms of both pollinator attraction and general plant vigor. Thus, there were no conflicting selection pressures between the pollinators and the other selective agents considered in this study.     

Shading by invasive shrub reduces seed production and pollinator services in a native herb

Plant invasions disrupt native plant reproduction directly via competition for light and other resources and indirectly via competition for pollination. Furthermore, shading by an invasive plant may reduce pollinator visitation and therefore reproduction in native plants. Our study quantifies and identifies mechanisms of these direct and indirect effects of an invasive shrub on pollination and reproductive success of a native herb. We measured pollinator visitation rate, pollen deposition, and female reproductive success in potted arrays of native Geranium maculatum in deciduous forest plots invaded by the non-native shrub Lonicera maackii and in two removal treatments: removal of aboveground L. maackii biomass and removal of flowers. We compared fruit and seed production between open-pollinated and pollen-supplemented plants to test for pollen and light limitation of reproduction. Plots with L. maackii had significantly lower light, pollinator visitation rate, and conspecific pollen deposition to G. maculatum than biomass removal plots. Lonicera maackii flower removal did not increase pollinator visitation or pollen deposition compared to unmanipulated invaded plots, refuting the hypothesis of competition for pollinators. Thus, pollinator-mediated impacts of invasive plants are not limited to periods of co-flowering or pollinator sharing between potential competitors. Geranium maculatum plants produced significantly fewer seeds in plots containing L. maackii than in plant removal plots. Seed set was similar between pollen-supplemented and open-pollinated plants, but pollen-supplemented plants exhibited higher seed set in plant removal plots compared to invaded plots. Therefore, we conclude that the mechanism of impact of L. maackii on G. maculatum reproduction was increased understory shade.     

Novel interactions of non-pollinating ants with pollinators and fruit consumers in a tropical forest

The tropical ants Ectatomma ruidum and E. tuberculatum (Formicidae) regularly patrol leaves, flowers, and fruits of the understory shrub, Psychotria limonensis (Rubiaceae), on Barro Colorado Island, Panama. Ant and pollinator exclusion experiments elucidated both positive and negative effects of ant attendance on plant reproductive success, including pollination, fruit set, fruit loss, and fruit removal. Ants did not pollinate flowers but did contribute to higher pollination success, probably by increasing the relocation frequency of winged pollinators and thus the rate of flower visitation. Ants also prevented fruit loss to herbivorous insects which were common during the early stages of fruit development. Thus, ant attendance strongly improved both pollination and fruit set whereby plants with ants set more fruit per flower and also lost fewer fruits during fruit maturation. In contrast, ants had a negative effect on the removal of ripe fruits by avian frugivores. Thus, ant attendance has a non-trivial influence on plant reproduction, this interaction being beneficial at some stages of the plant reproductive cycle and carrying costs at another stage. A tight ecological or co-evolved relationship between these Ectatomma spp. and P. limonensis is unlikely given that ant attendance of plants is detrimental to fruit removal. Received: 18 May 1998 / Accepted: 1 March 1999     

A REEXAMINATION OF THE POLLEN-DONATION HYPOTHESIS IN AN EXPERIMENTAL POPULATION OF ASCLEPIAS EXALTATA

The evolution of large floral displays in hermaphroditic flowering plants has been attributed to natural selection acting to enhance male, rather than female, reproductive success. Proponents of the “pollen-donation hypothesis” have assumed that maternal resources, rather than levels of effective pollination, limit fruit set. We investigated the pollen-donation hypothesis in an experimental population of poke milkweed, Asclepias exaltata, where effective pollination did not limit fruit set. Specifically, we examined the effects of flower number per plant, and flower number per umbel on male reproductive success (number of fruits sired) and female reproductive success (number of fruits matured). In 1990, a paternity analysis was performed on fruits collected from 53 plants whose inflorescences were not manipulated. Flower number per plant was significantly correlated with male success, but not with plant gender. Flower number per plant was also significantly correlated with female success, but umbel number and stem number per plant together explained more than half (58%) the variation in female success. The percentage of fruit set was not significantly correlated with flower number per plant. Plants with large floral displays did not disproportionately increase in male reproductive success, relative to female success, as predicted by the pollen-donation hypothesis. In 1991, the effect of flower number per umbel on male and female reproductive success was investigated. Flower number per umbel was manipulated on four umbels per plant by removing flowers to leave 6, 12, or 18 flowers in each umbel. Plants with the largest umbels effectively pollinated twice as many flowers on other plants, but produced only 1.35 times as many fruits as plants with 6 and 12 flowers per umbel. Relative maleness of plants with large umbels was nearly twice that of small and medium umbels. Although these observations are consistent with the pollen-donation hypothesis at the level of umbels, they are problematic, because much of the variation in flower number per umbel exists within, rather than among, plants in natural populations. Thus, plants consist of both reproductively male (large) and female (small) inflorescences, which act to increase total reproductive success. It is therefore inappropriate to explain the evolution of large floral displays in milkweeds solely in terms of potential male reproductive success.     

Floral morphology and reproductive success in the orchid Epipactis helleborine: regional and local across-habitat variation

 The terrestrial orchid Epipactis helleborine is a morphologically variable species with a wide distribution in Europe. It is pollinated by social wasps, and most populations show the morphological characteristics of outcrossing species. However, local predominantly selfing subspecies and varieties have been documented from drier habitats. To document geographic variation in floral morphology, ability to produce seeds through autogamy, and reproductive success in E. helleborine, we sampled 13 populations from three geographic regions along a latitudinal gradient of c. 1000 km from northern to southern Sweden. In the southernmost region, populations in dry and mesic habitats were compared. Supplemental hand-pollination was conducted to determine whether among-population variation in fruit set could be explained by differences in the natural level of pollination, and whether any relationship between floral morphology and fruit production could be explained by interactions with pollinators. Bagging experiments showed no evidence of autogamy in any of the study populations. Number of flowers, pollinia removal and fruit set varied significantly among populations but did not differ among regions. Pollinia removal was positively correlated with population size and both pollinia removal and fruit set were lower in dry than in mesic habitats. At the level of the individual plant, the number of pollinia removed increased more rapidly with flower number than did number of fruits produced. The hand-pollination experiment indicated that the positive relationship between number of flowers and fruit production was due to a higher degree of pollen limitation in plants with few flowers than in plants with many flowers. The experiment also showed that variation in the level of pollen limitation could only partly explain variation in fruit set among populations. Received November 6, 2001; accepted April 27, 2002 Published online: December 3, 2002     

INFLORESCENCE SIZE: TEST OF THE MALE FUNCTION HYPOTHESIS

One explanation for low fruit sets in plants with hermaphroditic flowers is that total flower production by a plant is controlled primarily by selection through male function. This male function hypothesis presupposes that success in pollen donation increases more strongly with flower number than does seed set. I tested this prediction by measuring male and female components of reproductive success as functions of flower number in natural populations of the self-incompatible, perfect flowered plant, Ipomopsis aggregata. Fruit set in this hummingbird-pollinated plant averaged 4.9 to 40.3% across the 4 years of study. Both the total amount of pollen donated and the total amount received, as estimated by movement of fluorescent powdered dyes, increased linearly with number of flowers on a plant. Total seed production, however, increased disproportionately quickly because plants with larger floral displays were more likely to set at least one fruit. An estimate of the functional femaleness of a plant, based on pollen donation and seed production, increased with flower number. These results do not support the male function hypothesis.     

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.     

Floral display and habitat quality affect cost of reproduction in Primula farinosa

Costs of reproduction should depend on resource availability and on reproductive effort, which in turn may depend on traits influencing reproductive success. Therefore, variation in both habitat quality and reproductive traits should be considered when assessing reproductive costs. We investigated the effect of habitat quality and floral display on the costs of reproduction in the perennial herb Primula farinosa. In the study area, P. farinosa occurs in habitats that differ in water availability, which strongly influences plant performance. Furthermore, it displays a scape length dimorphism, with two distinct scape morphs differing in attractiveness to pollinators and reproductive success. To test the hypothesis that the cost of fruit production is higher in the long‐scaped than in the short‐scaped morph, and depends on water availability, we manipulated reproductive investment in eight P. farinosa populations along a gradient of soil moisture. Fruit set was higher in long‐scaped individuals, and prevention of fruit set increased flower production in the following year among long‐scaped, but not among short‐scaped plants. Furthermore, costs of fruit production were evident at low and high moisture levels but not at intermediate levels. The results demonstrate an association between a genetically determined difference in floral display and cost of reproduction, and suggest that costs of reproduction are non‐linearly related to water availability. They thus indicate links between the evolution of plant reproductive traits and plant life histories, and between habitat quality and optimal life history.     

Is relative pollen production or removal a good predictor of relative male fitness? an experimental exploration with a wild strawberry (Fragaria virginiana,Rosaceae)

Understanding plant reproduction requires knowledge of genetic contributions through pollen and seeds. Since direct genetic assessments of fitness through pollen are often intractable, reproductive ecologists use components of male fitness such as pollen production and pollen removal as surrogates tot paternity. However, we know little of the strength of the relationship between these components and actual paternity. Here, I report on a study undertaken to examine the relationship of pollen production and removal with paternity in Fragaria virginiana, a wild strawberry. A morphological marker was used to track paternity in experimental arrays exposed to native pollinators. Relative pellen production proved to be a poor predictor of relative paternity in most arrays, and over all arrays there was no significant correlation between relative paternity and relative pellen production. In contrast, relative pollen removed correlated significantly and positively with proportion of seeds sired, suggesting that a plant's contribution to the pool of removed pellen is a good predictor of its male reproductive success. Deviations from expected paternity based on relative pollen removal suggest a systematic overestimation of the siring success of plants with low pollen removal. And, in at least one specific case, low pellen removal may be explained by delayed anther dehiscence, which could lower the effectiveness of the removed pellen.     

Reproductive biology and effect of nectar robbing on fruit production in Macleania bullata (Ericaceae)

Hummingbird-pollinated flowers are frequently subjected to nectar robbing. In this paper, I examine the impact of nectar robbing on plant reproductive success on a hummingbird-pollinated species. After studying the basic aspects of the floral morphology and reproduction of Macleania bullata (Ericaceae) in a tropical montane wet forest in southwest Colombia, I examined the percent of flowers robbed and the effect of nectar robbery on fruit set. The flowers of this species are typical for plants pollinated by long-bill hummingbirds. They are protandrous and open for four days. Fruit production requires a pollinator visit; fruit set following pollinator exclusion was zero. Fruit set following xenogamous pollen transfer (36.8%) differed significantly from that of population controls (11.9%) and of autogamous pollen transfer (6.3%). Nectar volume, sugar concentration and sugar production were measured at daily intervals from bud opening until the fading of flowers. Daily nectar production (both volume and amount of sugar) varied considerably with flower age. Sugar production peaked on the second day, coinciding with the male phase. The frequency of nectar robbing in the studied population was very high (75% of examined flowers) and was positively correlated with reduced fruit set. I discuss the probability of a relation between reduced fruit set on robbed flowers and an energetic investment. Robbing by non-pollinating visitors can suppose the plant to re-synthesize more nectar. The high incidence of nectar robbing impugns the advantage of specialization.     

云南草寇花序内不同部位的性分配

两性花植物花序内的性分配常存在差异,资源竞争、结构效应、交配环境(雌雄异熟、传粉者定向访花行为等)或授粉不均匀等几种假说可以解释这种现象。为验证上述假说,该研究以云南草寇两种表型(雄先熟型和雌先熟型)为材料,分析了其花序内不同部位(基部、中部和顶部)的每花花粉数、胚珠数、花粉/胚珠比、结实率和结籽率,花序内传粉者的定向访花行为,以及人工辅助授粉和去花处理对结实率和结籽率的影响。结果表明:两种表型花序内每花花粉数不随部位而变化,每花胚珠数、结实率和结籽率由基部到顶部依次降低,每花花粉/胚珠比由基部到顶部依次增加,表明顶部花存在偏雄的性分配。人工辅助授粉后,结实率、结籽率仍由基部到顶部依次降低,表明授粉不均匀假说不能解释云南草寇花序内不同部位结实率、结籽率的差异。去除基部和中部花后,顶部花人工辅助授粉条件下的结实率、结籽率与基部花人工辅助授粉条件下的结实率、结籽率无差异,表明云南草寇花序内不同部位结实率、结籽率的差异主要由资源竞争引起。雌先熟表型每花花粉数、花粉/胚珠比高于雄先熟表型,表明两种表型存在性分配差异。传粉者主要先访问云南草寇基部的花,然后向顶部移动。云南草寇花序内顶部偏雄的性分配可能是由资源竞争和传粉者定向访花造成的。