Effectiveness of nectarivorous birds and honeybees as pollinators of Banksia spinulosa (Proteaceae)

Abstract The effectiveness of nectarivorous birds and honeybees (Apis mellifera) as pollinators of Banksia spinulosa (Proteaceae) was investigated. Birds visited inflorescences in the early, mid and late flowering seasons. In contrast, honeybees visited only on days in the late flowering period when maximum temperatures exceeded 15°C. Self pollen remained on pollen presenters of flowers for up to 5 days in the early and mid periods. In the late period, when honeybees visited inflorescences, self pollen was removed within 2 days. Pollen removal was similar for caged (birds excluded) and open inflorescences in the late period, indicating that most pollen was removed by honeybees. In the early and mid periods, honeyeaters pollinated 22% and 27% of flowers on open inflorescences, respectively. In the late period, when both birds and bees visited inflorescences, 64–73% of flowers on open inflorescences were pollinated. Foraging by honeybees resulted in pollen deposition as 38% of flowers on caged inflorescences were pollinated. Throughout the flowering season a similar number of pollen grains was deposited per stigma. There were 3.0–3.7 pollen grains per stigma on open inflorescences in the late period, although only 2.0 grains per stigma on caged inflorescences. In the early and mid periods, fewer caged than open inflorescences produced fruits, indicating the importance of honeyeaters to reproductive success at these times. In contrast, in the late period when honeybees visited inflorescences, fruit-set was similar on caged and open inflorescences. Overall, these results indicate that honeybees were effective pollinators of B. spinulosa.     

The seasonal abundance and foraging behaviour of honeyeaters and their potential role in the pollination of Banksia menziesii

Seasonal changes in the abundances of five species of honey-eaters were assessed in relation to the flowering phenology of Banksia menziesii in banksia woodland near Perth, Western Australia. The total number of honeyeaters was significantly correlated to the number of inflorescences ofB. menziesii. New Holland Honeyeaters. Brown Honeyeaters and Western Spinebills were present throughout the year, whereas the larger honeyeaters (Red and Little Wattlebirds) were observed when B. menziesii was flowering. The foraging behaviours of the Little Wattlebirds, New Holland Honeyeaters, Brown Honeyeaters and Western Spinebills were similar and all were likely to effect pollination of B. menziesii florets. Differences in their foraging position at an inflorescence, number and direction of foraging probes, and the time spent at an inflorescence were minimal. Western Spinebills used inflorescences lower in the tree crown than the other species. Movements to inflorescences on different plants were inversely related to honeyeater size. Smaller honeyeaters were frequently chased from inflorescences by the larger species, increasing the proportion of distant foraging movements made by the smaller species. Also these interrupted visits were of shorter duration than uninterrupted visits. Visitation by smaller species, especially the Brown Honeyeater, may result in more cross pollinations although the effect on the reproductive success of B, menziesii is unknown.     

Evidence for stabilising selection acting on flowering time in Arum maculatum (Araceae): the influence of phylogeny on adaptation

The relationship between flowering time and reproductive success was investigated in the fly-pollinated, monoecious perennial herb Arum maculatum L. (Araceae). This species temporarily traps its principle pollinator, a psychodid midge. Probability of fruit set was analysed in relation to early, peak and late periods of the flowering phenology of four British populations between 1992 and 1997. In three out of five cases, plants which flowered during early and late periods were significantly less likely to set fruit. In addition, one population showed a similar relationship for percentage fruit set of individual inflorescences, and seeds from peak-flowering plants were significantly heavier. There was no variation in number of female flowers per inflorescence over the flowering season. Probability of fruit set appears to be mediated by the likelihood of trapping psychodid midges that have previously been trapped and picked up pollen, an unlikely event during early and late flowering periods when few inflorescences are open. The majority of plants in all populations produce only one inflorescence which means that timing of flowering may be crucial to reproductive success. We interpret our findings as evidence that stabilising selection may be acting on some populations and/or during some years. The ultimate cause, however, can be related to the very short (12–18 h) female phase of each inflorescence, a phylogenetically conservative trait within the Araceae. Received: 19 August 1998 / 15 February 1999     

Risk‐averse inflorescence departure in hummingbirds and bumble bees: could plants benefit from variable nectar volumes?

Most hermaphroditic, many-flowered plants should suffer reduced fitness from within-plant selfing (geitonogamy). Large inflorescences are most attractive to pollinators, but also promote many flower visits during a single plant visit, which may increase selfing and decrease pollen export. A plant might avoid the negative consequences of attractiveness through modification of the floral display to promote fewer flower visits, while retaining attractiveness. This report shows that increasing only the variance in nectar volume per flower results in fewer flower visits per inflorescence by wild hummingbirds ( Selasphorus rufus ) and captive bumble bees ( Bombus flavifrons ) foraging on artificial inflorescences. Inflorescences were either constant (all flowers contained the same nectar volume) or variable (half the flowers were empty, the other half contained twice as much nectar as in the constant flowers). Both types of inflorescence were simultaneously available to foragers. Risk-averse foraging behaviour was expressed as a patch departure preference: birds and bees visited fewer flowers on variable inflorescences, and this preference was expressed when resource variability could be determined only by concurrent sampling. When variance treatments were clearly labelled with colour and offered to hummingbirds, the departure effect was maintained; however, when preference was measured by inflorescence choice, birds did not consistently prefer to visit constant inflorescences. The reduced visitation lengths on variable inflorescences by both birds and bees documented in this study imply that variance in nectar production rates within inflorescences may represent an adaptive trait to avoid the costs of geitonogamy.     

Distribution of plant‐dwelling spiders: Inflorescences versus vegetative branches

Abstract The influence of the architecture of vegetative branches on the distribution of plant‐dwelling spiders has been intensively studied, and the effects on the aggregation of individuals in several spider species on plants include variation in prey abundance, availability of predator‐free refuges and smoother microclimate conditions. The emergence of inflorescences at the reproductive time of the plants changes branch architecture, and could provide higher prey abundance for the spiders. The distribution of spiders between inflorescences and vegetative branches was compared on four widespread plant species in a Brazilian savannah‐like system. Inflorescences attracted more spiders than vegetative branches for all plant species sampled. The influence of branch type (inflorescence and vegetative) on spider distribution was also evaluated by monitoring branches of Baccharis dracunculifolia DC. in vegetative and flowering periods for 1 year, and through a field experiment carried out during the same period where artificial inflorescences were available for spider colonization. Artificial inflorescences attached to B. dracunculifolia branches attracted more spiders than non‐manipulated vegetative branches for most of the year. However, this pattern differed among spider guilds. Foliage‐runners and stalkers occurred preferentially on artificial inflorescences relative to control branches. The frequencies of ambushers and web‐builders were not significantly different between treatment and control branches. However, most ambush spiders (65%) occurred only during the flowering period of B. dracunculifolia, suggesting that this guild was influenced only by natural inflorescences. The experimental treatment also influenced the size distribution of spiders: larger spiders were more abundant on artificial inflorescences than on vegetative branches. The hypothesis that habitat architecture can influence the spider assemblage was supported. In addition, our observational and experimental data strongly suggest that inflorescences can be a higher quality microhabitat than non‐reproductive branches for most plant‐dwelling spiders.     

Reproductive biology of the rare and endangered Banksia brownii Baxter ex R. Br. (Proteaceae)

Abstract Banksia brownii is an endangered species, now limited to ～ 15 disjunct populations in southwestern Western Australia. Data on flowering phenology, plant size, fruit set, pollination and the mating system were gathered for two of these populations between March and October 1993. Flowering for both populations followed a similar pattern, with open flowers first evident in April, and the number of inflorescences with open flowers peaking in June. At both locations, trees differed considerably with respect to their size, the total number of inflorescences produced and the length of their flowering season. Fruiting success was typically low, with approximately half of all inflorescences failing to develop into infructescences. Only 1. 8% of the flowers originally present on inflorescences developed into follicles. The distribution of follicles along each infructescence was non-random, with most forming in the middle third of the infructescence for reasons relating to nutrient supply and pollinator behaviour. More flowers opened during the day than at night, although pollen was lost from individual flowers during both periods. Honeyeaters such as Phylidonyris novaehollandiae were common at the two study sites, and often carried large loads of B. brownii pollen. Though less frequently caught, the nocturnal mammals Rattus fuscipes and Tarsipes rostratus also bore substantial amounts of pollen. Most inflorescences from which these mammals and birds were excluded remained barren. Fruiting success was further reduced when invertebrates such as Apis mellifera were also prevented from visiting inflorescences. The ability of B. brownii to set at least some fruit in the absence of biotic poli-nators indicates that the species is partially self-compatible. Honeyeaters foraged preferentially at inflorescences with one to two thirds of their flowers open, probing mainly along the ‘advancing front’ of open flowers. These animals moved more frequently between inflorescences on the same plant than between those on different plants, and were often recaptured in the same locations. Mammals also appeared to be sedentary. Both B. brownii populations had mixed mating systems, with genetically determined outcrossing rates of ～0.7. The unusually high level of selfing in each population is presumably a reflection of the species’ self-compatibility and the foraging behaviour of its pollinators.     

A test of the effect of floral color change on pollination effectiveness using artificial inflorescences visited by bumblebees

Floral color change has been recognized as a pollination strategy, but its relative effectiveness has been evaluated insufficiently with respect to other floral traits. In this study, effects of floral color change on the visitation pattern of bumblebees were empirically assessed using artificial flowers. Four inflorescence types were postulated as strategies of flowering behavior: type 1 has no retention of old flowers, resulting in a small display size; type 2 retains old flowers without nectar production; type 3 retains old flowers with nectar; and type 4 retains color-changed old flowers without nectar. Effects of these treatments varied depending on both the total display size (single versus multiple inflorescences) and the pattern of flower-opening. In the single inflorescence experiment, a large floral display due to the retention of old flowers (types 2–4) enhanced pollinator attraction, and the number of flower visits per stay decreased with color change (type 4), suggesting a decrease in geitonogamous pollination. Type-4 plants also reduced the foraging time of bees in comparison with type-2 plants. In the multiple inflorescence experiment, the retention of old flowers did not contribute to pollinator attraction. When flowering occurred sequentially within inflorescences, type-4 plants successfully decreased the number of visits and the foraging time in comparison with type-2 plants. In contrast, floral color change did not influence the number of visits, and it extended the foraging time when flowering occurred simultaneously within inflorescences but the opening of inflorescences progressed sequentially within a plant. Therefore, the effectiveness of floral color change is highly susceptible to the display size and flowering pattern within plants, and this may limit the versatility of the color change strategy in nature.     

Multi-cycle synchronous protandry in raceme-like inflorescences of a bumblebee-pollinated herb Aconitum grossedentatum

Multi-cycle synchronous dichogamy is expected to be a mechanism for reducing self-pollination and sexual interference. It is often found in plants with umbellate inflorescences where pollinator movement is unpredictable, but not in plants with raceme inflorescences that are pollinated by bumblebees. Plants with raceme inflorescences often acropetally open flowers, resulting in an arrangement of females at lower level and males at upper level. This is good enough to preclude geitonogamy because bees tend to move upwardly within the inflorescences. Furthermore, although the degree of segregation of sexes varies among species, their intraspecific variations within a population have rarely been examined. Here, we present a synchronous protandry in bee-pollinated Aconitum grossedentatum, which has a raceme-like inflorescence and opens flowers basipetally. To evaluate the functional significance of synchronous dichogamy in mating, we firstly observed the distribution of sex phases of open flowers. Then, we assessed the effect of each phase flower on foraging behavior by pollinators and seed-set success. The inflorescences tended to exhibit either male- or female-phase flowers at any moment early in the flowering season, but the degree of segregation of sexes declined over time within a population. The degree of the segregation did not affect bumblebee visits to flowers, but it decreased seed-set success of female-phase flowers at that time. Our results demonstrated that synchronous protandry was beneficial for pollination success in A. grossedentatum by avoiding geitonogamy. Nevertheless, we also found asynchronous protandry late in the season, suggesting that the benefits by synchronous protandry decreased over the season.

     

Large plant size counteracts early seed predation during the extended flowering season of a Silene uniflora (Caryophyllaceae) population

A population of the alvar race of the perennial herb, Silene uniflora (Caryophyllaceae), which grows on thin soils in open alvar habitats on the Baltic island of Oland SE Sweden, was found to have an extended and more or less bimodal flowering phenology Large individuals produced flowers during both periods, while small, and presumably young, individuals only produced flowers in either of the two periods, or in part of both In the early flowering period plants were heavily infested by the seed-predatory larvae of a noctuid moth, Hadena confusa , but in the late flowering period only a small proportion of the fruits was attacked by the seed predators The proportion of flowers developing into fruits was consistently high throughout the season Both the number of seeds per capsule and the mass of seeds decreased over the flowering season However, the germination success of early and late seeds did not differ Thus, although differing in number of seeds, both early and late flowers contributed to individual reproductive success Large individuals started to flower early in the season and despite their high loss of seeds in the early part of the summer they contributed a larger number of seeds to the seed pool than smaller and later-flowering individuals Although selection was acting to favour later flowering during a year with high early seed predation, consistency of date of flower initiation and of relative predation impact on individuals across years was low suggesting that recurrent selection by seed predators is weak Seed predation, although heavy, is therefore judged to be unlikely to cause a significant evolutionary response on flowering phenology in this plant     

The pollination ecology of Grevillea beadleana McGillivray,an endangered shrub from northern New South Wales,Australia

Grevillea beadleana (Proteaceae) is an endangered species known from five populations in northern New South Wales, Australia. The reproductive ecology of G. beadleana was compared in two populations with a ten-fold difference in the number of plants. Grevillea beadleana was found to be self-compatible in both populations and an examination of pollen viability and stigma maturation revealed that the species is protandrous. Flowering within inflorescences is acropetallous. In the first season plants in the largest population produced approx. ten-fold more inflorescences than those in the smaller population and, although the number of flowers per inflorescence did not vary significantly between populations the first season, the larger population produced more fruit per inflorescence than the smaller population. However, fruit to flower ratios were less than 0.2 in both seasons and populations. In both populations the number of fruit was significantly greater at the proximal end of the inflorescence, where flowers open first, compared with medial and distal positions. Several bird species were observed visiting flowers, although few birds were recorded foraging at plants in the smaller population. Within both populations, birds tended to make more within- than between-plant visits. Self-compatibility, acropetally and proximal fruit-set, combined with the predominantly within-plant movement of honeyeaters, suggests inbreeding may be common within both populations of G. beadleana. Pollination and fruiting success are discussed for G. beadleana and breeding systems among rare and common taxa in Grevillea are reviewed.     

Seasonal changes in a honeyeater assemblage in Banksia woodland near Perth,Western Australia

Abstract

Honeyeaters were the most numerous birds in banksia woodland near Perth, Western Australia, throughout the year. Numbers were greatest in a Banksia littoralis swamp, but only during those few months when it contained large amounts of nectar. In the surrounding woodland, numbers were lower but fairly constant during the year. This reflects the smaller amounts of nectar produced throughout the year, by the overlapping flowering patterns of several Banksia and Adenanthos species.

Large and medium-sized honeyeaters (wattlebirds and New Holland Honeyeaters) and flocking silvereyes dominated the swamp when it flowered. In contrast, small honeyeaters (spinebills and Brown Honeyeaters), many of whom were highly territorial residents, comprised the majority of the woodland assemblage throughout the year. These observations support a model based upon aggressive defence of rich nectar sources by the larger honeyeater species, and more efficient exploitation of dispersed flowers by smaller honeyeaters.     

Bumble-bee learning selects for both early and long flowering in food-deceptive plants

Most rewardless orchids engage in generalized food-deception, exhibiting floral traits typical of rewarding species and exploiting the instinctive foraging of pollinators. Generalized food-deceptive (GFD) orchids compete poorly with rewarding species for pollinator services, which may be overcome by flowering early in the growing season when relatively more pollinators are naive and fewer competing plant species are flowering, and/or flowering for extended periods to enhance the chance of pollinator visits. We tested these hypotheses by manipulating flowering time and duration in a natural population of Calypso bulbosa and quantifying pollinator visitation based on pollen removal. Both early and long flowering increased bumble-bee visitation compared with late and brief flowering, respectively. To identify the cause of reduced visitation during late flowering, we tested whether negative experience with C. bulbosa (avoidance learning) and positive experience with a rewarding species, Arctostaphylos uva-ursi, (associative learning) by captive bumble-bees could reduce C. bulbosa's competitiveness. Avoidance learning explained the higher visitation of early- compared with late-flowering C. bulbosa. The resulting pollinator-mediated selection for early flowering may commonly affect GFD orchids, explaining their tendency to flower earlier than rewarding orchids. For dissimilar deceptive and rewarding sympatric species, associative learning may additionally favour early flowering by GFD species.     

Equal and Opposite Effects of Floral Offer and Spatial Distribution on Fruit Production and Predispersal Seed Predation in Xanthosoma daguense (Araceae)1

Inflorescences of the terrestrial aroid Xanthosoma daguense in the Andes of Colombia are visited by Dinastinae and Nitidulidae beetles. Plants produce one inflorescence at a time, which is pollinated during the first night of opening. Dynastine beetles act as pollinators, whereas Nitidulids lay eggs in the inflorescence and the larvae damage the seeds. We explored the effects of floral offer and distance among inflorescences on the number of pollinator visits, fruit production, and predispersal seed predation. Number of Dynastine visits per inflorescence tended to increase with increasing distances among inflorescences, but fruit predation increased when inflorescences were more clumped. Both pollinator visitation rates and predispersal seed predation were low at high floral offer. Fruit set increased when inflorescences were visited by two or more Dynastines, but the proportion of fruits damaged by Nitidulid larvae was equivalent to the increase in fruit production due to more Dinastine visits. The net result was a similar number of undamaged fruits in all infructescences produced, independent of the number of Dinastine visits. Our results revealed that both pollinators and predators responded to the number of available inflorescences and their spatial distribution, but they had opposing effects on the infructescences. Thus, our study suggests that the interaction of two ecological processes, pollination and predispersal seed predation, may cancel each other's effects under natural conditions.     

Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set

The fecundity of insect-pollinated plants may not be linearly related to the number of flowers produced, since floral display will influence pollinator foraging patterns. We may expect more visits to plants with more flowers, but do these large plants receive more or fewer visits per flower than small plants? Do all pollinator species respond in the same way? We would also expect foragers to move less between plants when the number of flowers per plant are large, which may reduce cross-pollination compared to plants with few flowers. We examine the relationships between numbers of inflorescence per plant, bumblebee foraging behaviour and seed set in comfrey, Symphytum officinale, a self-incompatible perennial herb. Bumblebee species differed in their response to the size of floral display. More individuals of Bombus pratorum and the nectar-robbing B.?terrestris were attracted to plants with larger floral displays, but B. pascuorum exhibited no increase in recruitment according to display size. Once attracted, all bee species visited more inflorescences per plant on plants with more inflorescences. Overall the visitation rate per inflorescence and seed set per flower was independent of the number of inflorescences per plant. Variation in seed set was not explained by the numbers of bumblebees attracted or by the number of inflorescences they visited for any bee species. However, the mean seed set per flower (1.18) was far below the maximum possible (4 per flower). We suggest that in this system seed set is not limited by pollination but by other factors, possibly nutritional resources.     

Honeyeater foraging: A test of optimal foraging theory

Honeyeaters (Meliphagidae) were observed foraging for nectar from Lambertia formosa inflorescences, each of which has seven flowers. The frequency distribution of numbers of flowers probed per visit to an inflorescence was found to be bimodal, with one peak at two and the other at seven. It is hypothesized that this frequency distribution results from a rule of departure from inflorescences that maximizes the net rate of energy gain. Patterns of nectar distribution were determined for a large sample of inflorescences. In addition the extent to which the honeyeaters re-probe flowers during a visit to an inflorescence was estimated. From these data and from field measurements of the times required by the honeyeaters to perform the various foraging behaviours, computer simulations of honeyeater foraging were constructed. These simulations led in turn to optimal frequency distributions of numbers of flowers probed per inflorescence that were bimodal but had peaks at 1 and 7 instead of 2 and 7. Although the observed and predicted behaviour were consequently similar, the difference between them was nevertheless significant. This difference could have been due to the birds' transient occupancy of the study area.     

Pollinator preference and pollen viability mediated by flower color synergistically determine seed set in an Alpine annual herb

Gentiana leucomelaena manifests dramatic flower color polymorphism, with both blue‐ and white‐flowered individuals (pollinated by flies and bees) both within a population and on an individual plant. Previous studies of this species have shown that pollinator preference and flower temperature change as a function of flower color throughout the flowering season. However, few if any studies have explored the effects of flower color on both pollen viability (mediated by anther temperature) and pollinator preference on reproductive success (seed set) in a population or on individual plants over the course of the entire flowering season. Based on prior observations, we hypothesized that flower color affects both pollen viability (as a function of anther temperature) and pollen deposition (as a function of pollinator preference) to synergistically determine reproductive success during the peak of the flowering season. This hypothesis was tested by field observations and hand pollination experiments in a Tibetan alpine meadow. Generalized linear model and path analyses showed that pollen viability was determined by flower color, flowering season, and anther temperature. Anther temperature correlated positively with pollen viability during the peak of the early flowering season, but negatively affected pollen viability during the peak of the mid‐ to late flowering season. Pollen deposition was determined by flower color, flowering season (early, or mid‐ to late season), and pollen viability. Pollen viability and pollen deposition were affected by flower color that in turn affected seed set across the peak of the flowering season (i.e., when the greatest number of flowers were being pollinated). Hand pollination experiments showed that pollen viability and pollen deposition directly influenced seed set. These data collectively indicate that the preference of pollinators for flower color and pollen viability changed during the flowering season in a manner that optimizes successful reproduction in G. leucomelaena. This study is one of a few that have simultaneously considered the effects of both pollen viability and pollen deposition on reproductive success in the same population and on individual plants.     

Competition for bumblebee visitation between Melampyrum pratense and Viscaria vulgaris with healthy and Ustilago-infected flowers

Summary We studied bumblebee foraging on two sympatrically and simultaneously flowering species, Melampyrum pratense (Scrophulariaceae) and Viscaria vulgaris (Caryophyllaceae) during the flowering season of Viscaria in south-west Sweden. We distinguished between healthy and Ustilago-infected Viscaria plants. Both species shared the main insect visitor, queens of Bombus hortorum, which collected nectar from both species but pollen from Melampyrum only. The pattern of visitation changed over the season: bumblebees preferred Viscaria early on, but changed to Melampyrum later in the season, probably because of the higher sugar content of Melampyrum nectar and the possibility of collecting both nectar and pollen from the same flower. Pollen collecting is probably of increasing importance since the need of pollen for the developing larvae will increase with time. Flowers of Viscaria received fewer visits in plots with other species than in pure Viscaria plots during one year and received more visits early than late in the season during both years. Melampyrum flowers received similar amounts of visits in mixed and pure environments. They also received more visits early than late, although this was probably a result of pollinator satiation since Melampyrum became very abundant with time. Ustilago-infected plants received far fewer visits but because of its long flowering time the proportion of open flowers receiving visits was still high. Viscaria flowers received significantly more visits than flowers of other species when bumblebees made heterospecific flower visits from Ustilago-infected plants; thus Ustilago spores were probably effectively dispersed from infected to healthy plants by the pollinators. The mechanism behind competition for pollination in this system was competition through pollinator preference, since the visitation rate to Viscaria actually decreased, but also competition through improper pollen transfer (grains of both species were found on the bodies of bumblebees) since the incidence of switching between the two species increased, probably resulting in an increased misplacement of conspecific pollen grains with time.     

A Developmental Pattern of Flowering in Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp.: Effects of Bud Position and Gibberellin

The restricted flowering of colored cultivars ofZantedeschia is a consequence of developmental constraints imposed by apical dominance of the primary bud on secondary buds in the tuber, and by the sympodial growth of individual shoots. GA₃ enhances flowering inZantedeschia by increasing the number of flowering shoots per tuber and inflorescences per shoot. The effects of gibberellin on the pattern of flowering and on the developmental fate of differentiated inflorescences along the tuber axis and individual shoot axes were studied in GA₃ and Uniconazole-treated tubers. Inflorescence primordia and fully developed (emerged) floral stems produced during tuber storage and the plant growth period were recorded. Days to flowering, percent of flowering shoots and floral stem length decreased basipetally along the shoot and tuber axes. GA₃ prolonged the flowering period and increased both the number of flowering shoots per tuber and the differentiated inflorescences per shoot. Activated buds were GA₃ responsive regardless of meristem size or age. Uniconazole did not inhibit inflorescence differentiation but inhibited floral stem elongation. The results suggest that GA₃ has a dual action in the flowering process: induction of inflorescence differentiation and promotion of floral stem elongation. The flowering pattern could be a result of a gradient in the distribution of endogenous factors involved in inflorescence differentialtion (possibly GAs) and in floral stem growth. This gradient along the tuber and shoot axes is probably controlled by apical dominance of the primary bud. Online publication: 7 April 2005     

Effects of inflorescence size on visits from pollinators and seed set of Corydalis ambigua (Papaveraceae)

Female reproductive success (seed set) of a spring ephemeral plant, Corydalis ambigua Cham. et schlecht (Papaveraceae) was investigated in relation to inflorescence size and foraging behavior (frequency and duration of visitations) by pollinators (namely, overwintered queens of Bombus hypocrita sapporensis) by detailed daily observations of a natural population. Pollination experiments indicated that C. ambigua is self-incompatible and that seed set was significantly affected by the behavior of the pollinating queens. Plants with larger inflorescences were visited more often than those with fewer flowers. Fecundity also increased with increasing size of inflorescences. Visitation time (duration of foraging) rather than the frequency of visitations (number of visits) was critical for higher fecundity. Seed production was strongly enhanced by a few long visits (of more than 60 s), and seemed to be independent of large numbers of short visits (of less than 60 s). Hence, plants with larger inflorescences, which provide a conspicuous signal to pollinators and offer greater rewards in terms of nectar, received longer visits by B. hypocrita sapporensis queens and those plants exhibited higher fecundity.     

INTRASEASONAL VARIATION IN SEED PRODUCTION AMONG FLOWERS AND PLANTS OF THALICTRUM THALICTROIDES (RANUNCULACEAE)

Intraseasonal variation in ovule and seed production per flower and per plant of the perennial herb Thalictrum thalictroides (L.) Eames and Boivin was documented at three sites during three spring flowering seasons. Shading and pollen-addition experiments were used to assess the impact of variation in pollination and light availability on seed production. Mean seed number and percent seed set were lower in flowers open late in the season than in those open earlier. The lower percent seed set occurs primarily in flowers that are positioned laterally on the inflorescence and which open later than center flowers. Plants flowering early in the season produce more flowers, ovules and seeds than those flowering later. Mean percent seed set per plant did not change, however, indicating that temporal differences in total seed output can be traced largely to variation in total ovule number. Seed production may be limited by lack of pollination during periods of inclement weather. At least late in the season, seed output may also be resource-limited: the decline in percent seed set among flowers coincided with the decline in light availability, and hand-pollination of late-opening flowers did not increase percent seed set to earlier levels. Artificial shading led to a significant reduction in percent seed set of lateral flowers, and decreased the probability of flowering again the following year. Thus, the relative selective roles of resource and pollination limitations may vary during the flowering season.