Potential Pollen Donors Research Articles

Individual flowering phenology shapes plant-pollinator interactions across ecological scales affecting plant reproduction.

The balance of pollination competition and facilitation among co-flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant-pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio-temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi-scale plant-pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio-temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect-pollinated plant species (Cyanus segetum and Centaurea jacea, n=288) into agroecological infrastructure (10 sown wildflower and six legume-grass strips) across a farm-scale experiment (125 ha). We applied an individual-based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio-temporal heterogeneity in plant-pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra- and interspecific plant-pollinator interactions respond to spatio-temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co-occurrence of pollinator-mediated facilitative and competitive interactions among plant species and individuals.

Seed Paternity Analysis Using SSR Markers to Assess Successful Pollen Donors in Mixed Olive Orchards.

The olive tree (Olea europaea L.) is a wind-pollinated crop that exhibits an extreme alternate bearing habit. To improve fruit set, several methods have been used to determine the most successful compatible combinations of cultivars. In this study, priority is given to seed paternity analysis based on simple sequence repeats (SSRs), microsatellite markers used for the identification of potential pollen donors of cultivar ‘Oblica’ in a mixed olive orchard during two consecutive years. Seven microsatellite primers were successfully used to examine the paternity of olive embryos from ‘Oblica’ mother trees. Embryos were considered as a product of self-fertilization if only maternal alleles were present, but not a single case of self-fertilization was found among all the embryos analyzed. Two dominant pollen donors were not the closest nor the cultivars with the highest number of trees in the orchard, suggesting that cross-compatibility may have a key role in determining pollen donor success. In our earlier studies, pollen tube growth and fertilization success correlated with fruit set when controlled crosses between cultivars were performed; however, some discrepancy might appear compared to paternity analyses when mother trees have a free choice among different pollen sources from cultivars growing in their surroundings.

Registration of ‘CP 11‐1314’ sugarcane

Abstract‘CP 11‐1314’ (Reg. no. CV‐196, PI 693071) sugarcane, a complex hybrid of Saccharum spp., was developed through cooperative research conducted by the USDA‐ARS, the University of Florida, and the Florida Sugar Cane League, Inc.; it was released to Florida growers in June 2018 for cultivation on organic soils and further recommended in June 2019 for sand soils. This cultivar was derived from a polycross between CP 05‐1451 and a mixture of potential pollen donors. CP 11‐1314 was selected for release based on high cane yield and acceptable sucrose content. In addition, CP 11‐1314 displays good ratooning ability (particularly on muck soils), minimal sucrose decrease following freezing temperatures, and overall disease resistance. CP 11‐1314 is positive for the Bru1 locus, which confers resistance to brown rust, and is resistant or moderately resistant to all other sugarcane diseases of concern in Florida, including ratoon stunt disease, leaf scald, mosaic, and orange rust.

Determination of compatibility relationships between olive cultivars: an overview of available methods

Olive (Olea europaea L.) is a wind-pollinated, preferentially allogamous species. However, inconsistent results are often reported for the classification of some cultivars as either self-compatible or self-incompatible, probably because of the influence of management and environmental conditions during bloom. Cross- pollination significantly enhances fertilization and fruit set in most olive cultivars. However, this improvement partly depends on the compatibility relationships between recipient and pollinizer trees in the species, where a few cases of cross- incompatibility have been reported recently. Different methods have been used to evaluate the success and inter-compatibility in pollination assays. The most common method for the determination of inter-compatibility is based on the observation of pollen-pistil interactions in cross-pollination trials. This method requires the usage of pollination bags to prevent the access of unwanted pollen grains to the stigmas of recipient flowers and the subsequent observation of pollen-pistil interaction features (pollen adhesion, pollen germination, pollen tube growth in the pistil and fertilization) and/or initial and final fruit set in response to experimental crosses. Lately, simple sequence repeats (SSRs) or microsatellites are increasingly becoming the markers of choice for paternity analysis of seeds to measure the success of crosses, especially when potential pollen donors are multiple and not applied by hand. SSRs offer a reliable way to test the paternity of seeds by checking the presence of parental alleles in the progenies. Knowledge of self-compatibility and cross-compatibility relationships of olive cultivars is a vital factor in the success of olive crosses and, therefore, of breeding programs. The advantages and drawbacks of the different methods to evaluate compatibility relationships within and between olive cultivars are presented in this work.

Spatial genetic structure, genetic diversity and pollen dispersal in a harvested population of Astrocaryum aculeatum in the Brazilian Amazon.

BackgroundAstrocaryum aculeatum is a palm tree species native to the tropical regions of South America, exploited commercially by local farmers for the pulp extracted from its fruits. The objective of this research was to compare the genetic diversity between adult plants and seedlings from open-pollinated seeds, quantify the pollen flow and dispersal, the spatial genetic structure, and the effective size of a population that has been continuously harvested for its fruits. The study was carried out in a natural population of A. aculeatum distributed over approximately 8 ha in the State of Amazonas (Brazil), separated by 400 m from the closest neighboring population. In total, 112 potential pollen donors, 12 mother plants and 120 offspring were mapped and genotyped.ResultsGenetic diversity was high for parents and the offspring. The fixation indexes for adults (F = -0.035) and offspring (F = -0.060) were negative and not significant. A significant spatial genetic structure was detected for the adult plants (up to the distance of 45 m) indicating short-distance seed dispersal. Paternity analysis detected 9.2 % of pollen immigration and the average distance of pollination within the population was 81 m. The average effective pollination neighborhood area between plants was 1.51 ha.ConclusionsOur results indicate that substantial introduction of new alleles has occurred in the population through pollen immigration, contributing to the maintenance of genetic diversity. Conservation efforts aimed at maintaining the gene pool of the current population or establishing new populations should utilize offspring from mother plants selected to be spaced by at least 50 m to prevent collecting seeds from relatives.

Spatial dynamics of pollination in dioecious Shepherdia canadensis (Elaeagnaceae)

The immobility of plants and the need for third-party vectors to enact pollen transfer render pollination an inherently spatial process. Little is known about the spatial scales over which pollen transfer and plant reproductive success are dependent on mate availability in natural plant populations. Dioecious plants provide useful study systems for examining the dependence of reproductive success on spatial patterns of mate availability because potential pollen donors and recipients are unambiguous. We examined the spatial dynamics of pollination in the dioecious shrub Shepherdia canadensis (L.) Nutt. (Elaeagnaceae) at a range of spatial scales in Ivvavik National Park, Canada. On average, males occurred at higher frequencies than females across ten study sites with at least 60 S. canadensis plants per site. Although we detected substantial variation in sex ratios across sites, there was no association between sex ratio and fruit set. Instead, patterns of fruit set were significantly correlated with male density within sites, over distances of 4–6 m. The density of females and males was spatially cross correlated. Therefore, density effects appear to be driven by pollinator responses to the overall density of shrubs. A pollinator exclusion experiment indicated that syrphid flies (Family: Syrphidae) were the most important pollinators in these populations. This study demonstrates that female reproductive success in S. canadensis is limited by plant density over small spatial scales, corresponding with the short foraging distances characteristic of its fly pollinators.

Effects of forest fragmentation on the mating system of a cool-temperate heterodichogamous tree Acer mono

Pollination is a key process for reproduction and gene flow in flowering plants. Anthropogenic habitat fragmentation, however, can disrupt plant–pollinator interactions, and may have a negative impact on the reproductive success and population viability of entomophilous plants. Heterodichogamous plants containing protandrous and protogynous individuals within a population may be susceptible to habitat fragmentation due to a lack of available mating partners. In this study, we investigated the effects of forest fragmentation on the mating system in the heterodichogamous plant Acer mono, a major constituent of cool-temperate deciduous forests in Japan. Microsatellite analysis was applied to 212 adult trees and 17 seed families from continuous and fragmented forests. Dispersal kernel modeling using the neighborhood model indicated that pollen dispersal of A. mono was highly fat-tailed. The estimated parameters of the model suggested that the siring success of a pollen donor increased approximately fivefold, with a 100 cm increase in its diameter at breast height (DBH), and that disassortative mating was five times more frequent than assortative mating. The mating system parameters of each mother tree, outcrossing rate (tm), biparental inbreeding (tm−ts), and paternity correlation (rpm) varied among sites and conditions, depending on the local density of potential pollen donors. Whereas A. mono was effectively outcrossed (tm=0.901, tm−ts=0.052, and the number of effective sires was 1/rpm=14.93) in the continuous forest, clumped trees within the fragmented forest showed increased biparental inbreeding and reduced pollen pool genetic diversity (tm=0.959, tm−ts=0.245,1/rpm=1.742) as a result of localized mating combined with spatial genetic structures. In contrast, the isolated trees had a higher selfing rate, but the pollen pool diversity was maintained (tm=0.801, tm−ts=0.022, and 1/rpm=15.63) due to frequent long-distance pollination. These results suggest that although pollen limitation following habitat fragmentation could result in negative genetic consequences, enhanced long-distance pollination across a fragmented landscape could partly compensate for this limitation depending on the degree of forest fragmentation.

Paternity analysis of the olive variety "Istrska belica" and identification of pollen donors by microsatellite markers.

The leading olive variety in Slovenia is “Istrska belica” (Olea europaea L.), which currently represents 70% of all olive trees in productive orchards. Paternity analysis based on microsatellite markers was used for genotyping and identification of the potential pollen donors of “Istrska belica” and for assessing the proportion of self-fertilization in monovarietal olive orchards in the Slovene Istria. Seven microsatellite loci were used for genotyping thirty-one olive embryos from “Istrska belica” trees and for all potential pollen donor varieties, which are grown in the region and could participate as pollinators. Genotyping results and allele identification were performed using the FaMoz software. The most probable pollen donor was assigned to 39% of all analyzed embryos. Among all analyzed embryos no single case of self-fertilization was confirmed. According to the present results, the variety “Istrska belica” was in all cases fertilized by foreign pollen. The results will contribute to defining the new guidelines for farmers regarding the proper management and growing practice in monovarietal olive groves.

Mating between Echinacea angustifolia (Asteraceae) individuals increases with their flowering synchrony and spatial proximity

Although spatial distance is considered the primary factor in determining plant mating patterns, flowering time and synchrony are also likely to be important. We quantified the relationships of both distance and flowering phenology to the probability of mating between individual plants. In an experimental plot, we followed daily flowering phenology in Echinacea angustifolia, a self-incompatible perennial pollinated by solitary bees. We assigned paternity to 832 of 927 seedlings from 37 maternal plants using 11 microsatellite loci. Potential pollen donors included the experiment plot's 202 flowering plants and a nearby plot's 19 flowering plants. For each maternal plant sampled, we examined the pollen pool by quantifying correlated paternity and the effective number of pollen donors. Significantly more pollinations occurred between neighboring and synchronous plants than expected under random mating, with distance being more important than flowering synchrony. The distance pollen moved varied over the course of the season, with late flowering plants mating with more distant plants compared to early or peak flowering plants. All maternal plants had a diverse set of mates (mean number of effective pollen donors = 23.7), and the composition of the pollen pools overlapped little between maternal plants. Both distance and flowering synchrony influenced pollination patterns in E. angustifolia. Our results suggest that pollen movement between incompatible mates and flowering asynchrony could be contributing to the reduced seed set observed in small E. angustifolia remnants. However, we also found that individual plants receive pollen from a diverse group of pollen donors.

Establishment of full-sib families in Shorea platyclados using paternity analysis: an alternative to controlled pollination

Full-sib families are produced from artificially controlled pollination in tree breeding. This mating design is, however, very costly and logistically difficult as most tree species are large in size. With the development of molecular markers, known relationships among the offspring could be established from the maximum likelihood in paternity analysis. In this study, we successfully established full-sib families on the basis of paternity analysis of offspring derived from two Shorea platyclados seed parents from the natural forest. A total of 2,049 and 970 offspring were collected from two mother trees M01 and M06, respectively. Potential pollen donors surrounding the mother trees were sampled, specifically a total of 77 adult trees within the 10-ha plot for M01 and 28 adult trees within the 4-ha plot for M06. For mother tree M01, parentage was assigned to 45.4% of the offspring at the 95% confidence level with 59 full-sib families established. For mother tree M06, parentage was assigned to 19.9% of the offspring at the 95% confidence level with 19 full-sib families established. The establishment of full-sib families in S. platyclados enables breeding in dipterocarp tree species, once a decade-long proposition, to become more efficient in a cost-saving manner.

Sexual compatibility of the olive cultivar ‘Kalamata’ assessed by paternity analysis

Paternity analysis was used to assess the self-incompatibility of the olive (Olea europaea L.) cultivar ‘Kalamata’ and to identify some compatible pollenisers under a Mediterranean-type climate. Eight microsatellite markers were used for genotyping three ‘Kalamata’ mother trees, 120 embryos, and all potential pollen donors. The identified alleles were analysed using FaMoz software and showed that ‘Kalamata’ was highly self-incompatible. Only three ‘Kalamata’ embryos were assigned to ‘Kalamata’ self-fertilisation, even though it was the most available pollen donor. The alleles were also analysed using NTSYS-pc (version 2.02 k) software and identified 54 potential pollen donors in the study site; however, not all of them were located within the effective pollination distance of the mother trees (30 m in olive). According to the results of this study, ‘Kalamata’ (as a host) was compatible with ‘Barnea’, ‘Benito’, and ‘Katsourela’ (six ‘Kalamata’ embryos assigned in each) but incompatible with ‘Arbequina’, ‘Azapa’, and ‘Picual’ (zero ‘Kalamata’ embryos assigned in each). The olive growers could use some of these compatible pollenisers with ‘Kalamata’ to guarantee good fruit set.

Seed production is reduced by small population size in natural populations of the invasive grass Lolium multiflorum

As founders of new invasions or survivors of control efforts, small populations can play important roles in biological invasions. Theory and models have explored how population size can affect invasion probability and dynamics. In contrast, there have been few studies of small population size and invasions in the wild, especially for plants. Here, we identified 21 naturally-occurring small populations of the invasive annual grass Lolium multiflorum to elucidate the effects of population size on plant reproduction and potential invasiveness. We compared seed production of focal plants to three attributes of the population: population size, a focal plant’s floret neighborhood (a measure of the size and pollen production of nearby potential pollen donors), and focal plant size. Population size and floret neighborhood were weighted by inter-plant distance to account for variation in density and spatial arrangement. Variation among populations in the proportion of florets producing a seed was explained by population size and floret neighborhood. In contrast, the focal plant’s size did not significantly affect the seed production proportion. A decrease in reproduction due to the size and density of the population (while accounting for maternal size) is an Allee effect, which may limit the ability to predict invasion risks and rates early in the invasion process, particularly if the effect is not recognized.

Using Simple Sequence Repeats (SSRs) to Measure Pollen Flow in a Flowering Dogwood Orchard

Abstract Dispersal of flowering dogwood pollen in an orchard was evaluated by performing parentage analyses on open pollinated seedlings collected from a single maternal tree. Pollen sources for 45 seedling trees were established using three polymorphic simple sequence repeat (SSR) loci. Previous data suggest that Cornus florida pollen is typically limited to travel between closely situated trees. If this is true, breeding may be simplified by crossing trees through open pollination rather than more laborious current methods involving pollination cages. However, the expected leptokurtic distribution, in which the likelihood that pollen transfer will occur between trees decreases quickly as distance increases, was not seen. Although the majority of paternal trees were located within the 12 m (39.4 ft) radius of the study area, the trees most likely to have donated pollen were some of those located the greatest distance from the mother tree. Unusual pollen movement may be explained, in part, by asynchronous flowering times of trees within the study or by genetic similarities exhibited between the mother tree and potential pollen donors in the area. Additionally, andrenid and halictid bees, which were the predominant pollinators observed at this site, may have moved pollen farther than expected.

Gene flow and mating patterns in individuals of wych elm (Ulmus glabra) in forest and open land after the influence of Dutch elm disease

Dutch elm disease has severely reduced the number of large trees of U. glabra in Denmark. Consequently, the distance between large trees has increased and the overall density of the species has decreased. Patches of small trees with stem diameters up to 10 cm are, however, relatively frequent. With four microsatellites we studied potential differences in genetic diversity, mating patterns and pollen flow in trees of U. glabra that occur either in a continuous forest (Suserup Forest) or isolated in the open land. We found no indications of selfing in forest or open land but indications of biparental inbreeding in offspring of isolated trees. Estimates of effective pollen donors (N ep) and minimum number of pollen donors (N p) were alike in forest and open land (N ep of 31 and 34 and N p of 4 to >6 and 3 to >6, respectively). The number of alleles was also very similar. With indirect methods we found that average pollen dispersal was 104 m under forest conditions. The average distance between the isolated trees and their potential pollen donors was further, thus suggesting that effective pollen in the open land on average moves further than in a dense forest. Finally, 28% of small trees (diameters up to 10 cm) produced fruits. Reproduction at a young age may be a key stone in the survival of U. glabra as the vectors of the disease prefer older trees.

Pollen mediated gene flow in a native population of Malus sylvestris and its implications for contemporary gene conservation management

Pollen mediated gene flow was studied in a Danish population of Malus sylvestris with the objective to support the Danish genetic conservation and management activities. A total of 50 mature trees (potential pollen donors) along with 180 seedlings (originating from 12 of the mature trees) were genotyped at 10 nuclear microsatellite loci. Paternity could be established for 46 seedlings, and a pollination distribution curve based on these observations was developed. Further, two indirect methods, Kindist and Twogener were applied for estimation of gene flow parameters. Pollinations were mostly between nearby trees with a median of observed pollination distances of approximately 23 m. However, a few long distance pollinations were observed and this increased the average pollination distance to approximately 60 m. The Kindist analysis of the data seemed to underestimate the average pollination distance as the fat-tailed distribution of the pollen dispersal distribution was not taken into account. Application of the results in a conservation and domestication context is discussed.

Paternity analysis using microsatellite markers to identify pollen donors in an olive grove

Olive (Olea europaea L.) is a wind-pollinated, allogamous species that is generally not considered to be self-compatible. In addition, cross-incompatibilities exist between cultivars that can result in low fruit set if compatible pollinisers are not planted nearby. In this study, microsatellite markers were used to identify 17 genotypes that were potential pollen donors in a commercial olive orchard. DNA typing with the same primers was also applied to 800 olive embryos collected from five cultivars in the grove over 2 years of study. Pollen donors for the cultivars Barnea, Corregiola, Kalamata, Koroneiki, and Mission were estimated by paternity analysis, based on the parental contribution of alleles in the genotypes of the embryos. The exclusion probability for the marker set was 0.998 and paternity was assigned on the basis of the 'most likely method'. Different pollen donors were identified for each of the maternal cultivars indicating that cross-compatibilities and incompatibilities varied between the genotypes studied. Cross-pollination was the principal method of fertilization, as selfing was only observed in two of the embryos studied and both of these were from the cultivar Mission. This is the first report where these techniques have been applied to survey the pollination patterns in an olive grove. The results indicate that careful planning in orchard design is required for efficient pollination between olive cultivars.

Direct and indirect assortative mating: a multivariate approach to plant flowering schedules

This paper develops methods to partition the phenotypic correlation between mates for a focal trait--the standard measure for assortative mating--into a direct component and additional indirect components. Indirect assortative mating occurs when a nonassorting trait is correlated within individuals to a directly assorting trait. Direct and indirect assortative mating is assessed for flowering phenology in Brassica rapa. The flowering time of pollen recipients (mothers) was strongly correlated (rho=0.67) to that of potential pollen donors (fathers). Similarly, recipients and donors were correlated for duration of their flowering periods (rho=0.32) and stem diameters (rho=0.52). A partitioning of between-mate correlations revealed direct assortative mating for flowering time and period duration. However, assortment for stem diameter is explained solely through its correlation to flowering time. Examination of standard quantitative genetic theory shows that indirect assortative mating inflates genetic variance in a focal trait and the genetic covariance between focal and phenotypically correlated traits.

Patterns of pollen dispersal in a small population of Pinus sylvestris L. revealed by total-exclusion paternity analysis.

Patterns of pollen dispersal were investigated in a small, isolated, relict population of Pinus sylvestris L., consisting of 36 trees. A total-exclusion battery comprising four chloroplast and two nuclear microsatellites (theoretical paternity exclusion probability EP=0.996) was used to assign paternity to 813 seeds, collected from 34 trees in the stand. Long-distance pollen immigration accounted for 4.3% of observed matings. Self-fertilization rate was very high (0.25), compared with typical values in more widespread populations of the species. The average effective pollen dispersal distance within the stand was 48 m (or 83 m excluding selfs). Half of effective pollen was dispersed within 11 m, and 7% beyond 200 m. A strong correlation was found between the distance to the closest tree and the mean mating-distance calculated for single-tree progenies. The effective pollen dispersal distribution showed a leptokurtic shape, with a large and significant departure from that expected under uniform dispersal. A maximum-likelihood procedure was used to fit an individual pollen dispersal distance probability density function (dispersal kernel). The estimated kernel indicated fairly leptokurtic dispersal (shape parameter b=0.67), with an average pollen dispersal distance of 135 m, and 50% of pollen dispersed beyond 30 m. A marked directionality pattern of pollen dispersal was found, mainly caused by the uneven distribution of trees, coupled with restricted dispersal and unequal male success. Overall, results show that the number and distribution of potential pollen donors in small populations may strongly influence the patterns of effective pollen dispersal.

Ecological constraints on fruit initiation by Calyptrogyne ghiesbreghtiana (Arecaceae): FLORAL HERBIVORY, POLLEN AVAILABILITY, AND VISITATION BY POLLINATING BATS

Failure of a flower to initiate fruit can be attributed to an insufficiency of resources, genetic incompatibilities, or ecological constraints on pollination. Flower visitors can increase fruit initiation by pollinating flowers, or decrease fruit initiation by damaging them. If, however, resources limit fruit initiation, variation in visitation may be relatively unimportant. Visitors to Calyptrogyne ghiesbreghtiana, a rain forest understory palm, leave a record of their activity by feeding on floral tissue. I recorded variation in inflorescence visitation, floral display, and fruit initiation during 7 mo over two field seasons. On average, bats visited 50‐60% of flowers; katydids and scarab beetles visited and damaged the remaining 40‐50%. Four species of bat (Phyllostomidae) captured near inflorescences were found to be carrying C. ghiesbreghtiana pollen. Approximately 18% of flowers visited by bats and not damaged by insects initiated fruit, whereas insect‐damaged flowers set a negligible number. Some variation in fruit initiation was explained by nightly variation in the proportion of inflorescences at each site that were potential pollen donors (male‐phase inflorescences). Visitors responded to variation in floral display; katydids damaged more flowers on taller inflorescences, and in one season bats visited a greater proportion of flowers on inflorescences with many flowers. However, floral display only explained a small amount of variation in visitation. A similar amount of variation in flower visitation was explained by the degree to which inflorescences were obstructed by surrounding vegetation. I used path analysis to summarize the overall effect of floral display on visitation by both pollinators and florivores and the effect of visitation and site sex ratio on fruit initiation. Because almost all flowers were visited by bats or damaged by katydids, the amount of bat visitation was strongly negatively correlated with katydid damage. In spite of the low fruit set from bat‐visited flowers, variation in bat visitation explained 27‐37% of the variation in fruit initiation.

Seasonal Variation in Pollination Dynamics of Sexually Dimorphic Sidalcea Oregana SSP. Spicata (Malvaceae)

We explored the combined effects of seasonal variation in both pollinator assemblage and availability of pollen donors on pollination in a gynodioecious species, Sidalcea oregana ssp. spicata (Malvaceae). Hermaphrodites produced flowers with significantly larger petals but maintained fewer open flowers per inflorescence than females. Flowers of hermaphrodites produced 50% more nectar sugar in the 24 h after anthesis than the flowers of females. Nectar sugar production was also significantly and positively correlated with petal length. Pollinator visitation rates were influenced more by differences in petal length than by differences in flower number per inflorescence. Consequently, hermaphrodites experienced higher visitation rates on a per—flower basis. Female flowers tended to receive pollen at a lower rate than hermaphrodites, but remained in the receptive female—phase longer than hermaphrodites. On average, the length of the period of flower receptivitiy declined as pollen deposition rate increased. These opposing processes resulted in the sex morphs receiving equivalent levels of pollination. Seasonal variation in the rate of pollen reception was more strongly influenced by the efficiency of the available pollinator pool than by rates of visitation. Dramatic seasonal shifts in the composition of the pollinator assemblage and pollen availability were correlated with increased pollination intensity as the season progressed. Not only were more pollen grains received, but they arrived in a shorter period of time and the number of potential pollen donors (hermaphrodites) increased. These findings suggest that pollen competition in both sex morphs may be more intense late in the season.