Simulation analyses of the evolution of intra-inflorescence flowering patterns assuming selection on anthesis interval among individual flowers

What conditions select flowering patterns within inflorescences, or variation in the anthesis interval within inflorescences among plants? Under what conditions are gradual blooming and simultaneous blooming, both traits related to floral display size, advantageous? We constructed a simulation model in which the opening times and longevities of individual flowers within inflorescences, the sizes of attractive structures of individual flowers, and the numbers of ovules and pollen grains produced by individual flowers evolve. Individual plants in the population compete for pollinators, and plants are selected by pollinators according to their floral display sizes and amounts of resources allocated to attractive structures. We found that, if the proportion of pollen on a pollinator deposited on a stigma was low, gradual blooming did not evolve even if inbreeding depression was greater than 0.5. This is because the amount of outcross-pollen on pollinators decreased at a low rate during flower visits within a single inflorescence, and the selfing rate was suppressed to a low level even if the floral display size was large. On the other hand, if the proportion of pollen deposition was high, gradual blooming evolved even if inbreeding depression was smaller than 0.5. This may be because gradual blooming can enhance pollen delivery to other plants by reducing the loss of self-pollen by geitonogamy. On the other hand, allocation ratios among floral organs (female and male organs and attractive structures) were independent of the degree of simultaneous and gradual blooming within inflorescences. We concluded that the evolution of gradual blooming is more strongly affected by the proportion of pollen on a pollinator deposited on a stigma than by inbreeding depression.

Pollen production per flower increases with floral display size across animal-pollinated flowering plants

PREMISE

The number of open flowers on a plant (i.e., floral display size) can influence plant fitness by increasing pollinator attraction. However, diminishing marginal fitness returns with increasing floral display are expected as pollinators tend to visit more flowers per plant consecutively. An extended flower visitation sequence increases the fraction of ovules disabled by self-pollination (ovule discounting) and reduces the fraction of a plant's own pollen that is exported to sire seeds in other plants (pollen discounting). Hermaphroditic species with a genetic system that prevents self-fertilization (self-incompatibility) would avoid ovule discounting and its fitness cost, whereas species without such a genetically based barrier would not. Contrarily, pollen discounting would be an unavoidable consequence of a large floral display irrespective of selfing barriers. Nevertheless, the increasing fitness costs of ovule and pollen discounting could be offset by respectively increasing ovule and pollen production per flower.

METHODS

We compiled data on floral display size and pollen and ovule production per flower for 1241 animal-pollinated, hermaphroditic angiosperm species, including data on the compatibility system for 779 species. We used phylogenetic general linear mixed models to assess the relations of pollen and ovule production to floral display size.

RESULTS

Our findings provide evidence of increasing pollen production, but not of ovule production, with increasing display size irrespective of compatibility system and even after accounting for potentially confounding effects like flower size and growth form.

CONCLUSIONS

Our comparative study supports the pollen-discount expectation of an adaptive link between per-flower pollen production and floral display across animal-pollinated angiosperms.

Global analysis of floral longevity reveals latitudinal gradients and biotic and abiotic correlates

The length of time a flower remains open and functional - floral longevity - governs important reproductive processes influencing pollination and mating and varies considerably among angiosperm species. However, little is known about large-scale biogeographic patterns and the correlates of floral longevity. Using published data on floral longevity from 818 angiosperm species in 134 families and 472 locations world-wide, we present the first global quantification of the latitudinal pattern of floral longevity and the relationships between floral longevity and a range of biotic and abiotic factors. Floral longevity exhibited a significant phylogenetic signal and was longer at higher latitudes in both northern and southern hemispheres, even after accounting for elevation. This latitudinal variation was associated with several biotic and abiotic variables. The mean temperature of the flowering season had the highest predictive power for floral longevity, followed by pollen number per flower. Surprisingly, compatibility status, flower size, pollination mode, and growth form had no significant effects on flower longevity. Our results suggest that physiological processes associated with floral maintenance play a key role in explaining latitudinal variation in floral longevity across global ecosystems, with potential implications for floral longevity under global climate change and species distributions.

Context-dependency of resource allocation trade-offs highlights constraints to the evolution of floral longevity in a monocarpic herb

Floral longevity is a critical component of floral display, yet there is a conspicuous paucity of empirical research on its evolution within species. Evolutionary models of floral longevity are grounded in resource allocation theory and propose that selection acts on heritable variation to optimize longevity in light of competing floral construction and maintenance costs. Key assumptions remain untested within wild species. We measured maximum floral longevity alongside protandry, flower size, flower number and flowering rate across families of the monocarpic herb Sabatia angularis grown under high and low resources. We evaluated genetic variation, plasticity and correlations between display traits, including fundamental resource-allocation trade-offs and their interactions with resource availability. All display traits showed significant genetic variation. Resource availability influenced mean floral longevity and flower number, with genetic variation in these responses. Importantly, both floral longevity-flower number and flower number-size trade-offs were significant and stronger under low resources. This study reinforces the application of resource allocation theory to floral display trait evolution. Our work highlights the context-dependency of trade-offs and the potential importance of plasticity in resource allocation, with plants investing in the construction of new flowers at faster rates when resources are high rather than in the maintenance of longer-lived flowers.

Pollinators shift to nectar robbers when florivory occurs, with effects on reproductive success in Iris bulleyana (Iridaceae)

Studies have indicated that florivory and nectar robbing may reduce reproductive success of host plants. However, whether and how these effects might interact when plants are simultaneously attacked by both florivores and nectar robbers still needs further investigation. We used Iris bulleyana to detect the interactions among florivory, nectar robbing and pollination, and moreover, their effects on plant reproductive success. Field investigations and hand-pollination treatments were conducted on two experimental plots from a natural population, in which Experimental plot was protected from florivores and Control plot was not manipulated. The flower calyx was bitten by sawflies to consume the nectary, and three bumblebee species were pollinators. In addition, the short-tongued pollinator, Bombus friseanus, was the only robber when there was a hole made by a sawfly. The bumblebee had significantly shortened flower handling time when robbing, as compared to legitimate visits. Pollinator visitation and seed production decreased significantly in damaged flowers. However, seed production per flower after supplementary hand-pollination did not differ significantly between damaged and undamaged flowers. Compared to the Experimental plot, bumblebees visited fewer flowers per plant in a foraging bout in the Control plot. The flowers damaged by florivory allowed B. friseanus to shift to a nectar robber. Florivory and nectar robbing collectively decreased plant reproductive success by consuming nectar resources, which may reduce attractiveness to pollinators of the damaged flowers. However, the changes in pollinator behaviour might be beneficial to the plant by reducing the risk of geitonogamous mating.

Pollinator limitation on reproductive success in Iris tuberosa

Variation in plant and floral size can have conflicting effects on pollination and fruit production in flowering plants. This research examines the contributions of plant height, flower size and pollinator visitation to reproductive success in four populations of Iris tuberosa. The plants were pollinated exclusively by hymenopteran species, primarily during sunny days. Pollination supplementation increased the proportion of flowers that matured into fruit, with 95 % fruit set for hand-pollinated compared with 74.15 % for naturally pollinated flowers. The pollinator visitation rate and the proportion of fruit produced were not significantly different between tall and short plants or between small and large flowers. Furthermore, the increase in plant size and floral display did not increase the frequency of pollinator visitations and so did not increase the fruit set. Thus, despite the widespread effects of flowering plant size on pollinator attraction and plant reproduction in other species, these effects are lacking in I. tuberosa. This study quantifies the role of pollinators in the reproductive success of I. tuberosa. Pollinators visited tall/short plants and large/small flowers in equal proportion, suggesting that plant and floral display size do not affect pollinator attraction and reproductive success in I. tuberosa. These results suggest that sexual reproduction of I. tuberosa is fairly limited by pollinators and not by resource limitation.

Pollen packaging and dispensing: adaption of patterns of anther dehiscence and flowering traits to pollination in three Epimedium species

Pollen presentation theory (PPT) predicts that plant species typically pollinated by frequent and wasteful pollinators ought to be much more parsimonious and only gradually release pollen compared to plant species pollinated by infrequent pollinators that are efficient at delivering the pollen they remove. To test PPT, we compare the pollen presentation schedules and pollination systems in three related Epimedium species, having different pollinators. Results showed that differences in anther dehiscence and flowering traits resulted in different pollen packaging schedules. For E. sutchuenense and E. franchetii, a special 'roll-up' movement of the anther wall during anther dehiscence increased pollen removal compared to the dehiscence pattern in E. mikinorii, which lacked the 'roll-up' movement. Investigations revealed that honeybees had a higher pollen removal rate and lower stigmatic pollen load compared to bumblebees. In accordance with PPT, E. sutchuenense presents pollen sequentially and slowly for the frequent and wasteful honeybees. In comparison to E. sutchuenense, E. franchetii had a faster presentation rate and was adapted to the efficient and infrequent bumblebees. However, E. mikinorii was pollinated by both bumblebees and honeybees at high frequency and had the fastest pollen presentation. This pattern could reduce pollen wastage by honeybees and might be an adaptation to its short flower longevity (less than 1 day), to increase the chances of pollen deposition on stigmas. The study indicates that pollen presentation schedules can be a consequence of interactions among anther dehiscence, flowering traits and pollination environments for a given species.

Inflorescence architecture affects pollinator behaviour and mating success in Spiranthes sinensis (Orchidaceae)

• Despite the wide inflorescence diversity among angiosperms, the effects of inflorescence architecture (three-dimensional flower arrangement) on pollinator behaviour and mating success have not been sufficiently studied in natural plant populations. • Here, we investigated how inflorescence architecture affected inter- and intra-plant pollinator movements and consequent mating success in a field population of Spiranthes sinensis var. amoena (S. sinensis). In this species, the flowers are helically arranged around the stem, and the degree of twisting varies greatly among individuals. The large variation in inflorescence architecture in S. sinensis results from variation in a single structural parameter, the helical angle (the angular distance between neighbour-flower directions). • The numbers of visits per inflorescence and successive probes per visit by leaf-cutting bees decreased with helical angle, indicating that individual flowers of tightly twisted inflorescences received less visitations. As expected from pollinator behaviour, pollinia removal and fruit set of individual flowers decreased with helical angle. Meanwhile, geitonogamy decreased in tightly twisted inflorescences. • Our novel findings demonstrate that natural variation in inflorescence architecture significantly affects pollinator behaviour and reproductive success, suggesting that inflorescence architecture can evolve under pollinator-mediated natural selection in plant populations. We also discuss how diverse inflorescence architectures may have been maintained in S. sinensis populations.

Adaptive plasticity of floral display size in animal-pollinated plants

Plants need not participate passively in their own mating, despite their immobility and reliance on pollen vectors. Instead, plants may respond to their recent pollination experience by adjusting the number of flowers that they display simultaneously. Such responsiveness could arise from the dependence of floral display size on the longevity of individual flowers, which varies with pollination rate in many plant species. By hand-pollinating some inflorescences, but not others, we demonstrate plasticity in display size of the orchid Satyrium longicauda. Pollination induced flower wilting, but did not affect the opening of new flowers, so that within a few days pollinated inflorescences displayed fewer flowers than unpollinated inflorescences. During subsequent exposure to intensive natural pollination, pollen removal and receipt increased proportionally with increasing display size, whereas pollen-removal failure and self-pollination accelerated. Such benefit-cost relations allow plants that adjust display size in response to the prevailing pollination rate to increase their attractiveness when pollinators are rare (large displays), or to limit mating costs when pollinators are abundant (small displays). Seen from this perspective, pollination-induced flower wilting serves the entire plant by allowing it to display the number of flowers that is appropriate for the current pollination environment.

Bimodal pollination system in rare endemic Oncocyclus irises (Iridaceae) of Lebanon

Pollination systems based on indirect rewards, such as heat and shelter, have seldom been studied. Plant–pollinator interactions were characterized in Iris cedretii Dinsmore ex Chaudhary and Iris sofarana subsp. kasruwana Dinsmore ex Chaudhary, rare endemic Lebanese Oncocyclus irises exhibiting potential adaptation to shelter pollination. Despite a diversity of floral visitors (47 species), only two groups of Anthophoridae male bees ( Xylocopa spp. and Eucera spp.) could be considered as efficient pollinators on the basis of frequency of visits, visiting behaviour, and pollen load. Lebanese Oncocyclus irises showed a bimodal pollination system with (1) diurnal visits at low rates but with potentially large pollen transfers by Xylocopa bees, exhibiting a foraging-like behaviour, principally during warm periods of the day and (2) sheltering, especially Eucera male bees, during night and day when the weather is changeable. Refuge occurrence in flowers was more important during the night (27%) than during the day (12%) and, for daytime, during cloudy or windy than sunny conditions. It also varied depending on the exposure of floral tunnels. Visitation rates of both day-visiting Xylocopa and night-sheltering Eucera were negatively associated with an increase of the number of flowers per clump. No experimental evidence was found showing that the principal advantage for male bees to shelter in Iris flowers was to fly earlier in the morning.

Functional differentiation in pollination processes between the outer and inner perianths in Iris gracilipes (Iridaceae)

We examined the functional differentiation in pollination processes between the outer and inner perianths in Iris gracilipes A. Gray flowers. We manipulated the length of the outer and inner perianths and examined the effect on the following: number of pollinator approaches to, and landings on, flowers; pollen removal and deposition during a single pollinator visit; total number of pollen grains removed; and seed production. The outer perianths contributed to the functions of pollinator approach and landing, pollen removal, and seed production, but not to mechanical fitting of pollinators. Since flowers with shortened outer perianths gained less pollination success than control flowers, and since lengthening of outer perianths did not enhance pollination success, the length of the outer perianths might have evolved as an adaptive characteristic. On the other hand, the inner perianths contributed to the functions of pollinator approach and pollen removal but not to pollinator landing, mechanical fitting of pollinators, nor seed production. Since flowers with shortened inner perianths gained pollination success similar to that of control flowers, the adaptive length of the inner perianths may be shorter than what is found in present-day flowers. We propose that genetic correlation between the outer and the inner perianths might prevent shortening of the inner perianths.

Consequences of floral complexity for bumblebee-mediated geitonogamous self-pollination in Salvia nipponica Miq. (Labiatae)

I address how floral complexity influences geitonogamous self-pollination through manipulation of pollinator behavior in Salvia nipponica. The pivoting stamens of S. nipponica hinder nectar-collecting bumblebees from crawling into flowers, increasing the probing time per flower. I predicted that longer probing times would reduce the relative cost of moving between plants, causing bees to leave plants earlier. To test this prediction, I simplified S. nipponica flowers by removing the stamens from all open flowers within a 75-m2 quadrat. Bumblebees probed these flowers more quickly than intact flowers, but the stamen removal affected neither the frequency of flower revisitation nor the flight distance between plants. In response to the decrease in the probing time per flower, bees probed more flowers on these plants. Therefore, in S. nipponica, floral complexity reduces the opportunity for geitonogamous self-pollination. Stamen removal also increased bee visitation per flower, suggesting that this sort of complexity deters visitation. To keep complex flowers attractive, therefore, selection might increase floral rewards or longevity. Floral complexity might evolve in an integrative manner with the rest of the floral phenotype.

Selfed-seed production depending on individual size and flowering sequence in Iris gracilipes (Iridaceae)

To examine the influence of individual size and time of pollination on selfed and outcrossed seed production, we conducted hand-pollination experiments in 2000 and 2001 in the perennial herb Iris gracilipes A. Gray (Iridaceae). Small individuals produced outcrossed seeds to a greater extent than selfed seeds, whereas large individuals produced not only outcrossed seeds but also selfed seeds. On the other hand, when half of all flowering shoots of an individual were removed, both selfed and outcrossed seeds were produced independent of individual size. Flowers that opened earlier within the same flowering shoots were likely to produce outcrossed seeds, while those that opened later within the same flowering shoots produced relatively more selfed seeds than did the flowers which opened earlier. Thus, the ratio of the numbers of selfed to outcrossed seeds produced may differ between small and large individuals. These results suggest that plants regulate selfed- and outcrossed-seed production depending on their sizes and flowering sequence.Key words: self-fertilization, individual size, flowering sequence, Iris gracilipes.