Levels Of Pollen Limitation Research Articles

Inoculum source dependent effects of ericoid, mycorrhizal fungi on flowering and reproductive success in highbush blueberry (Vaccinium corymbosum).

Most terrestrial angiosperms form mutualisms with both mycorrhizal fungi and animal pollinators. Yet, the effects of mycorrhizae on pollinator behavior and plant reproduction are unknown for most species, and whether the source or type of mycorrhizal fungi affects reproductive success has rarely been examined. We examined whether inoculating highbush blueberry (Vaccinium corymbosum; Ericaceae) with ericoid mycorrhizal fungi enhanced investment in flowering and attractiveness to pollinators, and thus reduced their levels of pollen limitation over that of non-inoculated plants. We also examined the degree to which pollen limitation was dependent on inoculation source and the surrounding pollinator community context. Three-year-old saplings of Vaccinium corymbosum 'Bluecrop' or highbush blueberry (Ericaceae) were inoculated with a) ericoid mycorrhizal fungi within soil of the rhizosphere of plants growing at a local blueberry farm, b) a commercially available ericoid inoculant, c) both the local soils and commercial inoculum, or d) were not inoculated and served as controls. They were grown for one year in pots in a common garden and, in the following year, were moved to six farms in central Vermont that were known from prior studies to differ in pollinator abundance and diversity. We conducted a hand pollination experiment at each farm to examine if inoculation or pollinator abundance (i.e., farm context) affected reproductive success. Plants treated with all types of inoculums were more likely to flower, and produced more inflorescence buds than non-inoculated plants in 2018. However, in 2019, plants in the combination inoculum treatment, alone, produced more inflorescence buds than those in the other treatments. Neither the source of inoculum nor hand pollination affected fruit set (the proportion of flowers setting fruit), or fruit sugar content. Hand pollination, but not inoculation, increased berry mass and the average number of seeds produced/berry. Our results add to the growing body of evidence that mycorrhizal fungi can affect reproductive traits of their hosts but that the effects of mycorrhizal fungi depend on the mycorrhizal symbionts.

Large flowers can be short‐lived: Insights from a high Andean cactus

AbstractLong‐lived and large flowers signify high floral maintenance costs. Species of arid/semiarid climates with large flowers are expected to have short flower life spans and pollination‐induced flower longevity in order to curb high floral water and other maintenance costs. We explored the context‐dependent large flower/short flower longevity hypothesis in Eriosyce curvispina (Cactaceae), a large‐flowered species of the semiarid central Chilean Andes. We determined breeding system, flower visitation rates, and open‐pollination fruit set and quantified floral water content. In a temperature‐controlled field manipulative experiment, we measured potential flower longevity and tested for the presence of pollination‐induced floral senescence. We measured the time span of the complete flower cycle defined as from when the flowers began to open to when they were totally closed, and the time span of fully open flowers defined as from when they were totally open until they began to close. The potential flower life span averaged 2.8 days (complete flower cycle) to 2.3 days (fully open flower). The complete flower cycle lasted 21.5 h, but flowers were fully open for only 10.1 h across days. Flower longevity in days was far shorter than reported for a large sample of species in the area. No evidence was found for pollination‐induced flower senescence as a complementary means for reducing floral maintenance costs. Eriosyce curvispina is self‐incompatible and abundantly pollinated by two megachilid bees. The level of pollen limitation (L = 0.36) was lower than the average reported for self‐incompatible angiosperms. Thus, the short flower life span in E. curvispina is not an impediment for high fruit set. Flowers contain >5 g of water of which >2 reside in >40 petaloid tepals. The amount of water is far higher than in another large‐flowered, non‐cactus species in the area, but only about 15% of that reported in the giant saguaro cactus which has larger flowers than E. curvispina.

Functional role of long-lived flowers in preventing pollen limitation in a high elevation outcrossing species.

Low pollinator visitation in harsh environments may lead to pollen limitation which can threaten population persistence. Consequently, avoidance of pollen limitation is expected in outcrossing species subjected to habitually low pollinator service. The elevational decline in visitation rates on many high mountains provides an outstanding opportunity for addressing this question. According to a recent meta-analysis, levels of pollen limitation in alpine and lowland species do not differ. If parallel trends are manifested among populations of alpine species with wide elevational ranges, how do their uppermost populations contend with lower visitation? We investigated visitation rates and pollen limitation in high Andean Rhodolirium montanum. We test the hypothesis that lower visitation rates at high elevations are compensated for by the possession of long-lived flowers. Visitation rates decreased markedly over elevation as temperature decreased. Pollen limitation was absent at the low elevation site but did occur at the high elevation site. While initiation of stigmatic pollen deposition at high elevations was not delayed, rates of pollen arrival were lower, and cessation of pollination, as reflected by realized flower longevity, occurred later in the flower lifespan. Comparison of the elevational visitation decline and levels of pollen limitation indicates that flower longevity partially compensates for the lower visitation rates at high elevation. The functional role of flower longevity, however, was strongly masked by qualitative pollen limitation arising from higher abortion levels attributable to transference of genetically low-quality pollen in large clones. Stronger clonal growth at high elevations could counterbalance the negative fitness consequences of residual pollen limitation due to low visitation rates and/or difficult establishment under colder conditions. Visitation rates on the lower part of the elevational range greatly exceeded community rates recorded several decades ago when the planet was cooler. Current pollen limitation for some species in some habitats might underestimate historical levels.

Resource Allocation and Seed Size Selection in Perennial Plants under Pollen Limitation.

Pollen limitation may affect resource allocation patterns in plants, but its role in the selection of seed size is not known. Using an evolutionarily stable strategy model of resource allocation in perennial iteroparous plants, we show that under density-independent population growth, pollen limitation (i.e., a reduction in ovule fertilization rate) should increase the optimal seed size. At any level of pollen limitation (including none), the optimal seed size maximizes the ratio of juvenile survival rate to the resource investment needed to produce one seed (including both ovule production and seed provisioning); that is, the optimum maximizes the fitness effect per unit cost. Seed investment may affect allocation to postbreeding adult survival. In our model, pollen limitation increases individual seed size but decreases overall reproductive allocation, so that pollen limitation should also increase the optimal allocation to postbreeding adult survival. Under density-dependent population growth, the optimal seed size is inversely proportional to ovule fertilization rate. However, pollen limitation does not affect the optimal allocation to postbreeding adult survival and ovule production. These results highlight the importance of allocation trade-offs in the effect pollen limitation has on the ecology and evolution of seed size and postbreeding adult survival in perennial plants.

Earlier flowering did not alter pollen limitation in an early flowering shrub under short-term experimental warming

In animal pollinated plants, phenological shifts caused by climate change may have important ecological consequences. However, no empirical evidence exists at present on the consequences that flowering phenology shifts have on the strength of pollen limitation under experimental warming. Here, we investigated the effects of experimental warming on flowering phenology, flower density, reproductive success, and pollen limitation intensity in Caragana microphylla and evaluated whether earlier flowering phenology affected plant reproduction and the level of pollen limitation using warmed and unwarmed open top chambers in the Horqin Sandy Land of Inner Mongolia, northern China. The results of this study indicated that artificial warming markedly advanced flower phenology rather than extending the duration of the flowering. Additionally, warming was found to significantly reduce flower density which led to seed production reduction, since there were insignificant effects observed on fruit set and seed number per fruit. Experimental floral manipulations showed that warming did not affect pollen limitation. These results revealed the negative effects of advanced phenology induced by warming on flower density and reproductive output, as well as the neutral effects on reproductive success and pollen limitation intensity of long surviving plants.

Modest Pollen Limitation of Lifetime Seed Production Is in Good Agreement with Modest Uncertainty in Whole-Plant Pollen Receipt.

We recently introduced a model that predicts the degree to which a plant's lifetime seed production may be constrained by unpredictable shortfalls of pollen receipt ("pollen limitation"). Burd's comment in this issue criticized our analysis, first by arguing that the empirical literature documents much higher levels of pollen limitation than our model predicts and then suggesting that the apparent discrepancy stemmed from our (1) underestimating the costs of securing a fertilized ovule and (2) assuming too little unpredictability in whole-plant pollen receipt. We reply as follows. First, the empirical literature must be consulted carefully. Burd relies on pollen supplementation experiments performed on parts of plants or on whole plants but during only one reproductive season for polycarpic perennials; in both cases, resource reallocation often leads to gross overestimates of pollen limitation. We comprehensively review pollen limitation estimates that are free of these estimation problems and find strong agreement with our model predictions. Second, although cost estimates for different components of seed production are imprecise, errors are likely to be small relative to the >1,000-fold differences observed across plant species, the primary focus of our article. Finally, contrary to Burd's argument, pollen receipt by entire plants is much more predictable than that by individual flowers because the flower-to-flower variation "averages out" when summed across many flowers. Our model uses parameter values that are in broad agreement with the empirical record of modest plant-to-plant variation in pollen receipt and thus predicts the generally modest pollen limitation that is observed in nature.

Pollen limitation, species’ floral traits and pollinator visitation: different relationships in contrasting communities

Failures in the process of pollen transfer among conspecific plants can severely impact female reproductive success. Thus, pollen limitation can cause selection on plant mating systems and floral traits. The relationships between pollen limitation and floral traits might be partly mediated by the quantity and identity of pollinator visits. However, very little is known about the relationship between pollinator visits and pollen limitation. We examined the relationships between pollen limitation and floral traits at the community level to connect them to community ecology processes. We used 48 plant species from two contrasting communities: one species‐rich lowland community and one species‐poor alpine community. In addition, we calculated visitation rates and ecological pollination generalization for 38 of the species to examine the relationship between pollinator visitation and pollen limitation at the community level. We found low overall levels of pollen limitation that did not differ significantly between the alpine and the lowland community. In both communities, species with evolutionary specialized flowers were more pollen limited than species with unspecialized flowers. Species’ visitation rates and selfing capability were negatively related to pollen limitation in the alpine community, where pollinators are scarcer. However, flower size/number, ecological generalization of plants and flowering onset had greater effects on pollen limitation levels at the lowland community, indicating that the identity of the visitors and plant‐plant competitive interactions are more decisive for plant reproduction in this species‐rich community. There, pollen limitation increased with flower size and flowering onset, and decreased with ecological generalization, but only in species with evolutionary specialized flowers. Our study suggests that selection on plant mating system and floral traits may be idiosyncratic to each particular community and highlights the benefits of conducting community‐level studies for a better understanding of the processes underlying evolutionary responses to pollen limitation.

Parental optimism versus parental pessimism in plants: how common should we expect pollen limitation to be?

Many organisms exhibit parental optimism, producing more of the initial stages of offspring (e.g., eggs, embryos) than they can usually mature. For plants, parental optimism may be linked to the risk of seed production being limited by pollen receipt (pollen limitation). Here we elaborate a stochastic model of pollen limitation developed by Haig and Westoby ( 1988 ) and Burd ( 2008 ) and link it with published data on the magnitudes of prepollination costs versus postpollination costs of seed production in 80 plant species. We demonstrate that parental optimism should be expected when prepollination costs of seed production are small relative to postpollination costs. This was observed for most (62 of 80) of the plant taxa surveyed. Under parental optimism, plants overinvest in securing fertilized ovules, and consequently pollen limitation is predicted to be uncommon. However, for a sizable minority of plant species (18 of 80), prepollination costs approach or exceed postpollination costs. For these species, parental pessimism is instead optimal. Parents initiate fewer zygotes than they can usually mature, and pollen limitation is predicted to be severe. Because the relative magnitudes of prepollination and postpollination costs vary by more than 1,000-fold across plant taxa, parental outlook (optimism vs. pessimism) and levels of pollen limitation are predicted to vary widely.

Resource reallocation does not influence estimates of pollen limitation or reproductive assurance in Clarkia xantiana subsp. parviflora (Onagraceae)

Studies of pollen limitation and the reproductive assurance value of selfing are important for examining the process of floral and mating system evolution in flowering plants. Recent meta-analyses have shown that common methods for measuring pollen limitation may often lead to biased estimates. Specifically, experiments involving single- or few-flower manipulations per plant tend to overestimate pollen limitation compared to those involving manipulations on most or all flowers per plant. Little previous work has explicitly tested for reallocation within individual systems using alternative methods and response variables. • We performed single-flower and whole-plant pollen supplementation and emasculation of flowers of Clarkia xantiana subsp. parviflora to estimate pollen limitation (PL) and reproductive assurance (RA). We compared levels of PL and RA using the following response variables: fruit set, seeds/flower, and seeds/plant. We also assessed the germination and viability of seeds to evaluate potential variation in pollen quality among treatments. • Autonomous selfing in Clarkia xantiana subsp. parviflora eliminates pollen limitation and provides reproductive assurance. Estimates from single-flower manipulations were not biased, closely resembling those from whole-plant manipulations. All three response variables followed the same pattern, but treatments were only significantly different for seeds/flower. Pollen quality, as indicated by seed viability, did not differ among treatments. • Partial plant manipulations provided reliable estimates of pollen limitation and reproductive assurance. These estimates were also unaffected by accounting for pollen quality. Although whole plant manipulations are desirable, this experiment demonstrates that in some systems partial plant manipulations can be used in studies where whole-plant manipulations are not feasible.

Comparing the reproductive success and pollination biology of an invasive plant to its rare and common native congeners: a case study in the genus Cirsium (Asteraceae)

Previous studies have examined an association between reproductive success and pollination biology of rare versus widespread species through pair-wise comparisons of native and invasive congeners or rare and common congeners. To determine the importance of reproductive success and pollination biology for an invasive thistle, Cirsium vulgare, we compared it in its invaded range to five, co-occurring native Cirsium species that range from rare to common. Native study species include C. fontinale var. fontinale, C. andrewsii, C. brevistylum, C. occidentale, and C. quercetorum. We compared all species’ reproductive success, insect visitation rate and composition, autonomous self-pollination, and level of pollen limitation in multiple populations. Species differed in their reproductive success; the invasive C. vulgare produced more flower heads per plant than most native species. C. vulgare attracted more visitors than its congeners. In addition, reproductive success and insect visitation significantly varied between populations within species, mainly due to aphid infestation in one population of C. occidentale. Unlike the rare species (C. fontinale and andrewsii), C. vulgare did not require a pollinator for high-levels of seed production. The remaining native species set fewer seeds than C. vulgare without a pollinator. However, differences in insect visitation and autonomous self-pollination did not lead to differences in pollen limitation across species or between populations. This result suggests that factors other than pollination biology determine the difference in reproductive success of these species. However, high levels of autonomous self-pollination and generalist insect visitation may allow the invasive C. vulgare to easily establish new populations from low numbers of propagules. Our study provides one contrast that should build towards a larger comparative analysis to examine general patterns in the relationship between reproductive success, pollination biology, rare and invasive species, and our ability to predict biological invasions in introduced species.

Pollen Limitation and Reproductive Assurance in the Flora of the Coastal Atacama Desert

Pollen limitation (PL) in plant populations is supposed to be particularly strong in variable pollinator environments. Here, we examined the extent of PL in the coastal Atacama Desert, where low and unpredictable rainfall drives large interannual variation in plant cover and pollinator abundances. We estimated PL levels and the capacity for autonomous selfing (autofertility) in 16 annual and perennial species. In addition, we compared fruit set of emasculated and intact flowers to test whether selfing provides reproductive assurance. We also examined the relationships between autofertility and life forms, between PL and autofertility, and between PL and flower size. We found a low level of PL ($$\mathrm{mean}\,=0.28$$) and a high incidence of autofertility, which was negatively correlated with PL and closely related to life form. Our results suggest that the low PL levels in autonomous‐selfing species result from the reproductive assurance provided by selfing. Considering nonautonomous species only, the mean PL (0.44) was similar to that reported for other habitats. We detected no association between PL and flower size. In summary, high rates of PL were not a general pattern among plants in the Atacama Desert. The low levels of PL can be attributed to the high incidence of autofertility.

Pollen Limitation in the Alpine: A Meta-Analysis

Pollen limitation (PL) is thought to be an important factor driving the evolution of floral traits in alpine plants. However, results of pollen supplementation experiments in alpine plants do not always show high levels of PL, and a general review of the importance of PL on alpine-plant reproduction is needed. We assessed to what extent alpine plants are pollen limited, and whether the intensity of PL differs between alpine and lowland species. Moreover, we assessed whether or not PL of alpine species depends on their reproductive system and, finally, whether the outcome of PL studies depends on whether supplemental-pollination was done at the whole-plant level or within a subset of available flowers. We performed both classical and phylogenetic meta-analysis. Our results show that alpine plants are pollen limited. However, PL did not differ significantly between alpine and lowland species. In the alpine, self-incompatible and self-compatible species show similar levels of PL. We did not find differences in PL between different manipulation levels. These results will help understand the real importance of PL of seed production in the alpine. We identify gaps in our knowledge of PL in the alpine that could serve to guide future directions of research in this field.

Pollen Limitation in the Alpine: A Meta-Analysis

Pollen limitation (PL) is thought to be an important factor driving the evolution of floral traits in alpine plants. However, results of pollen supplementation experiments in alpine plants do not always show high levels of PL, and a general review of the importance of PL on alpine-plant reproduction is needed. We assessed to what extent alpine plants are pollen limited, and whether the intensity of PL differs between alpine and lowland species. Moreover, we assessed whether or not PL of alpine species depends on their reproductive system and, finally, whether the outcome of PL studies depends on whether supplemental-pollination was done at the whole-plant level or within a subset of available flowers. We performed both classical and phylogenetic meta-analysis. Our results show that alpine plants are pollen limited. However, PL did not differ significantly between alpine and lowland species. In the alpine, self-incompatible and self-compatible species show similar levels of PL. We did not find differences in PL between different manipulation levels. These results will help understand the real importance of PL of seed production in the alpine. We identify gaps in our knowledge of PL in the alpine that could serve to guide future directions of research in this field.

Seed production in a threatened Aloe is not affected by bird exclusion or population size

Small populations of many plant species have been shown to exhibit ecological Allee effects. These effects are expected to be pronounced in plants which are obligate outcrossers and rely on pollinators which forage preferentially in larger populations with greater nectar availability. We examined the breeding and pollination systems, level of pollen limitation and seed production in populations of a threatened “ornithophilous” species, Aloe pruinosa. Experimental hand-pollinations showed that A. pruinosa is genetically self-incompatible and thus an obligate outcrosser. Experimental exclusion of birds from inflorescences did not affect seed production, suggesting that insects are effective pollinators. Supplemental hand-pollinations in several populations showed that seed production in A. pruinosa is not pollen limited. Further, there were no significant relationships between population size and any measure of reproductive success in this Aloe species. Small populations of A. pruinosa are thus viable in terms of pollination processes and should be protected from more direct threats, such as habitat alteration.

Pollination performance and vulnerability to pollination breakdown of sixteen native shrub species from New Zealand

The reproductive biology of 16 native shrub species was studied in 34 populations to identify breeding systems, pollen limitation, local abundance, and population age structures. Seven of the study species are hermaphroditic, seven dioecious, and two gynodioecious. one of the 18 hermaphrodite populations (Alseuosmia macrophylla at Mamaku Plateau) had high levels of self‐incompatibility and pollen limitation and mutualism failure was evident. in the gender‐dimorphic taxa, two populations (Co‐prosma spathulata from Hakarimata and Cyathodes juniperina from Pukemokemoke) had high levels of pollen limitation and insect‐pollinated species consistently ranked higher in a vulnerability assessment compared with wind‐pollinated species. There was no significant relationship between natural fruit set and the distance to the nearest conspecific pollen in any of the study populations. Seedling recruitment was variable but evident in 32 of the 34 study populations and appeared to be related to availability of suitable habitat. The species we studied occur mostly on forest edges where they rely on disturbed soil and high light conditions for establishment, and edges may be important for successful reproduction of some native plants, especially shrubs. We have demonstrated that plants with self‐incompatibility mechanisms and pollinator specialisation are at greater risk from pollen limitation and mutualism failure than self‐compatible or generalist species.

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.