Variation in Floral Sex Allocation and Reproductive Success in Sequentially Flowering Inflorescence of Corydalis remota var. lineariloba (Fumariaceae)

In hermaphroditic plants, female reproductive success often varies among different positions within an inflorescence.However, few studies have evaluated the relative importance of underlying causes such as pollen limitation, resource limitation or architectural effect, and few have compared male allocation. During a 2-year investigation, we found that female reproductive success of an acropetally flowering species, Corydalis remota Fisch. ex Maxim. var. Iineariloba Maxim. was significantly lower in the upper late developing flowers when compared with the lower early flowers. Supplementation with outcross pollen did not improve female reproductive success of the upper flowers, while removal of the lower developing fruits significantly increased female reproductive success of the upper flowers in both years, evidencing resource limitation of the upper flowers. Female production in upper flowers was greatly improved by simultaneous pollen supplementation of the upper flowers and removal of the lower fruits, suggesting that, when resources are abundant, pollen may limit the female reproductive success of the upper flowers. The less seed mass in the upper flowers didn't increase in all treatments due to architecture. In the upper flowers, ovule production was significantly lower and the pollen : ovule ratio was significantly higher. These results suggest that male-biased sex allocation in the upper flowers may lead to increased male reproductive success, whereas the lower flowers have higher female reproductive success.     

Seed set variation in wild Clarkia populations: teasing apart the effects of seasonal resource depletion,pollen quality,and pollen quantity

In habitats where resource availability declines during the growing season, selection may favor early‐flowering individuals. Under such ephemerally favorable conditions, late‐blooming species (and individuals) may be particularly vulnerable to resource limitation of seed production. In California, a region prone to seasonal drought, members of the annual genus Clarkia are among the last to flower in the spring. We compared pollen limitation (PL) of seed set and outcrossing rates between early‐ and late‐flowering individuals in two mixed‐mating Clarkia taxa to detect whether flowering time is associated with changes in seed set due to resource depletion, PL, or increased selfing. In 2008–2010, we hand‐pollinated one flower on a total of 1855 individual plants either Early (near the onset of flowering) or Late (near the end of flowering) in the flowering season and compared seed set to adjacent, open‐pollinated flowers on the same stem. To assess the contribution of pollen quality to reproduction, we first (2008) used allozymes to estimate outcrossing rates of seeds produced by Early and Late open‐pollinated flowers. Second (2009), we conducted an anther‐removal experiment to estimate self‐pollen deposition. Seed set in Clarkia unguiculata was not pollen‐limited. Clarkia xantiana ssp. xantiana was pollen‐limited in 2008 and 2010, but not 2009. PL did not differ between Early and Late treatments. In both taxa, seed set of Early flowers was greater than Late flowers, but not due to PL in the latter. Reproduction was generally pollinator‐dependent. Most pollen deposition was xenogamous, and outcrossing rates were >0.7 – and similar between Early and Late periods. These results suggest that pollen receipt and pollen quality remain seasonally consistent. By contrast, the resources necessary to provision seeds decline, reducing the fitness benefits associated with resource allocation to ovules.     

Effects of flowering plant density on pollinator visitation, pollen receipt, and seed production in Delphinium barbeyi (Ranunculaceae)

Variation in flowering plant density can have conflicting effects on pollination and seed production. Dense flower patches may attract more pollinators, but flowers in those patches may also compete for pollinator visits and abiotic resources. We examined how natural and experimental conspecific flowering plant density affected pollen receipt and seed production in a protandrous, bumble bee-pollinated wildflower, Delphinium barbeyi (Ranunculaceae). We also compared floral sex ratios, pollinator visitation rates, and pollen limitation of seed set from early to late in the season to determine whether these factors mirrored seasonal changes in pollen receipt and seed production. Pollen receipt increased with natural flowering plant density, while seed production increased across lower densities and decreased across higher flower densities. Experimental manipulation of flowering plant density did not affect pollinator visitation rate, pollen receipt, or seed production. Although pollinator visitation rate increased 10-fold from early to late in the season, pollen receipt and seed set decreased over the season. Seed set was never pollen-limited. Thus, despite widespread effects of flowering plant density on plant reproduction in other species, the effects of conspecific flowering plant density on D. barbeyi pollination and seed production are minor.     

新疆郁金香营养生长、个体大小和开花次序对繁殖分配的影响

植物有性繁殖与资源分配的关系研究对于揭示植物生活史特征及繁育系统进化具有重要意义。新疆郁金香(Tulipa sinkiangensis)是新疆天山北坡荒漠带特有的一种多年生早春短命植物。在自然生境中,该物种仅以有性繁殖产生后代,每株能产生1-8朵花,且不同植株上的花数及果实数以及花序不同位置上的花与果实大小明显不同。本文通过对新疆郁金香有性繁殖与营养生长及植株大小的关系以及花序中不同位置花及果实间的资源分配研究,旨在揭示营养生长、个体大小及开花次序对其繁殖分配的影响。结果表明:在开花和果实成熟阶段,新疆郁金香植株分配给营养器官(鳞茎和地上营养器官)与繁殖器官的资源间均存在极显著的负相关关系(P<0.01),说明其植株的营养生长与生殖生长间存在权衡关系。多花是新疆郁金香的一个稳定性状,其植株上花数目、花生物量、果实生物量和种子数量与植株生物量间均呈极显著的正相关关系(P<0.01),说明新疆郁金香植株的繁殖分配存在大小依赖性。在具2-5朵花的新疆郁金香植株中,花序内各花的生物量、花粉数和胚珠数、结实率、果实生物量、结籽数、结籽率及种子百粒重按其开花顺序依次递减,说明花序内各花和果实的资源分配符合资源竞争假说。植株通过减少晚发育的花或果实获得的资源来保障早发育的花或果实获得较多的资源,从而达到繁殖成功。     

Pollen limitation and natural selection on floral characters in the yellow monkeyflower, Mimulus guttatus

In flowering plants, pollen limitation has been proposed to intensify selection on floral characters important in pollinator attraction, but may also select for traits that increase seed set through autonomous selfing. Here, a factorial design (+/- pollen addition, +/- pollinator removal) was used to investigate how the pollination environment affects selection on floral morphology via female fitness in a mixed-mating population of the yellow monkeyflower, Mimulus guttatus (Phrymaceae). Female fitness was strongly pollen-limited, with supplementally pollinated plants setting 37% more seeds than open-pollinated individuals. Strong positive selection was found on flower length, weak positive selection on flower width : length ratio and no selection on stigma-anther distance in both open-pollinated and supplementally pollinated treatments. By contrast, flowers with relatively narrow corollas and low stigma-anther distances were favored in the pollinator exclusion treatment. These results provide mixed support for the idea that pollen limitation intensifies selection on floral characters. Despite strong phenotypic selection, natural pollen limitation did not mediate selection on characters associated with either pollinator attraction or self-fertilization. However, the novel pattern of selection on severely pollen-limited plants suggests that reproductive assurance against pollinator loss may have been directly involved in the floral evolution of closely related selfing taxa.     

Early reproductive failure increases nectar production and pollination success of late flowers in south Andean Alstroemeria aurea

Fertile ramets of bumblebee-pollinated Alstroemeria aurea, a clonal perennial native to the temperate forests of the southern Andes, produce single terminal inflorescences that may bear two or more temporally non-overlapping whorls of flowers. While fruit set is commonly high (>80%) among early-opening flowers, it is usually low (<20%) among late-opening flowers within ramets. Using flowering ramets with two whorls of flowers, we examined experimentally the following related hypotheses. First, late flowers act as a reserve of ovaries, increasing their likelihood of setting seed when early fruits abort due to either pollen or resource limitation. Second, where early fruit abortion has occurred, plants may actively ensure pollination of late flowers by increasing their attractants and rewards. In a natural population, we simulated (1) lack of pollen deposition in early flowers, by excising their stigmas just before receptivity, and (2) resource limitation, by removing all the leaves from an experimental flowering ramet. Treatments were applied to individual ramets according to a 2 × 2 factorial design. We found that when early flowers failed to set fruit due to stigma excision, nectar secretion and particularly pollen receipt strongly increased in late flowers. Higher pollen deposition contributed significantly to the observed five-fold increase in seed output of late flowers. Fruit and seed set from early flowers were more negatively affected by defoliation than that from late flowers. Defoliation did not interfere with a ramet's capacity to increase late reproductive output when early reproduction failed. These results support the assertion that late flowers act as a reserve of ovaries helping a plant to cope with an unpredictable environment. These results also suggest that plants may actively increase pollinator visitation by opportunistically increasing flower rewards. Received: 23 June 1998 / Accepted: 3 May 1999     

Pollen limitation in a narrow endemic plant: geographical variation and driving factors

Pollen limitation may have important consequences for the reproduction and abundance of plant species. It may be especially harmful to endangered and endemic plants with small populations. In this study, we quantify the effect of pollen limitation on seed production and seedling emergence in an endangered narrow endemic crucifer, Erysimum popovii. We conducted a pollen addition experiment across the entire geographic distribution of the species, and explored the effect of pollinator assemblage, plant population size and density, and other habitat variables on pollen limitation intensity in 13 populations. We supplemented flowers in 20 plants per population with allogamous pollen. To account for potential resource reallocation, we used two types of control untreated flowers: internal control flowers from the same individual as the supplemented flowers, and external control flowers from other individuals. Our results indicate that E. popovii is pollen-limited in most of the populations studied, but only through seed production, since pollen supplementation did not enhance seedling emergence. Beefly abundance was associated with among-population differences in pollen limitation intensity. Populations in which beeflies were more abundant were less pollen-limited. In contrast, the abundance of other flower visitors, such as large bees or butterflies, was not associated with pollen limitation. Annual rainfall and bare soil cover were associated with the intensity of pollen limitation across populations.     

云南草寇花序内不同部位的性分配

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

Empirical models of pollen limitation, resource acquisition, and mast seeding by a bee-pollinated wildflower

Synchronous mast seeding is increasingly recognized as common in plant populations. Recent theoretical models show that synchronous mast seeding could be a consequence of resource allocation and storage within individual plants, coupled by pollen limitation in low-flowering years. We used long-term population and weather data to parameterize models of flowering based on stored resources and pollen limitation in Astragalus scaphoides, a bee-pollinated plant that flowers in alternate years. We used these models to test whether internal resource dynamics could explain mast seeding in A. scaphoides and, if so, whether synchrony was caused by pollen limitation and/or fluctuations in precipitation. We compared predictions of models that included all combinations of three factors: constant versus precipitation-dependent resource gain, uniform versus heterogeneous resource gain (among individual plants), and resource-dependent versus resource- and pollen-limited fruit set. Pollen limitation and heterogeneous resource gain were necessary and sufficient to explain alternate-year flowering, but precipitation increased the quantitative match between model predictions and flowering dynamics. Together, our results support the importance of density-dependent pollen limitation as an ultimate and proximate cause of mast seeding in A. scaphoides. Precipitation does not act as a direct cue for synchrony in this species but might affect long-term resource gain and fruiting dynamics.     

Many to flower, few to fruit: the reproductive biology of Hamamelis virginiana (Hamamelidaceae)

Hamamelis virginiana flowers from late September to late November. In 1977, we began studying the reproductive biology of this eastern North American arborescent shrub by examining floral phenology and rewards, pollen-ovule ratios, breeding system, pollination, pollinator and resource limitation, and seed dispersal. The homogamous, self-incompatible flowers emit a faint odor, bear nectar with sucrose ratios typical of bee- and fly-pollinated flowers, and produce abundant sticky pollen. Flowers were visited infrequently by insects representing six orders. Flies were the most common floral visitors, specifically members of the genus Bradysia, but small bees also carried high percentages of Hamamelis pollen. Despite high pollen/ovule ratios (11?445 grains/ovule), bees and flies are likely pollinators, as experiments indicate wind pollination is less likely. Pollen quantity and resource availability did not appear to limit reproductive output, but pollen quality did. Tests of >40?000 flowers showed natural fruit set to be <1%. The flowering time, breeding system, and clumped distribution of plants, likely due in part to limited seed dispersal, combine to yield this remarkably low fruit set. Because all other species of Hamamelis flower from late winter to early summer, it may be that H. virginiana evolved a fall flowering phenology to avoid competition for pollinators with the closely related H. vernalis.     

Temporal floral sex allocation in protogynous Aquilegia yabeana contrasts with protandrous species: support for the mating environment hypothesis

We tested one of the predictions of Brunet and Charlesworth (1995) that relative floral sex allocation will vary temporally with the mating environment and that the form of dichogamy (protandry vs. protogyny) will select for the pattern of variation in male versus female resource allocation. In many hermaphroditic plant species, allocation to female function (ovule number) decreases from early to late flowers within inflorescences as a result of resource limitation or ontogenetic changes. This pattern may obscure the effects of the mating environment and dichogamy on selection for allocation patterns in protandrous species (male allocation increases regardless). By examining a protogynous species the alternative pattern of temporal variation in resource allocation is predicted, namely that allocation to male function should decrease (or female allocation increase) throughout the flowering sequence. This pattern was observed in protogynous Aquilegia yabeana (Ranunculaceae), in which ovule number per flower remained constant whereas pollen number decreased in sequentially blooming flowers. These observations support the temporal sex allocation hypothesis of Brunet and Charlesworth (1995).     

露蕊乌头（Aconitum gymnandrum Maxim）花序内位置依赖的性分配

植物会采取什么样的繁殖对策来保障它的雌性和雄性繁殖成功,特别是那些生活在高寒地区的植物,已经引起了很多学者的关注.通过野外盆栽试验对青藏高原东部高寒草甸上的常见物种露蕊乌头(Aconitum gymnandrum Maxim)花序内的性分配模式进行了研究,发现同其它顶向式(从花序基部到顶部)开花的植物一样,随着花从基部到顶部的次序,露蕊乌头的心皮数、雌蕊重、种子数和种子重/果实减少;而花药数,花粉数/胚珠数以及雄蕊重/雌蕊重增大,顶部花表现出偏雄的性分配.结籽率在不同的位置间没有显著差异,不受花粉数量的限制;去花实验没有增加剩余部位花的种子数,只是增加了种子大小,这是花序内不可避免的位置效应导致了花序内位置依赖的性分配模式.     

Does pollen limitation affect population growth of the endangered Dracocephalum austriacum L.?

Reproductive strategies can have significant consequences for the viability of plant populations. Still, the effects of lower fruit set due to pollen limitation on plant demography and population persistence have rarely been explored. The objectives of this study were to assess the ecological factors determining female reproductive success and to study the impact of pollen limitation on population growth of Dracocephalum austriacum L. (Lamiaceae), a critically endangered species with a discontinuous distribution across Europe. Despite the significant background information gathered on the population dynamics and genetic diversity of D. austriacum, little is known about its reproductive strategy and the effect it has on population growth. Thus, the reproductive system, pollinator assemblage and pollen limitation were studied in natural populations and the impact of pollen-limited seed production on population growth was assessed using existing transition matrix models. The results revealed that D. austriacum is protandrous self-compatible species that produces very few seeds in the absence of pollinators. The flowers are visited by several insects, including legitimate pollinators (e.g., Bombus hortorum, Osmia spp.) and nectar robbers (other Bombus spp., O. aurulenta). Fruit and seed production was significantly pollen-limited in all populations studied. However, despite the positive effect of pollen supplementation on seed production, the resulting increase in seed number did not significantly increase population growth rates in any of the studied populations. Hence, we conclude that populations are demographically stable and current natural seed production is sufficient for the species’ persistence.     

Testing hypotheses for excess flower production and low fruit-to-flower ratios in a pollinating seed-consuming mutualism

Pollinator attraction, pollen limitation, resource limitation, pollen donation and selective fruit abortion have all been proposed as processes explaining why hermaphroditic plants commonly produce many more flowers than mature fruit. We conducted a series of experiments in Arizona to investigate low fruit-to-flower ratios in senita cacti, which rely exclusively on pollinating seed-consumers. Selective abortion of fruit based on seed predators is of particular interest in this case because plants relying on pollinating seed-consumers are predicted to have such a mechanism to minimize seed loss. Pollinator attraction and pollen dispersal increased with flower number, but fruit set did not, refuting the hypothesis that excess flowers increase fruit set by attracting more pollinators. Fruit set of natural- and hand-pollinated flowers were not different, supporting the resource, rather than pollen, limitation hypothesis. Senita did abort fruit, but not selectively based on pollen quantity, pollen donors, or seed predators. Collectively, these results are consistent with sex allocation theory in that resource allocation to excess flower production can increase pollen dispersal and the male fitness function of flowers, but consequently results in reduced resources available for fruit set. Inconsistent with sex allocation theory, however, fruit production and the female fitness function of flowers may actually increase with flower production. This is because excess flower production lowers pollinator-to-flower ratios and results in fruit abortion, both of which limit the abundance and hence oviposition rates, of pre-dispersal seed predators.     

Herbivory differentially affects male and female reproductive traits of Cucumis sativus

Herbivory is an important selection pressure in the life history of plants. Most studies use seed or fruit production as an indication of plant fitness, but the impact of herbivory on male reproductive success is usually ignored. It is possible that plants compensate for resources lost to herbivory by shifting the allocation from seed production to pollen production and export, or vice versa. This study examined the impact of herbivory by Helix aspersa on both male and female reproductive traits of a monoecious plant, Cucumis sativus. The effects of herbivory on the relative allocation to male and female flowers were assessed through measurements of the number and size of flowers of both sexes, and the amount of pollinator visitation. We performed two glasshouse experiments; the first looked at the impact of three levels of pre-flowering herbivory, and the second looked at four levels of herbivory after the plants had started to flower. We found that herbivory during the flowering phase led to a significant increase in the number of plants without male flowers. As a consequence there was significantly less pollen export from this population, as estimated by movement of a pollen analog. The size of female flowers was reduced by severe herbivory, but there was no affect on pollen receipt by the female flowers of damaged plants. The decrease in allocation to male function after severe herbivory may be adaptive when male reproductive success is very unpredictable.     

Flowering phenology influences seed production and outcrossing rate in populations of an alpine snowbed shrub, Phyllodoce aleutica: effects of pollinators and self-incompatibility

Background and Aims

Because of differences in snowmelt time, the reproductive phenologies of alpine plants are highly variable among local populations, and there is large variation in seed set across populations. Temporal variation in pollinator availability during the season may be a major factor affecting not only seed production but also outcrossing rate of alpine plants.

Methods

Among local populations of Phyllodoce aleutica that experience different snowmelt regimes, flowering phenology, pollinator availability, seed-set rate, and outcrossing rate were compared with reference to the mating system (self-compatibility or heterospecific compatibility with a co-occurring congeneric species).

Key Results

Flowering occurred sequentially among populations reflecting snowmelt time from mid-July to late August. The visit frequency of bumble-bees increased substantially in late July when workers appeared. Both seed set and outcrossing rate increased as flowering season progressed. Although flowers were self-compatible and heterospecific compatible, the mixed-pollination experiment revealed that fertilization with conspecific, outcrossing pollen took priority over selfing and hybridization, indicating a cryptic self-incompatibility. In early snowmelt populations, seed production was pollen-limited and autogamous selfing was common. However, genetic analyses revealed that selfed progenies did not contribute to the maintenance of populations due to late-acting inbreeding depression.

Conclusions

Large variations in seed-set and outcrossing rates among populations were caused by the timing of pollinator availability during the season and the cryptic self-incompatibility of this species. Despite the intensive pollen limitation in part of the early season, reproductive assurance by autogamous selfing was not evident. Under fluctuating conditions of pollinator availability and flowering structures, P. aleutica maintained the genetic composition by conspecific outcrossing.Key words: Alpine snowbed, autogamy, bumble-bee, cryptic self-incompatibility, flowering phenology, mixed pollination, outcrossing rate, Phyllodoce aleutica, pollination success, seasonality, self-pollination     

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.     

Determinants of seed production in Geranium maculatum

Summary We experimentally examined factors limiting seed production in two populations of the perennial woodland herb Geranium maculatum in central Illinois, USA. To test the pollinator-limitation hypothesis, we compared the seed production of plants whose flowers were supplementarily pollinated with outcross pollen to that of control plants receiving natural pollination only. To test if fruit production by early flowers suppresses fruit and seed formation by late flowers, a third group of plants was prevented from producing seed from the first 50% of the flowers to open (stigmas were excised at flower opening). Finally, to test if seed maturation and flower initiation are correlated with photosynthetic capacity, we performed a defoliation experiment in which either the stem leaves within the inflorescence, the stem leaves below the inflorescence, or the rosette leaves were removed during late flowering. Plants that reccived supplemental pollination produced 1.5–1.6 times more seeds than control plants. We found no difference between hand-pollinated plants and controls in mortality, flowering frequency or number of flowers produced in the year following the experiment. In both control and hand-pollinated plants, the fruit set and total seed production of early flowers were more than twice as high as those of late flowers. In one of the two populations, plants whose early flowers were prevented from setting seed produced significantly more seeds from their late flowers than did control plants. Seed predation was low and did not differ between early and late flowers. Leaf removal did not affect seed number or size in the year of defoliation, nor did it reduce survival or flower production in the subsequent year. This suggests that the plants were able to compensate for a partial defoliation by using stored resources or by increasing photosynthetic rates in the remaining leaves. Taken together, the results demonstrate that both pollinator activity and resource levels influence patterns of seed production in G. maculatum. While seed production was pollinatorlimited in both populations, a seasonal decline in resource availability was apparently responsible for the low seed production by late flowers.     

Pollination Rates and Pollen Tube Growth in a Vulnerable Plant,Liriodendron chinense (Hemsl.) Sarg. (Magnoliaceae)

The limited seed production of insect-depended plant, Liriodendron chinense was once considered to be pollen-limited and insufficient cross pollination. In this study, we counted pollen grains deposited on stigmas in three populations in Guizhou, Hunan and Zhejiang provinces of China respectively. Over 61.9% stigmas were pollinated. From 1994 to 1996, the mean number of pollen grains on each stigma ranged from 4.4 to 42.6, much more than ovules(2) in each pistil. Based on observations of three years, both the pollination rate and pollen quantity on stigmas significantly affected seed set. When flowers opened without stamens dehiscencing at the early stage of anthesis, stigmas received considerable quantity of pollen grains. Pollen grains from different sources were able to germinate and pollen tube growth rates were not greatly variable. It is very likely that pollen grains arrived first would fertilize eggs. Since only several pollen tubes went through the stylar canal, the potential pollen competition may exist. In this case, there would be strong selection on floral syndrome which benefit early insect visits. Pollen grains from the early visits would have a greater chance to fertilize ovules than those from later visits, which implies that cross pollination is the predominant breeding system of this plant. The conclusion was also confirmed by following four artificial experiments. Three treatments, including flowers bagged, netted or with the perianth removed, all reduced seed set clearly, but flowers with the stamens removed (emasculation)had a higher seed production by open pollination. As the rates of deposited stigmas in three populations were 6～8 times more than full seed set, we considerthat lower seed production in this plant may not mainly be due to pollen limitation.     

Pollen viability and limitation of seed production in a population of the circumpolar cushion plant, Silene acaulis (Caryophyllaceae)

Pollen viability among genders and limitation of female seed production in a natural trioecious population of the circumpolar cushion plant Silene acaulis was examined. Pollen viability was estimated by an in vitro pollen germination experiment. Both male and hermaphrodite flowers displayed large variation in pollen viability (0–53% in hermaphrodite and 0–54% in male flowers). There was a significant difference between genders in pollen viability: male plants had on average higher pollen viability than hermaphrodite plants. Resource and pollen limitation of seed production was studied by an experiment consisting of three treatments; (I) hand-pollination and removal of all other flowers on the cushion, (II) hand-pollination without removal of other flowers, and (III) open pollination without removal of flowers. Hand-pollination increased seed production, whereas removal of flowers had no effect on seed production. Abortion of pollinated ovules during seed development and seed mass did not differ among treatments. To control for effect of fruit number on seed production, data from naturally pollinated individuals was used. There was a positive correlation between both total number of seeds and fruit number, mean seed number per fruit and fruit number, respectively. These results indicate that seed production of 5. acaulis is mainly limited by pollen availability whereas resource competition between fruits is not important as a limiting factor. The possible role of male quality differences between genders and pollen limitation of seed production for maintenance of trioecious reproductive systems is discussed.