Pollination ecology and pollination system of Impatiens reptans (Balsaminaceae) endemic to China

BACKGROUND AND AIMS: China is one of the centres of geographical distribution of Impatiens L. Studies of the pollination ecology of this genus in China have, until now, been unreported. Impatiens reptans, a species endemic to China, was studied. The aims were to examine the pollination ecology and pollination system of this species, to compare its pollination ecology with other Impatiens species growing in Sumatra and Japan, and to discuss possible reasons for its limited distribution. METHODS: The pollination ecology of I. reptans was studied by carrying out continuous observations within three naturally growing populations. Its pollination system was studied using different pollination methods, marking and counting pollen grains, assessing pollen viability and observing pollinator behaviour. KEY RESULTS: The flowering phase of the protandrous I. reptans lasted for 89 d. The life span of an individual flower was 3.6 d. Primary pollinators were honey-bees and bumble-bees. Secondary pollinators were diurnal hawk moths and butterflies. Bombus briviceps and Bombus sp. were nectar gatherers. The mean nectar sugar concentration was 29.5 %, and the mean value of sucrose/glucose + fructose was 0.82. The proportion of seed set ranged from 0.857 to 0.873. Distances that seeds were ejected ranged from 0.58 to 1.17 m. Percentage seed germination under controlled conditions was 23.1. Pollen viability was highest on the day of anthesis and thereafter decreased. Ratios of pollen : ovules ranged from 958.8 to 970.6. CONCLUSIONS: Impatiens reptans reproduces by means of cross-pollination. Its dependence on a specialized habitat, a narrow environmental niche, a low percentage of seed germination, and habitat loss could be reasons for its limited distribution and endemism.     

益智传粉生物学的研究

 益智（Alpinia oxyphylla）的花期从2月底至4月下旬;单株花期（花序）约为16～26 d,通常为23～26 d;单花花期一般为1 d。正常情况下,益智和其它山姜属植物一样,具有花柱卷曲性促进异花授粉的机制,表现出两种类型：花柱上举型和花柱下垂型,其花柱卷曲运动的节律与其它已报道的山姜属植物基本一致。但观察发现,当遭遇低温天气时(日最高气温<18℃),单花花期延长为2 d,无论是上举型个体还是下垂型个体,均只表现出一种花型——下垂型：上午开花时花柱弯向上,柱头位于已开裂散发出花粉的花药上方,直到第二天上午6∶30～11∶00间,花柱才陆续地慢慢向下运动,柱头下降至与花药等高或位于花药下方。益智的主要传粉昆虫是蜜蜂（Apidae sp.）、木蜂（Xylocopa sp.）,绝大多数的访花者的访花目的是吸蜜。益智的花蜜分泌量（8.37～15.79 μl）和花蜜含糖量（30.12%～32.83%）较高,花蜜是益智作为传粉者访花的最主要的报酬。实验结果还表明,益智花部中唇瓣对昆虫访花有显著的招引作用;益智的花蜜对蜜蜂的访花频率有显著的影响,对木蜂有一定的招引作用,但并不显著。而花粉（花药）则对昆虫的访花频率影响不大。人工授粉实验结果表明益智存在自交亲和性,无论是上举型或下垂型个体自交和异交均有较高的结实率;人工自交和异交的结实率在上举型植株中存在较大的差异,而在下垂型个体中则差异不明显;去雄套袋、去柱头套袋和完全套袋不授粉等处理均不结实,表明益智不存在无融合生殖现象和自动自花授粉现象。益智的繁育系统是异花授粉交配系统。     

Floral morph composition and pollen limitation in the seed set of <Emphasis Type="Italic">Nymphoides indica</Emphasis> populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.     

APOMIXIS IN THE POA SECUNDA COMPLEX

Observation of cleared ovules of Poa secunda Presl showed that plants produce aposporous embryo sacs, and emasculation studies showed that they do not set seed without pollen. The ratio of sexual to asexual embryo sacs on a given plant does not correlate with percent stainable pollen, nor does pollen stainability correlate with meiotic regularity or with chromosome number. Percent apomictic ovules, percent pollen stainability, and regularity of meiosis all vary within a given population; they also vary if the same plant is moved from one site to another. Although there is morphological variation in the species, none of the reproductive features correlates with morphology. Pollen from any morph will stimulate seed set in any other morph, and hybrids have been produced in low frequency. Plants will also set seed using their own pollen. In its reproductive biology, P. secunda exhibits many similarities with P. pratensis L.     

Effects of self-, chase and mixed self/cross-pollinations on pistil longevity and fruit set in Ceiba species (Bombacaceae) with late-acting self-incompatibility

BACKGROUND AND AIMS: Late-acting self-incompatibility (LSI), in which selfed flowers fail to form fruits despite apparently successful growth of the pollen tubes to the ovules, is a contentious and still poorly understood phenomenon. Some studies have indicated pollen tube-pistil interactions, and major gene control. Others favour an early acting inbreeding depression explanation. METHODS: Experimental pollinations, including selfs (in a subsample of which the style was cut before pollen tubes reached the ovary), chase self/cross-pollinations, crosses, and mixed self/cross-pollinations were used to study floral/pistil longevity and effect on fruit set and seed yield in two Ceiba species known to have LSI. RESULTS: Self-pollinations, including those with a cut style, had extended floral longevity compared with unpollinated flowers. Chase pollinations in which cross-pollen was applied up to 3 h after selfing set fruits, but with reduced seed set compared with crosses. Those with cross-pollen applied at 4 and 8 h after self-pollination all failed to set fruits. Flowers subjected to 1 : 1 and 2 : 1 self/cross-pollinations all produced fruits but again with a significantly lower seed set compared with crosses. CONCLUSIONS: Extended floral longevity initiated with self-pollen tubes growing in the style indicates some kind of pollen tube-pistil interaction. Fruit set only in chase pollinations up to 3 h implies that self-pollen tubes either grow more slowly in the style or penetrate ovules more slowly on arrival at the ovary compared with cross-tubes. This agrees with previous observations indicating that the incidence of penetrated ovules is initially lower in selfed compared with crossed pistils. However, the low seed yield from mixed pollinations indicates that self- and cross-pollen tubes arrive at the ovary and penetrate ovules more or less simultaneously. Possible explanations for these discordant results are discussed.     

Floral display size and spatial distribution of potential mates affect pollen deposition and female reproductive success in distylous Pulmonaria officinalis (Boraginaceae)

In animal‐pollinated plants, both the spatial distribution of flowering individuals and the number of flowers that an individual displays affect pollen deposition rates and female reproductive success. Heterostylous species are likely to be particularly sensitive to the contingencies of spatial distribution, as they are reproductively subdivided into distinct mating groups, which usually exhibit self‐ and intra‐morph incompatibility and differ in floral morphology. In this paper, we explore the joint effects of both spatial distribution of potential mates and floral display size on morph‐specific pollen deposition rates and seed set patterns in two natural populations of Pulmonaria officinalis, a distylous species with a weak self‐incompatibility system. Both total stigmatic pollen load and the proportion of legitimate pollen decreased with increasing spatial isolation. Legitimate (intermorph) pollen transfer was, however, asymmetric and decreased more rapidly with decreasing proximity to a compatible legitimate mating partner in the S‐morph than in the L‐morph. Total stigmatic pollen loads per flower increased with increasing floral display size, indicating that large plants are disproportionately more visited than smaller individuals. However, because legitimate pollen deposition decreased with increasing floral display size, these results also suggest that larger numbers of flowers increase the degree of geitonogamous pollination. In both the L‐ and S‐morph, seed set significantly decreased with increasing isolation from a legitimate mating partner, but in the L‐morph seed set was less dependent on the spatial distribution of the S‐morph. In addition, seed set significantly increased with floral display size in the L‐morph, but not in the S‐morph. These findings indicate that the spatial distribution of potential mates and variation in floral display size may cause morph‐specific differences in pollen deposition rates and female reproductive success.     

Seed abortion in the sexual counterpart of <Emphasis Type="Italic">Brachiaria brizantha</Emphasis> apomicts (Poaceae)

Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three consecutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self- and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pollen viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self- and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression.     

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

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

Is Eucalyptus Cryptically Self-incompatible?

BACKGROUND AND AIMS: The probability that seeds will be fertilized from self- versus cross-pollen depends strongly on whether plants have self-incompatibility systems, and how these systems influence the fate of pollen tubes. METHODS: In this study of breeding systems in Eucalyptus urophylla and Eucalyptus grandis, epifluorescence microscopy was used to study pollen tube growth in styles following self- and cross-pollinations. KEY RESULTS: Pollen tubes from self-pollen took significantly longer than those from cross-pollen to grow to the base of the style in both E. urophylla (120 h vs. 96 h) and E. grandis (96 h vs. 72 h). In addition, both species exhibited reduced seed yields following self-pollination compared with cross-pollination. CONCLUSIONS: The present observations suggest that, in addition to a late-acting self-incompatibility barrier, cryptic self-incompatibility could be a mechanism responsible for the preferential out-crossing system in these two eucalypt species.     

Effects of pollen shortage and self-pollination on seed production of an endangered tree, Magnolia stellata

BACKGROUND AND AIMS: Pollen limitation is a significant determinant of seed production, and can result from both insufficient pollen quantity (pollen shortage) and quality (mainly relating to self-pollination). For animal-pollinated tree species with large floral displays, pollen limitation may be determined by a balance between increased pollen quantity due to increased attractiveness for pollinators, countered by increased self-pollination due to increased geitonogamy. The contributions of pollen shortage and self-pollination on seed production were quantitatively examined in the natural pollination of an insect-pollinated, dichogamous, endangered tree, Magnolia stellata, which has a large, showy floral display. METHODS: Manual self- and cross-pollinations were conducted to determine the effects of selfing on seed production. The outcrossing rate was measured using microsatellite analyses of open-pollinated seeds, and the embryo mortality rate caused by self-pollination was indirectly estimated. The frequency of ovule mortality due to pollen shortage was also inferred using the embryo mortality and ovule survival rates from natural pollination. KEY RESULTS: The average fruit set, seed set per fruit, and ovule survival rate per tree from hand cross-pollination were 1.37, 3.15, and 3.34 times higher than those from hand self-pollination, respectively, indicating that self-pollination causes inbreeding depression for fruit and seed set. The multilocus-outcrossing rate (t(m)) was intermediate, 0.632, and the primary selfing rate was 0.657. This indicates that frequent geitonogamous selfing occurs. The ovule mortality rate due to pollen shortage and the embryo mortality rate due to self-pollination were estimated to be 80.8 % and 45.9 %, respectively. CONCLUSIONS: It is concluded that seed production of M. stellata is strongly limited by both pollen shortage and self-pollination. Inefficient beetle-pollination and the automimicry system via asynchronous flowering might be responsible for the high level of pollen shortage and frequent geitonogamy. This is despite a large, showy floral display and the dichogamous system of the species.     

Adaptive significance of flexistyly in Alpinia blepharocalyx (Zingiberaceae): a hand-pollination experiment

BACKGROUND AND AIMS: Flexistyly is a sexual dimorphism where there are two morphs that differ in the temporal expression of sexual function and also involve reciprocal movement of the stigmatic surface through a vertical axis during the flowering period. The adaptive significance of flexistyly has been interpreted as a floral mechanism for outcrossing, but it may also function to reduce sexual interference in which styles and stigmas impede the pollen export. Here these two explanations of flexistyly were tested in Alpinia blepharocalyx through a hand-pollination experiment. METHODS: Hand-pollinations were performed in two temporal morphs and consisted of two sequential pollination treatments, namely self-pollination in the morning and inter-morph pollination in the afternoon (treatment 1) or conversely inter-morph pollination in the morning and self-pollination in the afternoon (treatment 2), and two simultaneous self- and inter-morph cross-pollination treatments either in the morning (treatment 3) or in the afternoon (treatment 4). Seed paternity was then determined to assess relative success of self- versus cross-pollen using allozyme markers. KEY RESULTS: In the sequential pollination treatments, whether the stigmas of recipients are receptive in the morning is crucial to the success of the pollen deposited. When the cataflexistylous (protandrous) morph served as pollen recipient, early-arriving pollen in the morning can sire only a very small proportion (<15%) of seeds because the stigmas were then unreceptive. However, when the anaflexistylous (protogynous) morph served as pollen recipient, early pollen did gain a large competitive advantage over the late pollen, particularly when cross-pollen arrived first. Simultaneous self- and inter-morph cross-pollination indicated that outcross-pollen is more competitive than self-pollen on receptive stigmas. CONCLUSIONS: Differential maturing of male and female organs in Alpinia blepharocalyx is sufficient for selfing avoidance, obviating the need for style movements. Instead, the upward style curvature of the cataflexistylous morph in the morning and the anaflexistylous morph in the afternoon most likely represents a means of reducing interference with pollen export.     

Pollen donation patterns in a tropical distylous shrub (PSYCHOTRIA SUERRENSIS; RUBIACEAE)

Selection on male function has been invoked to explain various floral features, including number of flowers, flower size, and flower color. Here I describe two experiments designed to examine the efficiency of distyly in promoting male floral function, as measured by successful pollen dispersal to stigmas. In both experiments, I performed emasculations to control the type of pollen locally available in a natural population of Psychotria suerrensis, a tropical shrub. In the “natural-recipients” experiment, I allowed each floral morph to donate pollen on alternate days to emasculated flowers of each morph. In the “paired-recipients” experiment, I attached paired cuttings of each morph to individual donor plants. The results of both methods were consistent. Pollen borne on low anthers (from pin plants) was transferred most efficiently to low stigmas (on thrum plants). Pollen borne on high anthers (from thrum plants) was dispersed in equal amounts to flowers of both morphs. The results suggest that distyly is only partially effective in achieving efficient pollen donation. Male function of pins is enhanced by the polymorphism, but male function of thrums is not. A supplemental pollination experiment illustrates that seed set in this species is predominantly pollen limited, reducing the importance of male function, in comparison with species where seed set is primarily resource limited.     

Heteromorphic incompatibility and efficiency of pollination in two distylous Pentanisia species (Rubiaceae)

BACKGROUND AND AIMS: Distyly has been hypothesized to promote cross-pollination by reducing intrafloral and geitonogamous self-pollination, and enhancing intermorph pollination. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. Distyly and its pollination consequences were examined in two Pentanisia species with long-tubed flowers which are pollinated almost exclusively by butterflies. METHODS: Anther and stigma heights were measured to quantify reciprocal herkogamy. The type of incompatibility system was determined by observing pollen tubes and seed production following controlled hand pollination. Pollen loads on pollinators and stigmas were also examined to assess the efficiency of intermorph pollen flow. KEY RESULTS: Pentanisia prunelloides and P. angustifolia exhibit reciprocal herkogamy and a host of ancillary dimorphisms, including pollen colour, exine sculpturing, stigmatic papilla shape and floral-tube pubescence. Controlled hand-pollinations revealed the presence of a strong heteromorphic incompatibility system in both species. The site of incompatibility differed between the morphs; intramorph pollen tubes were blocked in the style of the short-styled morph and on the stigmatic surface of the long-styled morph. Butterflies carried pollen from the short- and long-styled morphs primarily on their head and proboscis, respectively. Natural pollination resulted in a higher proportion of pollen transfer from long- to short-styled plants than vice versa. Nevertheless, fruit set did not differ between morphs. CONCLUSIONS: Both Pentanisia species are fully distylous. Reciprocal herkogamy results in pollen from the two morphs being carried on different locations on pollinators' bodies, which in turn promotes intermorph pollination. Intramorph pollination does not result in fertilization, because of an effective heteromorphic incompatibility system.     

菊科入侵植物三叶鬼针草的繁殖特征及其与入侵性的关系

三叶鬼针草(Bidens pilosa)是一种危害严重的菊科入侵植物。通过实验观察和人工控制套袋等方法, 对其花序开花动态、花粉胚珠比 (P/O)、自交亲和性、花粉活力、访花昆虫和种子 (瘦果)的萌发率等与繁殖相关的特征进行了研究, 探讨了这些繁殖特征与入侵性的关系。三叶鬼针草在10-11月开花, 单个花序的花期约为5-6天。每小花的花柱基部有圆筒状的蜜腺环绕。单个花序内可自交亲和, 自交结实率和花粉活力均较高, 其P/O值为1 7 54.1 2±29 .8 7。主要访花者为灰蝶科(Lycaenidae)、粉蝶科(Pieridae)和茧蜂科(Braconidae)昆虫。三叶鬼针草所具有的灵活交配机制是其入侵成功的重要因素。此外, 三叶鬼针草结实量大、种子产生迅速且适于传播, 以及种子萌发范围广和短期快速萌发等特性也增强了其入侵性。     

Distribution of flower morphs, ploidy level and sexual reproduction of the invasive weed Oxalis pes-caprae in the western area of the Mediterranean region

Background and Aims: Oxalis pes-caprae is a widespread invasive weed in regions witha Mediterranean climate. In its native habitat (southern Africa)this species has been reported as heterostylous with trimorphicflowers and a self- and morph-incompatible reproductive system.In most of the areas invaded, only a pentaploid short-styledmorphotype that reproduces mainly asexually by bulbils is reported,but this has only been confirmed empirically. This study aimsto analyse the floral morph proportions in a wide distributionarea, test the sexual female success, and explain the causesof low sexual reproduction of this species in the western areaof the Mediterranean Basin. Methods: Fifty-five populations of O. pes-caprae were sampled in theIberian Peninsula and Morocco to evaluate the floral morph ratioand individual fruit set. In plants from a dimorphic population,hand-pollination experiments were performed to evaluate theeffect of the pollen source on pollen tube growth through thestyle. The ploidy level and genome size of individuals of eachfloral morph were analysed using flow cytometry. Key Results: From the populations studied 89·1 % were monomorphic,with most of them containing the short-styled (SS) floral morph,and 10·9 % were dimorphic containing long-styled(LS) and SS morphs. In some of these, isoplethy was verifiedbut no fruit production was observed in any population. A sterileform was also recorded in several populations. Hand-pollinationexperiments revealed that pollen grains germinated over recipientstigmas. In intermorph crossings, pollen tubes were able todevelop and fruit initiation was observed in some cases, whilein intramorph pollinations, pollen tube development was sporadicand no fruit initiation was observed. All individuals withineach floral form presented the same DNA ploidy level: SS plantswere pentaploid and LS and the sterile form were tetraploid. Conclusions: The low or null sexual reproduction success of this speciesin the area of invasion studied seems related with the highfrequency of monomorphic populations, the unequal proportionof floral morphs in dimorphic populations and the presence ofdifferent ploidy levels between SS and LS morphs. The discoveryof the occurrence of an LS floral morph and a sterile form,whose invading capacity in these areas is as yet unknown, willbe valuable information for management programmes.     

菊科入侵植物三叶鬼针草的繁殖特征及其与入侵性的关系

三叶鬼针草(Bidens pilosa)是一种危害严重的菊科入侵植物.通过实验观察和人工控制套袋等方法,对其花序开花动态、花粉胚珠比(P/O)、自交亲和性、花粉活力、访花昆虫和种子(瘦果)的萌发率等与繁殖相关的特征进行了研究,探讨了这些繁殖特征与入侵性的关系.三叶鬼针草在10-11月开花,单个花序的花期约为5-6天.每小花的花柱基部有圆筒状的蜜腺环绕.单个花序内可自交亲和,自交结实率和花粉活力均较高,其P/O值为1754.12士29.87.主要访花者为灰蝶科(Lycaenidae)、粉蝶科(Pjerjdae)和茧蜂科(Braconidae)昆虫.三叶鬼针草所具有的灵活交配机制是其入侵成功的重要因素.此外,三叶鬼针草结实量大、种子产生迅速且适于传播,以及种子萌发范围广和短期快速萌发等特性也增强了其入侵性.     

Pollen-limited seed set in pseudogamous blackberries (Rubus L. subgen. Rubus)

Summary Pseudogamous blackberry species are polyploid and usually exhibit meiotic irregularities causing severe reduction in pollen viability. When species means were compared, relative seed set was strongly correlated with the number of good pollen grains produced per flower divided by the number of ovules (good-pollen/ovule ratio).Partial correlation analysis for percentage viable pollen and relative seed set, controlling for good-pollen/ovule ratio, revealed an almost significant relationship, presumably because meiotic irregularities have a similar effect on both pollen viability and the viability of meiotically derived embryo sacs. Seed germination, on the other hand, showed no relationship with relative seed set or with pollen viability.The automatic selfing capacity exhibited by most of these blackberry species does not appear to be related to either pollen production or relative seed set.     

Differential ovule development following self- and cross-pollination: the basis of self-sterility in Narcissus triandrus (Amaryllidaceae)

Self-pollination results in significantly lower seed set than cross-pollination in tristylous Narcissus triandrus. We investigated structural and functional aspects of pollen–pistil interactions and ovule–seed development following cross- and self-pollination to assess the timing and mechanism of self-sterility. Ovule development within an ovary was asynchronous at anthesis. There were no significant differences in pollen tube behavior following cross- vs. self-pollination during the first 6 d of growth, regardless of style morph type. Double fertilization was significantly higher following cross- vs. self-pollination. Aborted embryo development was not detected following either pollination type up to seed maturity. Prior to pollen tube entry, a significantly greater number of ovules ceased to develop following self- vs. cross-pollination. These results indicate that self-sterility in N. triandrus operates prezygotically but does not involve differential pollen tube growth typical of many self-incompatibility (SI) systems. Instead, low seed set following self-pollination is caused by a reduction in ovule availability resulting from embryo sac degeneration. We hypothesize that this is due to the absence of a required stimulus for normal ovule development. If this is correct, current concepts of SI may need to be broadened to include a wider range of pollen–pistil interactions.     

The functional significance of a stigma color polymorphism in Acer pictum subsp. mono (Aceraceae)

Aims Exploring the characteristics and function of a flower color polymorphism contributes to our understanding of floral evolution in angiosperms. However, little information is available on stigma color polymorphisms in flowering plants despite their important functional role in plant reproduction.Methods We studied a stigma color polymorphism at the individual level in Acer pictum subsp. mono (aka A. mono) by investigating stigma-color morph proportion and comparing stigma performance and components of female fitness (pollen adhesion, pollen germination, fruit set, seed set and fruit weight) between the two morphs (red and greenish-yellow stigma) within natural populations at Dongling Mountain in the north of China. In the flowering period, we conducted hand pollination in natural populations and then made microscopic observations using the aniline blue fluorescence method.Important findings Individuals in the studied populations did not change their stigma color between years, and flowers produced by a single tree were uniform in their stigma color. This strongly suggests that stigma color variation is genetically controlled. The percentage of the red stigma flowers with germinated pollen grains was significantly higher than that of the greenish-yellow stigma flowers when hand pollination was conducted in the early flowering period, but not so in the late flowering period. There was no significant difference in the percentage of flowers with pollen adhesion to the stigma between the two morphs. Fruit set of the red morph was significantly higher than that of the greenish-yellow morph. Our findings suggest that the red morph may be more advantageous for pollen germination and fruit set than the greenish-yellow morph, which may provide a functional explanation for the high incidence of the red stigma morph in the studied populations. Alternative explanations for the stigma color polymorphism are discussed to stimulate further work.     

Fertility differences among floral morphs following selfing in tristylous Eichhornia paniculata (Pontederiaceae): inbreeding depression or partial incompatibility?

Reduction in seed set following self- vs. cross-pollination in flowering plants can result from abortion of selfed offspring owing to inbreeding depression and/or partial self-incompatibility. Previous studies on tristylous Eichhornia paniculata (Pontederiaceae) indicate that reduced seed set following self-pollination generally occurs in the short- (S), but not the long-(L) or mid-styled (M) morphs. To determine whether this pattern results from morph-specific differences in inbreeding depression owing to the sheltering of deleterious alleles at the S locus and/or partial self-incompatibility, we conducted controlled hand-pollinations of the floral morphs and measured seed set and levels of seed abortion. There were no significant differences in fertilization success and seed set following self-, illegitimate, and legitimate pollinations in the L and M morphs. In contrast, in the S morph self-, intramorph and intermorph illegitimate pollinations resulted in significant reduction in seed set in comparison with legitimate pollination. This indicates that the reduced seed set observed in self-pollination is the result of partial incompatibility rather than inbreeding depression. Significantly reduced fertilization success and low levels of ovule abortion in illegitimate pollinations of S plants also supported this conclusion. Reduced fertility in the S morph may have implications for the observed loss of this morph from natural populations and the evolutionary breakdown of tristyly.