First evidence of putrescine involvement in mitigating the floral malformation in mangoes: A scanning electron microscope study

Floral malformation is the most destructive disease in mangoes. To date, the etiology of this disease has not been resolved. There are indications that stress-stimulated ethylene production might be responsible for the disease. Putrescine mediates various physiological processes for normal functioning and cellular metabolism. Here, the effect of putrescine in concentration ranging from 10^?1 to 10^?3 M was evaluated on disease incidence during mango flowering seasons of 2012 and 2013. In a scanning electron microscopy (SEM) study, putrescine (10^?2 M)-treated malformed floral buds bloomed into opened flowers with separated sepals and/or petals like healthy, whereas the untreated (control) malformed buds remained deformed. Further, malformed flowers recovered upon putrescine treatment, displaying clearly bilobed anthers, enclosing a large number of normal pollen grains and functional ovary with broad stigmatic surface as compared to control. The present findings provide the first report to demonstrate the role of putrescine in reducing various adverse effects of stress ethylene via decelerating the higher pace of its biosynthesis. It stabilizes the normal morphology, development, and functions of malformed reproductive organs to facilitate successful pollination, fertilization, and, thereby, fruit set in mango flowers. However, putrescine–ethylene-mediated cell signaling network, involving various genes to trigger the response, which regulates a wide range of developmental and physiological processes leading to normal cell physiology, needs to be investigated further.     

Pollen presenters in the flowering plants–form and function

Pollen presenters are specific floral structures, other than anthers, from which pollen is distributed for cross fertilization between flowers. They occur in only five families of monocotyledons and 20 families of the dicotyledons. Presenters in 15 families are described here. In the largest plant family (Asteraceae) all taxa have pollen presenters, while in others (e.g. Myrtaceae) they occur in only some species in a few genera. Most presenters are associated with the gynoecium and there is a wide range of forms involving hairs or outgrowths of the stylar tissue. Despite the placement of self pollen close to the stigma most taxa are outcrossing and avoid self fertilization by protandry, with the stigma being covered at anthesis, or by precise placement of pollen so that is does not contact the stigma. Likely selective advantages in the development of pollen presenters include greater accuracy in pollen transfer than in the normal anther to stigma movement and avoidance of interference between male and female organs in the flower. In some groups there is enhanced ability for female choice because effective pollen delivery enables a choice from among many pollen grains, while in others pollen presenters enhance male reproductive success. Study of the pollination biology of plants requires a knowledge of which plants have pollen presenters and a full understanding of the structures in the flower which are associated with the pollen presentation action.     

Does self-pollination provide reproductive Assurance in Aquilegia Canadensis (Ranunculaceae)?

The ability to produce seeds when pollinators or potential mates are scarce is thought to be one of the main advantages of self-fertilization in flowering plants. However, whether autonomous selfing increases seed set in natural populations has seldom been tested, and even fewer studies have evaluated the advantage of selfing across a gradient of pollen availability. This study examines the fertility consequences of autonomous selfing in Aquilegia canadensis (Ranunculaceae), a short-lived, spring-flowering perennial typically found in small, patchy populations on rock outcrops. We used a pollinator exclusion experiment to confirm reports that A. canadensis has a well-developed capacity for autonomous selfing resulting from incomplete protogyny and close proximity of stigmas and anthers during dehiscence. Flowers excluded from pollinators set 87% as many seeds per carpel (X +/- 1 SE = 7.1 +/- 1.4 seeds) as hand-pollinated flowers (8.1 +/- 1.3 seeds), and seed production in unpollinated flowers correlated negatively with the distance between stigmas and anthers (r = -0.46). Autonomous selfing could be potentially valuable in providing reproductive assurance because only 2.7 +/- 0.5 pollen grains were deposited on each stigma before anther dehiscence, compared to 134.1 +/- 17.9 pollen grains by the end of anther dehiscence. However, prevention of autonomous selfing by anther removal before dehiscence did not decrease seed set, even for plants at low plant densities where outcross pollen may have been in short supply. Emasculated flowers set as many seeds per carpel (9.3 +/- 0.9) as intact flowers (8.4 +/- 1.1), indicating that sufficient cross pollen is deposited for full seed set. These results do not support the hypothesis that autonomous selfing by A. canadensis has been selected because it provides reproductive assurance.     

大叶相思花部形态与开花生物学研究

对大叶相思(Acacia auriculiformis)栽培植株进行观察,用光学显微镜和扫描电子显微镜(SEM)对其花部形态和开花生物学特征进行研究。结果表明,大叶相思开花期为8～12月,穗状花序由114.9±11.9朵花组成,每朵花含124.1±16.5枚雄蕊,每雄蕊含有8粒16合花粉,单花花粉量为15885粒;雌蕊1枚,具14.4±1.8个胚珠,柱头凹陷,属于湿柱头类型。P/O值为1103.1;大叶相思花在夜间开放,开花可分为5个阶段。开花期间观察到雌蕊先于雄蕊伸出花冠和雄蕊先于雌蕊伸出花冠并存的现象,推测雌雄基本同熟。SEM观察花药在开花的第2阶段形成开裂线,第4阶段完全开裂并开始散粉。雄蕊于开花第1天的9: 00～12: 00大量散粉;雌蕊柱头在开花第1天上午出现分泌物,随后逐渐减少。开花后3~4 d柱头开始凋零,柱头可授期约为3 d,以开花第1天10: 00之后的可授性最佳。     

海滨锦葵花内五柱头裂片弯曲的独自调节(英文)

海滨锦葵花柱有5柱头裂片,若传粉失败,柱头裂片向下弯曲使柱头与自身花粉接触,发生延迟自交。传粉不足条件下发生的延迟自交可能提供繁殖保障。该研究定量分析了花粉搁置和授精胚珠数对花内未授粉柱头裂片运动的影响,并测定了花粉—胚珠比及柱头可授性和花粉生活力。结果表明,花内未授粉柱头裂片的弯曲不受其他柱头接受花粉量及授精胚珠数的影响,仅响应于其自身是否接受到花粉,花粉-胚珠比值显示海滨锦葵交配系统属兼性异交;未授粉柱头经弯曲与自身花粉授触时的强柱头可授粉和高花粉生活力为自交授粉的发生提供了可能。包括柱头裂片运动在内的多个花性状有机地展示了一种新花内混合交配系统,且花内柱头裂片弯曲的独自调节为从花水平验证被广泛接受的自交进化解释——繁殖保障假说提供了可能。     

雄蕊合生植物半边莲的花部综合征与繁育系统

 为了解花内雄蕊合生结构的繁殖适应意义, 初步研究了雄蕊合生植物半边莲(Lobelia chinensis)的花部综合征、传粉特性和繁育系统。半边莲花大且鲜艳, 花瓣中下部弯折并合生成背部有裂缝的不封闭的花冠筒。5雄蕊的花药紧密合生成花药筒, 花丝中上半部也合生在一起, 只有花丝基部分离插生于花冠筒上。柱头被包裹在花药筒内。半边莲单花寿命可达5 d左右。雄性先熟, 柱头在伸出花药筒之后才具活性。花的主要访问者为蚂蚁、食蚜蝇和苍蝇类等小型昆虫。半边莲单花花粉数约为(5 200±130)粒、胚珠数约为(55±6)颗, 花粉胚珠比为94.54, 应属于兼性自交繁育系统, 但异交指数和其它特征都显示其以异交为主, 部分自交亲和。套袋和人工授粉实验发现, 半边莲不存在无融合生殖与自发自交, 但自交亲和性高。雄蕊合生(尤其是花药的合生)能把花药中的花粉聚拢在一起在传粉者的一次访问中就被同时带出, 与同样具有较低花粉胚珠比的花粉聚联(Pollen aggregation)传粉过程近似。半边莲的雄蕊合生结构(花药合生成筒、花丝上部也合生)可能与一些特定的花部特征, 如花被合生成未完全封闭的筒、雌雄异熟以及低花粉胚珠比等联系在一起, 形成了适应小型传粉者的“花部综合征”     

The effect of wild radish floral morphology on pollination efficiency by four taxa of pollinators

The effects of floral morphology on rates of pollen removal and deposition by different pollinators in generalist plant species are not well known. We studied pollination dynamics in wild radish, Raphanus raphanistrum, a plant visited by four groups of pollinators: honey bees, small native bees, butterflies, and syrphyd flies. The effects of anther position and other factors on pollen removal during single visits by all four pollinator taxa were measured. Flowers with high anther exsertion (i.e., anthers placed higher above the opening of the corolla tube) tended to have the highest numbers of pollen grains removed, but this effect was strongest for honey bees and butterflies. For all pollinator taxa, pollen removal increased with the number of pollen grains available on a flower and whowed a positive, decelerating relationship with the duration of the visit. The effects of stigma position and other factors on pollen deposition during single visits by honey bees and butterflies were also studied. The nectar-feeding butterflies had a higher pollination efficiency (percentage of pollen grains removed from anthers that were subsequently deposited on a stigma) than the nectar- and pollen-feeding honey bees. Flowers with intermediate stigma exsertion had the highest numbers of pollen grains deposited on their stigmas by butterflies, but stigma exsertion had no effect on deposition by honey bees. For both butterflies and honey bees, pollen deposition on the recipient flower increased with the amount of pollen removed from the donor flower, and there was a positive, decelerating relationship between deposition and time spent at the flower; these results are analogous to those for pollen removal. The effects of anther and stigma exsertion on pollen removal and denosition did not fit predictions based on patterns of floral correlations, but results for morphology, pollen availability, time spent per visit, and pollinator efficiency are in broad agreement with previous studies, suggesting the possible emergence of some general rules of pollen transfer.     

Appendix: a novel type of homeotic mutation affecting floral morphology

A mutant with altered floral morphology, designated Appendix, was observed among the progeny of a regenerated transgenic tobacco (Nicotiana tabacum) plant. Appendix flowers had small outgrowths resembling pistils on the tips of fertile anthers. No Obvious morphological changes could be seen in any other part of the plant. A more careful analysis using histological methods and scanning electron microscopy revealed that the outgrowths were composed only of a style and a stigma and that these added organs can support pollen germination and pollen tube growth. Examination of anther ontogeny using scanning electron microscopy indicated that the anther style and stigma of the Appendix mutant developed from a small group of cells at the anther tip coincident with development of the carpel style and stigma. Genetic analysis indicated that this mutation is nuclear, recessive and linked to the ‘transferred DNA’ (T-DNA) inserted during the generation of transgenic plants using Agrobacterium tumefaciens. The Appendix phenotype does not correspond to any of the other types of floral homeotic mutations that have been described so far.     

棉花竹花形态与结构研究

棉花竹（Fargesia fungosa）是箭竹属重要经济竹种,有关其花器官的形态与解剖结构研究至今尚无报道。本文通过对棉花竹花器官的形态与解剖结构进行观察和分析,结果显示棉花竹花序为“混合花序”,小穗基部具潜伏芽,由单个叶片形成的“佛焰苞”包裹多枚小穗。小穗单次发生,均长3.05±0.390 cm,每枚小穗具有3~7朵小花,小花均长1.85±0.167 cm,颖片2枚。每朵小花皆具有内、外稃各1片,3枚浆片,雄蕊3枚,雌蕊1枚组成。未成熟花药具有4药室,花药壁由外到内依次为表皮、药室内壁、中层和绒毡层。花药成熟后纵裂散粉。成熟花粉粒为2或3细胞型,且具有1个萌发孔。子房1室,上位,侧膜胎座,倒生胚珠,雌蕊具2分叉羽毛状柱头。花药发育异常,导致出现大量败育现象。     

Anther and stigma morphology in mirror-image flowers of Chamaecrista chamaecristoides (Fabaceae): implications for buzz pollination

Enantiostyly (mirror-image flowers) is usually associated with buzz pollination. In buzz-pollinated flowers, pollen is released through terminal pores after bees vibrate the stamens. Several studies have evaluated the function of 'buzzing' in pollen release, but less attention has been paid to the effect of buzzing on pollen capture and deposition on stigmas. Evaluating the mechanism of pollen dispersion in buzz-pollinated flowers is important because it may affect mating patterns and reproductive success. In this study, we analysed the morphology of sexual organs (anther and stigma) using electron microscopy, and determined the relationship between sexual organ structure and pollen capture function through experimental manipulations of buzz-pollinated flowers of Chamaecrista chamaecristoides, as well as vibration frequencies on floral visitors. Pollen release occurs through two terminal pores at the tip of the stamens. However, unlike most angiosperms that have their stigmatic surface exposed, C.?chamaecristoides presents a stigmatic surface inside a cavity covered by trichomes. Experimental manipulations showed that effective fertilisation is only achieved when the style is vibrated, suggesting that buzzing is not only important for pollen release but also for pollen capture and deposition on the stigma. This result, in addition to vibration frequency analysis, suggests that although all floral visitors buzz flowers only those that buzz at higher frequencies achieve effective fertilisation. The anatomical features of sexual organs in flowers of C.?chamaecristoides demonstrate that this species possesses a highly specialised, elaborate morphology, with both genders selected for traits that promote buzz pollination.     

Shortened anther-stigma distance reduces compatible pollination in two distylous Primula species

Aims Distyly has been regarded as an adaptation to improve compatible pollination between two floral morphs with reciprocal herkogamy. The hypothesis that the different positions of anthers and stigmas within flowers as well as their reciprocal position between morphs, reduce the probability of self pollination raised by Darwin has been rarely tested. In this study, we measured stigmatic pollen loads in response to reduced reciprocal herkogamy in two Primula species.Methods To see whether reciprocal herkogamy can increase compatible and/or reduce incompatible pollen deposition, thus promoting compatible pollination, we shortened the distance between anthers and stigmas within the flowers by changing the position of the corolla tube, to which the anthers were fused, i.e. reduced herkogamy in natural populations of Primula secundiflora and P. poissonii and quantified stigmatic pollen loads in the field over 2 years.Important findings In both species, stigmatic pollen loads were significantly higher in the long-styled (L-morph) than in the short-styled morph (S-morph) in both control and manipulated flowers, but percentage of compatible pollen in S-morph were higher. Flowers manipulated to halve the anther–stigma distance showed a similar pattern for 2 years: total pollen grain counts on stigmas did not differ significantly but compatible pollen grains in L- and S-morphs were significantly decreased in both species. The percentage of compatible pollen loads was decreased by 68.7% in P. secundiflora and 65.3% in P. poissonii in L-morphs, while it decreased by 30.6% and 2.9% in S-morphs, respectively. Our manipulation of the relative position of anthers and stigmas in the two distylous species indicated that a lower degree of herkogamy reduced compatible but incompatible pollen transfer was likely to increase. The higher proportion of compatible pollen in the S-morph than in the L-morph in the two Primula species could be attributed to the accessibility of two-level sexual organs, floral orientations and pollinator behaviors. This is a first attempt to manipulate intraflower herkogamy for understanding adaptation of heterostyly, shedding insights into how the reciprocal herkogamy promotes compatible pollination.     

An Examination of the Function of Male Flowers in an Andromonoecious Shrub Capparis spinosa

The pollen donor and pollinator attractor hypotheses are explanations for the functions of the male flowers of andromonoecious plants. We tested these two hypotheses in the andromonoecious shrub Capparis spinosa L. (Capparaceae) and confirmed that pollen production and cumulative volume and sugar concentration of nectar do not differ between male and perfect flowers. However, male flowers produced larger anthers, larger pollen grains and smaller ovaries than perfect flowers. Observations on pollinators indicated that two major pollinators (Xylocopa valga Gerst and Proxylocopa sinensis Wu) did not discriminate between flower morphs and that they transferred pollen grains a similar distance. However, there were more seeds per fruit following hand pollination with pollen from male flowers than from perfect flowers. Individuals of C. spinosa with a larger floral display (i.e. bearing more flowers) received more pollen grains on the stigma of perfect flowers. Female reproductive success probably is not limited by pollen. These results indicate that male flowers of C. spinosa save resources for female function and that they primarily serve to attract pollinators as pollen donors.     

Morphological development of anthers induced by the dimorphic smut fungus<Emphasis Type="Italic"> Microbotryum violaceum</Emphasis> in female flowers of the dioecious plant<Emphasis Type="Italic"> Silene latifolia</Emphasis>

When inoculated with the dimorphic smut fungus Microbotryum violaceum (Pers.) G. Deml and Oberwinkler, the female flower of the dioecious plant Silene latifolia (Miller) E.H.L. Krause develops anther-like structures filled with spores instead of pollen grains. Using natural scanning electron microscopy, Nomarski interference microscopy, and fluorescence microscopy, we investigated the morphological modifications of the host plant resulting from this parasitism and the localization of smut hyphae in the flower bud. Flowers of infected plants lasted significantly longer than those of healthy plants, probably because the infection strengthened floral organs, such as the flower base and the anther filaments. Smut hyphae were observed throughout all organs of the young flower buds of infected plants, including sepals, petals, stamens, and pistil primordia. In healthy female flowers, anthers initiated sporogenous cell formation, but lacked parietal cell layers. By contrast, the parietal cell layers of infected female flowers differentiated into tapetal tissue, middle cell layers, and endothecial layers, as in the anthers of healthy male flowers. Smut spore formation in the infected anther was initiated in intercellular regions between the sporogenous cells, resulting in degeneration of premature sporogenous cells, tapetal tissue, and middle cell layers. The development of the endothecial layers and epidermis in the infected anther were morphologically normal.Abbreviations DAPI 4,6-diamidino-2-phenylidole - i infected - PMC pollen mother cell     

Reproductive ecology of distylous Palicourea Padifolia (Rubiaceae) in a tropical montane cloud forest. I. Hummingbirds' effectiveness as pollen vectors

The adaptiveness of distyly has been typically investigated in terms of its female function, specifically pollen receipt. However, pollen loads on stigmas can only provide moderate support for Darwin's hypothesis of the promotion of legitimate crosses. To determine the effectiveness of hummingbirds as pollen vectors between floral morphs and the consequences in terms of male (pollen transfer) and female function (pollen receipt) in Palicourea padifolia (Rubiaceae), floral visitors, their foraging modes, and temporal patterns of floral visitation were observed and documented. Differences in pollen and stigma morphology, pollen flow, rates of pollen deposition, and/or stigmatic pollen loads were then evaluated for their contribution toward differences in reproductive output between floral morphs. A pollination experiment with stuffed hummingbirds that varied in bill size was done to evaluate the contribution of bill variation toward differences between floral morphs in pollen receipt and pollen transfer and female reproductive output. Anthers of long-styled flowers contained significantly more and smaller pollen grains than those of short-styled flowers, independently of corolla and anther lengths. The shape and orientation of the stigma lobes differed between morphs and were significantly longer among short-styled flowers. Hummingbird visitation rates did not differ significantly between floral morphs, and foraging movements from focal plants towards neighboring plants were independent of floral morph. Stigmatic pollen loads under field conditions and those after controlled hummingbird visitation, along with rates of pollen accumulation through the day indicated that stigmas of short-styled flowers receive proportionately more legitimate (intermorph) pollen grains than did those of long-styled flowers. However, the species of hummingbird was marginally significant in explaining variation in pollen deposition on stigmas. Lastly, intermorph pollinations of P. padifolia resulted in significant differences in fruit production between floral morphs, independent of pollination treatment and pollinator species; short-styled flowers proportionately developed almost twice the number of fruits developed by long-styled flowers.     

Occurrence of 9.5 cellulase and other hydrolases in flower reproductive organs undergoing major cell wall disruption

The occurrence of enzymes associated with bean leaf abscission was investigated in bean (Phaseolus vulgaris) flower reproductive organs in which catabolic cell wall events are essential during anther and pistil development. Cellulase activity was detected in high levels in both pistil and anthers of bean flowers before anthesis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblotting with 9.5 cellulase antibody identified a protein in anthers and pistil with the same size (51 kilodaltons) and serologically closely related to the abscission cellulase. The accumulation of 9.5 cellulase protein in the anther is developmentally regulated and increases from undetectable levels at very young stages of anther development to high levels as the anther matures. In the pistil, the 9.5 cellulase was localized in the upper part of the pistil where the stigma and the stylar neck reside and was detected in the youngest developmental stage analyzed. Antibodies against basic chitinase, which accumulates to high levels in abscission zones after exposure to ethylene, identified a protein with the same size (33 kilodaltons) and serologically closely related, in both anthers and upper portion of the pistil. In contrast, a 45-kilodalton protein and the basic β-1,3-glucanase associated with abscission were undetected in bean reproductive organs. Interestingly, β-1,3-glucanase activity was detected in young bean anthers and decreased at anthesis, but the anther β-1,3-glucanase is serologically unrelated to the basic β-1,3-glucanase. Thus, it appears that the basic cellulase and chitinase occur in combination in many plant processes that require major cell wall disruption, whereas hemicellulases such as β-1,3-glucanase are specific to each process.     

Population-level variation in the expression of heterostyly in three species of Rubiaceae: does reciprocal placement of anthers and stigmas characterize heterostyly?

Heterostyly (i.e., reciprocal placement of anthers and stigmas between two or three floral morphs) is hypothesized to enhance outcrossing and reduce selfing. However, few studies have documented reciprocity among individual plants; instead, mean anther and stigma heights for floral morphs are usually reported, masking interindividual variation. We measured eight floral dimensions for individuals in five populations of three heterostylous Rubiaceae. The three methods used to quantify reciprocity yielded different conclusions regarding the degree to which populations conformed to expectations for heterostylous plants. Only Psychotria poeppigiana had stigma and, to a lesser degree, anther heights in discrete classes. Variation among plants of Bouvardia ternifolia and Psychotria chiapensis yielded a continuum of anther and stigma heights across populations. Comparison of distances between stigma and anthers indicated that only flowers of B. ternifolia had, as expected, a constant value for this distance. Finally, regression relationships between anther and stigma heights and corolla length showed that only in one population each of B. ternifolia and P. poeppigiana, and in P. chiapensis, was distance between anthers and stigmas the same across the range of corolla sizes for both floral morphs. Variation among these species in expression of heterostyly was not clearly linked to phylogenetic relationship or pollinator syndromes. Two approach herkogamous (AH) species were studied for comparison. Flowers of Psychotria brachiata were consistently AH, but flowers of P. pittieri were highly variable. Determining fitness consequences of population-level variation in sexual systems requires studies linking floral morphology to pollinator behavior and pollen transfer.     

Male function for ensuring pollination and reproductive success in <Emphasis Type="BoldItalic">Berberis lycium</Emphasis> Royle: A novel mechanism

In Berberis lycium anthers on alternate stamens dehisce, thus prolonging the male function so that pollination is affected and reproduction is ensured. The large pollen sac of each bithecous anther after the appearance of longitudinal dehiscence slit moves away from the filament while remaining attached at the tip of the connective and then orients in such a way that pollen-laden surface faces the stigma. No pollen is available to receptive stigma as pollen grains remain stuck to the anther sac. They do not get dispersed even by wind. Pollination and consequently reproduction is ensured through the intervention of insect, which does not affect pollen transfer to the stigma directly but by touching the base of the staminal filament while foraging nectar secreted by nectaries at the base of corolla, thus leading to staminal movement. This makes the dehisced anthers stick to the stigma and deposit pollen there.     

Anther structure and pollen development in Melicoccus lepidopetalus (Sapindaceae): An evolutionary approach to dioecy in the family

Anther and pollen development in staminate and pistillate flowers of dioecious Melicoccus lepidopetalus (Sapindaceae) were examined by light and electron microscopy. Young anthers are similar in both types of flowers; they consist of epidermis, endothecium, two to four middle layers and a secretory tapetum. The microspore tetrads are tetrahedral. The mature anther in staminate flowers presents compressed epidermal cells and endothecium cells with fibrillar thickenings. A single locule is formed in the theca by dissolution of the septum and pollen grains are shed at two-celled stage. The mature anthers of pistillate flowers differ anatomically from those of staminate flowers. The epidermis is not compressed, the endothecium does not develop fibrillar thickenings, middle layers and tapetum are generally persisting, and the stomium is nonfunctional. Microspore degeneration begins after meiosis of microspore mother cells. At anthesis, uninucleate microspores and pollen grains with vegetative and generative nuclei with no cytokinesis are observed. Some pollen walls display an abnormal exine deposition, whereas others show a well formed exine, although both are devoid of intine. These results suggest that in the evolution towards unisexuality, the developmental differences of anther wall tissues and pollen grains between pistillate and staminate flowers might become more pronounced in a derived condition, such as dioecy.     

Floral micromorphology and microsporogenesis of the gynodioecious herb Glechoma longituba (Lamiaceae)

Gynodioecy, the phenomenon of having both hermaphrodite and female (i.e. male‐sterile) individuals within the same population, is an important intermediate step in the evolution of separate sexes in flowering plants. In this study, we investigated the floral micromorphology and microsporogenesis of the gynodioecious herb Glechoma longituba from four natural populations in Korea. The floral micromorphological characters of the different sex morphs were examined and compared using scanning electron microscopy (SEM), and the ultrastructure of microspores during microsporogenesis was studied. We also examined the development of anthers and pollen grains in the three sexual morphs (i.e. hermaphrodites, females, and gynomonoecious, i.e. individuals with a mixture of female and hermaphroditic flowers) by embryological investigation. The major difference in anther development between the three phenotypes was the early disintegration of the tapetal cells in the anthers of female flowers. While mature fertile pollen grains were found in both hermaphrodite and gynomonoecious phenotypes, females did not produce any pollen grains. In addition, both fertile and sterile pollen grains in gynomonoecious phenotypes were frequently observed. The results of the present study indicate that floral micromorphological characters were not distinct between sexual morphs of G. longituba, except for the structure of the inner cell surfaces of the anther. The observed tapetum abnormalities and degeneration of pollen grains in both gynomonoecious phenotypes and females may be the consequence of inbreeding depression in hermaphrodites.     

Pollen flow in the distylous<Emphasis Type="Italic"> Palicourea fendleri</Emphasis> (Rubiaceae): an experimental test of the Disassortative Pollen Flow Hypothesis

The Disassortative Pollen Flow Hypothesis proposed by Darwin postulates that the relative position of anthers and stigmas in distylous flowers enhances pollen flow between flowers of different morphs (legitimate pollination), in comparison to flow between flowers of the same morph (illegitimate pollination). In order to test this hypothesis, we measured pollen transport, mediated by a trained Copper-rumped Hummingbird (Amazilia tobaci), between flowers of the distylous Palicourea fendleri under laboratory conditions. In individual tests, we offered to the hummingbird a pollen donor flower and two emasculated recipient flowers in a controlled sequence. After each foraging bout, we counted the number of pollen grains transported from the donor flower to the stigmas of both recipient flowers. In agreement with Darwin's hypothesis, we found that hummingbirds transport pollen of "pin" flowers in significantly higher numbers to legitimate "thrum" stigmas, even if previously visiting a "pin" flower. However, "thrum" pollen was deposited in greater numbers on illegitimate "thrum" stigmas. We interpret this asymmetry largely as the consequence of floral morphology; pollen flow was greater between anthers and stigmas that exhibit greater spatial matching. In P. fendleri, the position of floral organs along the corolla tube does not always precisely correspond. In our experimental system, the probability that the pollinator extracts a pollen grain from the anther and the probability of self-pollination were both dependent on the type of floral morph. We discuss the relevance of the latter findings in relation to other studies of pollen flow in heterostylous species.