Sex‐specific floral attraction traits in a sequentially hermaphroditic species

• ● Many angiosperms are hermaphroditic and produce bisexual flowers in which male (pollen export) and female (stigma receptivity) functions are separated temporally. This sequential hermaphroditism may be associated with variation in flower size, color, or pattern, all of which may influence pollinator attraction. In this study, we describe variation in these traits across discrete functional sex stages within and between 225 greenhouse‐grown individuals of Clarkia unguiculata (Onagraceae). In addition, to identify the effects of floral phenotype on pollinator attraction in this species, we examine the effects of these floral traits on pollen receipt in ~180 individuals in an experimental field array.
• ● Petal area, ultraviolet (UV)‐absorbing nectar guide area, and blue and green mean petal reflectance differ significantly across the functional sex stages of C. unguiculata. Male‐ and female‐phase flowers display significantly different pollinator attraction traits. Petal and UV nectar guide area increase as flowers progress from male phase to female phase, while blue reflectance and green reflectance peak during anther maturation.
• ● In field arrays of C. unguiculata, female‐phase flowers with large UV nectar guides receive more pollen than those with small nectar guides, and female‐phase flowers with high mean blue reflectance values are more likely to receive pollen than those with low blue reflectance. Female‐phase flowers with green mean reflectance values that differ most from background foliage also receive more pollen than those that are more similar to foliage. These findings indicate that components of flower color and pattern influence pollen receipt, independent of other plant attributes that may covary with floral traits. We discuss these results in the context of hypotheses that have been proposed to explain sex‐specific floral attraction traits, and we suggest future research that could improve our understanding of sexual dimorphism in sequentially hermaphroditic species and the evolution of features that promote outcrossing.

     

Analysis of amino acids in nectar from Silene colorata Poiret (Caryophyllaceae)

Nectar samples were collected from Silene colorata Poiret (Caryophyllaceae), in three different populations from south-western Spain: Zahara de la Sierra (Cádiz), Bornos (Cádiz) and Bormujos (Seville). Samples were analysed for amino acids by reverse-phase high-performance liquid chromatography with precolumn phenylisotiocyanate (PITC) derivatization. The method has the advantage of being highly sensitive, capable of detecting nanogram (ng) quantities of amino acids. Eighteen amino acids were identified and quantified. The mean number of amino acids in a nectar sample was 14 (SD = 2.8). Six amino acids (threonine, alanine, arginine, proline, tyrosine and methionine) were detected in all samples, accounting for 83% of the total amino acids content; proline and arginine were the most abundant amino acids, accounting for 40% and 20% of the total amino acids, respectively. The mean amounts of amino acids in nectar samples per population were 824, 782 and 356 µ m in Zahara de la Sierra, Bornos and Bormujos, respectively. Environmental variations such as temperature and sunlight are factors influencing the metabolic processes of nectar production. Our results may contradict the theory that the chemical constituents of floral nectar vary according to the kinds of pollinators.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 49–56.     

Scanning Electron Microscopic Studies on the Development of the Organs of Litchi Flower

The pattern of development of the floral parts of litchi (Litchi chinensis Sonn.) flower was followed using scanning electron microscopy. Before making scanning electron microscopic observations, specimens were tannin-osmium impregnated and critical point dried. In the bisexual flower, floral organogenesis starts with the formation of protrusions near the floral apex. The two to three protrusions present at the apical region of the floral apex later expand and fuse to form the ovary. At the upper middle region of the ovary another protrusion develops which later becomes the style and the stigma. When the flower matures the tip of the style not only splits but also becomes twisted. On the upper side of the stigma there are numerous papillate cells. These cells are covered with mucilage when fully mature. The development of the filament and anther begins a little bit earlier than the gynoecium. The first sign of androecium development begins when protrusions start to develop around the floral apex. Each litchi flower possesses 6 to 10 anthers. In addition to forming bisexual flowers, litchi also produces a large number of male and female unisexal flowers. But under the scanning electron microscope it is very difficult to distinguish accurately between male and female flowers, because both flowers invariably give rise to some poorly developed organs of the opposite sex. Thus it seems that all flowers in litchi are potentially bisexual and only at the final stage of development (i.e. about 50 days after floral initiation) sex organs fail to develop properly in some flowers.     

圆唇苣苔属(Gyrocheilos)花柱侧偏弯折现象及其传粉适应机制

圆唇苣苔属(Gyrocheilos)是苦苣苔科的中国特有属,有5种,全部狭域分布在我国西南及广东的高海拔山区。圆唇苣苔属所有物种的花柱侧偏且花柱顶端呈90°弯折,使得柱头位于花开口的中央位置。这种独特的侧偏弯折花柱结构,说明圆唇苣苔属可能有着特殊的演化历史和适应机制。为揭示这种特殊的花柱侧偏弯折现象的发生范围、发育过程及其传粉适应机制,该研究在圆唇苣苔(Gyrocheilos chorisepalus)、折毛圆唇苣苔(G. retrotrichus)和微毛圆唇苣苔(G. microtrichus)3个物种中开展了花部综合征观察,并研究了广东大雾岭保护区内的折毛圆唇苣苔花发育过程、花部特征和繁育系统以及传粉过程。结果表明:(1)微毛圆唇苣苔只有花柱左偏弯折现象,而圆唇苣苔和折毛圆唇苣苔虽然大部分花是花柱左偏弯折,但在部分个体中出现了少量的花柱右偏弯折现象(占种群总花数的2%～3%)。(2)传粉观察发现,折毛圆唇苣苔在花蕾期即出现了花柱弯折现象,2个可育雄蕊的花药合生、位于花冠筒喉部中央位置,与侧偏花柱不存在左右镜像对称关系。(3)折毛圆唇苣苔的花粉胚珠比(P/O)为456.98±15.55,属于兼性异交繁育系统。折毛圆唇苣苔存在一定的传粉限制,自交授粉可以结实,但异交种子萌发率更高,可能存在近交衰退。(4)折毛圆唇苣苔的访花昆虫较少,访花频率较低,主要访花昆虫有隧蜂、熊蜂、食蚜蝇等; 熊蜂体型较大,访花时降落在弯折花柱和花瓣下唇,胸部侧面及下部能有效接触到柱头。(5)反射率结果显示,折毛圆唇苣苔花瓣反射波长范围集中在紫光和蓝紫光区域,花冠的反射波长范围与蜂类视觉范围一致且花冠筒外侧和花瓣下唇的反射强度最大,更容易吸引蜂类落置在花冠宽大的下唇; 圆唇苣苔属的花柱侧偏弯折现象可能来自近缘的长蒴苣苔属(Didymocarpus)的花柱下弯现象或镜像花(mirror-image flowers)。综上认为,这种侧偏弯折的花柱,可能通过提供昆虫降落平台,使得柱头位于花开口中央和花瓣下唇的上方位置,提高了柱头接触访花昆虫的概率,是适应高海拔地区低频率访花者的一种机制。     

A bee’s eye view of remarkable floral colour patterns in the south-west Australian biodiversity hotspot revealed by false colour photography

Background and AimsColour pattern is a key cue of bee attraction selectively driving the appeal of pollinators. It comprises the main colour of the flower with extra fine patterns, indicating a reward focal point such as nectar, nectaries, pollen, stamens and floral guides. Such advertising of floral traits guides visitation by the insects, ensuring precision in pollen gathering and deposition. The study, focused in the Southwest Australian Floristic Region, aimed to spot bee colour patterns that are usual and unusual, missing, accomplished by mimicry of pollen and anthers, and overlapping between mimic-model species in floral mimicry cases.MethodsFloral colour patterns were examined by false colour photography in 55 flower species of multiple highly diverse natural plant communities in south-west Australia. False colour photography is a method to transform a UV photograph and a colour photograph into a false colour photograph based on the trichromatic vision of bees. This method is particularly effective for rapid screening of large numbers of flowers for the presence of fine-scale bee-sensitive structures and surface roughness that are not detectable using standard spectrophotometry.Key ResultsBee- and bird-pollinated flowers showed the expected but also some remarkable and unusual previously undetected floral colour pattern syndromes. Typical colour patterns include cases of pollen and flower mimicry and UV-absorbing targets. Among the atypical floral colour patterns are unusual white and UV-reflecting flowers of bee-pollinated plants, bicoloured floral guides, consistently occurring in Fabaceae spp., and flowers displaying a selective attractiveness to birds only. In the orchid genera (Diuris and Thelymitra) that employ floral mimicry of model species, we revealed a surprising mimicry phenomenon of anthers mimicked in turn by model species.ConclusionThe study demonstrates the applicability of ‘bee view’ colour imaging for deciphering pollinator cues in a biodiverse flora with potential to be applied to other eco regions. The technique provides an exciting opportunity for indexing floral traits on a biome scale to establish pollination drivers of ecological and evolutionary relevance.     

Reproductive structures and phylogenetic framework of the rosids - progress and prospects

With ca 70.000 species the rosids contain more than a quarter of the total angiosperm species diversity. This taxonomic richness is reflected in a tremendous variety of floral organization and architecture. Rosids have received extensive molecular phylogenetic study. As a result, the monophyly and taxonomic composition of the group are well established. In addition, many subclades at the order level are now apparent. Deeper relationships, however, are still largely equivocal. As in many other parts of the plant tree of life, it will be impossible to reach an adequate understanding of the evolutionary history of the rosids without taking into account information from comparative morphological studies of extant and, in particular, also of fossil taxa. The fossil record of rosids is rich in well-preserved reproductive structures, and together with recent results from comparative studies of extant rosids, provides a wealth of floral structural data. Although much remains to be done at all levels, fresh attempts to synthesize and possibly reconcile results from molecular phylogenetics, comparative floral morphology, and palaeobotany, seem timely.     

Floral anatomy of Paepalanthoideae (Eriocaulaceae, Poales) and their Nectariferous structures

BACKGROUND AND AIMS: Eriocaulaceae (Poales) is currently divided in two subfamilies: Eriocauloideae, which comprises two genera and Paepalanthoideae, with nine genera. The floral anatomy of Actinocephalus polyanthus, Leiothrix fluitans, Paepalanthus chlorocephalus, P. flaccidus and Rondonanthus roraimae was studied here. The flowers of these species of Paepalanthoideae are unisexual, and form capitulum-type inflorescences. Staminate and pistillate flowers are randomly distributed in the capitulum and develop centripetally. This work aims to establish a floral nomenclature for the Eriocaulaceae to provide more information about the taxonomy and phylogeny of the family. METHODS: Light microscopy, scanning electron microscopy and chemical tests were used to investigate the floral structures. KEY RESULTS: Staminate and pistillate flowers are trimerous (except in P. flaccidus, which presents dimerous flowers), and the perianth of all species is differentiated into sepals and petals. Staminate flowers present an androecium with scale-like staminodes (not in R. roraimae) and fertile stamens, and nectariferous pistillodes. Pistillate flowers present scale-like staminodes (except for R. roraimae, which presents elongated and vascularized staminodes), and a gynoecium with a hollow style, ramified in stigmatic and nectariferous portions. CONCLUSIONS: The scale-like staminodes present in the species of Paepalanthoideae indicate a probable reduction of the outer whorl of stamens present in species of Eriocauloideae. Among the Paepalanthoideae genera, Rondonanthus, which is probably basal, shows vascularized staminodes in their pistillate flowers. The occurrence of nectariferous pistillodes in staminate flowers and that of nectariferous portions of the style in pistillate flowers of Paepalanthoideae are emphasized as nectariferous structures in Eriocaulaceae.     

Reproductive biology and pollinators in two enantiostylous Qualea species (Vochysiaceae) in the Brazilian Cerrado

• Enantiostyly is a floral polymorphism in which two floral forms in the same species differ in deflection of the stigma to right or left position. In monomorphic enantiostylous plants, flowers of the two morphs occur within the same individual, usually in the same proportion. In self‐compatible species the function of monomorphic enantiostyly is proposed to increase outcrossing rates and offer a reproductive advantage under pollination limitation. Enantiostylous species are usually self‐compatible and show heteranthery, with poricide anthers and pollen as pollinator reward; however, there are families, such as Vochysiaceae, that have different characteristics.
• We analysed the reproductive system and pollination biology of Qualea parviflora and Q. multiflora, two enantiostylous species from the Brazilian Cerrado that have specific morphological and physiological traits. For this, we characterized flower traits, performed hand pollinations and studied floral visitors.
• We found no differences between morphs in the proportion of flowers, nectar produced or its concentration, pollen quantity and fruit set. Both species were self‐incompatible and quite generalist regarding floral visitors.
• Enantiostyly in self‐incompatible plants seems to confer a reproductive advantage by reducing self‐interference resulting from stigma clogging. This novel result helps to expand our knowledge on this complex floral polymorphism and opens new avenues for future research on this topic.

     

Are nectar guide colour changes a reliable signal to pollinators that enhances reproductive success?

Background: Ageing and post-pollination changes in floral colour occur widely in flowering plants, but it remains an open question as to whether or not colour changes in nectar guides are associated with the quantity of floral rewards that ultimately influence pollinator visitations and reproductive success.

Aims: To examine whether nectar guide changes should be considered as a reliable signal to pollinators and to assess the effects of nectar guide changes on reproductive success.

Methods: We studied the process and adaptive value of colour changes in the nectar guides of Arnebia szechenyi whose flowers typically display conspicuous nectar guides at the onset of anthesis, after which they begin to fade, and disappear completely on the second day.

Results: Changes in nectar guide colour in A. szechenyi were intrinsic and age-dependent, although pollination somewhat accelerated the change. By the time that the nectar guides disappeared completely, floral rewards were reduced almost to zero. Artificial removal of nectar guides decreased both fruit set and pollen export. Flowers without nectar guides do not appear to increase the overall attractiveness of the plants.

Conclusions: Nectar guides and their changes represent reliable signals to pollinators and enhance both male and female reproductive success.     

One for all and all for one: retention of colour‐unchanged old flowers increases pollinator attraction in a hermaphroditic plant

• Long‐lived flowers increase pollen transfer rates, but these entail high water and carbon maintenance costs. The retention of pollinated and reward‐free old flowers enhances pollinator visitation to young receptive flowers by increasing floral display size. This mechanism is associated with acropetal inflorescences or changes in flower colour and openness, but the retention of unchanging solitary flowers remains overlooked.
• We examined pollination‐dependent variation in floral longevity and determined stigmatic receptivity, pollen viability and pollen removal rates among flower ages in Kielmeyera regalis, a Neotropical savanna shrub. We also evaluated the effects of floral display size on pollinator visitation rates. Lastly, we determined whether old flowers are unvisited and exclusively increase pollinator attraction to young flowers through flower removal experiments.
• Regardless of pollination treatment, flowers lasted fully open with no detectable physical changes for 3 days. Over time, stigmas remained receptive but >95% of pollen was removed. Pollinator visitation significantly increased with floral display size and intermediate percentages (15–30%) of newly opened flowers. Accordingly, the retention of reward‐free and unvisited old flowers increased young flower–pollinator interaction.
• Our results reveal the importance of a prolonged floral longevity in increasing pollinator attraction toward newly opened receptive flowers without changes in flower colour and form. We conclude that the retention of pollinated, reward‐free and unvisited colour‐unchanged old flowers in K. regalis is a strategy that counteracts the water use costs associated with the maintenance of large flowers with increased mate opportunities in a pollen‐limited scenario.

     

Gymnosperm Orthologues of Class B Floral Homeotic Genes and Their Impact on Understanding Flower Origin

Class B floral homeotic genes play a key role in specifying the identity of male reproductive organs (stamens) and petals during the development of flowers. Recently, close relatives (orthologues) of these genes have been found in diverse gymnosperms, the sister group of the flowering plants (angiosperms). The fact that such genes have not been found so far, despite considerable efforts, in mosses, ferns or algae, has been taken as evidence to suggest that B genes originated 300–400 million years ago in a lineage that led to extant seed plants. Gymnosperms do not develop petals, and their male reproductive organs deviate considerably from angiosperm stamens. So what is the function of gymnosperm B genes? Recent experiments revealed that B genes from diverse extant gymnosperms are exclusively expressed in male reproductive organs (microsporophylls). At least for some of these genes it has been shown that they can partially substitute for the Arabidopsis B genes AP3 and PI in ectopic expression experiments, or even partially substitute these genes in different class B floral organ identity gene mutants. This functional complementation, however, is restricted to male organ development. These findings strongly suggest that gymnosperm and angiosperm B genes have highly related interaction partners and equivalent functions in the male organs of their different host species. It seems likely that in extant gymnosperms B genes have a function in specifying male reproductive organs. This function was probably established already in the most recent common ancestor of extant gymnosperms and angiosperms (seed plants) 300 million years ago and thus represents the ancestral function of seed plant B genes, from which other functions (e.g., in specifying petal identity) might have been derived. This suggests that the B gene function is part of an ancestral sex determination system in which B gene expression specifies male reproductive organ development, while the absence of B gene expression leads to the formation of female reproductive organs. Such a simple switch mechanism suggests that B genes might have played a central role during the origin of flowers. In the out-of-male and out-of-female hypotheses changes in B gene expression led to the origin of hermaphroditic flower precursors out of male or female gymnosperm reproductive cones, respectively. We compare these hypotheses with other recent molecular hypotheses on the origin of flowers, in which C/D and FLORICAULA/LEAFY-like genes is given a more prominent role, and we suggest how these hypotheses might be tested in the future.     

Do flowers wave to attract pollinators? A case study with Silene maritima

To answer the question whether flowers wave to attract pollinators, we determine: (1) the heritability of floral mobility; (2) whether wavy flowers attract more insects; (3) does the duration of pollination affect seed set; and (4) the relationship between seed set and floral mobility. The pollination ecology of Silene maritima was investigated. Flowers on stalks of different waviness were used to investigate the effect of floral movement on pollinator visits. There is heritable variation in both direct and indirect estimates of floral mobility. The highest insect total visitation times were associated with medium length thin stalks that were visited more frequently and by more insect species. Although mean individual visit durations were less than those of less mobile flowers, this was compensated for by increased visits. Observations of controlled pollinations show that when the visit times are low, so is seed set and therefore low and high mobility flowers might suffer from reduced fitness. Combining these observations provides a mechanism that could be driving stabilizing selection for flower stalk traits, with a trade-off applying between waving to attract pollinators and not being too mobile as to prevent effective pollination. Further evidence for stabilizing selection is provided by the relationship observed in the field between seed set and floral mobility where the highest levels of fitness was associated with intermediate levels of floral waving.     

In vitro development of angiosperm floral buds and organs

In the last twenty-five years, young inflorescences, floral buds and individual floral organs of a number of species have been cultured in vitro. There is considerable variability in the requirement of plant growth regulators and nutritional factors for flower development of different species. This variability is compounded by the fact that the hormonal and nutritional requirements are different at various stages of organ and floral development. Experimental studies on normal and mutant flowers in vitro have provided insights into some of the regulatory processes in floral organogenesis. The potential use of the in vitro technique in elucidating the various mechanisms in flower development is stressed.     

Bilabiate Flowers: The Ultimate Response to Bees?

BACKGROUND AND AIMS: Bilabiate flowers have evolved in many lineages of the angiosperms, thus representing a convincing example of parallel evolution. Similar to keel blossoms, they have obviously evolved in order to protect pollen against pollen-collecting bees. Although many examples are known, a comprehensive survey on floral diversity and functional constraints of bilabiate flowers is lacking. Here, the concept is widened and described as a general pattern. METHODS: The present paper is a conceptional review including personal observations of the authors. To form a survey on the diversity of bilabiate blossoms, a search was made for examples across the angiosperms and these were combined with personal observations collected during the last 25 years, coupled with knowledge from the literature. New functional terms are introduced that are independent of morphological and taxonomic associations. KEY RESULTS: Bilabiate constructions occur in at least 38 angiosperm families. They are characterized by dorsiventral organization and dorsal pollen transfer. They are most often realised on the level of a single flower, but may also be present in an inflorescence or as part of a so-called 'walk-around flower'. Interestingly, in functional terms all nototribic blossoms represent bilabiate constructions. The great majority of specialized bee-flowers can thus be included under bilabiate and keel blossoms. The syndrome introduced here, however, also paves the way for the inclusion of larger animals such as birds and bats. The most important evolutionary trends appear to be in the saving of pollen and the precision of its transfer. With special reference to the Lamiales, selected examples of bilabiate flowers are presented and their functional significance is discussed. CONCLUSIONS: Bilabiate blossoms protect their pollen against pollen-collecting bees and at the same time render their pollination more precisely. The huge diversity of realised forms indicate the high selection pressure towards the bilabiate syndrome. As bees are very inventive, however, bilabiate constructions will not represent the ultimate response to bees.     

Floral sexual dimorphism and flower choice by pollinators in a nectarless monoecious vine Akebia quinata (Lardizabalaceae)

We investigated the pollination system and movement patterns of pollinators among flowers of the nectarless, monoecious vine Akebia quinata in natural populations and experimental floral arrays. Female flowers did not offer any rewards for pollinators and were larger than male flowers. Pollinators of A.quinata , such as small solitary bees and hoverflies, clearly discriminated between male and female flowers. Hoverflies always visited male flowers and rarely visited female flowers. In contrast, solitary bees tended to visit female flowers first when entering the array, but then switched to male flowers within the same foraging bout. This tendency disappeared when the sepal size of female flowers was experimentally reduced to the size of male flowers. Thus, observed non-randomness in flower choice by solitary bees may be caused by female>male sexual dimorphism and may increase the chance of cross-pollination and lower the probability of geitonogamous pollination in a single visit to a plant. Therefore, floral sexual dimorphism in A.quinata is considered to be an adaptation for deceptive pollination associated with discriminating pollinators.