Ultrastruktur und Verteilung des Pollenkitts in der insekten- und windblütigen GattungAcer (Aceraceae)

Several insect- and wind-pollinated species of the genusAcer have been investigated and compared in regard to pollen stickiness. The considerable amount of very inhomogeneous pollenkitt inA. negundo remains on the loculus wall or is deposited inside the exine cavities; thus the pollen is powdery. The small amount of non-homogeneous, granular pollenkitt inA. campestre mostly disappears into the exine cavities; only small droplets appear on the tectum surface; the pollen stickiness therefore is only moderate. On the other hand,A. pseudoplatanus andA. opalus produce an average amount of granular pollenkitt, which is deposited partially inside the exine and partially as a slender film on the tectum surface; in both the pollen is sticky.A. platanoides contains a great deal of granular and homogeneous pollenkitt; it does not only fill up the exine cavities but also extends as a thick, ± homogeneous, non-granular layer of pollenkitt over the tectum surface; therefore, the pollen is very sticky.—The characteristics of pollen agglutination together with other aspects of floral biology illustrate the wide spectrum between unequivocal entomophily and anemophily within the genusAcer: WhileA. negundo is anemophilous andA. platanoides is entomophilous, the remaining species investigated have a pollination syndrome with entomophilous and anemophilous features and are thus amphiphilous. The evolution withinAcer is tending not only towards dioecy, but also towards anemophily.

Herrn Prof. Dr.L. Geitler zum 80. Geburtstag gewidmet.     

Abiotic pollen and pollination: Ecological, functional, and evolutionary perspectives

The transport and capture of pollen in ~20% of all angiosperm families occurs in air and water. In other words, pollination is abiotic and occurs via the fluid media, not an animal vector. Whereas some early concepts considered abiotic pollination to be largely a stochastic phenomenon, there is sufficient evidence to indicate that wind pollination (i.e. anemophily) and water pollination (i.e. hydrophily) have deterministic features and are sophisticated fluid dynamic solutions to the problem of pollen release, dispersal, and capture.An abiotic pollination syndrome is defined in which there is spatial or temporal separation of carpellate and staminate flowers, which are drab, a reduction in perianth parts, stigmas and anthers are exposed to the fluid, and typically unclumped pollen may be produced in large amounts. Separate pollination syndromes are defined for anemophilous (i.e. wind-pollinated), ephydrophilous (i.e. surface-pollinated), and hydrophilous (i.e. submarine-pollinated) plants. Distinctions are based on habitat and physical conditions for pollination, pollen size, shape, and ultrastructure, morphology and ultrastructure of stigmas, and outcrossing rates. For example, anemophilous pollen are spherical and small, ephydrophilous pollen are spherical or reniform and large, while hydrophilous pollen are filiform (i.e. filamentous) or functionally filiform. The pollination mechanisms and mechanics associated with these syndromes reveals a strong evolutionary relationship between plant morphology and fluid dynamics.     

Entwicklungsgeschichte und Ultrastruktur von Pollenkitt und Exine bei nahe verwandten entomophilen und anemophilen Sippen derOleaceae,Scrophulariaceae, Plantaginaceae undAsteraceae

Different members of the Angiosperm familiesOleaceae, Scrophulariaceae, Plantaginaceae andAsteraceae with variable modes of pollination were studied in relation to ultrastructure of pollenkitt and exine. In entomophilous representatives the mostly electron-dense and homogeneous pollenkitt is located on the outside layer of the exine, making the pollen sticky. In related anemophilous taxa the pollenkitt mostly is electrontransparent and inhomogeneous; it becomes inactive by remaining in the loculus or sinking to the bottom of the exine caves; this makes the pollen powdery. According to such criteria,Fraxinus ornus andRhinanthus serotinus are entomophilous, whileFraxinus excelsior is anemophilous. The pollen ofLathraea squamaria eventually becomes powdery and suited for wind dispersal. The broad transition field between entomo- and anemophily is demonstrated byPlantago andArtemisia. Plantago lanceolata is anemophilous, whilePl. serpentina andPl. media are amphiphilous.Artemisia nitida andA. mutellina are entomophilous,A. absinthium, A. pedemontana andA. gabriellae are amphiphilous, but tend towards entomo- (A. absinthium) or anemophily (A. gabriellae), whileA. chamaemelifolia and especiallyA. vulgaris are anemophilous.

     

Wind or insect pollination? Ambophily in a subtropical gymnosperm Gnetum parvifolium (Gnetales)

Gnetum (Gnetales) has long been regarded as insect pollinated due to its range‐restricted distribution in tropical rain forests, where wind pollination is supposed to be detrimental. However, ubiquitous pollen limitation in the tropics might cause transition to anemophily, or even ambophily for reproductive assurance, especially for gymnosperms such as Gnetum, which exhibit various anemophilous syndromes, including absence of petals, greenish color of strobili, dioecy and uniovulate flowers. Pollination treatments applied to Gnetum parvifolium in southern China revealed this rare and endangered species to be pollen limited and incapable of apomixis. Pollen grains of G. parvifolium did not adhere to each other and could be trapped by interception traps near the male and female strobili. Seed set in the netted treatment (anemophily) was significantly higher than in the bagged treatment (apomixis) but lower than open pollination (anemophily and entomophily), indicating that both wind and biotic pollination contributed to seed production of G. parvifolium. The occurrence of ambophily in Gnetum and the prevalence of anemophily in Ehpedra and Welwitschia suggest that wind may also play a role in the pollination of the ancient Gnetales.     

Dioecy and wind pollination in Pernettya rigida (Ericaceae) of the Juan Fernández Islands

Pernetttya rigida is endemic to the Juan Fernández archipelago. Although all flowers are complete, with seemingly fertile stamens and pistils, differences in fruit production and detailed field, anatomical, and morphological studies indicate they are functionally unisexual, and the species is consequently dioecious. A comparison of 15 features demonstrated sigdicant differences between the sexes. The populational sex ratio is 1:1. Nectaries located between the filaments produce small amounts of floral nectar with similar sugar composition in both sexes. There are =33,357 (or = 133,429) pollen tetrads (or pollen grains)/male flower and =109 ovules/female flower. No tetrads in either hand self-pollinated or open pollinated male flowers showed any pollen germination. Tetrads on open- and hand-pollinated female flowers germinate. Female flowers do not show autogamy or apomixis. During more than 80 hours of field observation, we recorded only seven floral visitors (representing three insect species). In spite of this, openpollinated female flowers have abundant fruit and seed-set. Thus, we conclude that pollen is transferred abiotically and the ever-present wind over the exposed ridges of the islands is the likely dispersal agent. A number of anemophilous features, such as dry pollen and exposed habitat, support this conclusion. Thus, dioecy and anemophily have evolved independently, in situ , in this species in this remote locality. Preservation of habitat and elimination of competitive invasives are the primary conservation challenges.     

Notes on the floral morphology in Vivianiaceae (Geraniales)

The functional floral morphology of the three genera of Vivianiaceae (= Ledocarpaceae, Geraniales), Rhynchotheca, Viviania and Balbisia, is compared. Likely pollination mechanisms are inferred from morphology and field observations. The flowers of Viviania are nectariferous and apparently zoophilous with nectar as the (primary) pollinator reward. Balbisia has pollen flowers without nectaries, its showy corolla indicates that it is also zoophilous with pollen as sole pollinator reward; bees were observed as flower visitors. One taxon (B. gracilis) may be anemophilous. Rhynchotheca has flowers without petals, with large, pendulous anthers and lacks nectaries. It shows synchronous mass flowering in its natural populations and is evidently anemophilous. A comparison with other Geraniales shows that nectar flowers with small anthers are likely the ancestral condition in Vivianiaceae. This suggests that the pollen flowers with larger anthers of Balbisia and Rhynchotheca may represent an apomorphic condition. The documentation of pollen flowers and anemophily in Vivianiaceae expands the range of known floral and pollination syndromes in Geraniales.     

Reproductive biology of Lactoris fernandeziana (Lactoridaceae)

Lactoris fernandeziana, monotypic in its family, is endemic to the cloud forests of Robinson Crusoe Island. Although there has been considerable study of the relationships of Lactoris, as a rare species and as a putative primitive paleoherb, little is known of its reproductive biology. Knowledge of the latter is essential for effective conservation programs. The species is gynomonoecious. The overall proportion of flowers is ∼1 female:1 hermaphrodite. The inconspicuous semipendulous green flowers, usually in mixed-gender inflorescences, do not produce rewards. Hermaphrodite flowers are herkogamous and protogynous. Pollen grains are shed from the extrorse anthers in permanent dry tetrads. There is a mean of 12879 tetrads per hermaphrodite flower. Both flower types bear an average of ∼18 ovules. The P/O (pollen/ovule) ratios imply facultative or obligate xenogamy, but hand pollinations show that Lactoris is self-compatible. No floral visitors were ever observed, but stigmata of open-pollinated flowers bore tetrads, and 64% of such styles had pollen tubes. Flowers enclosed in large mesh (1 mm) bags bore similar numbers of tetrads and pollen tubes. Thus, we conclude that Lactoris is anemophilous, a syndrome perhaps reflected by the P/O ratio. Low genetic diversity (isozymes and DNA) supports selfing and implies limited distance wind pollen dispersal. The small size of the island, the ± 1000 extant Lactoris plants, coupled with anemophily, self-compatibility, and pendant flower position, have yielded a geitonogamous system with high seed set and low genetic diversity. If inbreeding depression is expressed, it is in seed germination and seedling vigor, for Lactoris is very difficult to cultivate. For this species, effective conservation practices need to focus on habitat preservation and promotion of outcrossing.     

Variations of floral traits among different life forms illustrate the evolution of pollination systems in Potamogeton species from China

Four leaf morpha groups (floating-leaved, submerged broad-leaved, submerged linear-leaved and filiform-leaved) and three inflorescences type groups (erect, facultative floating and floating) can be found in the genus Potamogeton. Life forms in the genus correlate with pollination types (anemophily, epihydrophily and hydroautogamy). Floral traits vary greatly among different life forms and pollination types. Both floating and submerged broad-leaved species produce erect inflorescences that are suited for anemophilous pollination. These flowers also have the highest pollen/ovule (P/O) ratio (25,013) and the smallest pollen grain volume (6046 μm³). On the contrary, submerged filiform-leaved species produce only floating inflorescences and their flowers produce the largest pollen grains (17,567 μm³) and have the lowest P/O ratio (6752), which is typical of the syndromes of hydroautogamy and epihydrophily. The relatively prolific seed production (about 70%) and wide dispersal in the floating and submerged broad-leaved species and the submerged filliform-leaved species indicate that these two groups are well adapted to the aquatic environment. This is in contrast with the less well-adapted complex and unstable pollination system found in linear-leaved species. We propose that the pollination systems and life forms in the Potamogeton, have undergone modification in response to the demands imposed by the aquatic environment during the evolutionary process in the genus.     

Anemophilous plants select pollen from their own species from the air

In wind-tunnel experiments, Niklas (1985) has demonstrated the ability of anemophilous plants to select pollen from their own species from the airstream. However, there have been no field experiments to establish whether this operates in nature. We surveyed the pollents on the stigmas of four different, coextensive, dioecious anemophilous species surrounded by a Pinus radiata plantation. The alien pine pollen was overrepresented relative to background levels on only one of the four species. For all three indigenous species with both male and female plants in the area, the highest proportion — in all cases more than 40% — of the pollen found on their stigmas came from their own species. One indigenous species lacked male plants in the area; consequently, the results from this species are difficult to interpret. However, for all four species, there was at least 15% pine pollen, and some pollen of other indigenous species. These results suggest that there is some pollen selection, but that the mechanisms are may be not as effective as Niklas has suggested.     

Reproductive ecology and pollen representation among neotropical trees

Three years of pollen trapping data from Barro Colorado Island, Panama, are compared with local vegetation inventories. Two hypotheses relating pollen representation to ‘messy’ pollination and flower form are tested. Dioecious taxa were found to be over‐represented in pollen spectra compared with their occurrence in local forests. Similarly, anemophilous and ‘messy’ pollination types were found to be over‐represented. While anemophilous taxa were the best dispersed pollen types, zoophilous taxa were also well‐represented in dispersal classes of 20–40 m and > 40 m. Thus pollen arriving to lake sediments is most likely to be from anemophilous species or those zoophilous species exhibiting ‘messy’ pollination syndromes. Pollination mechanisms will predictably bias the fossil record against certain flower morphologies. These data are of significance to those using the fossil pollen record to reconstruct the timing and sequence of angiosperm evolution. These data help prioritize plants to be included in modern pollen reference collections and to focus the search for ‘unknown’ types on most‐likely candidate families.     

Entwicklungsgeschichte und Ultrastruktur von Pollenkitt und Exine bei nahe verwandten entomophilen und anemophilen Angiospermensippen derAlismataceae,Liliaceae, Juncaceae,Cyperaceae, Poaceae undAraceae

Some closely related members of the monocotyledonous familiesAlismataceae, Liliaceae, Juncaceae, Cyperaceae, Poaceae andAraceae with variable modes of pollination (insect- and wind-pollination) were studied in relation to the ultrastructure of pollenkitt and exine (amount, consistency and distribution of pollenkitt on the surface of pollen grains). The character syndromes of pollen cementing in entomophilous, anemophilous and intermediate (ambophilous or amphiphilous) monocotyledons are the same in principal as in dicotyledons. Comparing present with former results one can summarize: 1) The pollenkitt is always produced in the same manner by the anther tapetum in all angiosperm sub-classes. 2) The variable stickiness of entomophilous and anemophilous pollen always depends on the particular distribution and consistency of the pollenkitt, but not its amount on the pollen surface. 3) The mostly dry and powdery pollen of anemophilous plants always contains a variable amount of inactive pollenkitt in its exine cavities. 4) A step-by step change of the pollen cementing syndrome can be observed from entomophily towards anemophily. 5) From the omnipresence of pollenkitt in all wind-pollinated angiosperms studied one can conclude that the ancestors of anemophilous angiosperms probably have been zoophilous (i.e. entomophilous) throughout.

     

The biomechanics of pollen release: new perspectives on the evolution of wind pollination in angiosperms

Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence-induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal- and wind-pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.     

On the interpretation of fossil Poaceae pollen in the lowland humid neotropics

Poaceae pollen is abundant in fossil records and is often used as a paleoclimatic indicator. A common interpretation is to link increases in Poaceae pollen abundance to increased regional aridity. However, the representation of Poaceae pollen is influenced by a number of factors, such as the proportion of other plants in the flora that are anemophilous, the size of local marshes, and the influence of humans on the landscape. Abundant anemophilous trees are likely to mask the contribution of Poaceae pollen, whereas floras that are primarily entomophilous are likely to produce a pollen spectrum containing an over-representation of Poaceae. As most fossil pollen data are drawn from flooded settings, it is critically important that palynologists recognize the Poaceae pollen contribution derived from floating grasses and marshes that surround their coring site. Interpretations that ignore changes in effective lake size and assume that Poaceae percentage is a simple indicator of regional vegetation change are likely to overstate ‘dry’ episodes and transitions from wet forest to scrub environments. Human occupation of a site is sometimes manifested in increased Poaceae pollen abundance in lake sediments. The duration, intensity and land-use associated with occupation are all variables that influence Poaceae pollen representation. Very high (50–90%) abundances of Poaceae pollen provide a strong indicator of savanna habitats, but trying to determine transitional vegetation types between savanna and wet forest is best determined using other taxa. Indeed, reliance on Poaceae abundance as an indicator of paleoprecipitation is potentially very misleading when it is in the fossil record at moderate abundances.     

花粉形态与风媒植物母体地理分布间的关系

本文对部分风媒植物花粉形态特征,及其母体植物地理分布情况进行了对照研究。在此基础上发现,同一分类系统内,小个体花粉者,其母体植物地理分布趋向于较优越环境。对于具气囊花粉而言,具较大气囊者,或者花粉本体较小者,其母体植物趋向于分布在低纬度或较低海拔环境。并且,产大个体或球状花粉者,往往具有较强的单种成林倾向。本文首次指出了花粉形态与母体植物地理分布间存在着一定的统计学关系,并首次将动态概念引入植物生态学领域,认为花粉形态特征作为一项重要的参变量,通过种间生存竞争过程对母体植物适应倾向施加影响,进而影响物种的生活习性。那些在花粉的传播及授粉过程中稍占优势的物种,其自然分布域趋向于比较优越的环境。本文仅作为一种尝试,试图为动态植物生态学研究,乃至植物生态演化学研究提供一个新的研究思路。     

Breeding System and pollination of selected plants endemic to Juan Fernandez Islands

We conducted field studies on the Juan Fernández Islands flora on the breeding system of 25 endemic species from 17 families. We recorded data on flower features, pollen and ovule number, pollen/ovule ratio, pollen size, self-compatibility, floral visitors, and pollination. Flowers are mostly hermaphrodite, inconspicuous, small, and green. Six species are dioecious. Over 80% of the cosexual species are self compatible. However, many species are dichogamous (mostly protandrous); thus, even the self-compatible species may require pollen transfer. Selfing through geitonogamy seems to be the most common system, and several species express mixed breeding systems. Floral visitors are uncommon to rare, except for two hummingbird species (one native and one endemic) that visit five species we studied. In more than 300 h of observation of flowers over three field seasons, we detected only 23 native insect visits representing ten species (Diptera, Lepidoptera, and Coleoptera). One species each of an introduced ant and an introduced bee were also observed on some flowers, all near the single human settlement of San Juan Bautista. Wind directly moving pollen, or indirectly via shaking the flowers, is the most important pollen distribution mechanism. The majority of the wind-pollinated species bear some typical anemophilous features, but also others not characteristic of wind pollination, that presumably represent the condition of their biotically pollinated ancestors. Floral features often reflect ancestral reproductive systems, so floral biology studies of oceanic islands in particular must be done with cognizance of presumed ancestral forms, because the observed characters can be misleading.     

Neither insects nor wind: ambophily in dioecious Chamaedorea palms (Arecaceae)

Pollination of Neotropical dioecious trees is commonly related to generalist insects. Similar data for non‐tree species with separated genders are inconclusive. Recent studies on pollination of dioecious Chamaedorea palms (Arecaceae) suggest that species are either insect‐ or wind‐pollinated. However, the wide variety of inflorescence and floral attributes within the genus suggests mixed pollination mode involving entomophily and anemophily. To evaluate this hypothesis, we studied the pollination of Chamaedorea costaricana, C. macrospadix, C. pinnatifrons and C. tepejilote in two montane forests in Costa Rica. A complementary morphological analysis of floral traits was carried out to distinguish species groups within the genus according to their most probable pollination mechanism. We conducted pollinator exclusion experiments, field observations on visitors to pistillate and staminate inflorescences, and trapped airborne pollen. A cluster analysis using 18 floral traits selected for their association with wind and insect pollination syndromes was carried out using 52 Chamaedorea species. Exclusion experiments showed that both wind and insects, mostly thrips (Thysanoptera), pollinated the studied species. Thrips used staminate inflorescences as brood sites and pollinated pistillate flowers by deception. Insects caught on pistillate inflorescences transported pollen, while traps proved that pollen is wind‐borne. Our empirical findings clearly suggest that pollination of dioecious Chamaedorea palms is likely to involve both insects and wind. A cluster analysis showed that the majority of studied species have a combination of floral traits that allow for both pollination modes. Our pollination experiments and morphological analysis both suggest that while some species may be completely entomophilous or anemophilous, ambophily might be a common condition within Chamaedorea. Our results propose a higher diversity of pollination mechanisms of Neotropical dioecious species than previously suggested.     

Tafelerklärungen

The dogma that anemophily in the humid tropics is rare or even absent is challenged. Long pollen rain shadows, leptokurtic pollen distribution patterns, and electrostatic attractions between pollen and stigma may contribute to successful abiotic pollination among plants whose mechanisms bridge a continuum between the extremes of anemophily and zoophily. Representatives of 21 families of woody, neotropical plants are shown to possess some form of abiotic pollination or pollen dispersal, suggesting a greater role of anemophily in tropical reproductive biology and allergic responses than previously assumed.     

Pollination Ecophysiology of Mercurialis annua L. (Euphorbiaceae), an Anemophilous Species Flowering all Year Round

Mercurialis annua L. is a dioecious anemophilous species thatflowers all year round in central and southern Italy. The flowersof both sexes are dimorphic: the female flower has a vestigialcalyx; the male flower consists only of a calyx that opens atanthesis. The anthers always dehisce after anthesis. The anthesisof male flowers seems to be temperature dependent, whereas antherdehiscence is related to relative humidity. The pollen grainsvary in volume according to the season: they are smaller whenrelative humidity is low and vice versa. They always decreasein volume after anther dehiscence and have the capacity to varyin volume and reach equilibrium with a changing environment.Viability is high, but may drop suddenly during heavy rain orhail that damage the exposed male flowers. The number of pollengrains per stigma varies from 0 to 300. The data is discussedin relation to the type of pollination and environmental characteristics.Copyright1994, 1999 Academic Press Mercurialis annua, dioecism, anthesis, anther dehiscence, pollen volume, pollen viability, anemophilous pollination, pollination ecology