FLORAL PARAMETERS,BREEDING SYSTEM AND POLLINATOR TYPE IN TRICHOSTEMA (LABIATAE)

Diversity in flower size and degree of exsertion of anthers and stigma from the corolla in the California species of Trichostema appear to be correlated with breeding system and pollinator type (bee vs. bird). Autogamous (self-pollinating) species unlike xenogamous (cross-pollinating) species lack spatially separate anthers and stigma and nototribic pollination. The outcrossing species have significantly larger flowers, significantly larger nectar volumes and significantly higher pollen-ovule ratios than do selfing species. Because autogamous species are less reliant on flower visitors to facilitate pollination, there may be relaxed selection for large nectar producing flowers. Pollen-ovule ratios are correlated with breeding system and reflect the efficiency of pollen transport. Data on floral parameters suggest xenogamous species expend more energy per floral unit and are less efficient seed producers than closely related autogamous species.     

山莨菪(茄科)的传粉生物学

茄科的多数种类具有自交不亲和的特点, 主要通过异花传粉结实; 但是, 一些物种或者物种内的部分种群或者个体却高度自交亲合, 转变为自交的繁育系统。该科植物山莨菪(Anisodus tanguticus)主要分布在青藏高原, 开花较早, 比其他晚开花的植物种类更加缺少有效的异花传粉昆虫。我们选择了位于不同海拔高度的2个种群进行比较研究, 主要目的是检验该物种的繁育系统是否在极端环境下由于传粉者的缺乏而发生了部分改变。研究发现,山莨菪的花不完全雌性先熟, 柱头和花药间的平均距离随着花开放时间的延长而不断缩小, 但两者在多数花的单花花期结束时并没有发生接触。因此, 山莨菪花主要表现为适应异花传粉的雌雄异位特征。然而, 少数花 (4.9％)的柱头和花药发生接触, 为“自动自交”的传粉解除了空间隔离。2个种群的多数个体存在自交不亲和机制, 应具有异花传粉的繁育系统; 但是部分个体具有明显的自交亲和能力, 为自交提供了生理基础。高海拔种群的传粉昆虫主要是厕蝇(Fannia sp.), 它们在不同植株间的活动能够保证异花传粉结实; 同时该种群的部分个体存在“自动自交”。低海拔种群的主要访花昆虫是蚂蚁, 它们在花内的活动导致花粉在同一朵花内传递, 而引起“协助自交”; 而异花传粉昆虫厕蝇的访花频率则较高海拔种群低。两个种群的结实均由于异花传粉者不足而受到传粉限制。因此两种不同类型的自交机制为该早期开花植物异花访花昆虫的不足提供了一定程度上的繁殖补偿。     

Breeding system and floral visitors of Veratrum album subsp. oxysepalum (Melanthiaceae)

Veratrum album subsp. oxysepalum is an andromonoecious perennial woodland herb. In the present study, to clarify the essential life-history characteristics of this species, sexual reproduction was examined and breeding experiments were carried out along with observations of floral visitors in the field. The results of the breeding experiments showed that this species has a high level of self-incompatibility. As various Coleoptera and Diptera visited the flowers, the seed production of V. album subsp. oxysepalum possibly depends on outcrossing pollination by insects.     

Flower biology of the cactus Coryphantha elephantidens in the tropical dry forest of Central Mexico

Floral traits and sexual systems in angiosperms are strategies that enhance outcrossing within hermaphrodite flowers and among individuals in a population. Sexual systems with unisexual flowers have also evolved among angiosperms, resulting in sex specialization. Furthermore, the interaction of floral traits and floral visitors determines successful plant reproduction. Globose cacti are bee pollinated, and variation in the diversity of their pollinator assemblages is strongly associated with floral phenotype. Our objective was to describe the floral biology of the cactus Coryphantha elephantidens and to determine its relationship with pollinators. Floral traits were studied by direct observations in live and fixed flowers. The breeding system was determined using two estimators based on floral morphology: pollen grains to ovules per flower (P/O) ratio and outcrossing index. Pollination treatments were conducted to determine the mating system. Floral visitors were recorded using direct observation. Flowers of C. elephantidens are variable in color, protandric, herkogamous and nectarless. Estimators of the breeding system indicated xenogamy, which is consistent with the obligate outcrossing revealed by the pollination experiment. Thirty-seven percent of the plants have female flowers that do not produce pollen, making this population functionally gynodioecious. Both fruit and seed set were high compared to other globose cacti. Pollinators included eight species of native bees, a more diverse pollinator assemblage than other globose cacti. Given the high pressure on pollen due to functional gynodioecy, nectarless flowers, an outcrossing mating system, and the necessity of pollinators to set seeds, we concluded that native bees are highly efficient pollinators that play a crucial role in the sexual reproduction of C. elephantidens.     

Floral biology and pollination mechanisms of four Mexico-endemic Fuchsia species with contrasting reproductive systems

Aims The evolution of the outstanding variation of reproductive systems in angiosperms has been considered an important driver of lineage diversification. Closely related hermaphroditic and dioecious species with biotic pollination provide the opportunity to study and compare traits related to pollinator attraction and their consequences on reproductive components. A higher predictability of pollination syndromes is expected in dioecious species, which are dependent on pollinators, than in self-compatible hermaphroditic taxa. Dioecious species may suffer pollen limitation depending on the quality of floral rewards and the kind and abundance of pollinators, whereas no pollen limitation is expected in hermaphroditic species with autonomous self-pollination. Additionally, in the absence of pollen limitation, more or better seeds are expected in dioecious species, according to the sexual specialization hypothesis.Methods In natural populations of the hermaphroditic Fuchsia fulgens and Fuchsia arborescens and dioecious Fuchsia parviflora and Fuchsia obconica, all endemic to Mexico, we first described flower phenology, flower production and longevity and nectar volume and concentration. Then, we evaluated the correspondence between floral visitors and pollination syndromes. In hermaphrodite plants, we determined the level of herkogamy and the potential for autonomous self-pollination. Finally, we evaluated the effect of pollen limitation on fruit set and seed number and assessed seed germination for all species.Important findings In contrast to our prediction, dioecious species did not show a higher correspondence between pollination syndromes and floral visitors than did hermaphrodites; however, male flowers exhibited a higher correspondence than female flowers. No pollen limitation was detected in dioecious species, for which visitation rate did not differ between male and female flowers. The hermaphroditic F. fulgens showed pollen limitation for seed number, despite the presence of autonomous selfing. Fruit set from autonomous pollination was higher in F. arborescens, which showed a lower level of herkogamy compared with F. fulgens. Finally, dioecious species produced fewer but heavier seeds compared with hermaphrodite species. Although Fuchsia is classified as an outcrossing genus, both hermaphroditic species showed autonomous self-pollination. The heavier but lower number of seeds per fruit in dioecious species may be related to the more efficient resource allocation expected from sexual specialization. This could play an important role in the evolution of dioecy; however, a comparative phylogenetic approach is required to confirm this hypothesis.     

Pollination biology of the sclerophyllous shrub Pultenaea villosa Willd. (Fabaceae) in southeast Queensland, Australia

The pollination biology of the common shrub Pultenaea villosa Willd. was examined in a subtropical dry sclerophyll forest in eastern Australia. We determined floral phenology and morphology, the timing of stigma receptivity and anther dehiscence, nectar availability, the plant breeding system, and flower visitors. The shrub's flowers are typical zygomorphic pea flowers with hidden floral rewards and reproductive structures. These flowers require special manipulation for insect access. A range of insects visited the flowers, although bees are predicted to be the principle pollinators based on their frequency on the flowers and their exclusive ability to operate the wing and keel petals to access the reproductive structures. Nectar and pollen are offered as rewards and were actively collected by bees. Nectar is offered to visitors in minute amounts at the base of the corolla. In Toohey Forest, P. villosa flowers in spring and is the most abundant floral resource in the understory of the forest at this time. The breeding system experiment revealed that P. villosa requires outcrossing for high levels of seed set and that the overlap of stigma receptivity and pollen dehiscence within the flower suggests the potential for self-incompatibility.     

Pollination ecology of Gentiana siphonantha (Gentianaceae) and a further comparison with its sympatric congener species

In this study, we studied pollination ecology of Gentiana siphonantha (Gentianaceae), a late-autumn flowering alpine perennial in the northeastern Qinghai-Tibetan Plateau for two years for the first time. We also aimed to compare the pollination differentiation between this species and sympatrically distributed G. straminea that has a close phylogenetic origin but is flowering early. Flowers of G. siphonantha are characteristic of di-chogamy and herkogamy and this floral development prevents occurrence of autonomous self-pollination. This implication was confirmed by the breeding experiments, since no seed was produced when flowers were isolated. However, this gentian proved to be highly self-compatible when geitonogamous selfing was artificially induced. Each individual plant of this species has an average of 14.6 flowers ranging from 4 to 31, at both staminate and pistillate stages with a ratio of 1.2:1 in full anthesis. Both floral development and breeding experiments suggested that pollen vectors were indispensable for successful seed sets of this species. A great variety of insects were observed to visit this species, but the most common and only legitimate pollinator is Bumbus sushikini. A statistic observation suggested that this pollinator showed no preference to either staminate or pistillate flowers and visited them at random. We further calculated the frequency of their visits between and within individual plants. Among the pollinators’ bouts, the proportions of geitonogamous visits within an individual plant occupy about 87.8%. Such a case implied that geitonogamous selfing prevails in this species in spite of floral dichogamy and her-kogamy that were suggested to promote outcrossing. Compared with sympatric G. straminea, the total floral longevity and the male and female duration of this species are shorter. However, the number of flowers of each individual plant and branch increased when in full anthesis. It is interesting that both closely related species shared the same pollinator despite their distinct difference in flower morphology. This finding is inconsistent with the previous hypothesis that both flower color and corolla tube depth have coevolved with different pollinators during speciation and formation of reproductive isolation. Both visit frequencies of the individual flower and geitonoga-mous visits within the individual plant are higher in G. siphonantha than in G. straminea. This difference may result from their different inflorescence designs that actively act upon behaviors of pollinator. Although these two species differed in flowering phenology, their flowering stages overlapped for a few days, suggesting incomplete pollination isolation between them.     

露蕊乌头(毛茛科)不同海拔居群的传粉生态学

对青藏高原东北部两个不同海拔地点（互助,2460m和海北,3200m）的露蕊乌头Aconitum gymnandrum的传粉生态学进行了比较研究。研究结果表明高海拔居群的单花花期、雄性期持续时间、雌性期持续时间比低海拔长。露蕊乌头不存在无融合生殖现象,尽管高度自交亲和,但产生种子必须依赖传粉媒介。熊蜂是露蕊乌头的主要传粉昆虫,两个居群均存在传粉限制。熊蜂取食花蜜为主,在同一个花序上的访问顺序以由下向上为主,但在互助居群和海北居群有3．9％和2．7％的访花是由上向下进行的。统计发现有37．7％和29．3％的访问行为发生在同一植株内;因此,同株异花传粉造成的自交在露蕊乌头中仍然不能避免。低海拔居群的熊蜂种类和访花频率均高于高海拔居群,但自然状态下两个居群的结实率并没有显著差异。高海拔露蕊乌头居群可能具有较长的柱头持续时间,从而补偿了由于访花频率低带来的结实率降低。     

Reproductive biology of Butea monosperma (Fabaceae)

The reproductive biology encompassing phenology, floral biology, pollination and breeding systems, of Butea monosperma, a beautiful tree of the Indian subcontinent, was investigated in a protected dry, deciduous forest located in New Delhi. Phenological studies indicated that although the species shows a regular flowering season, all trees do not flower every year. Flowers are typically papilionaceous; the stigma is wet papillate and the style is hollow. The flowers show characteristics of bird pollination being large and bright orange-red in colour with copious amounts of nectar, and exhibiting diurnal anthesis. Although the flowers are frequented by as many as seven species of birds belonging to six families, only one species, the purple sunbird (Nectarinia asiatica), is the effective pollinator. The flowers are also pollinated by the three-striped squirrel (Funambulus tristiatus). Unlike other flower visitors, these two pollinators forage the nectar from the open side of the keel (legitimate path) during which pollen grains are deposited on their body parts. After the first visit of a sunbird or a squirrel, virgin flowers showed pollen load on the stigma and developed into fruits. B. monosperma shows a weak form of self-incompatibility. Fruit set following manual self-pollination (5.25 %) was comparable with open-pollination (approx. 5 %) but was significantly lower than manual cross-pollination (22.51 %). This indicates that there is a high degree of geitonogamous pollination in this species, which may lead to a weakening of self-incompatibility as a means of reproductive assurance. The results are analysed in the light of prevailing discussions on specialized vs. generalized pollination systems.     

Reproductive biology and natural hybridization between two endemic species of Pitcairnia (Bromeliaceae)

We investigated pollination biology and breeding systems in hybridizing populations of Pitcairnia albiflos and P. staminea; both species are endemic to rocky outcrops at Rio de Janeiro, Brazil. These species are morphologically distinct and easily recognized by floral color: white in P. albiflos and red in P. staminea. Putative hybrids show a large range of intermediate pink floral colors. The showy hermaphroditic flowers offer pollen and nectar that attract many visitors including bees, butterflies, hawk moths, and bats. Although the flowers of both parental species and hybrids open at night, only P. albiflos had other adaptations for nocturnal pollination. Flowering times overlapped during three consecutive years of observation. Bees visited both species and putative hybrids. Cross-pollinations were performed within and among parental species and hybrids in a greenhouse using plants transplanted from the field. Pitcairnia staminea and hybrids are self-compatible and could be spontaneously self-pollinated, whereas P. albiflos, though self-compatible, needs pollinators' services for self-pollination. Facultative agamospermy was found in the parental species. Prezygotic and postzygotic reproductive barriers between these taxa were weak. Reciprocal hand-pollinations between parental species and with hybrids yielded high fruit sets with viable seeds. Evaluations of fruit set, seed set, seed germination, and pollen viability were undertaken to compare the fitness of the hybrids relative to their parents. The hybrids showed equivalent fitness, except for lower pollen viability. Some conservation implications are noted.     

片断化生境中濒危植物黄梅秤锤树的开花生物学、繁育系统与生殖成功的因素

 研究处于片断化生境中濒危植物的繁育系统、传粉过程及生殖成功状况, 可为评估物种的受胁迫程度及制订相应的保护和管理策略提 供重要依据。通过野外观察和人工授粉实验等方法,对片断化生境中濒危植物黄梅秤锤树(Sinojackia huangmeiensis)的开花生物学和繁育系统 进行了初步研究, 并探讨了影响其生殖成功的因素。黄梅秤锤树的总状花序上通常有两性花2～6朵, 单花花期约5～7 d, 单花花药不同步开裂 使整朵花花粉活力持续时间延长。虽然花开放过程中柱头始终高于花药, 在单花水平上可在一定程度上避免同花自交的发生, 但由于花粉活力 和柱头可授期之间有一定的重叠期, 在自然情况下, 由于外界媒介动物和风媒的作用, 不可避免地会产生同株同花和同株异花传粉。实验结果 表明, 该种不存在无融合生殖现象, 繁育类型以异交为主, 部分自交亲和且需要传粉者。主要的传粉昆虫为黑带食蚜蝇(Epistrophe balteata) 、中华蜜蜂 (Apis cerana) 和中华回条蜂(Habropoda sinensis)。黄梅秤锤树自然状态下的结实率较低(10.33%), 平均每果种子数较少(0.387 个), 这主要与传粉昆虫少、花粉传递效率低, 以及柱头上自花及同株异花花粉的落置和自交亲和性低有关。但是单株较大的开花量保证了一定 的果实和种子产量, 使居群能自然更新。部分自交亲和性可能为该物种适应生境片断化提供了一定的生殖保障及进化潜力。对该残存居群的保 护, 应着眼于加强对传粉昆虫赖以生存的自然生态系统的恢复。     

铃铛子和赛莨菪（茄科）的传粉机制比较研究

通过野外交配系统试验和传粉昆虫观察,以铃铛子（Anisodus luridus）和赛莨菪（A．camiolicoides）为研究对象,探讨了山莨菪属内自交亲和系统的进化与传粉昆虫的转变。结果表明,铃铛子和赛莨菪均属于自交完全亲和的类群,但两个种的自动自交能力均不强,而且都存在传粉限制。北方黄胡蜂（Vespula rtfarufa）和石长黄胡蜂（Dolichovespula saxonica）分别是铃铛子和赛莨菪的主要传粉昆虫,自然状态下两种昆虫的传粉效率均比较高,但两种昆虫访问两种植物时传递花粉的方式不同,其中北方黄胡蜂主要促进了铃铛子的花间传粉,而石长黄胡蜂访花引起赛莨菪同一朵花内的传粉。通过与该属的另一种植物山莨菪（A．tanguticus）的传粉机制比较,发现在山莨菪属的物种分化过程中,由自交不亲和转变为自交完全亲和,传粉昆虫也发生了转变,证明了自交亲和系统存在于起源较晚的类群中。高山环境中频繁且不可预测的降雨可能降低了传粉昆虫的活动能力,进而导致铃铛子和赛莨菪均存在传粉限制。     

Reproductive biology of Echinopsis terscheckii (Cactaceae): the role of nocturnal and diurnal pollinators

The aim of this study was to analyse the reproductive biology of Echinopsis terscheckii, a species endemic to northwest Argentina that has nocturnal flowers. We expected that this species had a generalised pollination system, with moths and diurnal visitors as the primary pollinators. To test this, we studied the floral biology, breeding system and floral visitors of this species and the effectiveness of nocturnal and diurnal visitors. Floral biology was defined based on floral morphology, floral cycle and nectar production of the flowers. The breeding system and relative contributions of diurnal and nocturnal visitors to fruit and seed set were analysed through field experiments. E.?terscheckii flowers opened at sunset and closed the following day. The peak of nectar production occurred at midnight. Flowers were determined to be self-incompatible. Moths, bees and birds were identified as floral visitors. Moths were the most frequent visitors at night, whereas bees were the most frequent visitors during the day. Fruit production by diurnal pollinators was less than that by nocturnal pollinators; among all floral visitors, moths were the most effective pollinators. We have demonstrated for the first time that moths are the primary pollinators of columnar cacti of the genus Echinopsis. Our results suggest that moths might be important pollinators of columnar cactus species with nocturnal flowers in the extra-tropical deserts of South America.     

REPRODUCTIVE BIOLOGY OF THREE SPECIES OF EUPHORBIA SUBGENUS CHAMAESYCE (EUPHORBIACEAE)

The breeding system, pollen tube development, and floral biology of three species of Euphorbia subgenus Chamaesyce from southeastern Arizona have been examined in field and greenhouse studies. E. albomarginata is entirely dependent on insect vectors for pollination and seed production, E. capitellata requires insect visitors for maximum seed set but is capable of limited self-pollination, and E. hyssopifolia is self-pollinating and entirely independent of insect vectors. All three species are self-compatible. The differences in reliance on insect vectors, diversity of insects and probable amount of outcrossing are related to differences in cyathial size and arrangement. The reproductive biology of the three species is similar to that described for other weedy plant taxa.     

Plant sexual systems correlated with morphological traits: Reflecting reproductive strategies of alpine plants

Plant sexual systems can be affected by various environmental factors, and thus show a range of morphological differences. The associations between plant sexual systems and morphology are unique, for example, the subnival belt of the Hengduan Mountains has harsh climatic conditions, but hosts remarkable biodiversity. However, these associations have received surprisingly little attention. We analyzed the associations in this region, and compared the pattern of sexual systems with that of other floras, and we also reviewed the breeding system of 36 alpine species. Information on plant sexual systems and these traits were mainly based on published information, herbarium collections, and field observations. The results showed that there was a high proportion of hermaphroditic species and a low proportion of dioecious and monoecious species. Hermaphroditism was strongly associated with showy floral displays, specialist entomophily, and perennial forbs. Dioecy was strongly associated with inconspicuous, pale-colored flowers, anemophily, and shrubs, whereas monoecy was strongly associated with inconspicuous, pale-colored flowers, generalist entomophily, and perennial grasses. Although no association was found between sexual systems and fruit type, plants with dry fruits (98.1%) were predominant in our study region. With respect to the plant breeding systems in the alpine region of southwest China, we predicted that pollination service might be particularly low; most of the plants lacked enough outcrossing pollinators. Our data revealed that 97.1% of the hermaphroditic species were self-compatible, and 88.2% of them showed autonomous or facilitated selfing. The plant sexual systems identified might serve to optimize the probability of pollination and thus ensure reproductive success.     

铃铛子和赛莨菪( 茄科) 的传粉机制比较研

通过野外交配系统试验和传粉昆虫观察, 以铃铛子( Anisodus luridus) 和赛莨菪( A. carniolicoides)为研究对象, 探讨了山莨菪属内自交亲和系统的进化与传粉昆虫的转变。结果表明, 铃铛子和赛莨菪均属于自交完全亲和的类群, 但两个种的自动自交能力均不强, 而且都存在传粉限制。北方黄胡蜂( Vespula rufarufa) 和石长黄胡蜂( Dolichovespula saxonica) 分别是铃铛子和赛莨菪的主要传粉昆虫, 自然状态下两种昆虫的传粉效率均比较高, 但两种昆虫访问两种植物时传递花粉的方式不同, 其中北方黄胡蜂主要促进了铃铛子的花间传粉, 而石长黄胡蜂访花引起赛莨菪同一朵花内的传粉。通过与该属的另一种植物山莨菪(A. tanguticus) 的传粉机制比较, 发现在山莨菪属的物种分化过程中, 由自交不亲和转变为自交完全亲和, 传粉昆虫也发生了转变, 证明了自交亲和系统存在于起源较晚的类群中。高山环境中频繁且不可预测的降雨可能降低了传粉昆虫的活动能力, 进而导致铃铛子和赛莨菪均存在传粉限制。     

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass‐flowering species

Plant mating systems are driven by several pre‐pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass‐flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen‐supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass‐flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator‐mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.     

The role of a mixed mating system in the reproduction of a Mediterranean subshrub (Fumana hispidula, Cistaceae)

Knowledge about mixed mating systems can improve our understanding of the evolutionary dynamics of reproductive systems. Here we report a study of the pollination system (floral and reproductive biology, pollen limitation and stigmatic pollen load, floral visitors and inbreeding depression) of Fumana hispidula, a Mediterranean subshrub belonging to a species group with a strong selfing tendency. Autonomous self-pollination, hand self- and hand cross-pollination, open pollination and controls yielded fruits (0.28, 0.65, 0.68, 0.38, and 0.67, respectively); however, autonomous self-pollination resulted in the lowest fruit set. All individuals were fully self-compatible but we found great inter-individual variation in autonomous self-pollination ability, which was probably related to a variable expression of herkogamy degree. Inbreeding depression was low (0.040 for fruit set, 0.015 for seed set and ?0.026 for seed mass). The pollen supplementation experiment did not reveal pollen limitation and pollinators were seen visiting the flowers during the observation periods. These results support the idea that F. hispidula has a mixed mating strategy, which represents a successful reproduction mode in their patchy habitats.     

Wind-dragged corolla enhances self-pollination: a new mechanism of delayed self-pollination

BACKGROUND AND AIMS: Delayed self-pollination is a mechanism that allows animal-pollinated plants to outcross while ensuring seed production in the absence of pollinators. This study aims to explore a new mechanism of delayed self-pollination facilitated by wind-driven corolla abscission in Incarvillea sinensis var. sinensis. METHODS: Floral morphology and development, and the process of delayed self-pollination were surveyed. Experiments dealing with pollinator and wind exclusion, pollination manipulations, and pollinator observations were conducted in the field. KEY RESULTS: Delayed self-pollination occurs when the abscising corolla driven by wind drags the adherent epipetalous stamens, thus leading to contact of anthers with stigma in late anthesis. There is no dichogamy and self-incompatibility in this species. The significantly higher proportion of abscised corolla under natural conditions as compared with that in wind-excluding tents indicates the importance of wind in corolla abscission. When pollinators were excluded, corolla abscission significantly increased the number of pollen grains deposited on the stigma and, as a result, the fruit and seed set. Only half of the flowers in plots were visited by pollinators, and the fruit set of emasculated flowers was significantly lower than that of untreated flowers in open pollination. This species has a sensitive stigma, and its two open stigmatic lobes closed soon after being touched by a pollinator, but always reopened if no or only little pollen was deposited. CONCLUSIONS: This delayed self-pollination, which involved the movement of floral parts, the active participation of the wind and sensitive stigma, is quite different from that reported previously. This mechanism provides reproductive assurance for this species. The sensitive stigma contributes to ensuring seed production and reducing the interference of selfing with outcrossing. The pollination pattern, which combines actions by bees with indirect participation by wind, is also a new addition to ambophily.     

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

Self-compatibility in high arctic and alpine areas is regarded as an adaptation to low pollinator abundance. However, high genetic variability as a consequence of outcrossing is, with regard to population persistence, favorable in highly stochastic environments such as tundra habitats. To evaluate these contradictory scenarios, I performed in situ pollination experiments to examine the breeding system of the predominant outcrosser Saxifraga oppositifolia in ten populations at two different elevations in the Swiss Alps. Pollinator limitation was detected at both elevations, but fruit set in naturally pollinated flowers was only slightly less at the higher elevation. Increased pollinator limitation at high compared with low elevation thus could not be demonstrated in this experiment. Hand-crossings yielded equal mean proportion seed set at both elevations, and so did hand-selfings. This constant pattern of the breeding system in S. oppositifolia indicates selective factors that lead to the maintenance of a high level of outcrossing even in high-elevation populations. Based on sex allocation models, it was expected that a high ovule number should be selectively advantageous in a plant-pollinator system where chance visitation or selfing play important roles. However, female reproductive offer in terms of ovule number per flower did not change from low to high elevation. Since neither increased pollinator limitation nor increased seed set in selfed flowers was found at high compared with low elevation, the prerequisites for testing the hypothesis were not given. This study contradicts the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially outcrossing species like S. oppositifolia. Received: 28 April 1997 / Accepted: 20 October 1997