Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.     

Floral morphology and reproductive success in the orchid Epipactis helleborine: regional and local across-habitat variation

 The terrestrial orchid Epipactis helleborine is a morphologically variable species with a wide distribution in Europe. It is pollinated by social wasps, and most populations show the morphological characteristics of outcrossing species. However, local predominantly selfing subspecies and varieties have been documented from drier habitats. To document geographic variation in floral morphology, ability to produce seeds through autogamy, and reproductive success in E. helleborine, we sampled 13 populations from three geographic regions along a latitudinal gradient of c. 1000 km from northern to southern Sweden. In the southernmost region, populations in dry and mesic habitats were compared. Supplemental hand-pollination was conducted to determine whether among-population variation in fruit set could be explained by differences in the natural level of pollination, and whether any relationship between floral morphology and fruit production could be explained by interactions with pollinators. Bagging experiments showed no evidence of autogamy in any of the study populations. Number of flowers, pollinia removal and fruit set varied significantly among populations but did not differ among regions. Pollinia removal was positively correlated with population size and both pollinia removal and fruit set were lower in dry than in mesic habitats. At the level of the individual plant, the number of pollinia removed increased more rapidly with flower number than did number of fruits produced. The hand-pollination experiment indicated that the positive relationship between number of flowers and fruit production was due to a higher degree of pollen limitation in plants with few flowers than in plants with many flowers. The experiment also showed that variation in the level of pollen limitation could only partly explain variation in fruit set among populations. Received November 6, 2001; accepted April 27, 2002 Published online: December 3, 2002     

Evidence for widespread pollen limitation of fruiting success in Cape wildflowers

We used spot checks of stigmatic pollen deposition and hand-pollination experiments to test whether fruit production in Cape wildflower populations is limited by pollen availability. Natural levels of stigmatic pollen deposition were very low (median = 30.0% of flowers) in populations of 33 orchid species. We found similarly low levels of fruit set (median = 32% of flowers per plant) in six Orchidaceae and four Amaryllidaceae species. Experimental hand pollination at the whole plant level caused significant increases in fruit production in 11 of the 12 study populations. These results indicate that pollen limitation of fruit set may occur frequently among some plant families in the Cape flora. Received: 15 April 1996 / Accepted: 10 November 1996     

Genetic rescue by distant trees mitigates qualitative pollen limitation imposed by fine‐scale spatial genetic structure

Restricted seed dispersal frequently leads to fine‐scale spatial genetic structure (i.e., FSGS) within plant populations. Depending on its spatial extent and the mobility of pollinators, this inflated kinship at the immediate neighbourhood can critically impoverish pollen quality. Despite the common occurrence of positive FSGS within plant populations, our knowledge regarding the role of long‐distance pollination preventing reproductive failure is still limited. Using microsatellite markers, we examined the existence of positive FSGS in two low‐density populations of the tree Pyrus bourgaeana. We also designed controlled crosses among trees differing in their kinship to investigate the effects of increased local kinship on plant reproduction. We used six pollination treatments and fully monitored fruit production, fruit and seed weight, proportion of mature seeds per fruit, and seed germination. Our results revealed positive FSGS in both study populations and lower fruit initiation in flowers pollinated with pollen from highly‐genetically related individuals within the neighbourhood, with this trend intensifying as the fruit development progressed. Besides, open‐pollinated flowers exhibited lower performance compared to those pollinated by distant pollen donors, suggesting intense qualitative pollen limitation in natural populations. We found positive fine‐scale spatial genetic structure is translated into impoverished pollen quality from nearby pollen donors which negatively impacts the reproductive success of trees in low‐density populations. Under this scenario of intrapopulation genetic rescue by distant pollen donors, the relevance of highly‐mobile pollinators for connecting spatially and genetically distant patches of trees may be crucial to safeguarding population recruitment.     

Limitations to Reproductive Success in the Dioecious Tree Rhamnus davurica

The reproductive success of a female plant in a dioecious species may be affected by pollen limitation and resource limitation. This study presents evidence that the reproductive success of the dioecious understorey tree species, Rhamnus davurica, is affected by the distance to the nearest male. The sex ratios were female-biased, although showing fluctuations in the three years of conducting the study. The mortality rate of females was higher than that of males indicating a trade-off between reproduction and survival. Altogether 49 females, designated as “focal females”, were randomly selected for monitoring their reproductive status between April and October in 2010. But successful reproduction (meaning that the flowering female trees had fruit in the fruiting season) was observed only in 28 females in 2011 and 16 females in 2012. The method of path analysis was applied to determine the effect of topography, local competition and proximity to the nearest male on the fruit set of the females. In the three years of the study, elevation, competition and female size had no significant effect on the fruit set. The distance to the nearest male, however, had a significant effect on fruit set. Number of fruits and fruit set were decreased with increasing distance to the nearest male. It was possible to estimate maximum fruit set, based on the comparatively large dataset. The number of fruits and the fruit set are exponentially related to the distance to the nearest male and the relationships are described by an exponential model. The results of this study support the importance of pollen limitation on the reproductive success in Rhamnus davurica.     

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

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

Reproductive lability in pre-dispersal biology ofRhamnus alaternus L. (Rhamnaceae)

Summary Rhamnus alaternus L. is a dioecious, fleshy-fruited shrub, typical of Mediterranean vegetation, which has been recorded from the region since the tropical Tertiary. Reproductive biology of this species has been studied in Southern Italy over a period of 4 years with both field and laboratory experiments. In all the studied populations male plants predominate over the females, have a higher flower density and flower regularly every year while females are usually alternate-bearing. A strong negative association between fruit formation and vegetative growth has been found indicating that the presence of fruits on a branch reduces the growth of new shoots. This phenomenon, together with alternate-bearing, is evidence of a strong competition between vegetative growth and reproductive activity and indicates resource limitation. Flower morphology and quantity of pollen produced are typical of insect pollinated species, and flies of the Empididae and Muscidae were trapped bearing pollen at female plants but pollen is also dispersed by wind. Field experiments on pollination success showed that the exclusion of insects as pollen vectors produces a significant decrease in fruit development. Excess pollen provided by hand-pollination increases fruit set over open-pollination and reproductive success in the field is therefore pollen limited. The variability of sex expression, male fitness, fruit production, regrowth and pollination presented here suggests that these predispersal characteristics ofR. alaternus have been subject to evolutionary change since the tropical Tertiary climate was replaced by a Mediterranean one. This points to the possibility that reproductive, as well as morphological and physiological characteristics, may be equally selected for in extreme environments such as the Mediterranean.     

Pollinator Abundance and Pollen Limitation of a Solanaceous Shrub at Premontane and Lower Montane Sites

Populations at geographic range limits may frequently encounter changing biotic interactions as well as abiotic constraints. A conundrum in plant population biology, the frequent apparent overproduction of flowers, is thought to be a response by plant populations to unpredictable pollinator environments, such as those that may be expected at the range limit. We studied pollinator visitation rates and pollen limitation of fruit set in populations of Witheringia solanacea , a widespread bee-pollinated species. We compared two populations in the south and two in the north of Costa Rica, with each region encompassing a lower montane site at the upper elevational limit of the species range with small plant populations, and a premontane site where population densities were high. Populations in the south, where the climate was favorable, had higher pollinator visitation and higher natural fruit set, and showed only minor pollen limitation as compared with northern populations at windy sites near the Continental Divide. Lower montane populations had lower pollinator visitation rates than premontane populations but did not show more pollen limitation of fruit set. In general, pollen limitation was not inversely proportional to pollinator abundance but was higher where the climate is unpredictable.     

Pollen limitation in invasive populations of Solanum rostratum and its relationship to population size

Aims Small plant populations may be more likely to suffer more severe pollen limitation due to the lower number of potential mates or suitable pollinators. For invasive species, this phenomenon may be more common when an invading population colonizes a new habitat. Here, we investigated whether pollen limitation occurs in invasive populations of Solanum rostratum during its invasion from North America to China and evaluated the patterns between pollen limitation and population size.Methods Pollen addition experiments were performed on six invasive populations of S. rostratum. By comparing fruit set and seed production with open pollination treatment, we calculated the index of pollen limitation and regressed it to population size and density.Important findings Among the six sampled invasive populations of S. rostratum, the fruit set and seed production per fruit were 0.346±0.014 and 52.38±9.29, respectively, with open pollination treatment and 0.572±0.022 and 56.28±10.79, respectively, with pollen addition treatment. Compared with open pollination, pollen addition significantly increased fruit set and seed production by 65.3 and 7.4%, respectively. The standardized index of pollen limitation ranged from 0.022 to 0.125, with an average of 0.065, suggesting that invasive populations of S. rostratum do suffer from pollen limitation. The index of pollen limitation was negatively correlated with population size, which is consistent with the pattern that smaller populations suffer from more severe pollen limitation.     

Effects of population size on reproductive success of the endangered and endemic species Primula merrilliana

The reproductive ability related to the population size of the endangered and endemic species Primula merrilliana Schltr.was investigated.In 26 populations observed,only four contain more than 500 flowering individuals,whereas most of them(53.8%) consist of less than 100 flowering individuals.Though the ratio of pin and thrum plants keeps its balance at 1:1 for all populations,the frequency of pin and thrum flowers was significantly biased in most small populations.Population size strongly affected reproductive success; plants in small populations produced significantly fewer flowers and fruits per plant and fewer seeds per fruit,and therefore fewer seeds per plant.The floral density was another important factor that influenced the reproductive success of P.merrilliana,because four main reproductive success parameters (fruits per plant,seeds per fruit,seeds per plant,and the proportion of flowers setting fruit) were all positively correlated with floral density.The size of plants and the number of leaves per plant (measure of habitat quality) were not influenced by the variation of population size,suggesting that the reduced fecundity in small populations may not be a consequence of lower habitat quality,inbreeding depression and pollen limitation as a result of less attractiveness in small populations are therefore likely explanations for these patterns.     

Variation in sex ratio, morph-specific reproductive ecology and an experimental test of frequency-dependence in the gynodioecious Kallstroemia grandiflora (Zygophyllaceae)

An enduring puzzle in gynodioecious species is the great variation in female frequency seen among populations. We quantified sex ratio in 44 populations of gynodioecious Kallstroemia grandiflora. Then, we measured pollinator visitation, pollen deposition, autonomous selfing rate and pollen limitation of females. Finally, using experimental populations, we tested whether female fitness responds to the frequency of female plants. We found broad variability in sex ratio among populations (0-44% female). Hermaphrodite flowers received more pollinator visits and pollen grains than females, and bagged hermaphrodite flowers produced fruits. However, we found no evidence of pollen limitation in females. In experimental populations, female plants showed no evidence of frequency-dependent pollinator visitation, fruit set, seed set or total seed mass. These results do not support frequency-dependent variation in fitness as a major mechanism affecting female frequencies in K. grandiflora. Within the context of this study, pollinators are abundant and pollinator movement appears to operate at a large enough scale to overcome the potential reproductive disadvantages of producing solely female flowers.     

新疆产药用植物黑果枸杞有性生殖产出差异的繁殖生态学研究

由于黑果枸杞（Lycium ruthenicum）果实具有重要的保健价值,近年来倍受关注,但其野生资源正遭受着前所未有的采伐压力。目前关于影响黑果枸杞果实产量的有性生殖过程研究较少,本实验对新疆库尔勒地区6个不同生境的野生黑果枸杞居群开展了繁殖生态学观察,并对各居群的访花频率、访花昆虫种类、柱头花粉落置、胚珠数、座果率、结实率和单株果实产量等指标进行了比较分析。结果表明,访花频率与结实率、座果率具有显著正相关关系,说明传粉者是限制黑果枸杞有性生殖的关键因素之一;传粉者种类越丰富的居群其有性生殖产出越高,揭示了传粉昆虫的多样性和丰富度是影响黑果枸杞繁殖成功的重要限制因子。此外,和静县华侨农场和焉耆县霍拉山河沟的黑果枸杞居群生殖产出明显与其传粉成功率相偏离,进一步分析表明,风沙侵蚀和人为采伐对植株的破坏是影响这两个居群果实产量的重要原因。本研究结果对于野生黑果枸杞居群的保护及其人工规模栽培提供了重要的参考依据。     

Pollen limitation,fruit abortion,and autonomous selfing in three populations of the perennial herb Ruellia nudiflora

Multiple factors determine plant reproductive success and their influence may vary spatially. This study addresses several factors influencing female reproductive success in three populations of Ruellia nudiflora, specifically we: (i) determine if fruit set is pollen‐limited and if pollinator visitation rates are related to this condition; (ii) estimate fruit set via autonomous self‐pollination (AS) and relate it to the magnitude of herkogamy; and (iii) evaluate if fruit abortion is a post‐pollination mechanism that determines the magnitude of pollen limitation. At each site we marked 35 plants, grouped as: unmanipulated control (C) plants subjected to open pollination, plants manually cross‐pollinated (MP), and plants excluded from pollinators and only able to self‐pollinate autonomously (AS). Fruit set was greater for MP relative to C plants providing evidence for pollen limitation, while a tendency was observed for lower fruit abortion of MP relative to C plants suggesting that fruit set is influenced not only by pollen delivery per se, but also by subsequent abortion. In addition, although pollinator visits varied significantly among populations, the magnitude of pollen limitation did not, suggesting that pollinator activity was not relevant in determining pollen limitation. Finally, fruit set tended to decrease with the degree of herkogamy for AS plants, but this result was inconclusive. These findings have contributed to identify which factors influence reproductive success in populations of R. nudiflora, with potentially relevant implications for population genetic structure and mating system evolution of this species.     

Fertility of Vriesea gigantea Gaud. (Bromeliaceae) in southern Brazil

Plant fertility is a central subject of many questions in plant evolutionary and conservation biology. Pollen availability, abiotic resources, and flowering pattern can limit fruit and seed production. Open pollination and pollen supplementation studies are used to estimate any pollen limitation in natural populations. To study the impact of these factors on the reproductive success of Vriesea gigantea, an epiphytic bromeliad in the Atlantic Rainforest in Brazil, its fertility in four natural populations in Itapu? State Park was assessed by considering plant and inflorescence size, flower production, fruit and seed set, flower and fruit set pattern, and seed viability and germination rate. Supplemental pollination in adult plants was used to determine whether fruit production in V. gigantea is limited by reception of pollen. The results showed that V. gigantea has a high production of flowers, fruits, and seeds. Seeds are highly viable in all populations, presenting an average germination rate of 94% (SE ± 3.5). Plants of V. gigantea from Itapu? State Park are highly fertile. The high proportion of fruit and seed set after manual hand pollination indicates that the species is self-compatible. Pollination treatments showed evidence of pollinator limitation in the Itapu? State Park population.     

How flower biology and breeding system affect the reproductive success of the narrow endemic Polygala vayredae Costa (Polygalaceae)

Polygala vayredae is a narrow endemic species from the oriental pre-Pyrenees. Despite its conservation status and rarity, no information is available on its reproductive biology. As the flower is the structure directly involved in pollinator attraction, its morphological and functional traits have major effects on the reproductive success of the plant. In this work, the flower biology and breeding system of P. vayredae were studied to evaluate how they affect the reproductive outcome in natural populations. Flower morphology, flower rewards, and male and female functioning throughout the lifespan of the flower were assessed. Pollination experiments, involving pollinator exclusion and pollen from different sources, were conducted, and the pollen ovule index was determined. Female fitness and the occurrence of pollen limitation were assessed in three natural populations over 2 years by observing the presence of pollen on the stigma, pollen tube development, and fruit production. Polygala vayredae flowers are elaborate and long-lived with nectar rewards. The floral traits are well adapted to xenogamy and entomophily, which are in accordance with the observed breeding system and auto-incompatibility system. No mechanism of reproductive assurance was observed and P. vayredae strictly depends on pollinators to set fruit. Low fruit production was observed in the studied populations, which was largely the result of scarce, unreliable, and/or inefficient pollinators and poor pollen quality. In addition, available resources may be a limiting factor. The reproductive strategy of P. vayredae prevents inbreeding depression by a self-incompatibility system, which in years of scarce pollinators is overcome by the plant habit.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 157 , 67–81.     

A quantitative synthesis of pollen supplementation experiments highlights the contribution of resource reallocation to estimates of pollen limitation

Our understanding of pollen limitation depends on a realistic view of its magnitude. Previous reviews of pollen supplementation experiments concluded that a majority of plant species suffers from pollen limitation and that its magnitude is high. Here, we perform a meta-analysis and find evidence that publication bias, experimental design, and the response variable chosen all influence the magnitude of pollen limitation. Fail-safe numbers indicate that publication bias exists for some measures of pollen limitation; significant results are more likely to be published and therefore available for review. Moreover, experiments conducted on only a fraction of a plant's flowers and reproductive episodes report ~8-fold higher effect sizes than those on all flowers produced over the entire lifetime, likely because resource reallocation among flowers and across years contributes to estimates of pollen limitation. Studies measuring percentage fruit set report higher values of pollen limitation than those measuring other response variables, such as seeds per fruit, perhaps because many plant species will not produce fruits unless adequate pollen receipt occurs to fertilize most ovules. We offer suggestions for reducing the bias introduced by methodology in pollen supplementation experiments and discuss our results in the context of optimality theory.     

Florivory affects pollinator visitation and female fitness in <Emphasis Type="Italic">Nemophila menziesii</Emphasis>

While herbivory has traditionally been studied as damage to leaves, florivory – herbivory to flowers prior to seed set – can also have large effects on plant fitness. Florivory can decrease fitness directly, either through the destruction of gametes or through alterations to plant physiology during fruit set, and can also change the appearance of a flower, deterring pollinators and reducing seed set. In order to distinguish between these hypotheses, it is necessary to both damage flowers and add pollen in excess to study the effects of damage on pollen limitation. Very few studies have used this technique over the lifetime of a plant. Here I describe a series of experiments showing the effects of natural and artificial damage on reproductive success in the annual plant Nemophila menziesii (Hydrophyllaceae, sensu lato). I show that natural and artificial petal damage decreased radial symmetry relative to controls and that both types of damage deterred pollinator activity. Both naturally damaged flowers and artificially damaged flowers in the field set fewer fruit or seed relative to undamaged control flowers. Finally, in an experiment crossing artificial petal damage with pollen addition, petal damage alone over the lifetime of this plant decreased female fitness, but only after a threshold of damage was reached. The fitness effect appeared to be direct because there was no detectable effect of pollen addition on the relationship between florivory and fitness. This result implies that both damaged and undamaged plants show similar amounts of pollen limitation and suggests that pollinator-mediated effects contributed little to the negative effects of florivory on female fitness. Florivores may thus be an under-appreciated agent of selection in certain plants, although more experimental manipulation of florivory is needed to determine if it is important over a range of taxa.     

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.     

Variability in Pollen Load: Implications for Reproduction and Seedling Vigor in a Rare Plant, Silene douglasii var. Oraria

Habitat fragmentation can markedly influence the levels of pollen deposition and seed production in natural populations, and rare plants may be especially susceptible to any associated reductions in pollen quantity and quality. In order to ascertain the potential for pollen limitation of maternal fitness in a rare plant, Silene douglasii var. oraria, which is endemic to western coastal prairies, we counted ovules and measured conspecific and heterospecific pollen deposition on stigmas collected from open-pollinated plants. We further investigated the effect of increasing pollen intensity on fruit production, seed number and weight, as well as several measures of progeny vigor. Three levels of outcross pollen were added to plant stigmas for comparison with autogamous and open pollination in the largest naturally occurring population. Both seed and fruit production were significantly greater (P<0.05) for supplemented versus nonsupplemented stigmas, but flowers receiving different levels of pollen addition were statistically indistinguishable. Seed germination and seedling survival were also lowest for the offspring of nonsupplemented flowers; however, in natural populations, opportunities for pollen competition are very limited since open-pollinated flowers averaged fewer viable pollen grains than ovules. Seed production was equivalent for open- and autogamously pollinated flowers in 1996, indicating that natural pollen transfer may have involved mostly self pollen. Overall, the low reproductive success of var. oraria likely reflects both low pollen quantity and quality. Multiyear empirical studies of pollen intensity in field populations are needed so that we can better understand the fitness consequences of pollen limitation in rare perennials.     

Reproductive ecology of Cneorum tricoccon L. (Cneoraceae) in the Balearic Islands

This paper examines some aspects of the reproductive biology of Cneorum tricoccon L. (Cneoraceae), a Mediterranean relict shrub which evolved under tropical conditions during the Tertiary. The breeding system, the flowering and fruiting phenology, and the variability in fecundity within and among populations of this plant were examined in the Balearic Islands. The plant is andromonoecious, and pollen from male flowers appeared to be more fertile than pollen from hermaphrodite flowers. There was no limitation of pollen, and xenogamous crosses produced more fruit than geitonogamous ones. The efficiency of male flowers suggests that their use increases plant fitness, thus maintaining such a breeding system. Flowering time is unusual compared with other species in the habitat; there is synchrony among individuals, and flowers are produced almost continuously from November through June. Floral sex ratio (number of hermaphrodite flowers: total number of flowers) varied greatly both through the flowering season and between years and was not consistent through time, suggesting that sex expression may be determined more by factors such as resource status of the plant than by genetic factors. Fully developed fruits are observed from December to July. Fecundity varied among individuals depending upon plant size and was consistent during the three years of the study (1992–1994). Fruit crop also differed strongly among sites. The greatest, most vigorous and thus most fecund plants are found in some populations of Mallorca island, especially at high altitudes. In other Balearic islands with low altitudes (<200 m), such as Cabrera, individuals are always short, less leafy and less fecund.