Spatially inconsistent direct and indirect effects of herbivory on floral traits and pollination success in a tropical shrub

Investigations on plant–animal interactions have traditionally focused on single interactions at a time (e.g. herbivory, pollination), yet plant fitness is generally influenced in complex ways by several interactions operating concurrently, and very little is known on the degree of spatial consistency of the direct and indirect effects that link different interactions. This paper evaluates experimentally whether direct and indirect effects of herbivory on male and female flower size and pollination success of the monoecious tropical shrub Cnidoscolus aconitifolius (Euphorbiaceae) remain consistent at three distant regions in Yucatan (southeastern Mexico). Plants were subjected to different levels of leaf defoliation, and treatment effects on floral traits (corolla area, corolla tube length, pollen production), and male and female components of pollination success (percent pollen removal, number of pollen tubes) were subsequently measured to evaluate the indirect effect of herbivory on plant reproductive success via pollination. Defoliation had significant direct effects on floral traits, but its indirect effects differed between the male and female components of pollination success. Moreover, the relationships between defoliation, floral traits and male and female pollination success varied spatially (i.e. between regions), although they were frequently more spatially‐consistent for male success than for female success. Results from this study stress the importance of explicitly testing for spatial variation in direct and indirect effects arising from plant‐animal interactions.     

Functional implications of the specialized staminal appendages in alpine ginger (Roscoea spp.: Zingiberaceae)

Floral organs are widely believed to enhance the pollination and reproductive success of angiosperms. However, the functional implication of some floral structures is still unknown. In this study, we explored the functional role of staminal appendages on male and female reproductive success of Himalayan Roscoea spp. and tested if their function differed between species with biotic pollination and autonomous selfing. Phenotypic manipulation is a powerful approach to test the functional effect of a particular trait on plant fitness. We compared various proxies of pollination success between intact flowers and flowers with manually excised staminal appendages. We found that the rate of visitation did not differ between intact and manipulated flowers. Our results revealed that in outcrossing Roscoea spp., the staminal appendages act as triggering devices to facilitate pollen release and deposition and also to manipulate the foraging position of pollinators to ensure both male and female reproductive success. In contrast, in autonomously selfing Roscoea spp., the removal of staminal appendages did not affect any aspect of pollination processes. Our results suggest that the staminal appendages are an integral component of outcrossing in Roscoea spp. and are maintained by selection pressure through both male and female reproductive success. This study provides important insights on how variation in breeding systems can provoke changes in the structure and function of floral organs among congeners.     

Forest Fragmentation and Seed Germination of Native Species from the Chaco Serrano Forest

Habitat fragmentation is a widespread phenomenon that alters pollination and plant reproductive processes. These effects have demographic and genetic implications that determine offspring fitness and the long‐term viability of plant populations in fragmented systems. We evaluated fragmentation effects on early plant offspring fitness traits, individual seed mass, and percentage of seed germination in five native plant species (Acacia caven, Celtis ehrenbergiana, Croton lachnostachyus, Rivina humilis, Schinus fasciculatus) from the Chaco Serrano forest, a subtropical highly fragmented ecosystem. We found evidence of strong negative fragmentation effects on germination in the shrub C. lachnostachyus and the perennial herb R. humilis, after 30 d of controlled tests. No fragmentation effects were found in the studied traits on the remaining three tree species. We found significant maternal effects in offspring fitness traits in all five species. We discuss the relative magnitude of maternal vs. fragmentation effects taking into account both plant species' lifespan and the time elapsed in fragmentation conditions. We emphasize the need to increase the study of early and late plant offspring fitness produced in fragmented habitats coupled with analyses of genetic parameters and the pollination process in order to evaluate the conservation value of remnant forest fragments. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

The effects of nutrient addition on floral characters and pollination in two subalpine plants, <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis> and <Emphasis Type="Italic">Linum lewisii</Emphasis>

The availability of soil and pollination resources are main determinants of fitness in many flowering plants, but the degree to which each is limiting and how they interact to affect plant fitness is unknown for many species. We performed resource (water and nutrients) and pollination (open and supplemental) treatments on two species of flowering plants, Ipomopsis aggregata and Linum lewisii, that differed in life-history, and we measured how resource addition affected floral characters, pollination, and reproduction (both male and female function). We separated the direct effects of resources versus indirect effects on female function via changes in pollination using a factorial experiment and path analysis. Resource addition affected I. aggregata and L. lewisii differently. Ipomopsis aggregata, a monocarp, responded to fertilization in the year of treatment application, increasing flower production, bloom duration, corolla width, nectar production, aboveground biomass, and pollen receipt relative to control plants. Fertilization also increased total seed production per plant, and hand-pollination increased seeds per fruit in I. aggregata, indicating some degree of pollen limitation of seed production. In contrast, fertilization had no effect on growth or reproductive output in the year of treatment on L. lewisii, a perennial, except that fertilization lengthened bloom duration. However, delayed effects of fertilization were seen in the year following treatment, with fertilized plants having greater aboveground biomass, seeds per fruit, and seeds per plant than control plants. In both species, there were no effects of resource addition on male function, and the direct effects of fertilization on female function were relatively stronger than the indirect effects via changes in pollination. Although we studied only two plant species, our results suggest that life-history traits may play an important role in determining the reproductive responses of plants to soil nutrient and pollen additions.     

Consequences of nectar robbing for the fitness of a threatened plant species

The effect of nectar robbing on plant fitness is poorly understood and restricted to a few plant species. Furthermore, the available studies generally evaluate the effects of nectar robbing on female fitness, disregarding the male component. Here we measured the effects of the nectar-robbing bumblebees on male (measured as pollen analogue flow distance) and female (measured as seed production) reproductive success in the insect-dependent Polygala vayredae, a narrow endemic species from the pre-Pyrenees (Spain). Intense nectar robbing by bumblebees significantly reduced the nectar available to legitimate pollinators in the studied population, and this reduction affected both male and female fitness. Significant differences were observed in fluorescent dye dispersion between robbed and non-robbed flowers within the population. Fluorescent dyes from non-robbed flowers were dispersed to larger distances and over a larger number of flowers when compared with robbed ones. Moreover, significant differences were observed in both fruit set and seed ovule ratios between the two groups, with non-robbed flowers presenting higher reproductive outcomes. However, no effect on seed weight was detected among treatments. The data obtained suggest that in this species, nectar robbing has important indirect and negative effects on plant fecundity, through both male and female functions, due to a modification in the foraging behaviour of legitimate visitors.     

Effects of conspecific and heterospecific floral density on the pollination of two related rewarding orchids

Variation in within-population floral density can affect interactions between plants and pollinators, resulting in variable pollen export for plants. We investigated the effects of conspecific and heterospecific floral densities on pollination success both of two related, self-compatible, nectar-rewarding orchid species in Ireland, Spiranthes romanzoffiana (rare and listed as endangered) and its congener, S. spiralis (more abundant and not of conservation concern). Floral densities, insect visitation rates, and orchid pollen transport were recorded in multiple quadrats in four populations of both orchid species over their flowering season. We found that conspecific and heterospecific co-flowering plant density affected pollination in both orchid species. For S. romanzoffiana, higher heterospecific density increased pollen removal. For S. spiralis, higher conspecific visitation increased pollen removal and increased heterospecific density decreased pollen deposition. In addition, increased conspecific density increased pollen deposition in both species. This study shows that plants may interact to facilitate or compete for different components of the pollination process, namely; pollinator attraction, pollen removal and deposition. Such interactions have immediate consequences for endangered plant species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success.     

Reproductive consequences of mate quantity versus mate diversity in a wind-pollinated plant

Since most pollen travels limited distances in wind-pollinated plants, both the local quantity and diversity of mates may limit female reproductive success. Yet little evidence exists on their relative contribution, despite the importance of viable seed production to population dynamics.To study how variation in female reproductive success is affected by the quantity versus the diversity of surrounding mates contributing pollen, we integrated pollination experiments, data on natural seed set and seed viability, and AFLP genetic marker data in the wind-pollinated dioecious clonal forest herb Mercurialis perennis.Pollination experiments indicated weak quantitative pollen limitation effects on seed set. Among-population crosses showed reduced seed viability, suggesting outbreeding depression due to genetic divergence. Pollination with pollen from a single source did not negatively affect reproductive success. These findings were consistent with results of the survey of natural female reproductive success. Seed set decreased with the distance to males in a female plants’ local neighborhood, suggesting a shortage of pollen in isolated female plants, and increased with the degree of local genetic diversity. Spatial isolation to other populations and population size did not affect seed set. None of these variables were related to seed viability.We conclude that pollen movement in M. perennis is likely very limited. Both male proximity and the local degree of genetic diversity influenced female reproductive success.     

Variation in pollinator assemblages in a fragmented landscape and its effects on reproductive stages of a self-incompatible treelet, <Emphasis Type="Italic">Psychotria suterella</Emphasis> (Rubiaceae)

Few studies of plant–pollinator interactions in fragmented landscapes evaluate the consequences of floral visitor variation on multiple stages of plant reproduction. Given that fragmentation potentially has positive or negative effects on different organisms, and that self-incompatible plant species depend on pollinators for sexual reproduction, differences in floral visitor assemblages may affect certain plant reproductive stages. We evaluated how pollinator assemblage, availability of floral resources, pollination, reproductive output, and seed and seedling performance of Psychotria suterella Muell. Arg. varied among three fragmentation categories: non-fragmented habitats, fragments connected by corridors, and isolated fragments. Richness and frequency of floral visitors were greater in fragments than in non-fragmented sites, resulting mainly from the addition of species typically found in disturbed areas. Although 24 species visited Psychotria suterella flowers, bumblebees were considered the most important pollinators, because they showed the highest frequency of visits and were present in eight out of ten sites. Additionally, the number of pollen tubes per flower per visit was lower in areas without bumblebees. The increased visitation in fragments seemed to enhance pollination slightly. However, fruit and seed output, germination, and seed and seedling mass were similar in non-fragmented sites, connected sites, and isolated fragments. Our results suggested that, even for a self-incompatible species, responses to habitat fragmentation at different stages of plant reproduction might be decoupled from the responses observed in floral visitors, if fruit set is not pollen limited. If all reproductive stages were considered, variation on the small scale was more important than the variation explained by fragmentation category. In spite of its self-incompatible breeding system, this plant–pollinator system showed resilience to habitat fragmentation, mainly as a result of high availability of potential mates to P. suterella individuals, absence of pollen limitation, and the presence of bumblebees (Bombus spp.) throughout this highly connected landscape.     

VARIATION IN MALE AND FEMALE REPRODUCTIVE SUCCESS AMONG FLORAL MORPHS IN THE TRISTYLOUS PLANT LYTHRUM SALICARIA (LYTHRACEAE)

The purpose of this study was to assess variation in male and female reproductive success among the three morphs of the tristylous plant, Lythrum salicaria. Fluorescent dyes were used as pollen analogs to determine whether morphs differ in their abilities to donate and receive pollen, and actual and potential seed set was measured with a hand pollination experiment. Dye transfer among morphs was highly asymmetric, with more frequent transfer from the short-styled morph to the long- and mid-styled morphs. This suggests that shorts are performing better at pollen donation and longs and mids are performing better at pollen receipt. All flowers on 95 plants were hand pollinated to test whether female reproductive success is more pollen-limited in the short-styled morph than in other morphs. Hand-pollinated short-styled plants had significantly higher total seed mass and more seeds per capsule than short controls, whereas hand pollination failed to increase seed set in long and mid morphs. As predicted, short-styled morphs showed significant pollen limitation, whereas seed set in long- and midstyled morphs was not pollen-limited. Thus, in Lythrum salicaria asymmetrical pollen flow generates morph-specific differences in male and female fitness.     

Relative abundance of an invasive alien plant affects native pollination processes

One major characteristic of invasive alien species is their occurrence at high abundances in their new habitat. Flowering invasive plant species that are visited by native insects and overlap with native plant species in their pollinators may facilitate or disrupt native flower visitation and fertilisation by forming large, dense populations with high numbers of flowers and copious rewards. We investigated the direction of such a proposed effect for the alien invasive Rhododendron ponticum in Irish habitats. Flower visitation, conspecific and alien pollen deposition, fruit and seed set were measured in a self-compatible native focal plant, Digitalis purpurea, and compared between field sites that contained different relative abundances of R. ponticum. Flower visitation was significantly lower at higher alien relative plant abundances than at lower abundances or in the absence of the alien. Native flowers experienced a significant decrease in conspecific pollen deposition with increasing alien abundance. Heterospecific pollen transfer was very low in all field sites but increased significantly with increasing relative R. ponticum abundance. However, lower flower visitation and lower conspecific pollen transfer did not alter reproductive success of D. purpurea. Our study shows that indirect interactions between alien and native plants for pollination can be modified by population characteristics (such as relative abundance) in a similar way as interactions among native plant species. In D. purpurea, only certain aspects of pollination and reproduction were affected by high alien abundances which is probably a result of high resilience due to a self-compatible breeding system. Native species that are more susceptible to pollen limitation are more likely to experience fitness disadvantages in habitats with high relative alien plant abundances.     

宁夏枸杞的花粉呈现式样及交配系统研究北大核心CSCD

被子植物的花回馈、雌雄蕊时空分离特征和花粉呈现式样等花部特征及传粉者效率会影响雄性适合度和有性繁殖过程。宁夏枸杞(Lycium barbarum)是中国宁夏、新疆、内蒙等干旱和半干旱地区分布的特有种。该研究对新疆喀什地区宁夏枸杞自然种群的花部综合征、花粉呈现式样、花回馈、传粉者行为与交配方式的关系进行观察及统计分析,以探讨其花粉逐步呈现的适应性及其在提高雌雄繁殖过程中的意义。结果表明:(1)宁夏枸杞的单花寿命为(4.07±0.15) d,而雄性持续时间(0.07±0.01 d)比雌性持续时间短(4±0.01 d);花寿命内雌蕊长度比雄蕊长,属柱头探出式异位类型;花粉呈现式样为不完全逐步呈现。(2)花寿命不同阶段花回馈间存在显著差异。(3)意大利蜜蜂(Apis mellifera)、熊蜂(Bombus sp.)、食蚜蝇(Syrphidae sp.)是宁夏枸杞在自然居群的主要访花者,其中熊蜂和食蚜蝇是主要传粉者,但传粉者效率低,属于高移出低沉积类型。(4)宁夏枸杞的花粉胚珠比(2 448.11±448.32)及授粉实验结果均表现出兼性异交特征;自发自交及人工自花授粉花的座果率及结籽率很低,属于自交不亲和类型;自然传粉花的座果率及结籽率比人工去雄异花低,存在其较高的花粉限制(40.71%)。研究发现,宁夏枸杞花部综合征表现出雌雄异位和花粉不完全逐步呈现式样,这是避免雌雄功能及雄蕊各花药间的干扰、减少花粉同步移出及保障其雄性适合度的有效途径,但自然居群的传粉者种类限制、传粉效率低以及自交不亲性是导致宁夏枸杞花粉限制及降低雌性繁殖成功的主要原因。     

Reproductive interactions mediated by flowering overlap in a temperate hummingbird–plant assemblage

Pollinator‐mediated competition through shared pollinators can lead to segregated flowering phenologies, but empirical evidence for the process responsible for this flowering pattern is sparse. During two flowering seasons, we examined whether increasing overlap in flowering phenology decreased conspecific pollination, increased heterospecific pollination, and depressed seed output in the seven species composing a hummingbird–plant assemblage from the temperate forest of southern South America. Overall trends were summarized using meta‐analysis. Despite prevailing negative associations, relations between phenological overlap and conspecific pollen receipt varied extensively among species and between years. Heterospecific pollen receipt was low and presumably of limited biological significance. However, our results supported the hypothesis that concurrent flowering promotes interspecific pollen transfer, after accounting for changes in the abundance of conspecific flowers. Seed output was consistently reduced during maximum phenological overlap during the first flowering season because of limited fruit set. Responses varied more during the second year, despite an overall negative trend among species. Relations between estimated effects of phenological overlap on pollination and seed output, however, provided mixed evidence that conspecific pollen loss during pollinator visits to foreign flowers increases pollen limitation. By flowering together, different plant species might benefit each other's pollination by increasing hummingbird recruitment at the landscape level. Nevertheless, our results are mostly consistent with the hypothesis of pollinator‐mediated competition shaping the segregated flowering pattern reported previously for this temperate plant assemblage. The mechanisms likely involve effects on male function, whereby pollen‐transport loss during heterospecific flower visits limit pollen export, and more variable effects on female function through pollen limitation.     

Disentangling the pollination system of the food-deceptive orchid Anacamptis longicornu (Orchidaceae): from breeding system to spatio-temporal variation in reproductive success

Abstract

The deceptive breeding system in the Orchidaceae family has been studied for decades to disentangle how pollination service can occur. Moreover, the interference of introduced herbivores on population development has been reported to affect the reproductive output in different plant species, including orchids. In this study, the breeding system of the food-deceptive species Anacamptis longicornu has been assessed. The spatio-temporal variation in both male and female reproductive fitness and its relationship with plant features were also analyzed. The possible herbivory effects were investigated by an exclusion experiment using two types of cages. The results confirm that this species is pollinator dependent and mostly allogamous but is also self-compatible. This species showed high values of cumulative inbreeding depression index for fruit set and seed viability parameters and was highly pollen-limited. Male fitness was always higher than female fitness between years and locations, and both parameters showed spatio-temporal variation. The use of cages excluded herbivores but, eventually, also excluded pollinators, so that both male and female reproductive success parameters decreased significantly in the complete exclusion treatment. The strong dependence of the species’ fitness on pollinator-mediated pollen transfer must be considered for the long-term conservation of A. longicornu.     

Female and male fitness of a sexually deceptive orchid with a narrow distribution area: from phenotypic traits to spatial distribution patterns

• The Orchidaceae family presents one of the most extravagant pollination mechanisms: deception. While many studies on reproductive success have been performed on food‐deception orchids, less have been performed on sexually deceptive orchids. Here, we focused on Ophrys balearica P. Delforge, an endemic orchid of the Balearic Islands, to study its reproductive ecology, the spatio‐temporal variation of its reproductive success and the individual (floral display and geospatial position) and population parameters (patch size, shape and density) that affect its reproductive success.
• We performed hand‐pollination experiments, along with the recording of floral display parameters and GPS position of over 1,100 individuals from seven populations in two consecutive years. We applied, for the first time, GIS tools to analyse the effects of individual’s position within the population on the reproductive success. Reproductive success was measured both in male (removed pollinia) and female (fruit set) fitness.
• The results confirm that this species is pollinator‐dependent and mostly allogamous, but also self‐compatible. This species showed high values for the cumulative inbreeding depression index and high pollen limitation. Male fitness was almost equal to female fitness between years and populations, and reproductive success exhibited huge spatio‐temporal variation.
• Although we did not find strong correlations between floral display and reproductive success, patches with low‐plant density and individuals in the external portion of the population showed significantly higher plant fitness. These findings must be considered in conservation actions for endangered orchid species, especially considering that most orchids are strongly dependent on pollinators for their species’ fitness.

     

Phylogenetic Community Context Influences Pollen Delivery to <Emphasis Type="Italic">Allium cernuum</Emphasis>

Few studies have examined how the number and identity of species in the neighbouring community influences the reproductive success of particular focal species. Pollen delivery, an important component of fitness of sexual plants, is a function of not just the floral traits of any particular individual, but of features of the population and community as it depends on pollinator abundance and preferences. Many pollinators in flowering communities will prefer patches with high floral abundance or diversity yet may exhibit lower floral constancy when more flowering species are present. Thus, pollination efficiency could increase or decrease with increased species richness and this will alter the selection pressures placed upon the floral traits (such as floral colour or reward) of any member of a particular community. Moreover, recent studies have indicated that plant-pollinator networks are phylogenetically structured (pollinators visit related plant species more than expected by chance) and this may be an important factor structuring flowering plant communities. Thus, the sheer number of species within a patch may be less important than the number of closely-related species. We investigate whether species richness or phylogenetic proximity of coflowering species influences the amount and proportion of conspecific pollen delivered to nodding onion, Allium cernuum, in fragment patches of Garry Oak meadows in South Western British Columbia, Canada. We find that pollen delivery depended upon the presence of close relatives far more than on species richness or population density, indicating a central role of the community structure on pollination in flowering plant communities. Insofar as pollen delivery relates to the relative number of seeds produced by members of the community, pollination may in turn determine the community structure of the next generation.     

Size-dependent sex allocation in a monocarpic perennial herb, <Emphasis Type="Italic">Cardiocrinum cordatum</Emphasis> (Liliaceae)

The size-dependent sex allocation model predicts that the relative resource allocation to female function often increases with plant size in animal-pollinated plants. If size effects on reproductive success vary depending on the environmental conditions, however, the size dependency may differ among populations. We tried to detect site-specific variation in size-dependent sex allocation of a monocarpic hermaphrodite with reference to light availability. Multiple flowers and fruits were sampled from the individuals of Cardiocrinum cordatum, a monocarpic understory herb, and pollen, ovule and seed production were measured with reference to the plant size in two populations. Furthermore, frequency and foraging behavior of pollinator visitation was observed. Ovule production per flower increased with plant size in both populations, while pollen production per flower increased with size only in the population under sparse canopy. Therefore, proportional allocation to male function decreased with plant size in the population under closed canopy, but did not change in the population under sparse canopy. Pollinators usually visited only one flower per plant, indicating the negligible geitonogamous pollination in this species. Although seed production under closed canopy was lower than that under sparse canopy, seed-set rate per flower and seed mass per fruit were independent of plant size in either of the populations. Size-dependent sex allocation in this species was site-specific, suggesting that not only resource storage before reproduction (i.e., plant size) but also resource availability of environment throughout the reproductive process (i.e., light availability) affect reproductive performance in this species.     

Phenotype–fitness relationships and pollen-transfer efficiency of five orchid species with different pollination strategies

Premise

Deceptive pollination, a fascinating mechanism that independently originated in several plant families for benefiting from pollinators without providing any reward, is particularly widespread among orchids. Pollination efficiency is crucial in orchids due to the aggregated pollen in a pollinarium, which facilitates pollen transfer and promotes cross-pollination as pollinators leave after being deceived.

Methods

In this study, we compiled data on reproductive ecology from five orchid species with different pollination strategies: three deceptive-strategy species (shelter imitation, food deception, sexual deception), one nectar-rewarding species, and one shelter-imitation but spontaneously selfing species. We aimed to compare the reproductive success (female fitness: fruit set; male fitness: pollinarium removal) and pollination efficiency of species representing these strategies. We also investigated pollen limitation and inbreeding depression among the pollination strategies.

Results

Male and female fitness were strongly correlated in all species but the spontaneously selfing species, which had high fruit set and low pollinarium removal. As expected, pollination efficiency was highest for the rewarding species and the sexually deceptive species. Rewarding species had no pollen limitation but did have high cumulative inbreeding depression; deceptive species had high pollen limitation and moderate inbreeding depression; and spontaneously selfing species did not have pollen limitation or inbreeding depression.

Conclusions

Pollinator response to deception is critical to maintain reproductive success and avoid inbreeding in orchid species with non-rewarding pollination strategies. Our findings contribute to a better understanding of the trade-offs associated with different pollination strategies in orchids and highlight the importance of pollination efficiency in orchids due to the pollinarium.

     

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

• Pollen‐pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels).
• In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope.
• Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination.
• Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

     

Stigma closure and re-opening in Oroxylum indicum (Bignoniaceae): Causes and consequences

The study of plant responses to touch, particularly the responses of leaves, stems, and roots, has a long history. By contrast, floral responses are relatively unexplored. Stigma closure is common in the Bignoniaceae, but the factors influencing it are not well understood. We investigated factors influencing stigma closure and reopening and its effects on pollen receipt in seven Oroxylum indicum trees near Hat Yai, Thailand. The effects of pressure, conspecific and heterospecific pollen, and pollen load (the amount of pollen deposited) on stigma behavior were examined in 270 flowers (of the total 430 flowers evaluated in the entire study). Pressure alone resulted in faster closure than did conspecific pollination and faster reopening than did heterospecific pollination. Stigmas never reopened after conspecific pollination. Pollen load had no effect on stigma behavior. Stigmas discriminated between conspecific and foreign pollen; they reopened only after pollination with the latter. A manipulative experiment revealed that stigma closure did not affect the number of conspecific pollen grains received. We also counted pollen tubes in styles that were either hand-supplemented with outcross conspecific pollen or open-pollinated. Pollen tube numbers were highest after light pollination (～900 grains), indicating that interference among pollen grains may occur after pollination with very heavy loads (>6000 grains). Possible fitness consequences of these responses are discussed.     

Mating system,outcrossing distance effects and pollen availability in the wind-pollinated treeline species Polylepis australis BITT. (Rosaceae)

Fragmentation may negatively affect plant fitness through pollen limitation and increased levels of inbreeding. Effects of fragmentation may vary with regard to life form and breeding system, and few studies exist for wind-pollinated trees. We examined the effects of hand-selfing, varying outcrossing distances and pollen addition on seed mass and germination rate of Polylepis australis BITT. (Rosaceae), a wind-pollinated treeline species endemic to Argentina. We also investigated pollen germination on the stigma and pollen tube growth to determine compatibility resulting from selfing and outcrossing. Selfing reduced seed germination rates with significant differences between open pollination and outcrosses at 30 km. In addition, we found a tendency for pollen germination and pollen tube growth to decrease following selfing. Between-fragment crosses resulted in a trend of higher reproductive output than within-fragment crosses, whereas values were similar between open pollination and between-fragment crosses. Pollen addition did not increase reproductive success neither in small nor in larger fragments. Our results suggest that highly isolated P. australis forests have a potential for inbreeding depression through selfing and within-fragment crosses. However, the results also indicate that pollen flow between P. australis forest fragments is still effective at the current fragmentation level, counteracting negative effects on seed quality resulting from reproductive isolation.