Allocation to reproduction following experimental defoliation in Platanthera bifolia (Orchidaceae)

The roles of herbivory and pollination success in plant reproduction have frequently been examined, but interactions between these two factors have gained much less attention. In three field experiments, we examined whether artificial defoliation affects allocation to attractiveness to pollinators, pollen production, female reproductive success and subsequent growth in Platanthera bifolia L. (Rich.). We also recorded the effects of inflorescence size on these variables. We studied the effects of defoliation on reproductive success of individual flowers in three sections of inflorescence. Defoliation and inflorescence size did not have any negative effects on the proportion of opened flowers, spur length, nectar production or the weight of pollinia. However, we found that hand-pollination increased relative seed production and defoliation decreased seed set in most cases. Interactions between hand-pollination and defoliation were non-significant indicating that defoliation did not affect female reproductive success indirectly via decreased pollinator attraction. Plants with a large inflorescence produced relatively more seeds than plants with a small inflorescence only after hand-pollination. The negative effect of defoliation on relative capsule production was most clearly seen in the upper sections of the inflorescence. In addition to within season effects of leaf removal, defoliated P. bifolia plants may also have decreased lifetime fitness as a result of lower seed set within a season and because of a lower number of reproductive events due to decreased plant size (leaf area) following defoliation. Our study thus shows that defoliation by herbivores may crucially affect reproductive success of P. bifolia.     

Floral sex ratios and gynomonoecy in Solidago (Asteraceae)

Gynomonoecy is the sexual system in which individual plants bear both female and bisexual flowers. Little attention has been paid to the adaptive significance of this sexual system, which is particularly prevalent in the Asteraceae. We investigated one hypothesized advantage of having two flower types, namely that this arrangement permits flexibility in allocation of resources to male and female reproductive functions. We examined six species of goldenrod ( Solidago ), a genus of gynomonoecious, perennial herbs. In greenhouse experiments, we varied one or more of three environmental variables – light, nutrients and water – and/or examined heads in different positions on the plants. Most variables had little or no effect on the proportion of ray flowers. Significant effects were found for light in 0 of 5 experiments, for nutrients in 4 of 9 experiments and for water in 0 of 3 experiments. Heads in different positions in the inflorescence differed in the proportion of ray flowers in half of the experiments, though the differences were small. We also monitored temporal patterns in four species and found that the proportion of ray flowers increased significantly over the blooming period and the number of flowers per head declined. Because of the small number of significant effects and their modest magnitude, we conclude that the presence of two flower types in goldenrods is probably not advantageous in allowing flexibility in sex expression. It seems likely that this sexual system has been more important either in increasing pollinator attraction or in reducing pollen–pistil interference. The small observed changes in floral ratios were generally accompanied by changes in disc size in a manner consistent with an explanation based on allometry.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 413–422.     

Plant–pollinator population dynamics

We formulate and analyze a multi-generation population dynamics model for pollinators’ mutualism with plants. The centerpiece of our model is an analytical expression for population-level plant–pollinator interactions extrapolated from a model of individual-level flowers and bees interactions. We also show that this analytical expression can be productively approximated by the Beddington–DeAngelis formula—a function used to model trophic interactions in mathematical ecology.     

Influence of inflorescence size on sexual expression and female reproductive success in a monoecious species

Sex allocation theory forecasts that larger plant size may modify the balance in fitness gain in both genders, leading to uneven optimal male and female allocation. This reasoning can be applied to flowers and inflorescences, because the increase in flower or inflorescence size can differentially benefit different gender functions, and thus favour preferential allocation to specific floral structures. We investigated how inflorescence size influenced sexual expression and female reproductive success in the monoecious Tussilago farfara, by measuring patterns of biomass, and N and P allocation. Inflorescences of T.?farfara showed broad variation in sex expression and, according to expectations, allocation to different sexual structures showed an allometric pattern. Unexpectedly, two studied populations had a contrasting pattern of sex allocation with an increase in inflorescence size. In a shaded site, larger inflorescences were female-biased and had disproportionately more allocation to attraction structures; while in an open site, larger inflorescences were male-biased. Female reproductive success was higher in larger, showier inflorescences. Surprisingly, male flowers positively influenced female reproductive success. These allometric patterns were not easily interpretable as a result of pollen limitation when na?vely assuming an unequivocal relationship between structure and function for the inflorescence structures. In this and other Asteraceae, where inflorescences are the pollination unit, both male and female flowers can play a role in pollinator attraction.     

Survival and reproduction of plant species in the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) is the highest and largest plateau in the world. It covers correspondingly wide geological, topographical, and climatic gradients, and thus hosts greater biodiversity than surrounding lowlands and other high elevation regions. Due to its extreme environmental and biological diversity, the QTP is an ideal region for studying adaptations of plant species under harsh environmental conditions at multiple evolutionary levels. Many recent ecological studies have revealed functions of distinctive morphological features of various plants in the region that improve their reproductive success. Examples include large and showy bracts, hairy inflorescences, and drooping flowers. Numerous other investigations have examined QTP plants' sexual systems, patterns of biomass allocations, and biotic interactions. This paper summarizes recent advances in understanding of morphological adaptations, plant–plant interactions, plant–pollinator interactions, floral color patterns, pollination adaptations, and resource allocation patterns of alpine plants of the QTP. The overall aim is to synthesize current knowledge of the general mechanisms of plant survival and reproduction in this fascinating region.     

Fragmentation Effects on Diversity of Wasp Community and Its Impact on Fig/Fig Wasp Interaction in Ficus racemosa L.

Habitat fragmentation usually results in alteration of species composition or biological communities. However, little is known about the effect of habitat fragmentation on the fig/fig wasp system.In this study, we compared the structure of a fig wasp community and the interaction between figs and fig wasps of Ficus racemosa L. in a primary forest, a locally fragmented forest and a highly fragmented forest.Our results show that, in the highly fragmented forest, the proportion of pollinator wasps is lower and the proportion of non-pollinator wasps is higher compared with the primary forest and locally fragmented forest. The proportion of fruits without pollinator wasps in mature fruits is also greatly increased in the highly fragmented forest. The proportion of galls in all female flowers increases in the highly fragmented forest, whereas the proportion of viable seeds does not change considerably. The disruption of groups of fig trees results in a decrease in pollinator wasps and even might result in the extinction of pollinator wasps in some extreme cases, which may transform the reciprocal interaction between figs and fig wasps into a parasite/host system. Such an effect may lead to the local extinction of this keystone plant resource of rain forests in the process of evolution, and thereby, may change the structure and function of the tropical rain forest.     

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.     

Fragmentation Effects on Diversity of Wasp Community and Its Impact on Fig／Fig Wasp Interaction in Ficus racemosa L.

Habitat fragmentation usually results in alteration of species composition or biological communities. However, little is known about the effect of habitat fragmentation on the fig/fig wasp system.In this study, we compared the structure of a fig wasp community and the interaction between figs and fig wasps of Ficus racemosa L. in a primary forest, a locally fragmented forest and a highly fragmented forest.Our results show that, in the highly fragmented forest, the proportion of pollinator wasps is lower and the proportion of non-pollinator wasps is higher compared with the primary forest and locally fragmented forest. The proportion of fruits without pollinator wasps in mature fruits is also greatly increased in the highly fragmented forest. The proportion of galls in all female flowers increases in the highly fragmented forest, whereas the proportion of viable seeds does not change considerably. The disruption of groups of fig trees results in a decrease in pollinator wasps and even might result in the extinction of pollinator wasps in some extreme cases, which may transform the reciprocal interaction between figs and fig wasps into a parasite/host system. Such an effect may lead to the local extinction of this keystone plant resource of rain forests in the process of evolution, and thereby, may change the structure and function of the tropical rain forest.     

Effect of defoliation on leaf growth,sexual expression and reproductive success of Cnidoscolus aconitifolius (Euphorbiaceae)

Several studies on herbivory have shown that plants may have different responses to foliar damage. Most studies have been done using hermaphroditic species, here we present, using a defoliation experiment, different types of response to foliar damage by the monoecious Cnidosculus acontifolius. We evaluated four treatments: natural herbivory (control), and 25%, 50% and 100% artificial defoliation. The results suggest that this species is capable of showing the following compensatory responses: (a) undercompensation in total production of flowers and in fruit set (damage >25%); (b) full compensation in seed weight (all levels of damage); and (c) overcompensation in leaf growth and in the proportion of female flower production (damage 25%). The results show that C. aconitifolius can exhibit a very fast and plastic response depending on the intensity of damage and the time since defoliation. The increase in foliar growth suggests a long-term tolerance response to hervibory, whereas the change in the proportion of female flowers produced does not appear to have a long-term effect due to the decrease of fruit production in all levels of damage. We suggest that the response in foliar growth and the change in sexual expression by C. aconitifolius, are associated with the high resource availability conditions prevalent in the population studied.     

Sexes show differential tolerance to spittlebug damage and consequences of damage for multi-species interactions

Antagonists can play a role in sexual system evolution if tolerance or resistance is sex-dependent. Our understanding of this role will be enhanced by consideration of the effects of antagonists on other plant-animal interactions. This study determined whether the sex morphs of a gynodioecious Fragaria virginiana differ in their susceptibility and response to damage by spittlebugs and whether damage altered pollinator attraction traits or interactions with other antagonists. Tolerance, but not resistance, to spittlebugs differed between the sexes. Generally, spittlebugs were more damaging to hermaphrodites than females, a finding in accord with the hypothesis that the pollen-bearing morph is less tolerant of source-damage than the pollen-sterile morph when damage is incurred during flowering. In both sex morphs, spittlebugs reduced inflorescence height, increased petal size, but did not affect the number of open flowers per day, suggesting that the net effect of damage may be to increase pollinator attraction. Spittlebug infestation modified interactions with other antagonists in a sex-dependent manner: spittlebugs reduced attack by bud-clipping weevils in hermaphrodites but increased infection by leaf fungi in females. The complex interactions between plant sex, antagonists, and pollinator attraction documented here emphasize the importance of considering sex-differential multi-species interactions in plant sexual evolution.     

Costs of inflorescence longevity for an Asian fig tree and its pollinator

In ecological situations where interactions between two species are to their mutual benefit, traits are expected to evolve to maximise the value of the timing of their encounters. Plants that depend on animals for pollination vary in the longevity of their flowers and also in how the quality of the rewards they offer varies in flowers of different ages. However, costs of floral longevity are rarely studied. Using field experiments with Ficus semicordata, a SE Asian dioecious fig tree, we examined sexual differences in syconium ageing, how the reproductive success of the plant and its pollinator change with syconium age and whether these changes are reflected in pollinator preferences. Un-pollinated syconia remained receptive to their host-specific pollinators for long periods, but eventually abort. Compared with male syconia, un-pollinated female syconia aborted more quickly and lost their ability to attract pollinators more quickly. Older pollinated female syconia were also more likely to abort. Further, declines in productivity with syconium age were also more apparent in female syconia, though older male syconia also produced fewer, smaller wasp offspring. The longevity costs are reflected in pollinator preferences. This suggests that sexual differences in duration of receptivity may be adaptive and a component of the reproductive strategies. It also indicates that placing fig wasps onto older syconia over-estimates their likelihood of being pollinated under natural conditions and prolonged receptivity increases the likelihood of pollination at the cost of reduced productivity with syconium age. This opens interesting perspectives on the co-evolution of this inter-specific interaction.     

Experimental defoliation affects male but not female reproductive performance of the tropical monoecious plant Croton suberosus (Euphorbiaceae)

Background and Aims

Monoecious plants have the capacity to allocate resources separately to male and female functions more easily than hermaphrodites. This can be advantageous against environmental stresses such as leaf herbivory. However, studies showing effects of herbivory on male and female functions and on the interaction with the plant''s pollinators are limited, particularly in tropical plants. Here, the effects of experimental defoliation were examined in the monoecious shrub Croton suberosus (Euphorbiaceae), a wasp-pollinated species from a Mexican tropical dry forest.

Methods

Three defoliation treatments were applied: 0 % (control), 25 % (low) or 75 % (high) of plant leaf area removed. Vegetative (production of new leaves) and reproductive (pistillate and staminate flower production, pollen viability, nectar production, fruit set, and seed set) performance variables, and the abundance and activity of floral visitors were examined.

Key Results

Defoliated plants overcompensated for tissue loss by producing more new leaves than control plants. Production of staminate flowers gradually decreased with increasing defoliation and the floral sex ratio (staminate : pistillate flowers) was drastically reduced in high-defoliation plants. In contrast, female reproductive performance (pistillate flower production, fruit set and seed set) and pollinator visitation and abundance were not impacted by defoliation.

Conclusions

The asymmetrical effects of defoliation on male and female traits of C. suberosus may be due to the temporal and spatial flexibility in the allocation of resources deployed by monoecious plants. We posit that this helps to maintain the plant''s pollination success in the face of leaf herbivory stress.     

Pollinator responses to variation in floral display and flower size in dioecious Sagittaria latifolia (Alismataceae)

In animal-pollinated plants with unisexual flowers, sexual dimorphism in floral traits may be the consequence of pollinator-mediated selection. Experimental investigations of the effects of variation in flower size and floral display on pollinator visitation can provide insights into the evolution of floral dimorphism in dioecious plants. Here, we investigated pollinator responses to experimental arrays of dioecious Sagittaria latifolia in which we manipulated floral display and flower size. We also examined whether there were changes in pollinator visitation with increasing dimorphism in flower size. In S. latifolia, males have larger flowers and smaller floral displays than females. Visitation by pollinators, mainly flies and bees, was more frequent for male than for female inflorescences and increased with increasing flower size, regardless of sex. The number of insect visits per flower decreased with increasing floral display in males but remained constant in females. Greater sexual dimorphism in flower size increased visits to male inflorescences but had no influence on the number of visits to female inflorescences. These results suggest that larger flower sizes would be advantageous to both females and males, and no evidence was found that females suffer from increased flower-size dimorphism. Small daily floral displays may benefit males by allowing extended flowering periods and greater opportunities for effective pollen dispersal.     

Plant–pollinator networks: adding the pollinator's perspective

Pollination network studies are based on pollinator surveys conducted on focal plants. This plant-centred approach provides insufficient information on flower visitation habits of rare pollinator species, which are the majority in pollinator communities. As a result, pollination networks contain very high proportions of pollinator species linked to a single plant species (extreme specialists), a pattern that contrasts with the widely accepted view that plant–pollinator interactions are mostly generalized. In this study of a Mediterranean scrubland community in NE Spain we supplement data from an intensive field survey with the analysis of pollen loads carried by pollinators. We observed 4265 contacts corresponding to 19 plant and 122 pollinator species. The addition of pollen data unveiled a very significant number of interactions, resulting in important network structural changes. Connectance increased 1.43-fold, mean plant connectivity went from 18.5 to 26.4, and mean pollinator connectivity from 2.9 to 4.1. Extreme specialist pollinator species decreased 0.6-fold, suggesting that ecological specialization is often overestimated in plant–pollinator networks. We expected a greater connectivity increase in rare species, and consequently a decrease in the level of asymmetric specialization. However, new links preferentially attached to already highly connected nodes and, as a result, both nestedness and centralization increased. The addition of pollen data revealed the existence of four clearly defined modules that were not apparent when only field survey data were used. Three of these modules had a strong phenological component. In comparison to other pollination webs, our network had a high proportion of connector links and species. That is, although significant, the four modules were far from isolated.     

Variation in inflorescence size in a dioecious fig tree and its consequences for the plant and its pollinator fig wasp

The host-specific relationship between fig trees (Ficus) and their pollinator wasps (Agaonidae) is a classic case of obligate mutualism. Pollinators reproduce within highly specialised inflorescences (figs) of fig trees that depend on the pollinator offspring for the dispersal of their pollen. About half of all fig trees are functionally dioecious, with separate male and female plants responsible for separate sexual functions. Pollen and the fig wasps that disperse it are produced within male figs, whereas female figs produce only seeds. Figs vary greatly in size between different species, with female flower numbers varying from tens to many thousands. Within species, the number of female flowers present in each fig is potentially a major determinant of the numbers of pollinator offspring and seeds produced. We recorded variation in female flower numbers within male and female figs of the dioecious Ficus montana growing under controlled conditions, and assessed the sources and consequences of inflorescence size variation for the reproductive success of the plants and their pollinator (Kradibia tentacularis). Female flower numbers varied greatly within and between plants, as did the reproductive success of the plants, and their pollinators. The numbers of pollinator offspring in male figs and seeds in female figs were positively correlated with female flower numbers, but the numbers of male flowers and a parasitoid of the pollinator were not. The significant variation in flower number among figs produced by different individuals growing under uniform conditions indicates that there is a genetic influence on inflorescence size and that this character may be subject to selection.     

SEX POLYMORPHISM IN SILENE ACAULIS (CARYOPHYLLACEAE) AND THE POSSIBLE ROLE OF SEXUAL SELECTION IN MAINTAINING FEMALES

Pollen accumulation rates and their relationship to stigma morphology and pollinator visitation behavior were compared between female and hermaphrodite sex morphs of the alpine plant Silene acaulis var. subacaulescens (Caryophyllaceae). Although stigmas of female and perfect flowers collected comparable pollen loads, stigmas of female flowers became receptive earlier in anthesis and therefore recruited a larger number of pollen tubes. Together with early receptivity young female flowers had a larger stigmatic area with longer papillae than did perfect flowers. Pollinator behavior also differed between morphs, with bumble bees spending more time probing female than perfect flowers. Differences in stigma receptivity schedules of female and perfect flowers have consequences for different opportunities for sexual selection in the two sex morphs. Female flowers, by providing a more effective gametophytic screen, have the potential to produce higher quality offspring. This is proposed as a further compensatory advantage maintaining females, with a single fitness function, in populations containing hermaphrodites which have both male and female fitness functions.     

Fine-scale temporal dynamics of flower visitors sheds light on insect-assemblage overlap between sexes in a dioecious Ecuadorian palm

Dioecious plants generally display sexual dimorphism in male and female floral traits, potentially attracting slightly different pollinator communities. The sharing of common floral visitors between male and female flowers and their timing of visits to both sexes is of critical importance to ensure plant's reproductive success. Palm inflorescences are visited by abundant and diverse insect communities, yet the temporal patterns of insect visits on both sexes remain poorly known. We characterized the composition of a community of flower-visiting arthropods associated with the dioecious ivory palm (Phytelephas aequatorialis, Spruce) in a pre-montane forest of Ecuador. We monitored the temporal variations in insect visits along the flowering of 12 inflorescences (eight female and four male) using interception traps recovered every 4 h. We report 59 morphospecies in the arthropod community, dominated by three beetle families: Staphylinidae, Nitidulidae, and Curculionidae. Male inflorescences were more abundantly visited than female, but visitors of the later were taxonomically more diverse. Among the 16 pollinator candidates identified, nine visited both inflorescence sexes synchronously at dusk /night whereas the others did so asynchronously during the day. Our study provides new insights into the pollination mechanism of P. aequatorialis. We found evidence of differential pollinator attraction between floral sexes, which may be explained by the sexual dimorphism of both flowers. Synchronicity in dusk/night visits of both inflorescence sexes suggests a sexual synchronization of the signal used to attract pollinators.     

SEX ALLOCATION IN THE MONOECIOUS HERB BEGONIA SEMIOVATA

Sex-allocation models predict that the evolution of self-fertilization should result in a reduced allocation to male function and pollinator attraction in plants. The evolution of sex allocation may be constrained by both functional and genetic factors, however. We studied sex allocation and genetic variation for floral sex ratio and other reproductive traits in a Costa Rica population of the monoecious, highly selfing annual Begonia semiovata. Data on biomass of floral structures, flower sex ratios, and fruit set in the source population were used to calculate the average proportion of reproductive allocation invested in male function. Genetic variation and genetic correlations for floral sex ratio and for floral traits related to male and female function were estimated from the greenhouse-grown progeny of field-collected maternal families. The proportion of reproductive biomass invested in male function was low (0.34 at flowering, and 0.07 for total reproductive allocation). Significant among-family variation was detected in the size (mass) of individual male and female flowers, in the proportion of male flowers produced, and in the proportion of total flower mass invested in male flowers. Significant among-family variation was also found in flower number per inflorescence, petal length of male and female flowers, and petal number of female flowers. Except for female petal length, we found no difference in the mean value of these characters between selfed and outcrossed progeny, indicating that, with the possible exception of female petal length, the among-family variation detected was not the result of variation among families in the level of inbreeding. Significant positive phenotypic and broad-sense genetic correlations were detected between the mass of individual male and female flowers, between male and female petal length, and between number of male and number of female flowers per inflorescence. The ratio of stamen-to-pistil mass (0.33) was low compared to published data for autogamous species with hermaphroditic flowers, suggesting that highly efficient selfing mechanisms may evolve in monoecious species. Our results indicate that the study population harbors substantial genetic variation for reproductive characters. The positive genetic correlation between investment in male and female flowers may reflect selection for maximum pollination efficiency, because in this self-pollinating species, each female flower requires a neighboring male flower to provide pollen.     

Yield improvement through female homosexual hybrids and sex genetics of sweet gourd (<Emphasis Type="Italic">Momordica cochinchinensis</Emphasis> Spreng.)

The present research aimed to evaluate the effect of silver nitrate (AgNO₃) on sex modification in sweet gourd (Momordica cochinchinensis Spreng.), and to explore the possibility of sexual crossing between two genetically female plants. Spray applications of AgNO₃ on 30 days’ old female plants induced hermaphrodite flowers. Male plants were insensitive to the AgNO₃ sprays. Application of 500 mg/l AgNO₃ on female plants produced the maximum proportion of induced hermaphrodite flowers. Hermaphrodite flowers appeared 17–21 days after AgNO₃ spray and continued up to 8–17 days, depending upon the concentration of AgNO₃. Pollen grain viability of induced hermaphrodite flowers (93.5%) was similar to pollen grain viability of normal male (95.0%) flowers. Because of higher fruit weight, progenies from female homosexual cross recorded higher yield. The hybrids from such crosses produced only female plants while the hybrids between female and normal male segregated into male and female in an equal proportion, indicating that sex in sweet gourd is controlled by a single factor, male being heterozygous and female being homozygous recessive. Through this technique, elite characters of female genotypes could be combined into a single plant.     

Novel nectar robbing negatively affects reproduction in Digitalis purpurea

With many plant–pollinator interactions undergoing change as species’ distributions shift, we require a better understanding of how the addition of new interacting partners can affect plant reproduction. One such group of floral visitors, nectar robbers, can deplete plants of nectar rewards without contributing to pollination. The addition of nectar robbing to the floral visitor assemblage could therefore have costs to the plant´s reproductive output. We focus on a recent plant colonist, Digitalis purpurea, a plant that in its native range is rarely robbed, but experiences intense nectar robbing in areas it has been introduced to. Here, we test the costs to reproduction following experimental nectar robbing. To identify any changes in the behavior of the principal pollinators in response to nectar robbing, we measured visitation rates, visit duration, proportion of flowers visited, and rate of rejection of inflorescences. To find the effects of robbing on fitness, we used proxies for female and male components of reproductive output, by measuring the seeds produced per fruit and the pollen export, respectively. Nectar robbing significantly reduced the rate of visitation and lengths of visits by bumblebees. Additionally, bumblebees visited a lower proportion of flowers on an inflorescence that had robbed flowers. We found that flowers in the robbed treatment produced significantly fewer seeds per fruit on average but did not export fewer pollen grains. Our finding that robbing leads to reduced seed production could be due to fewer and shorter visits to flowers leading to less effective pollination. We discuss the potential consequences of new pollinator environments, such as exposure to nectar robbing, for plant reproduction.