EFFECTS OF POLLINATION INTENSITY IN CAMPSIS RADICANS

I examined effects of pollination intensity on fruit, seed and seedling characteristics in Campsis radicans, and joint effects of pollen donor and pollination intensity on fruit production. Large pollen loads were more likely to initiate fruit production than small pollen loads, and the former fruits contained more seeds and a greater total seed mass. No further increases in seed number or mass occurred for pollen loads above 4,000 grains. The weight of individual seeds was unaffected by pollen load. Effects of pollen donor were generally larger than effects of pollen load, and fruit production from small loads of pollen from one donor were sometimes equal to fruit production from larger pollen loads from another donor. The ratio of pollen grains deposited to resultant seeds increased with pollen load, and several explanations are proposed. Seeds from heavy pollinations emerged better than seeds from light pollinations, but did not differ in speed of germination or in the performance of seedlings up to 126 days. The emergence differences are probably due to differing intensities of pollen tube competition.     

Seed germination patterns of Salvia mellifera in fire-prone environments

Summary I examined the effects of pollen loads containing pollen from one, three and five donors on fruit production and fruit quality in Campsis radicans. Number of pollen donors had no significant effect on % fruit production, seed number, seed weight or seed germination. In singledonor pollinations the identity of the donor did have a strong effect on the above parameters. Furthermore, the best single donor sired fruits with more seeds and heavier seeds than any mixture containing this donor. This pattern indicates interference of pollens or preemption of some ovules by the inferior pollen. In Campsis, therefore, the number of pollen donors contributing to a pollen load is less important than the identity of these donors in determining fruit production and fruit quality. Seeds from fruits resulting from mixed pollination were slightly more variable than seeds from fruits resulting from single-donor pollinations.     

Reproductive versatility in legumes: the case of amphicarpy in Trifolium polymorphum

Amphicarpy is a fascinating reproductive strategy, defined as fruit produced both below the soil surface and as aerial fruit on the same plant. Trifolium polymorphum is a grassland species subject to herbivory that combines amphicarpy with vegetative reproduction through stolons. Underground flowers have been described as obligate autogamous and aerial ones as self‐compatible allogamous, with aerial floral traits favouring cross‐pollination. In the present work we performed different pollination treatments on aerial flowers to analyse rates of pollen tube development and offspring fitness, measured as fruit set, seed production and germination percentage. This last variable was compared to that of seeds produced underground. No significant differences were found between fruit set in self‐ and cross‐pollinations. Seed production was higher in self‐pollinations, which is consistent with the higher rate of pollen tube development observed in self‐crosses. Spontaneous self‐pollination is limited in aerial flowers; thus pollen transfer by means of a vector is required even within the same flower. Germination tests showed that aerial seeds produced after self‐ and cross‐pollination did not differ in fitness, but underground seeds had higher germination percentage than aerial ones. Thus, we conclude that T. polymorphum has a mixed mating system. In grasslands with heavy grazing pressure, clonal propagation and underground seed production ensure persistence in the field. An intermediate level of selfing in aerial flowers ensures offspring, but morphological (herkogamy) and functional (dicogamy) floral traits maintain a window to incorporate genetic variability, allowing the species to tolerate temporal and spatial pressures.     

Pollen competition as a reproductive isolating mechanism between two sympatric Hibiscus species

Differences in pollen tube growth rates (certation) between heterospecific (foreign) and conspecific pollen may strongly influence whether hybrid offspring are produced after mixed pollen loads are delivered to a stigma. For both members of a sympatric species pair, Hibiscus moscheutos and H. laevis, pollination by pure loads of foreign pollen resulted in fruit set that was not significantly different from conspecific pollination, indicating that pure loads of foreign pollen could readily result in hybrid offspring. However, the number of seeds per fruit from pure foreign pollinations was significantly less than that of pure conspecific pollination. Simultaneous mixed pollination resulted in a proportion of hybrid seeds (detected by an electrophoretic marker enzyme) that was significantly lower than expected based upon the capacity of foreign pollen to effect fertilization when applied in pure pollinations. After these 50/50% pollen mixtures were applied to stigmas, 8.0 and 7.4% hybrids were produced when H. moscheutos and H. laevis were the ovule parents, respectively. For these Hibiscus species, pollen competition appears to function as a barrier to hybridization that is of moderate intensity compared with similar barriers occurring between other recently studied sympatric species pairs.     

Mixed pollen load and late‐acting self‐incompatibility flexibility in Adenocalymma peregrinum (Miers) L.G. Lohmann (Bignonieae: Bignoniaceae)

• Mixed cross and self‐pollen load on the stigma (mixed pollination) of species with late‐acting self‐incompatibility system (LSI) can lead to self‐fertilized seed production. This “cryptic self‐fertility” may allow selfed seedling development in species otherwise largely self‐sterile. Our aims were to check if mixed pollinations would lead to fruit set in LSI Adenocalymma peregrinum, and test for evidence of early‐acting inbreeding depression in putative selfed seeds from mixed pollinations.
• Experimental pollinations were carried out in a natural population. Fruit and seed set from self‐, cross and mixed pollinations were analysed. Further germination tests were carried out for the seeds obtained from treatments.
• Our results confirm self‐incompatibility, and fruit set from cross‐pollinations was three‐fold that from mixed pollinations. This low fruit set in mixed pollinations is most likely due to a greater number of self‐ than cross‐fertilized ovules, which promotes LSI action and pistil abortion. Likewise, higher percentage of empty seeds in surviving fruits from mixed pollinations compared with cross‐pollinations is probably due to ovule discounting caused by self‐fertilization. Moreover, germinability of seeds with developed embryos was lower in fruits from mixed than from cross‐pollinations, and the non‐viable seeds from mixed pollinations showed one‐third of the mass of those from cross‐pollinations.
• The great number of empty seeds, lower germinability, lower mass of non‐viable seeds, and higher variation in seed mass distribution in mixed pollinations, strongly suggests early‐acing inbreeding depression in putative selfed seeds. In this sense, LSI and inbreeding depression acting together probably constrain self‐fertilized seedling establishment in A. peregrinum.

     

Pollen tube growth in the self‐compatible sweet cherry genotype, ‘Cristobalina’, is slowed down after self‐pollination

Sweet cherry is a self‐incompatible fruit tree species in the Rosaceae. As other species in the family, sweet cherry exhibits S‐RNase‐based gametophytic self‐incompatibility. This mechanism is genetically determined by the S‐locus that encodes the pollen and pistil determinants, SFB and S‐RNase, respectively. Several self‐compatible sweet cherry genotypes have been described and most of them have mutations at the S‐locus leading to self‐compatibility. However, ‘Cristobalina’ sweet cherry is self‐compatible due to a mutation in a pollen function modifier that is not linked to the S‐locus. To investigate the physiology of self‐compatibility in this cultivar, S‐locus segregation in crosses involving ‘Cristobalina’ pollen, and pollen tube growth in self‐ and cross‐pollinations, were studied. In the crosses with genotypes sharing only one S‐haplotype, the non‐self S‐haplotype was inherited more frequently than the self S‐haplotype. Pollen tube growth studies revealed that the time to travel the whole length of the style was longer for self‐pollen tubes than for cross‐pollen tubes. Together, these results suggest that ‘Cristobalina’ pollen tube growth is slower after self‐pollination than after cross‐pollination. This reproductive strategy would allow self‐fertilisation in the absence of compatible pollen but would promote cross‐fertilisation if cross‐compatible pollen is available, a possible case of cryptic self‐incompatibility. This bet‐hedging strategy might be advantageous for an ecotype that is native to the mountains of the Spanish Mediterranean coast, in the geographical limits of the distribution of this species. ‘Cristobalina’ blooming takes place very early in the season when mating possibilities are scarce and, consequently, self‐compatibility may be the only possibility for this genotype to produce offspring.     

Pollination biology of the hexaploid self‐compatible species Turnera velutina (Passifloraceae)

• The evolution of monomorphisms from heterostylous ancestors has been related to the presence of homostyly and the loss of self‐incompatibility, allowing the occurrence of selfing, which could be advantageous under pollinator limitation. However, flowers of some monomorphic species show herkogamy, attraction and rewarding traits that presumably favour cross‐pollination and/or a mixed mating system. This study evaluated the contributions of pollinators, breeding system and floral traits to the reproduction of Turnera velutina, a herkogamous monomorphic species.
• Floral visitors and frequency of visits were recorded, controlled hand cross‐pollinations were conducted under greenhouse and natural conditions, and individual variation in floral traits was characterised to determine their contribution to seed production.
• Apis mellifera was the most frequent floral visitor. Flowers presented approach herkogamy, high variation in nectar features, and a positive correlation of floral length with nectar volume and sugar concentration. Seed production did not differ between manual self‐ and cross‐pollinations, controls or open cross‐pollinations, but autonomous self‐pollination produced, on average, 82.74% fewer seeds than the other forms, irrespective of the level of herkogamy.
• Differences in seed production among autonomous self‐pollination and other treatments showed that T. velutina flowers depend on insect pollination for reproduction, and that approach herkogamy drastically reduced seed production in the absence of pollen vectors. The lack of differences in seed production from manual cross‐ and self‐pollinations suggests the possible presence of a mixed mating system in the studied population. Overall, this species was possibly derived from a distylous ancestor but appears fully capable of outcrossing despite being monomorphic.

     

Self-pollen interference is absent in wild radish (Raphanus raphanistrum, Brassicaceae), a species with sporophytic self-incompatibility

Explaining the diversity of mating systems and floral forms in flowering plants is a long-standing concern of evolutionary biologists. One topic of interest is the conditions under which self-pollination can interfere with seed set for flowering plants with a self-incompatibility system. We investigated the effect of self-pollen interference for wild radish, Raphanus raphanistrum, which has sporophytic self-incompatibility. We performed pollinations and determined seed set for plants grown in the greenhouse, using pollen mixtures representing either self- with outcross-pollen or outcross-pollen alone. Stigmas were collected for a subset of pollinated flowers to determine the number of pollen grains applied. Average seed set for the self/cross (5.13 seeds/pollination) and cross treatments (5.09 seeds/pollination) did not differ significantly. Stigmatic pollen loads averaged around 700 grains, an amount close to observed natural pollen loads on R. raphanistrum. We concluded that for R. raphanistrum in natural populations, self-pollen is unlikely to interfere with outcross-pollen success. This study is the first to investigate effects of self-pollen interference on seed set for a homomorphic species with sporophytic self-incompatibility where rejection occurs at the stigmatic surface.     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

INDIVIDUAL VARIATION IN THE VIGOR OF SELF POLLEN AND SELFED PROGENY IN HIBISCUS MOSCHEUTOS (MALVACEAE)

In some self-compatible species, self pollen tubes grow more slowly than outcross pollen, presumably leading to low selfing rates when mixtures of self and outcross pollen reach the stigma simultaneously. Here we show that the competitive ability of self pollen differed among individuals of Hibiscus moscheutos. Self pollen tubes grew slower than outcross pollen in three plants, faster than outcross pollen in four plants, and showed no difference in five other plants (based on rates of callose plug formation). Levels of inbreeding depression were examined by comparing progeny from self and outcross pollinations in seven maternal families. Self pollination led to reduced seed number in only one maternal family, and a slight decrease in seed size was seen in two maternal families. Considerable inbreeding depression occurred later in the life cycle, and the degree of inbreeding depression varied among maternal families of 6-week-old plants. Our results demonstrate the potential for unpredictable effects of pollen competition on individual selfing rates, which in turn may affect progeny vigor. This complex situation contrasts with previous reports of species in which outcross pollen consistently outcompetes self pollen (cryptic self-incompatibility).     

Early-acting inbreeding depression in three species of Vaccinium (Ericaceae)

Greenhouse pollinations were performed to determine whether early-acting inbreeding depression is contributing to low levels of self-fertility in three Vaccinium species: V. myrtilloides Michaux, V. angustifolium Aiton, and V. corymbosum L. All three species showed a significant reduction in self fruit set and in the proportion of fertilized ovules that developed into mature seed in self compared to outcross fruit. Reductions were more severe in V. myrtilloides and V. angustifolium than in V. corymbosum; however, early-acting inbreeding depression appeared to be the primary factor limiting self-fertility in all three species. Evidence for early- acting inbreeding depression included the presence and higher proportion of aborted ovules in self fruit than in outcross fruit, a correlation between levels of self and outcross seed set, and pollen chase experiments demonstrating that self-pollen does fertilize the ovules. Self-fertility in the three species was probably influenced by levels of genetic load, ploidy level and outcrossing rates. Received: 10 December 1999 / Revision accepted: 20 June 2000     

Asymmetric reproductive interference: The consequences of cross‐pollination on reproductive success in sexual–apomictic populations of Potentilla puberula (Rosaceae)

Apomixis evolves from a sexual background and usually is linked to polyploidization. Pseudogamous gametophytic apomicts, which require a fertilization to initiate seed development, of various ploidy levels frequently co‐occur with their lower‐ploid sexual ancestors, but the stability of such mixed populations is affected by reproductive interferences mediated by cross‐pollination. Thereby, reproductive success of crosses depends on the difference in ploidy levels of mating partners, that is, on tolerance of deviation from the balanced ratio of maternal versus paternal genomes. Quality of pollen can further affect reproductive success in intercytotype pollinations. Cross‐fertilization, however, can be avoided by selfing which may be induced upon pollination with mixtures of self‐ and cross‐pollen (i.e., mentor effects). We tested for reproductive compatibility of naturally co‐occurring tetraploid sexuals and penta‐ to octoploid apomicts in the rosaceous species Potentilla puberula by means of controlled crosses. We estimated the role of selfing as a crossing barrier and effects of self‐ and cross‐pollen quality as well as maternal: paternal genomic ratios in the endosperm on reproductive success. Cross‐fertilization of sexuals by apomicts was not blocked by selfing, and seed set was reduced in hetero‐ compared to homoploid crosses. Thereby, seed set was negatively related to deviations from balanced parental genomic ratios in the endosperm. In contrast, seed set in the apomictic cytotypes was not reduced in hetero‐ compared to homoploid crosses. Thus, apomictic cytotypes either avoided intercytotype cross‐fertilization through selfing, tolerated intercytotype cross‐fertilizations without negative effects on reproductive success, or even benefitted from higher pollen quality in intercytotype pollinations. Our experiment provides evidence for asymmetric reproductive interference, in favor of the apomicts, with significantly reduced seed set of sexuals in cytologically mixed populations, whereas seed set in apomicts was not affected. Incompleteness of crossing barriers further indicated at least partial losses of a parental genomic endosperm balance requirement.     

Pollination ecology of Ruellia angustiflora (Ness) Lindau ex Rambo (Acanthaceae) in the Serra da Bodoquena,Mato Grosso do Sul,Brazil

In Brazil, studies focusing on reproductive biology and aspects of pollination in the genus Ruellia have demonstrated common characteristics such as pollination by hummingbird, the predominance of self-fertility and spontaneous self-pollination. The objectives of this study were to describe the floral biology, nectar production, the reproductive system, the effect of pollination intensity and flowering phenology of Ruellia angustiflora, as well as keeping a record of the hummingbirds that visit the flowers of this species. Data collection was performed in a stretch of the Salobrinha stream in the Serra da Bodoquena, Mato Grosso do Sul, from August 2005 to July 2006. R. angustiflora flowered throughout most of the study period; it has diurnal anthesis and has increased nectar production in the morning hours with decreasing production rates during the day. The results of the reproduction experiments indicate self-fertility and pollen limitation in R. angustiflora. Among experiments with different pollination intensity there was no significant difference in fruit formation. Four hummingbird species were observed visiting the flowers of R. angustiflora, and Phaethornis pretrei was the most frequent followed by females of Thalurania furcata. According to its behavior and frequency of visits, P. pretrei can be considered the main pollinator of R. angustiflora in the study area. The experiments on flowers treated with different pollen loads indicate that most successful fruit formation in this species is obtained in flowers that received greater pollen loads on their stigmas. Besides, data suggest that major success in fruit set could be reached with more than three visits of P. pretrei.     

POLLEN FLOW WITHIN A TRISTYLOUS SPECIES: LYTHRUM SALICARIA

Lythrum salicaria L. is a classic example of tristyly. Pollen flow within a natural population of this species was studied by analyzing pollen loads on stigmas of both intact and emasculated plants. The data obtained indicated that a significant amount of self pollination occurs within flowers but that between flowers or plants pollinations are predominantly legitimate, i.e. between stamens and styles of equal length. It is concluded that while the multiallelic system excels in reducing frequency of incompatible pollinations between plants, the heteromorphic system is relatively more effective in reducing self pollinations within a plant.     

The bees don't know and the flowers don't care: the effect of heterospecific pollen on reproduction in co‐occurring Thysanotus species (Asparagaceae) with similar flowers

Species integrity relies on the maintenance of reproductive isolation, particularly between closely related species. Further, it has been hypothesized that the presence of heterospecific pollen on flower stigmas adversely affects plant reproduction with increasing effect in closely related species. Using two pairs of co‐occurring buzz‐pollinated Thysanotus spp. in the Mediterranean climate region of Perth, Western Australia, we quantified the effect of heterospecific pollen on fruit and seed set. We first determined the mating systems of the two focal species using self‐ and outcross pollen, followed by separate treatments with heterospecific pollen within each species pair. Of the two species receiving pollen, Thysanotus triandrus had a mixed mating system, but with significantly lower fruit and seed set from self‐pollen relative to outcross pollen. Thysanotus tenellus was autogamous with no difference in fruit or seed set between autogamous, self‐ or outcross pollinations. Heterospecific pollen had no effect on fruit or seed set of either focal species. These outcomes point to post‐pollination reproductive isolation, consistent with a floral morphology that causes low specificity of pollen placement and thus a poor capacity for pre‐pollination discrimination. Negative effects of heterospecific pollen, therefore, do not appear to play a role in the reproduction in this group of buzz‐pollinated flowers.     

Pollen source and resource limitation to fruit production in the rare species Eremosparton songoricum (Fabaceae)

Eremosparton songoricum (Litv.) Vass. is a rare, central Asian desert species which shows lower fruit set and seed set (<16%) than most hermaphroditic species. We hypothesized that fruit production was limited by pollen and resources. To evaluate potential fruit abortion due to pollen limitation, supplemental hand‐pollination was undertaken, the mating system was investigated and the foraging behavior of pollinators was recorded. To investigate possibile resource limitation, flowers and young pods were artificially removed and fertilization were manipulated. The results showed that under natural pollination, the number of pollen deposited on the stigma greatly exceed the number of ovules per ovary. Mating system experiments showed that the species is self‐compatible, but depended on pollinators to set seeds. Supplemental outcross pollination increased fruit set significantly. The most frequent effective pollinator Megachile terminate Morawitz, was observed pollinating many flowers of the same individual plant (74.5±1.3%). These results suggested that fruit production is affected by insufficient outcross pollen rather than by pollen quantity. Removal of 2/3 of the flowers and young pods led to significantly higher fruit set, as did addition of fertilizers (N–P–K: 0.025–0.05–0.013 g, N–P–K: 0.05–0.1–0.025 g) showing that reducing resource acceptors and increasing inorganic resources both helps to improve fruit set. We therefore conclude that reproductive success of E. songoricum is limited by both outcross pollen and available nutrients.     

Breeding systems in Cyrtanthus (Amaryllidaceae): variation in self‐sterility and potential for ovule discounting

• Breeding systems of plants determine their reliance on pollinators and ability to produce seeds following self‐pollination. Self‐sterility, where ovules that are penetrated by self‐pollen tubes that do not develop into seeds, is usually considered to represent either a system of late‐acting self‐incompatibility or strong early inbreeding depression. Importantly, it can lead to impaired female function through ovule or seed discounting when stigmas receive mixtures of self and cross pollen, unless cross pollen is able to reach the ovary ahead of self pollen (‘prepotency’). Self‐sterility associated with ovule penetration by self‐pollen tubes appears to be widespread among the Amaryllidaceae.
• We tested for self‐sterility in three Cyrtanthus species – C. contractus, C. ventricosus and C. mackenii – by means of controlled hand‐pollination experiments. To determine the growth rates and frequency of ovule penetration by self‐ versus cross‐pollen tubes, we used fluorescence microscopy to examine flowers of C. contractus harvested 24, 48 and 72 h after pollination, in conjunction with a novel method of processing these images digitally. To test the potential for ovule discounting (loss of cross‐fertilisation opportunities when ovules are disabled by self‐pollination), we pollinated flowers of C. contractus and C. mackenii with mixtures of self‐ and cross pollen.
• We recorded full self‐sterility for C. contractus and C. ventricosus, and partial self‐sterility for C. mackenii. In C. contractus, we found no differences in the growth rates of self‐ and cross‐pollen tubes, nor in the proportions of ovules penetrated by self‐ and cross‐pollen tubes. In this species, seed set was depressed (relative to cross‐pollinated controls) when flowers received a mixture of self and cross pollen, but this was not the case for C. mackenii.
• These results reveal variation in breeding systems among Cyrtanthus species and highlight the potential for gender conflict in self‐sterile species in which ovules are penetrated and disabled by pollen tubes from self pollen.

     

SELF-INCOMPATIBILITY SYSTEMS IN THE RUBIACEAE OF A TROPICAL LOWLAND WET FOREST

A program of experimental field pollinations was carried out on 14 species of woody Rubiaceae in a Costa Rican wet forest in order to test for the presence of self-incompatibility systems. Species of Cephaelis, Coussarea, Faramea, Hamelia, Posoqueria, Psychotria, Rudgea, and Warszewiczia were investigated in the study. Ten of the species are distylous, and of these, nine were found to be self-incompatible. The site of the incompatibility barrier varied between and within species. Short style plants always had stigmatic inhibition of pollen tubes following self and intraform pollinations, but the site of the rejection response in long style plants was quite variable. In several species, pollen tubes, resulting from incompatible pollinations of the long style flowers, penetrated to the base of the style. Fruit set was followed in two of these species, and despite deep penetration of self and intraform pollen tubes in the long style morph, only interform pollinations resulted in fruit set. Four of the species tested are florally monomorphic and each was found to be self-incompatible on the basis of fruit set patterns. Pollen tubes in the styles of self-pollinated flowers of two of the monomorphic species, penetrated to the ovary, but no fruits resulted from selfpollinations.     

Pollen-pistil interactions inBrassica oleracea: Cell calcium in self and cross pollen grains

Summary The levels of calcium in pollen grains on the stigma, after self vs. cross pollinations, were compared inBrassica oleracea, a species showing sporophytic self-incompatibility. Self pollen was characterized by higher levels of chlorotetracycline fluorescence and by higher calcium signals in energy-dispersive analysis of X-rays than cross pollen. Cellular integrity of pollen grains was maintained after rejection, and self pollen could be rescued from the stigma to germinate 4 h after pollination, suggesting that the rejection response was not irreversible.abbreviations CTC chlorotetracycline - EDAX energy dispersive analysis of x-rays - FDA fluorescein diacetate - RH relative humidity - SSI sporophytic self-incompatibility - SLSG S locus-specific glycoproteins     

The presence of heterospecific pollen on stigmas of nectariferous and nectarless orchids and its consequences for their reproductive success

Summary The widespread occurrence of nonorchid, heterospecific pollen grains on the stigmatic surfaces of a range of nectariferous and nectarless European orchids (Dactylorhiza. Orchis, Goodyera, andGymnadenia species) is reported for the first time, and the impact of heterospecific pollination on orchid reproductive success is experimentally investigated. There are three main components of stigmatic contamination by heterospecific pollen: the frequency of contamination, the diversity of foreign species present on the stigma, and the amount of pollen deposited. Six out of seven of the species examined have greater than 85% of stigmas contaminated with wind and insect-dispersed pollen. From one to nine insect-dispersed foreign pollen species are present per stigma, including pollen of members of the families Apiaceae, Asteraceae, Caryohpyllaceae, Ericaceae, and Primulaceae. Average loads per stigma vary from 13 to 176 grains, with individual stigma loads ranging from one to 909. Whether or not the orchid provides nectar has a major impact on these three components. Nectarless orchids have the greatest frequencies of contamination, diversity of species, and average load per stigma. Insect-dispersed pollen is deposited both by pollinators and visitors but, in spite of low levels of pollination, nectarless orchids still exhibit higher frequencies of heterospecific pollen contamination. The effect of the presence of heterospecific pollen on the reproductive success of orchids is tested in this study for the first time. Average-to-high, naturally occurring loads of heterospecific pollen derived from a mixture ofArmeria maritima,Caltha palustris,Cochlearia officinalis,Cytisus scoparius, andPrimula vulgaris and consisting of 50–250 grains per load are placed onto stigmas ofDactylorhiza purpurella which are subsequently self-pollinated with half of a pollinium. All pollinations produce capsules indicating that heterospecific pollen does not affect fruit set. Although experimental and control fruits are similar in size, they differ in total seed weight and composition. Total seed weight is reduced and the proportion of seeds with normal embryos decreased while the proportion of unfertilised ovules increased following pollination with heterospecific pollen, indicating a detrimental effect on fertilisation. Lower reproductive success caused by fertilisation failure is likely to be most severe in nectarless species because of their generally higher levels of contaminated stigmas. As nectarless orchids are known to have lower fruit set compared with nectariferous ones, this finding suggests that the reproductive success of nectarless orchids may be even lower than previously realised.Abbreviations RS reproductive success