Pollen biology in four Mediterranean Quercus species

Potential pollen production, viability and germination were studied in the most important species of Quercus in the mountains of Córdoba to determine the contribution of each species to the total amount of airborne pollen. The results were compared over two consecutive years with different rainfall patterns. The viability of pollen grains was determined at anther opening, and during the pollination period, in order to determine potential pollination capacity. Results indicated that there were differences in the number of pollen grains produced by stamen in the four species. Equally, there were differences in the number of flowers among the species, being Q. suber the species with higher number of catkins groups and flowers per individual tree. Total pollen production per tree can be summarized in the following proportion: 1: 3: 3: 6 (Q. coccifera, Q. ilex ssp. ballota, Q. faginea and Q. suber). Potential pollen viability was estimated using the Fluorochromatic Reaction (FCR) and a germination assay. The results have shown that Quercus potential pollen viability is high and declines slowly with time. Q. coccifera was the species with the highest percentage of germination, with Q. suber being the lowest.     

Effects of relative humidity and temperature stress on pollen viability of Cistus incanus and Myrtus communis

Cistus incanus and Myrtus communis are two species commonly found in Mediterranean shrublands, flowering respectively in spring and summer. Pollen of single flowers of C. incanus is available for dispersal for 8-10 hours, during which time viability decreases from 91% to 61%. Field measurements of the variation in the percent pollen viability during the anthesis of single flowers of M. communis shows that, immediately after anthesis, pollen viability is very high; by the end of the first day it significantly declines and 36 hours after anthesis almost all grains are unviable. Experiments carried out under controlled environment showed that, in C. incanus, the greatest loss of viability occurs at 100% humidity and high temperatures, while under dry conditions pollen viability remains high throughout a wide temperature range. Similarly, pollen viability of M. communis remains high for several hours at high temperature and dry conditions, while it rapidly decreases at high humidity. In conclusion, the temperature experiments indicate that in C. incanus and M. communis, pollen viability does not appear to be drastically reduced if the relative humidity of the environment is low. Therefore, humidity is a far more important factor determining pollen viability loss in both species.     

Morphological studies of pollen grains of the Polish endemic species of the genus <Emphasis Type="Italic">Rubus</Emphasis> (Rosaceae)

The pollen of six taxa of the genus Rubus endemic to Poland (R. capitulatus, R. chaerophylloides, R. ostroviensis, R. posnaniensis, R. seebergensis and R. spribillei)was studied using light and scanning electron microscopy. The study objective was to extend the knowledge of the species pollen morphology and to investigate whether pollen morphology may help to taxonomically distinguish a very rare bramble species. Study material was derived from six natural localities where the species occur in the Wielkopolska region (Poland). Ten quantitative pollen grain features were analysed as well as the following qualitative ones: outline, shape, type of bridge and exine sculpture. Only mature, correctly formed pollen grains (30 per specimen) were measured and 180 pollen grains were examined in total. It was found that R. capitulatus and R. seebergensis had the highest mean values of P, E and Le features. In addition, pollen grains of R. capitulatus were most flattened (mean P/E ratio equals 1.14), while those of R. seebergensis were most elongated (mean P/E ratio equals 1.27). R. spribillei was the only species with striae of similar width as muri, whereas striae in the remaining species were wider. R. chaerophylloides and R. posnaniensis were characterised by specific, similar exine sculpture. Nevertheless, it should be stated that differences between pollen features of the species studied are so small and the variability of these features are so large that it is difficult to clearly identify the species studied. Therefore, pollen grain morphology can serve only as an auxiliary feature for the determination of these species.     

Stylar trimorphism in four functionally andromonoeciousProsopis species (Mimosaceae)

The reproductive system ofProsopis chilensis, P. pugionata, P. flexuosa, andP. torquata is described. Observations were made of floral morphology, stigmatic receptivity, pollen grain viability, and the nature of pollen reserves. Scanning electrone microscopy and statistical analysis were carried out. The results allow us to conclude that: 1) The studied species have three distinct floral morphs characterized by significantly different style lengths and, to a lesser extent, different stamen lengths; 2) style precocity found in all species does not indicate protogyny; 3) flowers with shortest styles are not receptive; 4) the pollen grains of all floral morphs showed high viability; 5) peroxidase is present in both the pollen and pistil; 6) viable pollen grains invariably have both starch and pollenkitt; 7) the studied species are andromonoecious.     

Floral biology and pollination in Cucumis melo L., a tropical andromonoecious cucurbit

We studied floral biology and the role of flower visitors in fruit set of muskmelon (Cucumis melo L.). Muskmelon produced Staminate (hereafter ‘S’) and Hermaphrodite (hereafter ‘H’) flowers with a ratio of 18:1. Flowers opened around 40 days after planting and flowering continued for another 40 days. Longevity of both types of flowers was one day. Anthesis in S flowers was earlier (0545–0700 h) than that of H flowers (0700–0830 h). Anther dehiscence occurred 10–15 min after anthesis in both the flowers. The number of pollen grains in H flowers was significantly lower (2715 ± 143) than S flowers (4968 ± 398). Pollen viability and stigma receptivity was maximum between 1600 and 1800 h. The pollen tube of pollen grains from H flowers grew very slowly and never reached the ovary. There were two peaks in nectar production in a day (1000–1200 h and 1400–1800 h). Sixteen species of insects belonging to Hymenoptera (81.25%), Diptera (12.50%) and Lepidoptera (6.5%) visited muskmelon flowers. Major visitors were honeybees, Apis cerana F. and A. florea F. and both were efficient pollinators. Diversity and abundance of flower visitors positively correlated with floral abundance. In pollination exclusion experiments, no fruit set was observed in bagged, self-pollinated and the flowers artificially crossed only with other H pollen. But, those flowers hand pollinated with pollen from S flowers and ones that were allowed for open pollination set maximum fruits. The crop needs a minimum of 8–10 bee visits per flower to achieve the same level of pollination efficiency provided with natural/unrestrained visits.     

Maturation timing of stamens and pistils in the dioecious plant Silene latifolia

The dioecious plant Silene latifolia depends on nocturnal insects for pollination. To increase the chance of cross-pollination, pollen grains seem to be released and stigmas seem to be receptive simultaneously at night. We divided the floral development of S. latifolia into 1–20 stages, and determined the timetables of male and female function. The corolla of both male and female flowers opens at sunset (1900 hours) and closes at sunrise (0900 hours). To investigate the period of the reproductive phase of male and female function, we measured the germination rate on a pollen medium and the pollen germination rate on stigma during the period when stamens and stigmas were viable in the timetable. Male flowers had early- and late-maturing stamens that had the highest pollen viability, germination rate and pollen tube growth at midnight (0000 hours) at 1 day after flowering (DAF) and 0000 hours at 2 DAF. In contrast, female flowers maintained a germination rate of nearly 100 % from 1800 hours at 1 DAF to 1200 hours at 3 DAF. These results suggested that S. latifolia transferred the matured pollen grains from male flowers to female flowers only at night.     

Assessment of pollen reward and pollen availability in Solanum stramoniifolium and Solanum paniculatum for buzz‐pollinating carpenter bees

The two widespread tropical Solanum species S. paniculatum and S. stramoniifolium are highly dependent on the visits of large bees that pollinate the flowers while buzzing them. Both Solanum species do not offer nectar reward; the rewarding of bees is thus solely dependent on the availability of pollen. Flower visitors are unable to visually assess the amount of pollen, because the pollen is hidden in poricidal anthers. In this study we ask whether and how the amount of pollen determines the attractiveness of flowers for bees. The number of pollen grains in anthers of S. stramoniifolium was seven times higher than in S. paniculatum. By contrast, the handling time per five flowers for carpenter bees visiting S. paniculatum was 3.5 times shorter than of those visiting S. stramoniifolium. As a result foraging carpenter bees collected a similar number of pollen grains per unit time on flowers of both species. Experimental manipulation of pollen availability by gluing the anther pores showed that the carpenter bees were unable to detect the availability of pollen by means of chemical cues before landing and without buzzing. Our study shows that the efficiency of pollen collecting on S. paniculatum is based on large inflorescences with short between‐flower search times and short handling time of individual flowers, whereas that of S. stramoniifolium relies on a large amount of pollen per flower. Interestingly, large carpenter bees are able to adjust their foraging behaviour to drastically different strategies of pollen reward in otherwise very similar plant species.     

Systematic importance of pollen morphological features of selected species from the genus Rosa (Rosaceae)

The aim of this study is verification of the taxonomic usefulness of the pollen grain features studied, based on pollen morphology of 32 wild species from all 4 subgenera and all 10 sections of the genus Rosa, mainly for delimitation of subgenera, sections, and species. The measurements and observations were carried out with both light microscopy and scanning electron microscopy. Only correctly formed pollen grains (30 per specimen) were measured, and 960 pollen grains were examined in total. They were analyzed for 14 quantitative features of pollen grains and exine sculpturing and the following qualitative ones: outline, shape, and operculum structure. Our study revealed that the diagnostic features of pollen grains studied were: exine sculpture, length of polar axis, and pollen shape (P/E ratio). On the basis of the above characters, five species were isolated and the remaining ones were included in several groups isolated on the basis of exine sculpture types. The following three exine sculpture types occurred in the species studied: granular-verrucate (in R.?stellata), striate-psilate (in R.?multibracteata and R.?multiflora), and striate (the remaining species). R.?banksiae is characterized by small pollen grains, while R.?setigera has strongly elongated pollen with P/E ratio >1.5. Exine sculpture features considered to be diagnostic should be treated as auxiliary because they fail to differentiate individual species, although they can be helpful in distinguishing groups of species of similar exine sculpture. The arrangement of the species examined on a dendrogram only slightly corroborates division of the Rosa genus into subgenera and sections currently adopted in taxonomy (Rehder 1940). An interesting result was reported for the species studied from the Caninae (R.?agrestis, R.?canina, R.?dumalis, R.?jundzillii, and R.?rubiginosa) section which, despite hybrid nature, with the exception of R.?villosa, grouped in the same, most separated group of species.     

An estimate of pollen carryover by ants in a natural population of Scleranthus perennis L. (Caryophyllaceae)

Summary Pollen carryover by ants was estimated in a natural population of Scleranthus perennis L. (Caryophyllaceae) using fluorescent dye as a pollen analogue.A highly significant exponential decrease in deposition of dye grains in a sequence of flowers visited by the main flowr visitor, Formica (S.) rufibarbis F., was observed with most grains being deposited in the first few flowers visited. At most 12 flowers were visited in a sequence.The mean between-flower transfer distance was estimated to be 6.2 cm, within-plant pollen transfers being recorded as zero. When all within-plant movements were excluded the mean was 22.5 cm. The distribution of transport distances differed from what I observed for honey-bees which occasionally foraged on dense stands of S. perennis. Calculations made on the deposition of dye grains transferred by ants showed that 7% of the dye grains deposited in the first flower visited after the dye source may reach a flower situated c. 30 cm from the dye source.     

Effect of flower orientation on the male and female traits of Myrtillocactus geometrizans (Cactaceae)

• Intra‐individual variation in the production and size of reproductive traits has been documented in columnar cacti, being higher in equator‐facing flowers. Such variation is attributed to the high amount of PAR intercepted by stems oriented towards the equator. Most studies focused on this phenomenon have documented the existence of intra‐individual variation on traits associated with the female function; however, its impact on traits associated with the male function has been neglected. We tested the hypothesis that equator‐facing flowers of Myrtillocactus geometrizans exhibit higher values on traits associated with both male and female functions than flowers facing against it.
• Number and size of anthers and ovaries, pollen:ovule ratio and number and quality of pollen grains (diameter, germinability, viability and pollen tube length) were estimated from reproductive structures facing north and south, and compared with t‐tests between orientations.
• Number of anthers per flower, number of pollen grains per anther and per floral bud; pollen size, viability and germinability; pollen tube length; ovary length and pollen:ovule ratio were significantly higher in reproductive structures oriented towards the south (i.e. equator).
• These findings suggest that intra‐individual variation in floral traits of M. geometrizans might be associated with different availability of resources in branches with contrasting orientation. Our results provide new evidence of the existence of a response to an orientation‐dependent extrinsic gradient. To our knowledge, this is the first study documenting the existence of intra‐individual variation on pollen quality and P:O ratio in Cactaceae species.

     

Dioecism of the genusCallicarpa (Verbenaceae) in the Bonin (Ogasawara) Islands

The morphology and function of the flowers of three species of the genusCallicarpa, C. glabra, C. nisnimurae andC. subpubescens, endemic to the Bonin Islands, revealed that all three species were dioecious. Male plants had short-styled flowers with sterile ovaries, while female plants had long-styled flowers with non-germinating inaperturate pollen grains. Such dioecism has never been reported from the family Verbenaceae. This curious sexual system with non-germinating pollen grains may have been maintained as a reward to insect pollinators.     

Flowering and morphological characterization of Dendrocalamus asper androecium and pollen grains

Abstract

Little is known about the reproduction of Dendrocalamus asper because it flowers only every 100 to 120 years. In the present work we describe some reproductive features of this bamboo and characterise flowers and pollen at various developmental stages. Number of pollen grains and ovules per flower, pollen/ovule ratio, in vitro twinning and pollen grain viability in vivo were evaluated and the different stages of floral development identified. Further, we performed a morphological analysis of androecium and pollen development. Seven distinct stages of flower development were identified; four initial stages, a pre-anthetic stage, and two stages of anthetic. Dendrocalamus asper pseudospikelets avoid inbreeding by means of protogyny. The floral and pollen characteristics suggest that the species is anemophilous. The ultrastructural characteristics of free microspores (stage two of floral development), vacuolated microspores (stage five) and mature pollen (anthetic) were analysed. During maturation, pollen grains accumulate larger and more numerous amyloplasts and organelles such as mitochondria. Pollen disperse in the tricellular development stage. Pollen is monoporate with an operculum-like pore, with a rugulate structure and a spinose tectum.     

Heterospecific pollen transfer between sympatric species in a midsuccessional old-field community

The cumulative (season-long) incidence of heterospecific pollen transfer (HPT) was examined using nine sympatric species in a midsuccessional old field. Inflorescences were collected weekly during the flowering season, and the proportion of foreign pollen/stigma was recorded. Flowering phenologies of sympatric species and ovule and seed counts of study species were also recorded. Heterospecific pollen was detected on some stigmas of each species. Medicago sativa (Fabaceae) received the most foreign pollen; in some cases, all of the grains on a stigma were heterospecific. Lotus corniculatus (Fabaceae) received the least amount of foreign pollen; the incidence of heterospecific pollen was near zero in most cases. The mean and range of foreign pollen received varied by as much as an order of magnitude between species. The six species with zygomorphic flowers, all Fabaceae, received more heterospecific pollen than the three species with actinomorphic flowers, Potentilla recta and P. simplex (Rosaceae) and Ranunculus acris (Ranunculaceae). This probably reflects a bias because our data were analyzed on a cumulative basis and the Fabaceae had longer flowering phenologies. HPT was not correlated with the species' relative abundance within the community. Proportion of foreign pollen received varied temporally within species, and this variation generally was not related to phenology of any sympatric taxa or the species' own phenology. Pollen grain diameter was positively related to levels of foreign pollen received by species. This might be caused by poor adhesion of large pollen grains to small stigmatic papillae or if generalist pollinators carrying large amounts of heterospecific pollen visit the large-grained species and specialists with little foreign pollen visit the small-grained species. The large proportions of heterospecific pollen on stigmas of many species indicate that HPT occurs frequently in the community we studied and the implications may include reduced seed set because of occlusion by foreign grains. As yet, however, it is unclear how important a factor HPT is in mediating pollen limitation of reproductive success.     

Andromonoecy in an Old World Papilionoid legume,Erophaca baetica

Andromonoecy (i.e. the occurrence on individual plants of hermaphroditic and male flowers) is a rare sexual system among the angiosperms, regarded by some authors as a transitional stage from hermaphroditism to monoecy. Having discovered the occurrence of andromonoecy in Erophaca baetica (a Mediterranean shrubby legume with two subspecies), a novelty for Old World papilionoid legumes, we investigated the morpho‐functional correlates and the geographical distribution of this phenomenon in the species. The relative frequencies of hermaphrodite and male flowers were determined in two field and 111 herbarium populations. Biomass allocation within flowers, pollen production and viability, pollen tube growth, nectar production and the temporal pattern of male flower production were also studied in two nearby southern Spanish populations. Virtually all of the studied populations were andromonoecious. Male flowers tended to appear at apical positions within the inflorescence, and became more abundant by the end of the flowering season. Male flowers were externally similar to hermaphroditic flowers (although with less biomass and smaller parts) and released equivalent amounts of pollen and nectar; however, their pollen germinated significantly better. Erophaca is the first example of an andromonecious Papilionoid in the Old World. Since the main difference among floral morphs in this species is functional (i.e. pollen germination rate) rather than morphological, andromonoecy is not readily noticeable, and very careful inspection may be required to reveal it. The potential effect of andromonoecy in enhancing outcrossing rate in this species is discussed.     

COSTS OF SELF-POLLINATION IN A SELF-INCOMPATIBLE PLANT,POLEMONIUM VISCOSUM

Plants of Polemonium viscosum are strongly self-incompatible, yet have floral features that enforce spatial and temporal separation of pollen and stigma presentation. In this study, we address the widely held view that such barriers to selfing are maintained because they reduce interference by self pollen in performance of outcross grains. In the first year of the study we found that 24 hr precedence of self pollen on the stigma (24S), reduced outcross pollen germination by 32%, and seed set by 40% in flowers of P. viscosum. In the second year significant negative effects of self pollen on compatible pollen germination were seen when self pollen was applied four hours before (4S) or immediately prior (S×) to outcross pollen. Surprisingly, however, in the second year of study, seed set was not reduced with as much as 50% inhibition of compatible pollen germination. Results suggest that by interfering with normal functioning of outcross grains, incompatible self pollen can increase pollen limitation in P. viscosum. Reduction in seed production as a result of this phenomenon should select for displacement of pollen presentation from stigma receptivity.     

Interspecific pollen transfer as a mechanism of competition: Consequences of foreign pollen contamination for seed set in the alpine wildflower,Polemonium viscosum

Summary Earlier studies showed that under natural conditions foreign pollen receipt is correlated with loss of seed set in populations of P. viscosum. Here, we report on experimental pollinations using foreign pollen from two co-flowering species that share pollinators with P. viscosum. Experiments were designed to ascertain whether improper pollen transfer causes reduced seed set under controlled conditions and, if so, to determine the stage at which interference occurs. In flowers pre-treated by pollination with either Mertensia or Castilleja pollen, adherence of subsequently deposited conspecific pollen grains was unaffected. However, conspecific grains had significantly lower germination and ovule fertilization success in flowers receiving foreign pollen than in controls. These results suggest that pollinator infidelity in high alpine plant communities can reduce female reproductive success of P. viscosum by inhibiting the performance of conspecific pollen grains. Negative effects of Mertensia and Castilleja pollen were comparable despite large interspecific differences in pollen grain morphology. Accordingly, susceptibility to foreign pollen interference appears to be an intrinsic property of P. viscosum that is not dependent on the unique structural properties of pollen produced by competitive species.     

Relationship between floral longevity and sex allocation among flowers within inflorescences in Aquilegia buergeriana var. oxysepala (Ranunculaceae)

Understanding the fitness of plants with inflorescences requires examining variation in sex allocation among flowers within inflorescences. We examined whether differences in the duration of the male and female phases of flowering lead to variation in sex allocation and reproductive success among flowers within inflorescences. In 2002 and 2003, we quantified floral longevity, floral sex allocation, and reproductive success between the first and the second flowers within inflorescences in a protandrous species, Aquilegia buergeriana var. oxysepala. Floral longevity was greater in the first flowers than in the second ones in both years. The male phase lasted longer, and the initial number of pollen grains and the number of pollen grains removed were greater in the first flowers than in the second ones in both years. Within first flowers, the number of pollen grains removed was greater in flowers that had longer male phases, thus duration of the male phase may positively affect male reproductive success in the first flowers. The female phase lasted longer and the number of ovules was greater in the first flowers than in the second only in 2002. However, seed production per flower and female phase duration in both years were not significantly related. The variation in the number of pollen grains among flowers in this species may be caused by the variation in male phase duration.     

Pollen morphology and localization of Ni in some Ni-hyperaccumulator taxa of Alyssum L. (Brassicaceae)

Pollen morphology of seven Alyssum L. taxa growing on serpentine soils in different places in the European Mediterranean macrobioclimate territory were studied, described and compared. Cluster analysis was performed to show similarity between species and their populations. The shape of the pollen grains varies among the species and among the grains within the same anther. The pollen grains are 3-colpate, prolate, with long and narrow colpi reaching the poles. The ornamentation of the exine varies from micro-reticulate to reticulate between the species. Pollen sterility/fertility was also calculated. The highest percentage of sterile pollen (73.76%) was calculated for Alyssum murale subsp. murale and the lowest (9.54%) for A. bertolonii subsp. bertolonii. All species are representatives of sect. Odontarrhena (C.A. Meyer) Koch well known as Ni-hyperaccumulators. Nickel and other elements present in pollen and stamen were studied by inductively coupled plasma-mass spectrometry. The stamen parts of all species were micromorphologically analyzed by scanning electron microscopy coupled to an energy-dispersive X-ray probe. Accumulation of Ni was detected in the stamens of all studied species and rarely in the pollen grains. The distribution patterns of Ni were similar among the species examined.