Dry Pollen Research Articles

Development of breeding populations from interspecific hybrids between Trifolium repens L. and T. occidentale Coombe

AbstractTrifolium occidentale is a diploid wild relative of T. repens (white clover) with adaptation to dry, saline coastal habitats. Transfer of drought and salt‐tolerant adaptive traits to white clover could be potentially valuable if interspecific hybridization can be achieved efficiently and leads to fertile hybrid populations. To achieve hybridization, 4x plants of T. occidentale were generated. Efficient techniques for generation of 4x plants and their identification using dry pollen shape are described. Interspecific 4x F1 plants were achieved without embryo rescue. F2 populations and first backcross hybrids to white clover were also efficiently achieved. Although male and female fertility were lower than in white clover, they were adequate to produce large amounts of seed from small numbers of inflorescences. Thus, early generation interspecific hybrid prebreeding populations can be readily developed, opening the way for transfer of traits from T. occidentale to white clover.

Terminal settling velocity and physical properties of pollen grains in still air

Numerical simulation of wind pollination requires knowledge of pollen grain physical parameters such as size, shape factor, bulk density, and terminal settling velocity. The pollen grain parameters for Japanese cedar, Japanese cypress, short ragweed, Japanese black pine, and Japanese red pine were assessed for dry condition. Terminal settling velocities of dry pollen grains in still air were measured using image analysis of scattered light tracks in a dark settling tube. The measurement system was validated by comparing results to those obtained for standard microspheres of known size and density. Dry pollen grain shape factors indicate the resemblance of particles to spheres, except for pine pollen. Circularity factors of dry pine pollen grains were 0.90–0.86, suggesting more irregular shape than those of other pollen species. Aerodynamic diameters of dry pollen grains were calculated based on the terminal settling velocity. Aerodynamic diameters of Japanese cedar, Japanese cypress, and short ragweed closely resembled the projected area equivalent diameters, suggesting that aerodynamic behaviors of these pollen grains can be managed simply in numerical simulations. However, aerodynamic diameters of dry pine pollen grains were nearly 30 % smaller than projected area equivalent diameters. Sacci on dry pine pollen can reduce the terminal settling velocity through low density and shape effects attributed to their non-sphericity, engendering aerodynamic diameter smaller by more than 10 µm from area equivalent diameters.

Paleoenvironmental and climatostratigraphic insights for Marine Isotope Stage 19 (Pleistocene) at the Montalbano Jonico succession, South Italy

Quantitative analyses on pollen and ostracods were performed on the Montalbano Jonico succession (MJS) through Marine Isotope Stages (MIS) 18–21 in order to acquire details of paleoenvironmental changes and climate pattern during MIS 19, which includes the Matuyama–Brunhes paleomagnetic boundary (MBB). While the MJS does not record the MBB, which is the main criterion for defining the Early–Middle Pleistocene boundary, it is a candidate Global Boundary Stratotype Section and Point (GSSP) for the Ionian Stage and Middle Pleistocene Subseries because of its excellent exposure, continuous deposition, and astronomically tuned record between MIS 37 and MIS 16. The new 40Ar/39Ar age of 773.9 ± 1.3ka for volcaniclastic layer V4 allows the age-model for the succession to be refined. Ostracod assemblages show significant paleodepth fluctuations at the glacial–interglacial scale, in agreement with the pollen distality index (Pinus vs. Caryophyllaceae, Amaranthaceae and Ephedra) which also suggests higher values during MIS 19. Pollen analysis provides supplementary paleoclimate data that are valuable to reconstruct temperature variation during the identified phases. A major expansion of steppic vegetation at 790ka in the uppermost MIS 20 correlates with a significant cold and arid Northern Hemisphere climate phase, as documented by a concurrent peak both in North Atlantic ice rafted debris and Mediterranean dust records. Climate conditions recorded in MIS 19.3 were similar to those of the present interglacial, whereas a dry climate phase marks MIS 19.2 at 771.84ka, slightly above the V4 layer. Millennial scale climate variability occurred during MIS 19 as evidenced by two abrupt short-term moderately dry pollen events at 783.5ka and 774.8ka that could be related to a period of reduced geomagnetic field intensity through the MBB transition. Additional environmental/chronological events accompanied this stratigraphic interval: i) the volcaniclastic layers V3 and V4, radiometrically dated at 801.2 ± 19.5ka and 773.9 ± 1.3ka, respectively; ii) the co-occurrence of maximum flooding and thermal maximum in agreement with the previously recorded co-occurrence of the Neopycnodonte community and tropical–subtropical mesopelagic teleostean Bonapartia pedaliota, close to the increase of Quercus; and iii) the maximum depth (MD) between 777.3 and 773.25ka as recorded by outer shelf/upper slope macroinvertebrate fauna, close to the highest value of pollen distality index in MIS 19. Results enhance the understanding of orbital-suborbital paleoenvironmental change during the mid-Pleistocene, and particularly of the climate pattern during MIS 19 and across the MBB.

Pollen as a possible pathway for the dissemination of Pseudomonas syringae pv. actinide and bacterial canker of kiwifruit

Pollen collected in a kiwifruit orchard with symptoms of bacterial canker and naturally contaminated by Pseudomonas syringae pv. actinidiae (Psa), was used to pollinate an experimental orchard, in order to confirm its role, under commercial orchard conditions, in disseminating the pathogen and, possibly, contributing to disease spread. A pollen lot, certified free from Psa, was used with the same methods as a control. Two pollination techniques were used: dusting (dry pollen) and spraying (pollen suspension in water). The orchard was monitored during 2 years from experimental pollination, with regular sampling of flowers, fruits, leaves, and vines, to check for Psa as an epiphyte or endophyte, and for bacterial canker symptoms. Psa was recovered from flowers, fruitlets and leaves during the first season, mainly in plots where contaminated pollen had been sprayed in water suspension. From early August until harvesting time (mid-October), Psa detection was possible only on leaves. No symptoms developed during the first season after pollination. No endophytic Psa was detected in pruned vines in the following winter. During the second season, detection and isolation of Psa was erratic, but direct isolation was achieved from four plots. During the second season after pollination, typical leaf symptoms were observed on a few vines, and Psa was isolated and identified. Our results suggest that Psa could be disseminated via contaminated kiwifruit pollen as a pathway for spread of bacterial canker. However, further pollination experiments are needed to establish, beyond any doubt, whether contaminated pollen may contribute to possible disease outbreaks.

Isolation and DNA-Marker Based Genotyping of Individual Pollen Grains

Dry pollen of diploid Solanum clone 9507-04, Cotton chromosomal substitution line and Echinacea purpurea „Magnus‟ were isolated and their cells contents released in individual tubes containing water or pollen germination media, respectively.Six polymorphic markers from individual pollen grains of the diploid potato were amplified using two random amplified polymorphic DNA (RAPD) primers, i.e., UBC291 and UBC504. The procedures for primer extension pre-amplification (PEP) for genome and subsequent amplified fragment length polymorphism (AFLP) analysis with 13 primers pairs were tested on the upland cotton line. The cotton parental AFLP markers were scored for their presence or absences in the 35 pollen samples and majority of the markers were found monomorphic as expected from the gametes of a near isogenic line.PEP and AFLP procedures for linkage mapping were standardized usingpurpleconeflower pollen grains. From 64 possible AFLP primer pairs,37.5% amplified 104 markers for coneflowerthat were linked in 11 linkage groupsranging from 13 to 134 centimorgans (cM). The 409 cM sizedlinkage map constructed forE. purpurea „Magnus‟genome had average distance of 2.4 cM to 22 cMfor the two to 55 polymorphic markers identifiedwithin the 11 groups.The protocols described in this report would serve to enhance breeding efficiencies in plantsimportant for food,fiber, ornamental and medicinal industries.

Autofluorescence of atmospheric bioaerosols: spectral fingerprints and taxonomic trends of pollen

Abstract. Primary biological aerosol particles (PBAP) are important factors in atmospheric cycling, climate, and public health. Pollen is a major fraction of PBAP and is receiving increasing attention due to its high allergenic potential and the associated impacts on personal life quality and economy. Recently, autofluorescence-based techniques have proven to be valuable tools for real time, in situ quantification and classification of PBAP. First studies suggest that the autofluorescence of pollen may be sufficiently selective to be utilized for an automated and real-time monitoring of pollen in ambient air. However, the degree of selectivity autofluorescence can provide is still in question and actively debated. This study addresses the origin, properties, and selectivity of autofluorescence from natural pollen by fluorescence microscopy and spectroscopy measurements along with a systematic synthesis of related literature. We show that dry pollen reveals characteristic and reproducible autofluorescence signatures which are shaped by cell wall associated fluorophores, such as phenolic compounds and carotenoid pigments. In addition, fluorescence signals from proteins and chlorophyll a were observed in some species. The abundance and intensity of the individual fluorescence signals show certain taxonomic trends and allow systematic differentiation from bacteria and fungal spores due to the lack of proteins on the grain surface. Principal component analysis was used to explore the discrimination potential of pollen autofluorescence, in combination with size and shape, revealing a differentiation of pollen on family level. Our results help explore the levels of selectivity that autofluorescence-based techniques can provide to PBAP analysis and will support the development and application of autofluorescence-based detectors for monitoring of allergenic pollen in the atmosphere.

Pollen morphology and taxonomy of Atraphaxis (Polygoneae, Polygonaceae)

Pollen grains of 32 species of Atraphaxis L. including six species previously attributed to Polygonum L. s.str. were investigated with LM (acetolysed pollen) and SEM (dry pollen). Pollen grains of all studied species are spheroidal to oblong-spheroidal, mostly tricolporate, rarely 4-6-loxocolporate, ellipsoidal in equatorial view, rounded-trilobed or trilobed in polar view, with striate or striato-perforate, reticulato-foveolate, or reticulato-perforate sporoderm ornamentation. Exine sculpture is the most variable feature, differing in width, distinctness, and spacing of the striae, which are divided by grooves with small pits or perforations in rows. The most distinct variants of sporoderm ornamentation are connected by a full range of transitions. Striato-perforate ornamentation is common in species with the perianth typical for Atraphaxis, whereas reticulato-foveolate or reticulato-perforate sporoderm ornamentation was found in species with the perianth of Polygonum-type.

NAD+ Accumulation during Pollen Maturation in Arabidopsis Regulating Onset of Germination

Although the nicotinamide nucleotides NAD(H) and NADP(H) are essential for various metabolic reactions that play major roles in maintenance of cellular homeostasis, the significance of NAD biosynthesis is not well understood. Here, we investigated the dynamics of pollen nicotinamide nucleotides in response to imbibition, a representative germination cue. Metabolic analysis with capillary electrophoresis electrospray ionization mass spectrometry revealed that excess amount of NAD+ is accumulated in freshly harvested dry pollen, whereas it dramatically decreased immediately after contact with water. Importantly, excess of NAD+ impaired pollen tube growth. Moreover, NAD+ accumulation was retained after pollen was imbibed in the presence of NAD+-consuming reaction inhibitors and pollen germination was greatly retarded. Pollen deficient in the nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) gene, encoding a key enzyme in NAD biosynthesis, and a lack of NAD+ accumulation in the gametophyte, showed precocious pollen tube germination inside the anther locule and vigorous tube growth under high-humidity conditions. Hence, the accumulation of excess NAD+ is not essential for pollen germination, but instead participates in regulating the timing of germination onset. These results indicate that NAD+ accumulation acts to negatively regulate germination and a decrease in NAD+ plays an important role in metabolic state transition.

Secretions from the female gametophyte and their role in spermatozoid induction in Cycas revoluta

In cycads, spermatozoids are released from pollen tubes and swim in fluid toward the archegonia. The source of this fluid was examined using Cycas revoluta Thunb. ovules placed in culture. Dissected female gametophytes just before fertilization produced copious fluid on their upper surface. The fluid first appeared around the archegonial chamber and then on the inside of the archegonial chamber. When this fluid was applied to dry turgid pollen tubes, they discharged spermatozoids 12 h later. The archegonial neck appeared as two semi-spherical swellings, whereas the four neck cells later became visible and they separated in a schizogenous manner. Many globose particles appear on the top of the archegonial neck cells when the fluid is present. The contents of pollen tubes, spermatozoids and surrounding liquid intermingle with the secreted fluid. The female gametophyte differs in ultrastructure during the stages before and after fluid secretion, the latter showing changes suggestive of fluid secretion from the female gametophyte.

Pollen morphology of the genus Iris L. (Iridaceae) from Croatia and surrounding area: taxonomic and phylogenetic implications

In Croatia and the surrounding area, the genus Iris is represented with about 20 Alpine-Dinaric, Mediterranean and Pannonian taxa from the subgenera Iris and Limniris. We researched pollen morphology of all taxa by using scanning electron microscopy. All pollen grains are sulcate with few palynological features, which could have taxonomic importance on different classification levels: shape of dry pollen grains; shape, outline and approximate size of hydrated pollen grains, approximate size of the sulcus, and ornamentation of the sulcus membrane and of the exine. At least four pollen types were recognised and taxonomically delimited to the series level: two characterise the subgenus Iris, section Iris (series Elatae and Pumilae), and two the subgenus Limniris, section Limniris (series Laevigatae, Sibiricae and Spuriae). Taxonomic implications of pollen morphology of their critical groups and taxa have been evaluated. Possible phylogenetic implications of pollen morphology of the genus Iris on the Southern European area were suggested: the subgenus Iris is more advanced than the subgenus Limniris; I. graminea seems to be the most primitive member of the subgenus Limniris on the Southern European territory; and pollen morphology of the population of the subspecies I. sibirica subsp. erirrhiza from the Mountain Bjelolasica could represent a link between the subgenera Limniris and Iris in the territory of Southern Europe. The possible pathway of evolution of the genus Iris on the territory of Southern Europe was suggested: from the subgenus Limniris, through the “linked taxon” I. sibirica subsp. erirrhiza from the Bjelolasica Mountain, to the subgenus Iris, series Pumilae, and finally to the series Elatae.

In vitro pollination in poplar of section Populus

Two methods of in vitro pollination of aspen were tested and compared, first with moist pollen by transferring swollen pollen grains from a wet agar surface to the stigma and, second, using dry pollen grains. In vitro pollination with dry pollen grains appeared to be more efficient and this method was applied in three subsequent years. Additionally, pollination was performed with selected pollen grains in order to obtain triploid plants. Diploid pollen was induced by heat-treatment of pollen mother cells, but the enrichment of unreduced pollen from natural pollen mixtures was also successful. The selection of diploid pollen, which is larger in diameter than haploid pollen, was performed by sieving using micro sieves. Five triploid plants from different poplar combinations were obtained from 1,227 zygotic embryos cultured after in vitro pollination of 2,676 ovaries with dry pollen over 3 years. Embryo rescue was used to facilitate the development of immature zygotic embryos. The results of this study demonstrate the possibility to effect pollination using selected single pollen grains with the help of in vitro techniques.

The unique design of pollen tetrads in Dionaea and Drosera

The heteropolar pollen grains of Dionaea and Drosera (Droseraceae) are united into permanent tetrads. Dionaea and Drosera tetrads have unique, complex internal set-ups. In both genera, apertures (pores) are situated equatorially (stephanoporate); neither Dionaea nor Drosera can be designated as examples for proximal polar situated apertures. In turgescent state, the tetrads show protrusions along the borders of adjoining pollen grains, which are enclosed by a thick intine and emerge from the apertures. One of these protrusions finally forms the pollen tube during germination. Both genera produce Ubisch bodies. In Drosera, the distal pollen wall above the apertures of each monad invaginates towards the centre of the pollen grain, forming an incomplete septum with a central opening. This septum divides the pollen grain into a larger distal and a smaller proximal chamber containing cytoplasm. In the literature, the septal opening was erroneously addressed as aperture (‘pore’). The proximal chambers collapse in the dry state (or after acetolysis) due to the harmomegathic effect. The septum becomes folded and partially in contact with the proximal wall, thus forming some sort of channels. The thick equatorial pollen walls of Dionaea muscipula show a row of pores, i.e. the apertures, from where the protrusions extend. In contrast to Drosera, there is no inner septum. Each protrusion in Dionaea comprises a massive operculum, which acts as a plug in dry pollen grains.

Anthecology and reproductive system of Mourera fluviatilis (Podostemaceae): Pollination by bees and xenogamy in a predominantly anemophilous and autogamous family?

The Podostemaceae are sessile hydrophytes restricted to habitats with waterfalls and seasonal flow variability. Mourera fluviatilis belongs to the family and has a disjunct distribution in the Amazon and northeastern Brazil. The following hypotheses were tested: (i) both autogamy and anemophily act on the pollination system of M. fluviatilis, and (ii) the highest reproductive success is achieved by cross-pollination. Natural populations were monitored in the rivers Pirangi and Ipojuca, northeastern Brazil. Floral visitors are the polylectic bees Apis mellifera, Trigona spinipes (Apidae), Augochlora sp. and Augochloropsis sp. (Halictidae), though only the latter three are effective pollinators. In experiments on the reproductive system and pollination there was a high percentage of fruit formation (>86%), both in treatment and control plants. Although the index of self-incompatibility is 0.86, there was a significant difference in the number of seeds produced among reproductive system treatments, and xenogamy produced more seeds. There was no significant difference in the number of penetrated ovules between control and manual cross-pollination (xenogamy) treatments, 24 h after pollination. However, there were differences among treatments, 48 h after pollination, except for the control and xenogamy treatments. The pollen tubes from cross-pollination treatments usually penetrate the micropyle in shorter time and in higher quantity. We found differences among pollination experiments, except for the control and non-emasculated treatments, and between emasculated treatments. In spite of the ambiguity of floral attributes (pedicel, filaments and anthers colors, sweet odor, reduced perianth, absence of nectar, small and dry pollen grains), wind pollination is less important for this species; what predominates instead are first allogamy mediated by bees and second self-pollination at the end of anthesis.

Tolerância à dessecação de pólen de berinjela

A alta demanda por sementes híbridas tem estimulado pesquisas que asseguram a oferta destas, como é o caso da manipulação de grãos de pólen. Assim, em estudo conduzido na Embrapa Hortaliças, no ano de 2006, objetivou-se determinar a tolerância à dessecação dos grãos de pólen berinjela 'Ciça'. Para isso foi utilizado pólen fresco (testemunha) e pólen seco em recipiente de alumínio fechado com 300 gramas de sílica gel e em dessecador, ambos por períodos de 24 e 48 horas em ambiente de laboratório. Os respectivos grãos de pólen foram os polinizadores da linhagem feminina. Foi avaliado o pegamento de fruto, a produção e a qualidade fisiológica de sementes através de: massa de 100 sementes, teste de primeira contagem, germinação, emergência de plântulas e envelhecimento acelerado. Concluiu-se que grãos de pólen toleram dessecação até 4,7% de umidade e com teores de água entre 4,7% a 47,2% proporcionam boa produção de sementes sem afetar sua qualidade fisiológica.

The gynoecium of male Anchietea pyrifolia (Violaceae): Preserved structure with a new function

Structure of inflorescences and flowers and flowering behaviour are reported for the woody liana Anchietea pyrifolia (Violaceae) from Brazil. The specimen studied is grown for some decades now in the greenhouses of Halle Botanical Garden and turned out unisexually male, which adds a further example of dioecism to the family Violaceae, in which this type of sex distribution is rarely encountered. The flowers are exceptional also for the strongly asymmetric anterior petal, which represents a rare case of a species with enantiomorphic flowers pollinated by Lepidoptera. They have a fully developed gynoecium with a complicated architecture comparable to the pistil of bisexual Violaceae flowers, though without ovules. The style head is capable to release viscose liquid on tactile stimulation or pressure, which is known to act as pollen-gathering mechanism in bisexual Violaceae species with usually dry pollen and buzz-pollination. This function has switched in male A. pyrifolia to a mechanism for efficient pollen release mediated by insect pollinators from its short-lived flowers.

The role of pollen in the changing environmental conditions of Scots pine

Rapid climate warming poses a challenge to forest trees adapted to cooler conditions. However, a genetic safety net may exist for wind pollinated tree species in the form of long distance gene flow mediated by pollen containing alleles that are (pre)adapted to a warmer climate. In order to examine long distance gene flow in Scots pine (Pinus sylvestris L.), we monitored flowering phenology in central, northern, and northernmost Finland and measured the amount and viability of airborne pollen over four years. Viable airborne pollen grains were detected during female flowering periods and prior to local pollen shedding in all study sites. Depending on the year and study site, female flowers were exposed only to pollen originating elsewhere for between one and four days. Foreign airborne pollen represented 2.3% of all viable grains detected and varied between years and study sites. Artificial pollination trials were conducted to evaluate the effect of pollen origin on seed siring success. Pollen genotypes originating from southern Finland sired 76% and 48% of seeds in competition experiments where pollen from both populations was introduced simultaneously into the female strobilus. Furthermore, we investigated the importance of pollen arrival order by introducing pollen of northern and southern genotypes in to the strobilus at two-hourly intervals. Northern genotypes sired 76% of the analysed seeds when injected first but when southern genotypes were supplied first, both pollen types enjoyed equal success. The first pollen grain in the pollen chamber did not always fertilize the ovum. Instead there is likely a more subtle and complex form of competition between genotypes and their pollen grains. To examine chemically-mediated pollen interactions we conducted an in vitro germination experiment where different genotypes were in chemical but not physical contact. Both positive and negative interactions were found. We found highly negative effects in the germinability of northern pollen grains when in contact with southern pollen, and an increase in the germinability of southern pollen when in contact with northern pollen. There was no variation in the size of dry pollen grains between sources, but after hydration and germination, northern pollen grains were larger than southern pollen. Genotypes with high hydration rates had low germinability and slow pollen tube growth rates. This suggests that early germination and growth of the pollen tube is more dependent on pollen storage materials and less dependent on water intake. Pollen source affected the outcome of competition experiments and therefore is an important factor in long distance gene flow. However, genotype also plays a role and must be taken into account.

Pollen Morphology of the Species Belonging to the Genus Dontostemon (Andrz.) C. A. Mey. (Brassicaceae Jussj)

Pollen grains of fi ve species of the genus Dontostemon (Andrz.) C. A. Mey are studied in detail by using light and scanning electron microscopes. Pollen description of each taxon is given. Dry pollen grains are oval, tricolpate with average size of 27.4-38.1 μm in length, and 22.6-29.1 μm in width, light yellowish in color, reticulate on surface, which made of amorphous polygons, mostly pentagonal units of variable in size.

Breaking the cells of rape bee pollen and consecutive extraction of functional oil with supercritical carbon dioxide

Supercritical carbon dioxide was applied to break the cell wall of rape bee pollen and consecutively extract lyzed bee pollen oil. The supercritical CO 2 rapid depressurization was carried out on the bee pollen. The conditions of consecutive extraction of lyzed bee pollen oil were optimized using response surface methodology and performed at the range of pressure 13.2–46.8 MPa, temperature 33.2–66.8 °C and CO 2 flow rate 6.6–23.4 L/h. Scanning electron microscopy studies revealed that supercritical CO 2 treatment was an effective way to break cell of rape bee pollen and higher CO 2 pressure was found to be more effective. The extraction pressure, temperature and CO 2 flow rate significantly affected the yield of lyzed bee pollen oil in supercritical CO 2 extraction. Optimum oil yield was obtained as 5.98 g/100 g dry pollen at the following predicted conditions: extracted at 39.2 MPa and 54.7 °C with CO 2 flow rate of 17.1 L/h after bee pollen was treated with supercritical CO 2 at 45 MPa for 10 min and then depressurizing. Polyunsaturated fatty acids were the predominant fatty acids in the extracted oil, indicating its potential in the nutraceutical industry. The application of supercritical CO 2 technology in food industry has received much attention in recent years. This study investigates the feasibility of breaking pollen cell wall by supercritical CO 2 . The procedure is under low temperature with short time. From an industrial point of view, it is a promising method for breaking bee pollen cell wall. The optimum parameter of supercritical CO 2 extraction of functional pollen oil simultaneously obtained from the process may also be attractive to the food and nutraceutical industries.

Bundles of hexagonally arranged tubules in timothy grass pollen: Detection of a novel pollen component using anhydrous fixation and image analysis techniques in transmission electron microscopy

Pollen from timothy grass (Phleum pratense L.) was subjected to various aqueous and non-aqueous fixation and preparation protocols for transmission electron microscopy. Only in the cytoplasm of anhydrously prepared pollen grains were conspicuous inclusions observed that range in size from less than 1 mum up to 8 or 10 mum. These bodies have so far not been described in the literature. Higher magnifications show that these inclusions consist of bundles of hexagonally arranged small tubules. In order to obtain details of the ultrastructure of this novel pollen component, TEM micrographs of ultrathin sections of hexagonally arranged tubules were analyzed using Fourier transform techniques of image analysis. It was found that the tubules form groups with quasi-periodic hexagonal arrangement, with an average centre-to-centre spacing between the neighbouring tubules of approximately 42 nm. Individual tubules are formed by 12 or 13 particles. The outer diameter of the tubules ranges between 22 and 24 nm. From our experiments, we conclude that the quasi-periodic hexagonally arranged tubules forming conspicuous cytoplasmic inclusions in dry timothy grass pollen grains are structurally similar to microtubules.

The biomechanics of Cornus canadensis stamens are ideal for catapulting pollen vertically

Summary Rapid movements in fungi and plants have evolved in different species to facilitate the dispersal of spores and seeds. The mechanisms of action can differ among species, but the effectiveness of these movements has rarely, if ever, been tested. Here we show through a quantitative biomechanical analysis that the stamens of Cornus canadensis L. (bunchberry) are ideal for catapulting pollen vertically at high speeds. We develop a biomechanical model to describe the explosive launch of pollen from the flowers of bunchberry. The model determines the equation of motion for the stamens based only on the morphology and measurements of the parts of the stamens. To measure the motion of the stamens to compare with our model, we analysed individual frames of a video taken at 10 000 fps. The thecae of adjacent stamens dehisce in bud so that the stomia face each other, retaining pollen between neighbouring anthers. As the flowers open, pollen is accelerated vertically as long as the thecae remain in contact. Pollen is released only when the anthers move horizontally and separate. The observed motion of the stamens matches the results from our model through release of the pollen. The model reveals that pollen release (horizontal movement of the anthers) occurs only after the vertical speed is at its maximum. Thus, for this particular catapult mechanism, the morphology of the stamens is optimal for launching light, dry pollen straight upwards at high speed. Pollen launched vertically at high speed both enhances insect pollination by helping to making pollen stick on visiting insects, and also allows for successful wind pollination by propelling pollen into the air column. Seed set by inflorescences in pollinator‐exclosure cages further supports the ability of this flower to use wind as a pollination mechanism.