Why does some pollen lack apertures? A review of inaperturate pollen in eudicots

Apertures are key characters of pollen grains with systematic importance in angiosperms. They function as sites for pollen tube exit, water uptake, transfer of recognition substances and accommodation of volume changes. Not all pollen has apertures; inaperturate pollen (lacking obvious apertures) characterizes many angiosperm groups, especially in early divergent angiosperms and monocots, but also eudicots. In order to expand our knowledge of the systematic distribution, possible functional significance and development of inaperturate pollen in angiosperms, this review focuses on inaperturate and cryptoaperturate (with hidden apertures) pollen in the large eudicot clade, which comprises about 75% of present‐day angiosperm species. It includes new TEM observations of inaperturate pollen from four exemplar taxa selected from different parts of the eudicot phylogeny. Two categories of inaperturate (including cryptoaperturate) pollen occur in eudicots. (1) Sterile attractant or feeding pollen associated with functional dioecy has evolved iteratively at least six times in conjunction with complex breeding systems in the core eudicots. (2) Fertile pollen has evolved numerous times independently throughout eudicots, though generally in a relatively small number of individual taxa. Notable exceptions are the petaliferous crotonoid Euphorbiaceae s.s., in which fertile inaperturate pollen occurs in c. 1500 species, and two subfamilies of Apocynaceae s.l. (Secamonoideae and Asclepiadoideae) with c. 2500 species with fertile inaperturate pollen in pollinia. Fertile inaperturate pollen is sometimes (but not always) associated with an aquatic habit, parasitism, insectivory, heterostyly, anemophily or pollinia. Most fertile inaperturate pollen has a thin exine, or the exine is largely restricted to isolated components (muri, protuberances, subunits) separated by thinner areas which probably function as apertures. In cryptoaperturate pollen, the aperture is covered by continuous exine which probably has a protective function, similar to an operculum. Developmentally, inaperturate pollen is not associated with any particular tetrad type or meiotic spindle orientation (unlike some apertures) due to the absence of a colpal shield of endoplasmic reticulum or other organelles and hence is independent of microsporogenesis type. The lack of a colpal shield during the tetrad stage of development permits complete deposition of first primexine and then exine around each microspore, possibly mediated by the action of the DEX1 protein. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155 , 29–48.     

Airborne pollen in Bahía Blanca,Argentina: seasonal distribution of pollen types

The composition and seasonal distribution of airborne pollen grains in the atmosphere of Bahía Blanca, Argentina, has been studied between June 2001 and December 2003 using the Rotorod sampler (model 40). The results show that the main pollen types during this period were Cupressaceae, Fraxinus, Myrtaceae, Poaceae, Amaranthus/Chenopodiaceae, Pinus, Urticaceae, Ulmus, Olea and Styphnolobium. The highest concentrations occurred from August to December (end of winter and spring), accounting for 80% of the total annual pollen count. The greatest diversity was found in the spring, with the major of pollen coming from short-flowering plant types, such as Populus, Acer, Platanus, Juglans, Tamarix, Ailanthus and Typha. The potential sources of pollen from woody ornamental species are Cupressus sempervirens, Eucalyptus camaldulensis and Fraxinus pennsylvanica. whereas those from herbaceous species are the Chenopodiaceae and Poaceae, which are found within the city and also in the surrounding natural vegetation, and the Urticaceae, which are only present in the city. Marked annual differences were noted during the study period. The increase in 2002 may have been due to the abundant rainfall that occurred prior to the spring season, which would have favored the vegetative stage and flower development of plants. The decrease in pollen concentration in 2003 was mainly due to low rainfall throughout the year.     

How much pollen can thrips destroy?

ABSTRACT. 1. A laboratory technique for measuring the number of pollen grains consumed by thrips is described.
2. Thrips imaginis Bagnall and Thrips obscuratus (Crawford) (Thripidae) were studied particularly on pollen of the kiwifruit Actinidia deliciosa (A. Chevalier) Liang & Ferguson in New Zealand, and Echium plantagineum L. in Australia.
3. Mean daily feeding rates (in grains per thrips per day) ranged from 29 to 843, with an individual rate as high as 1626 for T.imaginis larvae II on E. plantagineum.
4. The time taken to feed on a single grain was proportional to grain volume, and decreased with temperature.
5. Daily feeding rates were significantly different between pollens, and were higher for smaller grains. The total volume of pollen contents consumed and the total time spent ingesting this volume per thrips per day may be constant with respect to pollen species.
6. Daily feeding rates were equivalent to 0.2–0.7% of the average total pollen production of a flower per thrips per day.
7. Extrapolation of the daily feeding rates suggests that pollen damage by thrips could sometimes be reducing crop yield or plant fitness.     

A practical approximation of pollen dispersal–deposition for calculation of pollen productivity and quantification of vegetation

We compared simple mathematical pollen dispersal–deposition models with Gaussian plume models. The simple mathematical models proved equal or better approximations of real world pollen dispersal–deposition. We concluded that for most standard applications, such as estimating pollen productivity or quantitative vegetation reconstructions, simple mathematical models would perform satisfactory. Such easy-to-use models may lower the threshold to employ quantitative tools to palaeoecological questions.     

Cyclic pollen production in Cedrus deodara

Based on a seven-year study of pollen production and release in two different natural populations of Cedrus deodara from Garhwal Himalaya, India, we determined that pollen output follows a two-year cycle. Pollen productivity determinations considered various sources of variability, including the number of pollen strobili per branch, strobili per tree, microsporophylls per tree and pollen grains per tree. Each of these parameters revealed significant year-to-year and population effects. Microsporangium dehiscence took from 2.5 to 3.5 days. Maximum dehiscence was observed between 12:00 and 14:00 h, which coincides with diurnal highest temperature and lowest relative humidity. Annual production of pollen per tree varied from averages of 4.7 × 10⁹, 7.2 × 10⁹ and 5.1 × 10⁹ in years of low production, with alternate high years, producing 12.6 × 10⁹, 14.1 × 10⁹, 13.3 × 10⁹ and 14.0 × 10⁹ pollen grains per tree. Annual pollen production in individual trees of C. deodara ranged from 1.4 × 10⁹ to 22.3 × 10⁹.     

Variation in annual pollen accumulation rates of Fagus along a N–S transect in Europe based on pollen traps

Annual pollen-accumulation rates (PAR) of Fagus (beech) obtained within the framework of the Pollen Monitoring Programme (PMP) were analyzed in pollen traps along a N–S transect from the Baltic Sea to the Black Sea in different European vegetation units. The study regions are situated in the lowlands of northern Poland, the uplands of SE Poland, the Czech Krkono?e Mts, the Czech ?umava Mts, the Swiss Jura Mts, the Swiss Alps, the Bulgarian Rila Mts and the Bulgarian Strandzha Mts. Most time series are 10 or 11 years long, some are 5–16 years long. Inter-annual fluctuations in Fagus PAR were analyzed and compared with seed mast years. Years with high Fagus PAR and others with low Fagus PAR occurred most frequently in parallel within each region and often in two neighbouring regions. 2006 was exceptional as it had a very high Fagus sylvatica pollen deposition in all study regions and it was also a mast year. In Bulgaria, the trend in the 5 years of Fagus orientalis PAR in the Strandzha Mts differed from that of F. sylvatica PAR in the Rila Mts. Aiming at establishing the relationship between average Fagus PAR and tree cover, differences in Fagus PAR (averaged per pollen trap) were related in each region to the proportion of beech trees in the vegetation within 2 km of the pollen traps, the distance to the nearest pollinating Fagus tree, regional or local presence of beech forests, the degree of landscape openness, and the size of forest opening in which a trap is situated. Average Fagus PAR was found to track the regional abundance of beech trees in the vegetation, not the distance of the nearest Fagus tree. Regional occurrence of beech-dominated forests was reflected by a Fagus PAR of ca. 1,400 grains cm^?2 year^?1, local abundance very close to pollen traps by ca. 2,400 grains, small patches of forest with admixture of Fagus by ca. 170–220 grains, and scarcity or absence of Fagus by ca. 40 grains or less.     

The relationship between airborne pollen and fungal spore concentrations and seasonal pollen allergy symptoms in Cracow in 1997–1999

The investigation of airborne pollen and fungalspore concentrations was carried out in Cracowbetween 1997–1999. For this study thevolumetric method has been employed (Burkard).At the same time the clinical diagnosis ofpollen allergy in 40 patients was obtained onthe basis of an interview, positive skin pricktests with pollen extracts and increasedspecific IgE level. An increase in seasonalallergy symptoms in all patients occurred fromthe middle of May to the middle of August.Eighty eight percent of the patients (35 out of40) were sensitive to Poaceae pollen and about50% of them were additionally sensitive totree and herb pollen excluding grasses. Forpatients with additional allergy to tree pollenthe seasonal symptoms started at the end ofMarch (Betula) while for patients withadditional allergy to herb pollen it wasextended to the middle of September (Artemisia).Five people out of 40 revealed positive skinreactions to Alternaria spores and anincrease in specific IgE level. Positive skinreaction to Cladosporium spores with noincrease in specific IgE level occurred in 2patients. The increase in seasonal allergysymptoms in people sensitive to Alternariaspores noted in July and August could becaused not only by these spores but also byPoaceae pollen.     

Intergeneric pollen – megagametophyte relationships of conifers in vitro

 Germinating pollen from larch (Larix occidentalis), Sitka spruce (Picea sitchensis) and white pine (Pinus monticola) were co-cultured with megagametophytes dissected from cones of other genera (Pseudotsuga menziesii, Larix×eurolepis and Pinus monticola). Pollen was presented to megagametophytes possessing archegonia which were either alive, degenerating or dead. In addition, pollen was presented to fertilized megagametophytes and to megagametophytes that had been cut in half. Megagametophyte penetration by pollen tubes and male gamete release into archegonia were verified by serial sections of glycomethacrylate-embedded specimens. Pollen tubes penetrated through any part of the apex of the megagametophyte. Division of the body cell into the two gametes was regularly observed. Delivery of gametes was confirmed between spruce and larch. Pollen tubes also penetrated fertilized megagametophytes, dead or degenerating archegonia as well as wounded and/or cut surfaces. This demonstrates the inability of the male gametophyte to optimize its mating efforts, since it is unable to differentiate between healthy and unhealthy archegonia. The megagametophyte cells are unable to optimize male selection. They may produce secretions of a generally attractive nature, as pollen is attracted to the apex of the megagametophyte, but archegonia themselves do not produce pollen-specific signals of either a promotive or inhibitory nature. These results open new avenues for the development of novel breeding strategies where natural breeding barriers may be bypassed. Received: 19 March 1998 / Accepted: 29 April 1998     

Airborne pollen flora of Jabalur – the central India

The fortnightly aerobiological survey of pollengrains at five different sites at two differentheights (ground level and at 10 m) was carriedout at Jabalpur city during January toDecember, 1996. Air sampling was done for 30minutes at each sampling point by using aRotorod air sampler. A total of 84 pollen typesbelonging to 34 families of Angiospermae andone family of Gymnospermae were identified. Themaximum mean concentration (1.04/m³ air) ofpollen was contributed by Asteraceae followedby Amaranthaceae/Chenopodiaceae (0.77/m³ air)and the minimum (0.001/m³ air) byCucurbitaceae, Convolvulaceae and Sapotaceae.Maximum numbers of pollen grains werecontributed by herb species (62.07%) followedby shrubs (18.47%) and trees (14.96%). Asmall fraction, i.e., 4.50% of total pollenflora remained unidentified. Morphologicalanalysis revealed maximum concentration(1.81/m³) of triporate pollen grains. There wasa significant difference in the pollenconcentration at ground level and at 10 mheight. The ratio observed was 1.7 : 1.0.There was non-significant negative correlation(r = –0.196) between mean pollen concentrationand mean atmospheric temperature. May, thehottest month, showed minimum mean pollenconcentration (1.65/m³ air), whereas October,with moderate temperature, the maximum(10.86/m³ air).     

Does a lateral gradient of membrane potential on the plasma membrane of growing pollen tube of germinating pollen grain exist?

The data presented in the article by Breigina et al. (2009) "Changes in the membrane potential during pollen grain germination and pollen tube growth" (Tsitologiya. 51 (10): 815-823) and concerning the measurement of electric membrane potential (Delta Psi) on the plasma membrane of growing pollen tube of germinating pollen grain with the use of fluorescent potential-sensitive dye, di-4-ANEPPS, were critically analyzed in order to clarify whether a lateral gradient of Delta Psi on this membrane indeed exists. This analysis showed that the main conclusion of the authors of the above article on the existence of polar distribution of Delta Psi along the pollen tube plasma membrane is not in accordance with a number of known peculiarities of di-4-ANEPPS behavior in biological membranes and requires a significant revision. The findings in question reported by the authors, in my opinion, might be interpreted as evidence for the presence on the plasma membrane of growing pollen tube not only the membrane potential Delta Psi but also lateral gradient of so called intra-membrane dipole potential. Based on the comments made, another interpretation of the experimental results described by Breigina et al. has been offered. In addition, some drawbacks in the methodology used by the authors for measurement of Delta Psi with other fluorescent potential-sensitive dye, DiBAC3(3), are also shortly considered.     

Erdtmanitheca portucalensis,a new pollen organ from the Early Cretaceous (Aptian–Albian) of Portugal with Eucommiidites-type pollen

A new lignitised, slightly compressed pollen organ, Erdtmanitheca portucalensis, with affinities to extinct Erdtmanithecales from the Early Cretaceous (Aptian–Albian) of Vale de Água (Lusitanian Basin, western Portugal), is described. The pollen organ is composed of loosely arranged microsporophylls radiating from a central core. The estimated number of microsporophylls is about 100–150. The microsporophylls are sessile and ellipsoidal to barrel-shaped with a flattened or slightly apically depression containing about ten narrow sporangia. The sporangia enclose abundant well-preserved pollen grains of Eucommiidites-type. Pollen grains found in situ are elliptical in equatorial outline, about 16.0–27.2 μm long and 11.9–16.4 μm wide. The main (distal) colpus is long with expanded rounded ends. It is flanked by two subsidiary colpi in an almost equatorial position. The surface of the pollen wall is psilate and occasionally punctate. The ektexine is composed of a distinct tectum, granular infratectal layer and a thin foot layer. The endexine is thick and laminar. The new Early Cretaceous Portuguese pollen-organ is similar in several respects to that of Erdtmanitheca texensis described from the Late Cretaceous of Texas, USA. The new fossil species further documents the importance of the Bennettitales-Erdtmanithecales-Gnetales group in the Early Cretaceous floras of Portugal extending the stratigraphic and geographical distribution of the genus with regard to systematic and phylogenetic significance of the Eucommiidites-producing plants that may have been co-occurring with the Early Cretaceous diversification of angiosperms. It is ascertained that perforate tectum occurs in pollen grains with a well-developed foot layer as well as in pollen grains in which a foot layer is poorly developed or lacking, and that pollen features do not support a separation of the Erdtmanithecales seeds and pollen organs.     

Do pollen donors with fastest-growing pollen tubes sire the best offspring in an anemophilous tree, Betula pendula (Betulaceae)?

The relationship between pollen and progeny performance has been a subject of many studies but the evidence for pollen-tube growth rate as an indicator of progeny fitness is equivocal. We used an anemophilous tree, Betula pendula, to examine the relationship between pollen-tube growth rate and seed and seedling performance. We crossed nine maternal plants with pollen from six pollen donors in a clonal B. pendula seed orchard, measured the pollen-tube growth rates for every cross, and analyzed the performance of the resulting seeds and seedlings. The only significant positive correlation was found between pollen-tube growth rate and seed mass when we controlled for seed number per inflorescence. Using seed mass as a covariate, we found that only maternal parent had a significant effect on the number of seeds per inflorescence, the percentage of germinable and embryonic seeds, and early seedling growth. Both maternal and paternal parents had significant effects on seedling height after 85 d of growth. These results are in concordance with the general view that maternal effects are usually most apparent in seed characters and during early plant growth. This study does not provide strong evidence for the theory of pollen-tube growth rate as an indicator of progeny quality.     

Does aluminum inhibit pollen germination via extracellular calmodulin?

The effect of aluminum (Al) on pollen germination and its mechanism of action were investigated. Pollen germination and pollen tube elongation were inhibited by Al at pH 4.5. This inhibitory effect was reversed by the addition of purified calmodulin (CaM), whereas neither the calcium binding-protein S-100 nor Al chelator citric acid at the same concentrations had any obvious effect on Al-inhibited pollen germination. The presence of either the membrane-impermeable CaM inhibitor anti-CaM antiserum or Ca2+ chelator EGTA completely suppressed the effect of exogenous CaM. These results indicate the involvement of extracellular calmodulin in the short-term effects of Al on pollen germination and pollen tube elongation.     

Effects of γ-ray treatment onCannabis saliva pollen viability

The viability and thein vitro germination capability of hemp pollen (cv. Carmagnola) were studied. Viability tests were based on the microscopic observation of the fluorescence of loaded fluorescein diacetate (FDA), while, for germinability tests, five different media were tested. The effects of irradiation with γ-rays on pollen viability and germination and on seed set were also studied, at three different irradiation doses (20, 60 and 100krad). The results show that in one of the media tested, about 85–90% of the pollen grains are viable and able to germinate in control samples, and that while viability measured by FDA test is not affected by increased γ-ray doses, the pollenin vitro germinability drops to about one-half of the controls at the maximum γ-ray dose employed, 100krad. Seed set of hemp plants pollinated with the irradiated pollen dropped to less than 1% of that of plants pollinated by untreated pollen for the higher dose used. The different media suitable forin vitro germination of hemp pollen, and the observed lack of correspondence between viability and germination capacity tests are discussed.     

Does pollen competition reduce the cost of inbreeding?

We hypothesize that floral features promoting pollen competition in angiosperms may have evolved, in some cases, in response to selection generated by the negative effects of inbreeding, at least in plants with mixed-mating systems. Screening of haploid genotypes through pollen competition may purge recessive (or additive) deleterious alleles that are expressed in haploid pollen and hence may reduce the fitness cost of self-pollination, geitonogamy, or biparental inbreeding. We tested one prediction of this hypothesis, that offspring produced by more intense competition among self-pollen have higher fitness than offspring produced by less intense competition. Dalechampia scandens (Euphorbiaceae) flowers were pollinated with pollen from other flowers on the same plant (geitonogamous self-fertilization). Those flowers experiencing more intense pollen competition as a result of low pollen dispersion (positional variance) on the stigma produced heavier seeds and seedlings with faster-growing radicles than flowers experiencing less intense pollen competition (high pollen dispersion), as predicted by our hypothesis.     

Birch pollen abundance in Reykjavík,Iceland

A ten-year birch pollen record for Reykjavík, Iceland, has been analyzed with respect to fluctuations in the annual pollen sum, the starting date, and the duration of the birch pollen season. A three-year cycle is observed for the annual birch pollen sum, which could be of value in predicting the severity of the next birch pollen season. The annual birch pollen sum is regressed on six climatic variables, only one of which turned out to be statistically significant. The number of days with temperatures above +7.5°C in the year of inflorescence initiation shows a significant correlation with the annual birch pollen sum of the following flowering year (P =0.009). For the starting date of the birch pollen season the accumulated thermal sum on May 15 is shown to have the best predictive value, estimating it to within one week. The mean duration of the birch pollen season in southwestern Iceland is 17 days, starting May 29 and ending June 14.     

Airborne pollen calendar of Buca–Izmir, Turkey

This communication presents the level ofpollen in the atmosphere of Buca–Izmir, Turkey.The evaluation includes qualitative as well asquantitative results. Studies were carried outduring 1996–1997 using gravimetric method. Datawas recorded from lower (BUL. 1.60 m) andhigher (BHL. 20 m) levels. Aeropalynologicalresults revealed that the atmosphere in thisarea abounds in pollen from 55 taxa out ofwhich 24 are woody and 31 herbaceous. At bothlevels plant groups with higher pollendensities per square centimeter were trees(BHL. 89%, BUL. 73.7%), other herbs (BHL.6.6%, BUL. 20.4%), Gramineae (BHL. 3.8%,BUL. 4.9%) and unidentified taxa (BHL. 0.6%,BUL. 1%) respectively. However, at lowerlevel Pinus, Quercus, Oleaceae,Morus, Centaurea, Gramineae,Chenopodiceae/Amaranthaceae showed highestconcentration; whereasCupressaceae/Taxaceae, Quercus,Gramineae, Chenopodiaceae/Amaranthaceae andPlantago were dominant at higher level.The concentration of allergic taxaChenopodiaceae/Amaranthaceae, Cupressaceae/Taxaceae; Gramineae, Morus, Oleaceae, Pinus, Plantago and Quercus was highin the atmosphere of the study area. The pollencalendar of the area has been prepared inrelation to the atmospheric parameters.     

One hundred years of Quaternary pollen analysis 1916–2016

We review the history of Quaternary pollen analysis from 1916 to the present-day, with particular emphasis on methodological and conceptual developments and on the early pioneers of the subject. The history is divided into three phases—the pioneer phase 1916–1950, the building phase 1951–1973, and the mature phase 1974–present-day. We also explore relevant studies prior to Lennart von Post’s seminal lecture in 1916 in Kristiania (Oslo) in an attempt to trace how the idea of Quaternary pollen analysis with quantitative pollen counting and stratigraphical pollen diagrams developed.