Numerous Pollen Research Articles

Strong pollen limitation in genetically uniform hybrid chestnut orchards despite proximity to chestnut forests

Key messageIn arboriculture, the number and diversity of pollen donors can have a major impact on fruit production. We studied pollination insurance in hybrid chestnut orchards (C. sativa × C. crenata) provided by nearby wild European chestnuts (C. sativa) in southwestern France. Most fruits were sired by hybrid pollenizers rather than by wild chestnuts. When these hybrid pollenizers were too scarce, a frequent situation, pollen produced by wild chestnut trees did not compensate for the lack of compatible pollen and fertilization rates and fruit production collapsed.ContextThe demand for chestnuts has been increasing in recent years in many European countries, but fruit production is not sufficient to meet this demand. Improving pollination service in chestnut orchards could increase fruit production.AimsInvestigate pollination service in chestnut orchards. Evaluate the contribution to pollination of trees growing in chestnut woods and forests.MethodsWe investigated five orchards planted with hybrid chestnuts (C. sativa × C. crenata) cultivars in southwestern France. We combined fruit set data, which provide information about pollination rate, with genetic data, which provide information about pollen origin. We used this information to estimate the contribution of nearby C. sativa forest stands to the pollination of each orchard.ResultsPollination rates vary considerably, being fivefold higher in orchards comprising numerous pollen donors than in monovarietal orchards. Because of asymmetric hybridization barriers between hybrid and purebred cultivars, the surrounding chestnut forests provide very limited pollination insurance: less than 14% of the flowers in these monovarietal orchards had been pollinated by forest trees.ConclusionBecause chestnut orchards are now increasingly relying on hybrid cultivars, surrounding wild European chestnut trees are no longer a reliable pollen source. To achieve maximal fruit set, efforts must therefore concentrate on orchard design, which should include enough cultivar diversity.

52 million years old Eucalyptus flower sheds more than pollen grains.

PREMISEFossils provide fundamental evidence of the evolutionary processes that crafted today’s biodiversity and consequently for understanding life on Earth. We report the finding of Myrtaceidites eucalyptoides pollen grains preserved within the anthers of a 52‐million‐year‐old Eucalyptus flower collected at Laguna del Hunco locality of Argentinean Patagonia and discuss its implications in understanding the evolutionary history of the iconic Australian genus Eucalyptus.METHODSPollen grains were extracted from the flower’s anthers and were then observed under light microscopy and scanning electron microscopy. The phylogenetic position of the fossil was investigated by adding pollen data to a previously published total‐evidence matrix and analyzing it using parsimony.RESULTSWe erect the species Eucalyptus xoshemium for the fossil flower. Pollen extracted from E. xoshemium belongs to the species Myrtaceidites eucalyptoides, which, until now, was only known as dispersed pollen. The numerous pollen grains recovered from the single flower allowed estimation of M. eucalyptoides’ variability. Results of the phylogenetic analysis reinforce the position of this fossil within crown group Eucalyptus.CONCLUSIONSThe discovery of these pollen grains within a Patagonian Eucalyptus fossil flower confirms the hypothesis that Myrtaceidites eucalyptoides represents fossil pollen in the Eucalyptus lineage, extends the geographic and stratigraphic fossil pollen record, and supports an earlier age for crown‐group eucalypts.

Alder pollen concentrations in the air during snowfall

The focus of our study was airborne alder pollen because it is one of the main causes of inhalant allergies in many countries in the Northern Hemisphere. The main research setback was pollen concentrations during snowfall. Analyses from a 21-year database showed that the hourly patterns of occurrence of airborne Alnus pollen during snowfall differ. Snowfall can cause a decrease in pollen concentrations in the air that may persist for several hours. However, during the snowfall period of 2018, an increase in pollen concentrations was observed. High temperatures during the days preceding snowfall stimulate thermal currents, and pollen could become airborne. During snowfall, airborne pollen grains are supposedly washed out of the atmosphere and numerous pollen grains are deposited on the snow surface. Hypotheses about the long-distance transport of pollen have also been verified. Back-trajectory analysis has revealed that air masses from Belarus and Ukraine were transported to Rzeszow. We found that the influence of snowfall on pollen concentrations is ambiguous and individuals prone to allergies also notice symptoms on days with snowfall and temperatures below zero.

Scanning electron microscope and light microscope investigations of pollen from an atypical mid-Eocene coal facies in Stolzenbach mine (PreußenElektra) near Borken (Kassel, Lower Hesse, Germany)

Light microscope (LM) and scanning electron microscope (SEM) investigations of palynomorphs from a mid-Eocene coal lens at the Stolzenbach brown-coal mine (“Borken brown coal field”) yielded a remarkable microflora assemblage, with 76 pollen and spore taxa, dominated by monolete fern spores. Numerous pollen and pollen clumps of Myrtaceae and Salicaceae might represent in-situ growth of these families, which were associated with pollen of Aralia-type, Eotrigonobalanus spp., Flueggea-type, Nyssa-type, three Pentapollenites taxa, and Spinaepollis spinosus, plus many others. Ericaceae such as Erica-type, Kalmia-type, and Rhododendron-type pollen are described from here for the first time; the last represents the earliest known occurrence of the genus. Additionally, five tropical south Asian Sapotaceae taxa (Mimusopeae- and Isonandreae tribes), and one tropical South American bombacoid Eriotheca-type occur. Several previously undescribed pollen taxa for the Eocene have been found; some are amongst the oldest examples of these pollen taxa: the Nuphar-type pollen represents the earliest occurrence in Europe and, whilst the Acalypha-type and Trochodendron-type are the oldest pollen occurrences in general, the Viburnum-type, affiliable to Viburnumclemesiae probably represents a tropical south Asian lineage.

Novel computed tomography-based tools reliably quantify plant reproductive investment.

The flower is a bisexual reproductive unit where both genders compete for resources. Counting pollen and ovules in flowers is essential to understand how much is invested in each gender. Classical methods to count very numerous pollen grains and ovules are inefficient when pollen grains are tightly aggregated, and when fertilization rates of ovules are unknown. In this study we have therefore developed novel counting techniques based on computed tomography. In order to demonstrate the potential of our methods in very difficult cases, we counted pollen and ovules across inflorescences of deceptive and rewarding species of European orchids, which possess both very large numbers of pollen grains (tightly aggregated) and ovules. Pollen counts did not significantly vary across inflorescences and pollination strategies, whereas deceptive flowers had significantly more ovules than rewarding flowers. The within-inflorescence variance of pollen-to-ovule ratios in rewarding flowers was four times higher than in deceptive flowers, possibly demonstrating differences in the constraints acting on both pollination strategies. We demonstrate the inaccuracies and limitations of previously established methods, and the broad applicability of our new techniques: they allow measurement of reproductive investment without restriction on object number or aggregation, and without specimen destruction.

A palynological study of natural honeys from the Bongaigaon district of Assam, northeast India

ABSTRACTPalynological examination of 18 honey samples (BG-1–18) procured from nine localities in the Bongaigaon District, Assam, India, produced a assemblage of pollen in terms of quantity and diversity. According to palynological assessment of the honey samples, 12 were unifloral (five in terms of Brassica campestris, two from Elaeocarpus rugosus and one each from Mimosa pudica, Xanthium strumarium, Syzygium cumini, Salmalia malabaricum and Meliaceae). Six were multifloral; the dominant taxa include Mimosa, Brassica, Coriandrum, Ziziphus mauritiana and Salmalia. Taxa such as Cassia fistula, Acacia catechu, Toona ciliata, Butea monosperma, Lagerstroemia parviflora, Lamiaceae, Malvaceae and Oleaceae were minor sources of nectar and bee forage, as indicated by low frequencies of their pollen. The numerous pollen types and their diversity show that bees travel considerable distances to collect nectar for honey production. The study could be helpful in evaluating the allergenicity of some honeys in the area of investigation, causing hay fever, asthma, dermatitis and other disorders, where composition is mainly of ‘Compositae allergic’ pollen such as Xanthium (BG-2; 57.78%; unifloral) and Eupatorium (BG-4; 37.64%; unifloral).

Generative reproduction of Antarctic grasses, the native species Deschampsia antarctica Desv. and the alien species Poa annua L.

AbstractThe embryology of two species, Deschampsia antarctica, a native species, and Poa annua, an alien species in the Antarctic we studied. Flowering buds of plants growing in their natural habitats on King George Island and generative tissues of both plant species grown in a greenhouse were analyzed. Adaptations to autogamy and anemogamy were observed in the flower anatomy of both species. The microsporangia of the evaluated grasses produce a small number of three−celled pollen grains. Numerous pollen grains do not leave the microsporangium and germinate in the thecae. Deschampsia antarctica and P. annua plants harvested in Antarctica developed a particularly small number of microspores in pollen chambers. In D. antarctica, male gametophytes were produced at a faster rate: generative cells in pollen did not become detached from the wall of the pollen grain, they were not embedded in the cytoplasm of vegetative cells, and they divided into two sperm cells situated close to the wall. The monosporous Polygonum type of embryo sac development was observed in the studied species. The egg apparatus had typical polarization, and the filiform apparatus did not develop in synergids. Large antipodals with polyploidal nuclei were formed in the embryo sacs of D. antarctica and P. annua. Poa annua was characterized by numerous antipodal cells which formed antipodal tissue in the chalazal region of the embryo sac. Three distinct antipodals with atypical, lateral position in the vicinity of the egg apparatus were observed in D. antarctica. The diaspores of the investigated grass species were characterized by small size, low weight and species-specific primary and secondary sculpture of the testa and caryopsis coat.

The Pollen Spectrum of Apis mellifera Honey from Reconcavo of Bahia, Brazil

Background: The use of pollen grains to establish the geographical and botanical origin of honey samples, and for supplying information concerning to the floral class of a honey produced in a region, has been employed since 1895. Pollen analysis has relevant significance for the quality control of a honey that may include numerous pollen grains and elements of honeydew. The objective of this work was to determine the botanical origin of pollen from plants contributing in the composition of honey from the region of Reconcavo of Bahia, Brazil. Methodology: Form each honey sample, a 10 g subsample was diluted in 20 mL of distilled warm water (40oC) and centrifuged before the supernatant was drawn out. After centrifugation the pollen sediment was acetolysed following for better observation of the pollen grains. Subsequently, this sediment was mounted in microscopy slides with glycerin jelly for latter pollen grains counting and identification. The identification of the pollen grains found in the samples was determined by comparison with a reference collection from the Palynotheca of the Nucleus of Insect Studies from the Federal University of Reconcavo da Bahia and with descriptions obtained from specialized literature. A total of 70 honey samples of Apis mellifera L. were obtained from beekeepers of 17 Original Research Article Nascimento et al.; JSRR, 6(6): 426-438, 2015; Article no.JSRR.2015.167 427 counties in the Reconcavo of Bahia region, from March 2009 to February 2010, and palynologically analysed. Acetolyzed pollen were count and identified. Results: One hundred and twelve pollen types were identified, distributed within 35 families. Among these pollen types, 67.00% occurred with low frequency (rare) within the samples analyzed. Inside abundance class, 63.00% occurred as minor pollen. Mimosaceae was the richest accounting for 13.00% of the pollen types. There was similarity among the sources of trophic resources used by A. mellifera in the counties studied, with the highest similarity index of trophic resources found between the counties of Cabaceiras do Paraguacu and Sao Felipe (Cs = 0.68). Conclusion: Pollen analysis demonstrated that honey produced by A. mellifera in the region of Reconcavo of Bahia, Brazil, is predominantly multifloral, with emphasis the Mimosaceae.

Pollen Tube Growth and Double Fertilization in Camellia oleifera

Camellia oleifera is an important plant species that produces edible oils. Understanding the double fertilization of this plant is critical for studies concerning crossbreeding, self-incompatibility, and the biological mechanisms underlying hybridization. We aimed to characterize pollen tube growth and double fertilization in C. oleifera. The female and male parent cultivars (Huashuo and Xianglin XLC15, respectively) were used for artificial pollination. Growth of the pollen tube in the style, ovary, and ovule from pollination to fertilization and the cytological characteristics of female and male gamete fusion during double fertilization were observed using fluorescence and scanning electron microscopy (SEM). Numerous pollen grains germinated 2 to 4 hours after pollination. The pollen tubes entered the interspaces between the papillar cells, grew along the stylar canal, and aggregated at the one-third site of the style. They grew in the gradually narrowing stylar canal, entering the locule. The tubes turned 90° and entered the embryo sac through the micropyle; subsequently, they entered a degenerated synergid, where the spermatids were released. One sperm nucleus fused with the polar nucleus, forming the primary endosperm nucleus, whereas the other sperm fused with the egg, forming the zygote. The polar nucleus was fertilized earlier than the egg. Double fertilization of C. oleifera is characterized as pre-mitotic gametogony. The current results lay a theoretical foundation for studies concerning the crossbreeding and embryology of C. oleifera and provide fundamental data concerning the reproductive biology of the genus Camellia.

PECTIN METHYLESTERASE48 is involved in Arabidopsis pollen grain germination.

Germination of pollen grains is a crucial step in plant reproduction. However, the molecular mechanisms involved remain unclear. We investigated the role of PECTIN METHYLESTERASE48 (PME48), an enzyme implicated in the remodeling of pectins in Arabidopsis (Arabidopsis thaliana) pollen. A combination of functional genomics, gene expression, in vivo and in vitro pollen germination, immunolabeling, and biochemical analyses was used on wild-type and Atpme48 mutant plants. We showed that AtPME48 is specifically expressed in the male gametophyte and is the second most expressed PME in dry and imbibed pollen grains. Pollen grains from homozygous mutant lines displayed a significant delay in imbibition and germination in vitro and in vivo. Moreover, numerous pollen grains showed two tips emerging instead of one in the wild type. Immunolabeling and Fourier transform infrared analyses showed that the degree of methylesterification of the homogalacturonan was higher in pme48-/- pollen grains. In contrast, the PME activity was lower in pme48-/-, partly due to a reduction of PME48 activity revealed by zymogram. Interestingly, the wild-type phenotype was restored in pme48-/- with the optimum germination medium supplemented with 2.5 mm calcium chloride, suggesting that in the wild-type pollen, the weakly methylesterified homogalacturonan is a source of Ca(2+) necessary for pollen germination. Although pollen-specific PMEs are traditionally associated with pollen tube elongation, this study provides strong evidence that PME48 impacts the mechanical properties of the intine wall during maturation of the pollen grain, which, in turn, influences pollen grain germination.

The Impact of Olive Orchard Abandonment and Rehabilitation on Pollen Signature: An Experimental Approach to Evaluating Fossil Pollen Data

For millennia the olive was an important cultivated tree in the southern Levant, as evidenced by numerous archaeological finds and Holocene pollen assemblages. However, the impact of abandonment and rehabilitation of olive orchards (a recurrent historical process) on the fossil pollen record has not been studied. We documented quantitative differences in the olive pollen signature in a well-managed traditional olive orchard, an abandoned orchard, and an orchard rehabilitated after decades of abandonment, establishing the biological basis for understanding the olive pollen signature. The results indicate a strong decline in flowering and pollen production for decades following the cessation of cultivation and a rapid increase following rehabilitation. This strong response suggests that the fossil pollen curves are a reliable marker for determining the extent of olive oil production in ancient times. In terms of agricultural/economic efficiency, rehabilitation of an orchard takes much less time than establishing a new orchard. This could have been one of the reasons why the same sites were reoccupied during peaks of settlement activity in antiquity. The recent field results are compared to fossil pollen data from the Sea of Galilee during the Bronze and Iron Ages.

Eocene fossil is earliest evidence of flower-visiting by birds.

Birds are important pollinators, but the evolutionary history of ornithophily (bird pollination) is poorly known. Here, we report a skeleton of the avian taxon Pumiliornis from the middle Eocene of Messel in Germany with preserved stomach contents containing numerous pollen grains of an eudicotyledonous angiosperm. The skeletal morphology of Pumiliornis is in agreement with this bird having been a, presumably nectarivorous, flower-visitor. It represents the earliest and first direct fossil evidence of flower-visiting by birds and indicates a minimum age of 47 million years for the origin of bird-flower interactions. As Pumiliornis does not belong to any of the modern groups of flower-visiting birds, the origin of ornithophily in some angiosperm lineages may have predated that of their extant avian pollinators.

Decreased Pollen Viability and Thicken Pollen Intine in Antisense Silenced Brassica campestris Mutant of BcMF19

Brassica campestris male fertility 19 (BcMF19; GenBank accession number GQ902048.1), a gene that is specially expressed in tapetum cells and microspores during anther development in B. campestris ssp. chinensis, which is learned from the previous in situ hybridization study. In the present study, we constructed antisense-silenced plants of BcMF19 using B. campestris ssp. chinensis to validate this prediction. The morphology of the pistils, long anthers, and short anthers was significantly affected in 35sbcmf19 compared with the control samples. 4′-6-Diamidino-2-phenylindole staining revealed that two generative nuclei and one large vegetative nucleus were not affected in the mutant compared with control. Statistical analysis of Alexander's staining results showed that 96% of the control pollen grains had vitality, whereas only 86% of the mutant pollen grains did. Under scanning electron microscopy, the mutant demonstrated numerous abnormal pollen grains and resembled dried persimmon. The frequency of normal pollen grains was approximately 18%. Under transmission electron microscopy, the pollen intine during the binucleate and mature pollen stages in 35sbcmf19 exhibited abnormal thickening, especially at the germinal furrows, compared with control. In vitro pollen germination test showed that the tips of the mutant pollen tubes transformed into globular alveoli and stopped growing compared with control. On the other hand, in vivo pollen germination test suggested that BcMF19 affected the pollen tube extension in the pistil. These findings indicate that BcMF19 is essential to the pollen development and pollen tube extension of B. campestris ssp. chinensis.

Relationship between vegetation and modern pollen-rain along an elevational gradient on Kilimanjaro, Tanzania

Modern pollen-rain studies are crucial for the calibration and interpretation of fossil pollen records. In East Africa, numerous pollen records provide important insights into the impact of human populations and climate change on tropical ecosystems. However, the confident use of fossil pollen data to reconstruct vegetation and climate dynamics requires well-founded knowledge regarding the relationship between pollen deposition and modern-day vegetation, which is still deficient for large parts of the African continent and non-existent for the tropical East African Mountains. In this study, we investigated the relationship between vegetation and modern pollen-rain along the elevational gradient of Kilimanjaro. We apply multivariate data analysis to assess the relationship between vegetation and modern pollen-rain and quantify the representativeness of forest zones. We further assessed the taxonomic level needed for differentiation between forest zones based on the modern pollen-rain assemblage, biodiversity patterns and pollen and spore drift. In the montane vegetation of Kilimanjaro, it is sufficient to analyse the pollen-rain on plant family level in order to derive the forest zone of the surrounding vegetation. Along this elevational gradient, pollen and spore dispersal is strongly influenced by regional wind patterns, but their deposition reflects the diversity patterns of the surrounding vegetation. This study represents the first statistical analysis of pollen vegetation relationship along an elevational gradient in Africa. Hence, this paper improves confidence in interpretation of palynological records from the tropical East African Mountains and may refine past climate reconstructions for a more accurate comparison of data and modelling.

Factors affecting the production of seeds in fully fertile tomatoes (Lycopersicon esculentum L. Mill.) and those showing a tendency to parthenocarpy

Comparative studies on the development of the female gametophyte, pollination and fertilization in two lines of <em>Lycopersicon esculentum</em>, Kholodostoykye (Kh, fertile) and A33 (with a tendency to parthenocarpy) have revealed that seed production is affected by disturbances in embryo sac formation but mainly by its degeneration after anthesis, which is especially visible in line A33. Moreover, delayed development of some embryo sacs and incomplete pollination due to various stigma levels seem to be responsible for the diminution of seed number in line A33. Deep fluorescence of numerous pollen grains as well as whole pollen tubes in 83.3 per cent of A33 stigmas and only 24.1 per cent in the Kh line points to the heterogeneity of pollen. This could be one more reason for reduced fertility. The results of application of plant growth regulators (auxin, PCIB) which affect seed production in tomato of line A33 remain inconclusive.

Melissopalynological studies of western part of Almora District, Uttarakhand

The study of pollen contents has been carried out from the honey samples procured from the western zone of Almora District, Uttarakhand. A total of ten honey samples were collected of which four were unifloral and two bifloral. Sapindus rarak, Brassica campestris, Salmalia malabaricum, Caesalpinia pulcherrima, Coriandrum sativum, Syzygium cumini and Mimosa pudica are the dominant pollen types observed in the samples. Besides, the other reliable nectar sources were also identified from the study area. The numerous pollen types and their diversity showed that bees travel considerable distance for collecting nectar for honey production.

Increasing taxonomic resolution in pollen identification: Sample size, spatial sampling bias and implications for palaeoecology

Many studies proposing methods for differentiating fossil pollen of different species derive diagnostic identification criteria from small sample sizes and/or modern pollen collected from limited areas within species ranges. Here, we evaluate the importance of sample size and population representation in pollen identification methods, in an examination of morphological measurements on over 21,000 pollen from three alder species (Alnus viridis subsp. sinuata, Alnus incana subsp. tenuifolia and Alnus rubra) in western North America. We assess the sensitivity of morphometric analyses and multi-trait classification models of Alnus pollen to changes in sample size and spatial sampling breadth, by randomly reducing our large dataset in size and by splitting the dataset into regional subsets. Sensitivity analyses demonstrate that using small sample sizes to identify diagnostic morphological traits for pollen identification can result in misleading conclusions about interspecific morphological differences in pollen types and ultimately in misidentification of pollen. Pollen identification methods based on reference samples collected from limited regions are prone to bias, due to regional differences in modern pollen morphology. Our study reveals the importance of using large sample sizes from across species ranges when attempting to determine the diagnostic morphological features for fossil pollen identification. We recommend basing identification criteria on pollen from as many individual plants per species as possible, as opposed to numerous pollen grains from only a small number of individuals, because at least in Alnus, individual plants produce morphologically similar pollen.

A tribute to Prof. DSc Elissaveta Bozilova on the occasion of her 80th birthday

One of the outstanding European palynologists and palaeoecologists, Prof. DSc Elissaveta Bozilova celebrates her 80th birthday on March 17th 2012. For more than 50 years she has been actively involved in science and teaching in the Faculty of Biology at Sofia University St. Kliment Ohridski. Her interest in botany, and subsequently in palynology, was encouraged and supported by her master’s degree supervisor Prof. Boris Kitanov in the mid 1950s and also by her family members. The remarkable scientific and teaching career of Elissaveta Bozilova started in 1958 when she joined the academic staff of the Department of Botany as an Assistant Professor. The initial years of her research were devoted mostly to the investigation of the pollen morphology of various gymnosperm and flowering plant taxa, but subsequently this knowledge was applied to the study of the late Quaternary history of the Bulgarian flora and vegetation, and gradually expanded to aeropalynology, melissopalynology, archaeobotany, marinopalynology and pollen monitoring. Numerous pollen analytical studies were conducted by her on lake, mire and peat sediments in the glaciated and nonglaciated Bulgarian mountains, along the Black Sea coast, the Danube valley and inland plains as well as on marine cores. Elissaveta Bozilova and her co-workers postulated for the first time the main chronological stages in the lateglacial and Holocene vegetation development in different parts of the country and contributed to the elucidation of important scientific issues such as location of glacial tree refugia and migration routes, fluctuations in the tree line, formation of recent vegetation belts and specific plant communities, pollen dispersal mechanisms, climate changes, sea-level fluctuations, human impact on the natural environment since prehistoric times, etc. She also laid the foundations of contemporary aeropalynological and melissopalynological studies in Bulgaria. An important premise for the recognition of Elissaveta Bozilova as a leading Bulgarian palynologist was the establishment of personal and fruitful long standing scientific contacts with outstanding foreign specialists, including Prof. H. -J. Beug, Prof. G. Lang, Prof. B. E. Berglund, Dr. A. Smit, Prof. K. Faegri, Prof. H. J. B. Birks, Prof. M. Ralska-Jasiewiczowa, Prof. Y. Vasari, Prof. B. Ammann, Prof. L. -K. Konigsson, Prof. S. Hicks, Prof. J. -L. de Beaulieu, Prof. K. -E. Behre, Prof. N. Athanasiadis, Prof. C. -A. Haeggstrom, Prof. H. Jungner, Prof. A. Gerasimidis, and many others, who shared generously their knowledge and expertise in palaeoecological and botanical research, both in the field and in the laboratory. One of the most important professional merits of Elissaveta Bozilova, in addition to her intensive research and teaching activities, was the organization of the Laboratory of Palynology at the Department of Botany, which she administrated successfully until 2000. The laboratory established itself as the leading national centre in fundamental and applied palynology and palaeoecology, where Elissaveta Bozilova supervised over 25 master and doctoral students from several generations, most of them being engaged today either in research or teaching in the broad Communicated by F. Bittmann.

Driving forces of mid-Holocene vegetation shifts on the upper Tibetan Plateau, with emphasis on changes in atmospheric CO 2 concentrations

Numerous pollen records across the upper Tibetan Plateau indicate that in the early part of the mid-Holocene, Kobresia-rich high-alpine meadows invaded areas formerly dominated by alpine steppe vegetation rich in Artemisia. We examine climate, land-use, and CO 2 concentration changes as potential drivers for this marked vegetation change. The climatic implications of these vegetational shifts are explored by applying a newly developed pollen-based moisture-balance transfer-function to fossil pollen spectra from Koucha Lake on the north-eastern Tibetan Plateau (34.0°N; 97.2°E; 4540 m a.s.l.) and Xuguo Lake on the central Tibetan Plateau (31.97°N; 90.3°E; 4595 m a.s.l.), both located in the meadow-steppe transition zone. Reconstructed moisture-balances were markedly reduced (by ∼150–180 mm) during the early mid-Holocene compared to the late-Holocene. These findings contradict most other records from the Indian monsoonal realm and also most non-pollen records from the Tibetan Plateau that indicate a rather wet early- and mid-Holocene. The extent and timing of anthropogenic land-use involving grazing by large herbivores on the upper Tibetan Plateau and its possible impacts on high-alpine vegetation are still mostly unknown due to the lack of relevant archaeological evidence. Arguments against a mainly anthropogenic origin of Kobresia high-alpine meadows are the discovery of the widespread expansion of obviously ‘natural’ Kobresia meadows on the south-eastern Tibetan Plateau during the Lateglacial period indicating the natural origin of this vegetation type and the lack of any concurrence between modern human-driven vegetation shifts and the mid-Holocene compositional changes. Vegetation types are known to respond to atmospheric CO 2 concentration changes, at least on glacial–interglacial scales. This assumption is confirmed by our sensitivity study where we model Tibetan vegetation at different CO 2 concentrations of 375 (present-day), 260 (early Holocene), and 650 ppm (future scenario) using the BIOME4 global vegetation model. Previous experimental studies confirm that vegetation growing on dry and high sites is particularly sensitive to CO 2 changes. Here we propose that the replacement of drought-resistant alpine steppes (that are well adapted to low CO 2 concentrations) by mesic Kobresia meadows can, at least, be partly interpreted as a response to the increase of CO 2 concentration since 7000 years ago due to fertilization and water-saving effects. Our hypothesis is corroborated by former CO 2 fertilization experiments performed on various dry grasslands and by the strong recent expansion of high-alpine meadows documented by remote sensing studies in response to recent CO 2 increases.

Changes in growth and cell wall extensibility of maize silks following pollination.

In response to pollination maize silks undergo an accelerated process of senescence which involves an inhibition of elongation. To gain insight into the mechanism underlying this growth response, the relationships among silk elongation kinetics, cell wall biophysical properties, pollen tube growth, and expansin protein abundance were investigated. The inhibition of silk elongation became apparent beyond 12 h after pollination. Pollinated walls were less responsive in assays of extension induced by pollen β-expansin. Expansin protein abundance and endogenous expansin activity were not considerably reduced after pollination. Silk wall plastic compliance was significantly reduced 6 h post-pollination and beyond, suggesting that the wall undergoes structural modifications leading to its rigidification in response to pollination. The reduction in the plastic compliance occurred locally and progressively, shortly after pollen tubes traversed through a region of silk. Though numerous pollen grains germinated and initiated pollen tubes at the silk tip, the density of pollen tubes gradually declined along the length of the silk and only 1–2 reached the ovary even 24 h after pollination. These results support the notion that pollination-induced cell wall rigidification plays multiple roles in maize reproduction, including inhibition of silk growth and prevention of polyspermy.