Multifunctional bracts enhance plant fitness during flowering and seed development in Rheum nobile (Polygonaceae), a giant herb endemic to the high Himalayas

Specialized bracts are thought to be important for the successful reproduction of some plants and are regarded as adaptations to diverse driving forces. However, few empirical studies have quantified the adaptive significance of bracts within a cost–benefit framework. We explored the adaptive significance of large and showy bracts for reproduction in Rheum nobile, a giant herb endemic to the high Himalayas. We examined whether the bracts enhance reproductive success during flowering and seed development. Bracts increased flower and fruit temperature on sunny days, greatly decreased the intensity of ultraviolet-B (UV-B) radiation reaching flowers and fruits, and prevented pollen grains being washed away by rain. Experiments indicated that high temperature could promote pollen germination, while pollen grains exposed to rain and UV-B radiation at ambient levels were seriously damaged. Furthermore, bract removal decreased the number of pollinators visiting flowers. When bracts were removed before or after flowering, fecundity and progeny quality were adversely affected, but seed predation by larvae of pollinators decreased. A cost–benefit analysis demonstrated that the cost of bracts, i.e., increased seed predation, is modest. Our results suggest that the bracts of R. nobile promote pollen germination, protect pollen grains from rain and intense UV-B radiation, enhance pollinator visitation during flowering, and facilitate the development of fertilized ovules during seed development. We conclude that multifunctional bracts of R. nobile are an effective adaptive strategy in alpine environments and might have been selected for because of abiotic environmental conditions as well as for enhancing pollination success.     

Selective light transmittance of translucent bracts in the Himalayan giant glasshouse plant Rheum nobile Hook. f. & Thomson (Polygonaceae)

Translucent bract transmittance of ultraviolet (UV) to infrared (IR) radiation (between 320 and 800 nm) and leaf anatomy were examined in a glasshouse plant, Rheum nobile Hook. f. & Thomson (Polygonaceae) to assess the function of avoiding injury by UV radiation while keeping the inflorescence warm by photosynthetically active (PA) and IR radiation. Although the translucent bracts and rosulate leaves transmitted little UV radiation, the former always transmit more PA and IR radiation. Additionally, the bracts transmit much more scattered solar radiation than direct radiation. The bracts are also anatomically different from the rosulate leaves. They have two or three layers of mesophyll cells with neither palisade nor spongy parenchymatous cells; in addition, the uppermost layer of mesophyll and the epidermis stain easily, and both are thought to play a role in attenuating UV radiation. The leaf epidermis of many land plants has UV absorbing pigments such as flavonoids, which absorb almost all UV radiation. Thus the role of the bracts of R. nobile is to protect the reproductive organs by absorbing UV radiation and to keep them warm by transmitting PA and IR radiation. The bracts are believed to have adapted function and form to the environment, in particular, to the weather conditions of the eastern Himalaya.     

Optical and anatomical characteristics of bracts from the Chinese "glasshouse" plant,Rheum alexandrae Batalin (Polygonaceae), in Yunnan,China

Bracts that lacked chlorophyll were compared with rosette leaves on the Chinese glasshouse plant Rheum alexandrae Batalin. The structures were analyzed anatomically and with photospectrometry. Histological features were significantly different between the bracts and the rosette leaves. Epidermal pavement cells and palisade cells were larger in the bracts than in the rosette leaves, but the ratio of the intercellular spaces to the cells in the palisade layer was the same in the two structures. Absorption spectrum analyses of the half-translucent bracts showed that, despite their thinner structure, they absorbed ultraviolet (UV) radiation more efficiently than did the green rosette leaves. The characteristics of the R. alexandrae bracts were different from those of the Himalayan glasshouse plant Rheum nobile. These results suggest that R. alexandrae evolved bracts as an adaptive strategy to protect the inflorescence from UV radiation.     

Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) inferred from chloroplast DNA trnL-F sequences

BACKGROUND AND AIMS: Rheum, a highly diversified genus with about 60 species, is mainly confined to the mountainous and desert regions of the Qinghai-Tibetan plateau and adjacent areas. This genus represents a good example of the extensive diversification of the temperate genera in the Qinghai-Tibetan plateau, in which the forces to drive diversification remain unknown. To date, the infrageneric classification of Rheum has been mainly based on morphological characters. However, it may have been subject to convergent evolution under habitat pressure, and the systematic position of some sections are unclear, especially Sect. Globulosa, which has globular inflorescences, and Sect. Nobilia, which has semi-translucent bracts. Recent palynological research has found substantial contradictions between exine patterns and the current classification of Rheum. Two specific objectives of this research were (1) to evaluate possible relationships of some ambiguous sections with a unique morphology, and (2) to examine possible occurrence of the radiative speciation with low genetic divergence across the total genus and the correlation between the extensive diversification time of Rheum and past geographical events, especially the recent large-scale uplifts of the Qinghai-Tibetan Plateau. METHODS: The chloroplast DNA trnL-F region of 29 individuals representing 26 species of Rheum, belonging to seven out of eight sections, was sequenced and compared. The phylogenetic relationships were further constructed based on the sequences obtained. KEY RESULTS: Despite the highly diversified morphology, the genetic variation in this DNA fragment is relatively low. The molecular phylogeny is highly inconsistent with gross morphology, pollen exine patterns and traditional classifications, except for identifying all samples of Sect. Palmata, three species of Sect. Spiciformia and a few species of Sect. Rheum as corresponding monophyletic groups. The monotypic Sect. Globulosa showed a tentative position within the clade comprising five species of Sect. Rheum. All of the analyses revealed the paraphyly of R. nobile and R. alexandrae, the only two species of Sect. Nobilia circumscribed by the possession of large bracts. The crude calibration of lineages based on trnL-F sequence differentiation implied an extensive diversification of Rheum within approx. 7 million years. CONCLUSIONS: Based on these results, it is suggested that the rich geological and ecological diversity caused by the recent large-scale uplifts of the Qinghai-Tibetan Plateau since the late Tertiary, coupled with the oscillating climate of the Quaternary stage, might have promoted rapid speciation in small and isolated populations, as well as allowing the fixation of unique or rare morphological characters in Rheum. Such a rapid radiation, combined with introgressive hybridization and reticulate evolution, may have caused the transfer of cpDNA haplotypes between morphologically dissimilar species, and might account for the inconsistency between morphological classification and molecular phylogeny reported here.     

ENVIRONMENTAL EFFECTS ON THE PERCENTAGE OF STAINABLE AND PRESUMED NORMAL POLLEN IN ASTER (COMPOSITAE)

Pollen stainability is often equated with pollen viability or fertility and is used by botanists to evaluate degree of hybridity among individuals of a plant population. Under certain conditions, however, the effects of meiotic irregularities resulting from hybridization may be masked by those of the environment. In the genus Aster high percentages of pollen stainability were obtained in most samples from plants flowering in the greenhouse, including hybrids of distantly related species. When grown outdoors, plants of the same clones frequently suffered significant reduction in the amounts of normal pollen produced. During the period of flower development, two climatic parameters were assessed, relative humidity and maximum/minimum temperatures. Fluctuations in relative humidity were considerable but of nearly equal amplitude both in the greenhouse and in the open field. Diurnal temperature fluctuations, however, were much less drastic in the greenhouse than outdoors. The differential percentages of stainable pollen observed are probably attributable to the sum of environmental factors, including hard rains and strong winds, rather than any single parameter. In the genus Aster high amounts of aborted pollen grains appear to be largely a reflection of sensitivity to unfavorable microclimatic conditions during the post-meiotic period of flower development, and data on pollen stainability cannot be employed as a measure of fertility or degree of hybridity.     

Investigations cytochimiques,marquage immunocytochimique et effets destructeurs des U. V. sur le pollen de Dactylis glomerata L

The elucidation of the ultrastructural cytochemistry, coupled with chemical stabilizing procedures during fixation and embedding, make a significant contribution to the understanding of pollen-exine ontogenesis. A combination of the Con-A agglutination and lanthanum precipitation methods proved particular advantageous during the intine formation stage. By using immunogold techniques, it is possible to demonstrate that only the mature exine of the atmospheric pollen grains reacted positively after sectioning, not the intine. An important difference appears when immature pollen grains are treated under the same conditions: in this case, both the microsporal cytoplasm of the future pollen grain and the immature exine react, but not the intine.

In untreated pollen grains, the exine is a biological material of exceptionally high density with a very low stainability: staining for proteins and lipids is only moderate. A degradation of exine structures by U. V. radiation of 254 nm can be easily proved and the framework obtained is comparable to that induced by some chemical attacks. When sporopollenin is degraded from filamentous sub-units of the exine, stainability increases, and the cytochemical tests for acid-mucopolysaccharids are positive. It is clear that glycocalyx units within exines are chemically bound to the sporopollenin matrix. Attention can also be profitably directed to the future investigation of the function of exine frameworks, through allergen fixation in living pollen grains.     

<Emphasis Type="Italic">Hieracium</Emphasis> subgen. <Emphasis Type="Italic">Pilosella</Emphasis>: pollen stainability in sexual,apomictic and sterile plants

Two pollen stainability tests (Alexander’s stain and acetocarmine) were used to detect differences in pollen viability of the sexual, apomictic and sterile plants of Hieracium subgen. Pilosella. In sexual taxa (Hieracium bauhini and H. densiflorum), the average stainability was 93.7–98.4%. Similarly high stainability (92.2–97.2%) was found in the apomictic Hieracium pilosellinum and in the majority of the apomictic populations (or plants) of the pentaploid and hexaploid H. bauhini. In some apomictic plants of Hieracium bauhini the average pollen stainability was 49.0–75.4%. The lowest pollen stainability was found in the sterile plants, i.e. the triploid H. pistoriense (33.6%) and the pentaploid H. brachiatum (29.6%).     

Multifunctional bracts in the dove tree Davidia involucrata (Nyssaceae: Cornales): rain protection and pollinator attraction

Although there has been much experimental work on floral traits that are under selection from mutualists and antagonists, selection by abiotic environmental factors on flowers has been largely ignored. Here we test whether pollen susceptibility to rain damage could have played a role in the evolution of the reproductive architecture of Davidia involucrata, an endemic in the mountains of western China. Flowers in this tree species lack a perianth and are arranged in capitula surrounded by large (up to 10 cm x 5 cm) bracts that at anthesis turn from green to white, losing their photosynthetic capability. Flowers are nectarless, and pollen grains are presented on the recurved anther walls for 5-7 days. Flower visitors, and likely pollinators, were mainly pollen-collecting bees from the genera Apis, Xylocopa, Halictus, and Lasioglossum. Capitula with natural or white paper bracts attracted significantly more bees per hour than capitula that had their bracts removed or replaced by green paper. Experimental immersion of pollen grains in water resulted in rapid loss of viability, and capitula with bracts lost less pollen to rain than did capitula that had their bracts removed, suggesting that the bracts protect the pollen from rain damage as well as attracting pollinators.     

Optical and anatomical characteristics of bracts from the Chinese "glasshouse" plant, Rheum alexandrae Batalin (Polygonaceae), in Yunnan, China

Bracts that lacked chlorophyll were compared with rosette leaves on the Chinese glasshouse plant Rheum alexandrae Batalin. The structures were analyzed anatomically and with photospectrometry. Histological features were significantly different between the bracts and the rosette leaves. Epidermal pavement cells and palisade cells were larger in the bracts than in the rosette leaves, but the ratio of the intercellular spaces to the cells in the palisade layer was the same in the two structures. Absorption spectrum analyses of the half-translucent bracts showed that, despite their thinner structure, they absorbed ultraviolet (UV) radiation more efficiently than did the green rosette leaves. The characteristics of the R. alexandrae bracts were different from those of the Himalayan glasshouse plant Rheum nobile. These results suggest that R. alexandrae evolved bracts as an adaptive strategy to protect the inflorescence from UV radiation. Received: August 22, 2001 / Accepted: October 15, 2001     

Pollen flow in a population of Primula vulgaris Huds.

A British population of Primula vulgaris was visited four times between 1971 and 1976 for study of the nature and functioning of distyly. The stainability and production of pollen from pins exceeded those of thrums. About 20% of pin pollen and 48% of thrum pollen was removed from anthers by insect visitors. Pins and thrums were present in a 1:1 ratio in 1971 but pins outnumbered thrums in 1976. Thrums produced more seeds per flower than pins though the number of ovules produced by each was similar. Two methods for collecting stigmas from open flowers and analyzing them for pollen loans produced somewhat different results. For pin stigmas, the pollen load consisted of 2–23% thrum pollen; for thrum stigmas, the pollen load consisted of 0–71% pin pollen with most stigmas having less than 50% pin pollen. In general, intermorph pollen flow is less than would be expected if pollen flow were random. It is probable that most intramorph pollen on stigmas is a result of self- or geitonogamous pollination. The extensive literature concerning the natural pollinators of the primrose is reviewed. Although Darwin's hypothesis concerning the functional significance of distyly in promoting intermorph pollination was never quantified, the pollen flow patterns observed in P. vulgaris are unexpectedly deviant and are similar to those patterns observed in several unrelated heterostylous species in other plant families.     

Production of total and stainable pollen grains in Olea europaea L

The production of the total and stainable number of pollen grains per flower and per inflorescence of 13 olive cultivars (Olea europaea L.) has been investigated. Grains stainability was evaluated by using the acetic carmine staining method whereas the production of pollen grains was calculated by using a Bürker haemocytometer. All pollen characteristics taken into account varied significantly among the cultivars. The number of flowers per inflorescence ranged from a maximum of 31.3 (‘Cipressino’) to a minimum of 13.0 (‘Leccino’), the number of pollen grains per anther ranged from 123?346 (‘Arbequina’) to 40?975 (‘Sant'Agostino’), and the pollen stainability from 97.6% (‘Cipressino’) to 48.0% (‘Cellina di Nardò’). The number of stainable pollen grains per inflorescence averaged 2?559?512, ranging from a maximum of 3?913?678 (‘Nociara’) to a minimum of 940?143 (‘Sant'Agostino’). All parameters were positively correlated, whereas a linear negative correlation was found between stainability and total pollen grains both per anther and per inflorescence.     

Effect of Sporophytic Heterozygosity on the Male Gametophyte of the Tetraploid Potato (Solanum tuberosum L.)

Quantitative variation for pollen stainability was examinedin both tetraploid x diploid and tetraploid x tetraploid familiesof potato (Solanum tuberosum L.). The diploid parents, whichwere highly heterozygous, produced first division restitution(FDR) 2n pollen. The families used were not affected by cytoplasmic-geneticmale sterility. Significant differences between families werefound. FDR 2n pollen producing diploid clones had progenieswith a significantly higher pollen stainability than those derivedfrom crosses between tetraploid parents. This was related tothe level of heterozygosity in the sporophyte. In FDR 2n pollen,80% of the heterozygosity and a large fraction of epistasispresent in the diploid parent was transmitted to its tetraploidprogeny. This is important for the production of open pollinated,true potato seed, because pollen stainability is significantlycorrelated with fruit set (r = 0·85).Copyright 1994,1999 Academic Press FDR 2n pollen, gametophytic heterozygosity, true potato seed     

Hybridization between Viola canina and V. persicifolia in Norway

Hybridization in Norway between Viola canina and V persicifolia was investigated by analysis of root tip mitosis, pollen stainability and morphological variation. The chromosome number was proved to be 2n = 40 (tetraploid) for V. canina ssp. montana , 2n = 20 (diploid) for V. persicifolia and 2n = 30 (triploid) for the hybrid, V. canina × persicifolia . No deviating chromosome numbers were found. The tetraploids and the diploids had a high frequency, and the triploids a low frequency, of apparently good pollen grains. Canonical variate analysis were used to investigate the pattern of morphological differentiation in the material. The specimens clearly separate into three groups in accordance with the three ploidy levels. No introgression was demonstrated.     

Fructokinase and hexokinase from pollen grains of bell pepper (Capsicum annuum L.): possible role in pollen germination under conditions of high temperature and CO2 enrichment

The processes of pollen grain development and germination depend on the uptake and metabolism of pollen sugars. In pepper (Capsicum annuum L.), initial sugar metabolism includes sucrose hydrolysis by invertase and subsequent phosphorylation of glucose and fructose by hexose kinases. The main objective of this study was to investigate changes in fructokinase (EC 2.7.1.4) and hexokinase (EC.2.7.1.1) activities in pepper flowers during their development, and to study the possible roles of these enzymes in determining pollen germination capacity under high temperature and under CO(2) enrichment, previously shown to modify sugar concentrations in pepper pollen (Aloni et al., 2001 Physiologia Plantarum 112: 505-512). Fructokinase (FK) activity was predominant in pepper pollen, and increased during pollen maturation. Pollen hexokinase (HK) activity was low and did not change throughout pollen development. High-temperature treatment (day/night, 32/26 degrees C) of pepper plants reduced the percentage of pollen that germinated compared with that under normal temperatures (26/22 degrees C), and concomitantly reduced the activity of FK in mature pollen. High temperature also reduced FK and HK activity in the anther. Under high ambient CO(2) (800 micro l l(-1)) pollen FK activity was enhanced. The results suggest that pollen and anther FK may play a role in the regulation of pollen germination, possibly by providing fructose-6-phosphate for glycolysis, or through conversion to UDP-glucose (UDPG) to support the biosynthesis of cell wall material for pollen tube growth. High temperature stress and CO(2) enrichment may influence pollen germination capacity by affecting these pathways.     

Effects of Temperature on Pollen Viability in Mango cv. 'Kensington'

The response of pollen development to low or high temperatureregimes was studied to determine the conditions suitable forthe formation of fertile pollen in the mango cv. 'Kensington'.The phase most sensitive to the degree and duration of temperaturestress was that from meiosis to the pre-vacuolate microspore(about 3 d duration at 25/20 °C) though vacuolated microsporeswere also sensitive to low temperature. Night temperatures below10 °C resulted in pollen grains with a low viability (<50%). A temperature between 15 and 33 °C during the phasefrom meiosis to the pre-vacuolate microspore was optimum forpollen development (70-85% pollen viability). Analysis of field records showed a linear negative correlationbetween percentage of pollen viability and number of days whichhad a mean night temperature lower than 10 °C during theperiod from meiosis to early mature stage (y = 77·7-3·4x,r² = 0·60). The temperature sensitive phase was estimatedto begin 155 degree days D = [(T_max + T_min)/2 - 10] before anthesisand to end 78 degree days before anthesis. This equation maybe useful as a means of predicting pollen viability in the fieldfrom temperature records and thus fruit set, date of maturityand yield. It may also aid in the selection of areas for growingmangoes in marginal climates.Copyright 1994, 1999 Academic Press Mangifera indica L. mango, microsporogenesis, pollen development, viability, sterility, temperature     

HEAT INJURY DURING FLORAL DEVELOPMENT IN COWPEA (VIGNA UNGUICULATA,FABACEAE)

High night temperatures during floral development induce male sterility in cowpea (Vigna unguiculata [L.] Walp.). The objectives of this study were to determine: the possible causes of the male sterility; the stage of floral development when damage due to heat stress occurs; and whether specific tissues are damaged during the period of sensitivity to heat. Plants were grown under controlled temperatures in both greenhouses and growth chambers in separate experiments. Floral development was normal under a night temperature of 20 C, whereas flowers developed under high night temperature (30 C) set no pods due to low pollen viability and anther indehiscence. Anthers developed under 33/30 C day/night temperatures did not exhibit endothecial formation, whereas anthers developed under 33/20 C day/night temperatures exhibited normal development of the endothecial layer. Reciprocal transfers of plants between chambers with high or optimum night temperature demonstrated that the stage of floral development most sensitive to heat stress occurs 9 to 7 d before anthesis. Anthers developed under either optimal or high night temperatures were compared cytologically. Development was similar through meiosis, but after tetrad release, which occurred 8 d before anthesis, the tapetal layer degenerated prematurely under high night temperature. Premature degeneration of the tapetal layer and lack of endothecial development may be responsible for the low pollen viability, low anther dehiscence, and low pod set under high night temperatures.     

Effect of genome-plastome interaction on meiosis and pollen development in Oenothera species and hybrids

 Oenothera villaricae Dietrich and Oe. picensis Dietrich, complete translocation heterozygotes, are fully interfertile, giving rise to six discrete classes of true-breeding hybrids from a reciprocal cross. Associated with each parent and hybrid is a characteristic abortive non-staining pollen fraction easily distinguished from fully developed pollen under the light microscope. Pollen abortion has been associated with translocation rings in other angiosperm species, and may characterize such systems. The abortive pollen fraction is significantly different between reciprocal Oenothera hybrids, however (P<0.001), indicative of partial cytoplasmic control. Pollen abortion is most severe in the F₁ hybrid generation, and ameliorates with successive generations of hybrid self-fertilization. Three-way analysis of variance shows significant effects on pollen stainability (a measure of the non-abortive fraction) for nucleus, cytoplasm and selfed hybrid generation, individually or in combinations. This result suggests a combined nucleocytoplasmic basis for the pollen abortion. Correlated with the observation of increased pollen abortion in Oenothera hybrids are meiotic findings of broken chromosome rings (chains, univalents), asymmetric anaphase chromosome distributions and trinucleate tetrads. To test the hypothesis that such anomalous meiotic events play a role in the mechanism of pollen abortion, meiotic disjunction frequency was determined for each parent, F₁ and F₉ selfed hybrid accessions. Three-way analysis of variance shows levels of significance comparable to those noted for pollen stainability (P<0.001) for effects of nucleus, cytoplasm and selfed hybrid generation on disjunction frequency. A high degree of correlation (r ²=0.984) is noted between disjunction frequency and pollen stainability. We conclude that the abortive pollen grains are indeed the products of nondisjunctional meiotic events, which themselves are consequences of hybrid nucleocytoplasmic incompatibility. Received: 28 May 1997 / Revision accepted: 23 July 1997     

Presence and behaviour of B-chromosomes in Acanthophyllum laxiusculum (Caryophyllaceae)

Two karyotypic variants were recognized in populations of Acanthophyllum laxiusculum Schiman-Czeika. Variant A had 2N = 30 and variant B showed the presence of 0 to 3 B-chromosomes in addition to 2N = 30 chromosomes. Analysis of chromosome behaviour at meiosis showed that the presence of B-chromosomes increases chiasma frequency in A-chromosomes; this effect was higher for plants with odd numbers of B-chromosomes compared with plants with even numbers of B-chromosomes. Comparisons of variants A and B, suggests that B-chromosomes have an effect pollen stainability and seed production. It seems that the presence of B-chromosomes may increase pollen stainability and seed production in variant B.