ROLE OF CALCIUM IN THE CALLOSE RESPONSE OF SELF-POLLINATED BRASSICA STIGMAS

In Brassica oleracea, sporophytic self-incompatibility prevents germination of self pollen, or normal growth of self pollen tubes. After self-pollination, the papillae of stigmas synthesize callose. The role of Ca⁺⁺ in the formation of stigmatic callose was tested by adding compounds that interact with Ca⁺⁺ to suspensions of pollen that were known to induce callose formation in self stigmas. The calcium channel antagonist, lanthanum, and the calcium chelating agent, EGTA, reduced or abolished the callose response to self-pollen suspensions. In the presence of Ca⁺⁺, the calcium ionophore, A23187, induced callose in stigmatic papillae when added to pollen suspensions, or alone. Therefore, callose deposition in response to incompatible pollinations appears to be a calcium-dependent process. Pretreatment of pistils with 100 μm 2-deoxy-D-glucose abolished the callose response to self-pollination, while self pollen remained inhibited and cross pollen grew normally in treated pistils. Thus, callose formation in the stigma is not an essential part of the self-incompatibility mechanism preventing the growth of self pollen in Brassica.     

Pollen tube reallocation in two preanthesis cleistogamous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae)

Pollen deposition on stigmas and pollen tube growth in two apocarpous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae), were examined with fluorescence microscopy. The reallocation of pollen tubes among pistils was observed in both species. The percentage of pollinated stigmas per flower was only 22.0% in R. rostratum and 51.0% in S. guyanensis, though the seed/ovule ratios are higher than 65% in both species. The number of pollen grains on each single stigma ranged from 0 to 96 in R. rostratum, and from 0 to 125 in S. guyanensis. When more than one pollen grain deposited on a stigma, all pollen tubes grew to the ovary, but only one of them turned towards the ovule and finally entered the nucleus. The other tubes grew through the receptacle tissue into ovules of adjacent carpels whose stigmas were unpollinated or pollinated later. The intercarpellary growth of pollen tubes could be a mechanism to increase the efficiency of sexual reproduction in an apocarpous gynoecium with low pollination on the pistils.     

Pollen tubes enter neighbouring ovules by way of receptacle tissue,resulting in increased fruit-set in Sagittaria potamogetifolia Merr

Using fluorescence microscopy, deposition of pollen on stigmas and pollen tube growth in the gynoecium of Sagittaria potamogetifolia Merr., a monoecious species with an apocarpous gynoecium, were observed. The maximum rate of pollination averaged 83.9 +/- 4.7 %, and the number of pollen grains per stigma ranged from zero to 30. Pollen tubes grew through one stigma to the base of the ovary at almost the same speed, but generally only one of the pollen tubes then turned towards the ovule and finally entered the nucellus through the micropyle. The other pollen tubes grew through the ovary base and the receptacle tissue into ovules of adjacent carpels whose stigmas were not pollinated or which had been pollinated later. This phenomenon is termed pollen tube 'reallocation' by the authors. To verify the direct effect of the phenomenon on fruit set, artificial pollination experiments were conducted in which two or more pollen grains were placed onto only one stigma in each gynoecium; frequently more than one fruitlet was obtained from each flower treated. The reallocation of pollen tubes among pistils in the gynoecium could effect fertilization of ovules of unpollinated pistils and lead to an increase in sexual reproduction efficiency. It would, to some extent, also increase pollen tube competition among pistils of the whole gynoecium.     

Different regulatory processes control pollen hydration and germination in Arabidopsis

To elucidate the functional differences in how Arabidopsis stigmas regulate pollen hydration and germination, we analyzed receptivity of stigmas, epidermal surfaces (leaves, stems of inflorescence bolts, and floral organs), and an abiotic surface (cover glass) for pollen hydration and germination. Using 65% relative humidity (RH), we found that mature pollen grains were able to hydrate and germinate on stigmas at flower developmental stages 9–13, but not on the distal end of pistils at stage 8, epidermal surfaces, or glass. Furthermore, under 100% RH, pollen grains could hydrate on all tested surfaces, but pollen germination was observed only on the young floral organs (stages 9–12) and the stigmas at stages 9–13. The distal ends of pistils at stage 8, the epidermal surfaces, and the cover glass did not support pollen germination even under 100% RH. Our results indicate that pistil factors regulating pollen hydration and germination are synthesized at stage 9 when stigmatic papillar cells begin to develop. Although pistil factors involved in pollen hydration may only be present on the stigma, the factors involved in pollen germination may localize on both the stigma and surfaces of unopened floral organs.     

野菊与菊花杂交中花粉活力和柱头可授性及胚胎发育研究

应用石蜡制片、活体压片、光学显微镜及扫描电子显微镜观察等方法,研究了四倍体河南云台山野菊(Dendranthema indicum)与栽培菊花'钟山金山'(D.grandiflorum 'Zhongshanjinshan')种间杂交中父本花粉活力、花粉在柱头萌发、花粉管生长及胚胎发育情况等.结果发现,父本云台山野菊的花粉活力在授粉时为12%左右.人工授粉后的不同时间,在柱头上都观察到正常萌发的花粉粒,且花粉管都能进入柱头,其中,在授粉后0.5 h时,平均每柱头有5.9粒花粉萌发;12 h时,为59.9粒;而24和48 h时,则分别降为47.1和35.7粒.此外,在授粉后8、10、12和15 d时,分别在49.1%、40.8%、39.7%和38.5%子房内观察到正常发育的胚胎,最终杂交结实率为44.8%,而母本自然开放结实率为52.3%.研究表明,授粉前其多数母本雌蕊发育良好、授粉后多数花粉能在柱头正常萌发和花粉管正常生长,在受精后大部分胚胎发育正常是野菊与栽培菊种间杂交较高结实率的重要保证,而授粉前父本较低的花粉活力对杂交结实率影响不大.     

Sunny-side up: flower heliotropism as a source of parental environmental effects on pollen quality and performance in the snow buttercup, Ranunculus adoneus (Ranunculaceae)

Floral traits affect mating success via their influence on the microenvironment in which sexual reproduction occurs as well as their impact on pollinator attraction. Here we investigate the importance of flower heliotropism as a source of parental environmental effects on pollen quality and performance. Flowers of the snow buttercup, Ranunculus adoneus, closely track the sun's rays. We experimentally restrained flowers to test for effects of heliotropism on pollen quality and performance after pollination. When equivalent amounts of pollen were transferred to recipient pistils, pollen from solar-tracking donor flowers exhibited a 32% advantage in germination compared to pollen from stationary (tethered) donor flowers. By the end of anthesis, pistils of tracking flowers contained 40% more germinating pollen grains and 44% more pollen tubes midway down the style than pistils of stationary ones. Solar tracking had no direct effect on pollen tube growth. The greater amount of germinating pollen in tracking flowers accounted for the treatment effect on pollen tube density. A survey of pollen receipt and pollen germination in naturally tracking flowers indicated that solar tracking primarily affects pollen tube density by promoting pollen germination rather than pollen deposition. We conclude that flower heliotropism, by enhancing the paternal environment for pollen development and the maternal environment for pollen germination, represents a source of positive parental environmental effects on pollen performance in snow buttercups.     

Pollen competition in a natural population of Cucurbita foetidissima (Cucurbitaceae)

The pollen competition hypothesis predicts that when the number of pollen grains deposited onto stigmas exceeds the number of ovules, selection can operate in the time frame between deposition and fertilization. Moreover, because of the overlap in gene expression between the two phases of the life cycle, selection on microgametophytes may alter the resulting sporophytic generation. The extent to which pollen competition occurs in nature has been unclear, because tests of the predictions of the pollen competition hypothesis have used cultivars and/or artificial growth conditions and hand-pollination techniques. In this study we used a wild species, Cucurbita foetidissima, in its natural habitat (southern New Mexico) to determine the amount and timing of the arrival of pollen onto stigmas, the relationship between pollen deposition and seed number, and the effects of the intensity of pollen competition on progeny vigor. We found that ～900 pollen grains are necessary for full seed set and that a single visit by a pollinator results in the deposition of 653.0 ± 101.8 pollen grains. About 29% of the flowers receiving a single pollinator visit had 900 or more pollen grains on its stigma. Moreover, within 2 h of anthesis, >4000 pollen grains were deposited onto a typical stigma, indicating that multiple pollinator visits must have occurred. Fruits produced by multiple visits had greater seed numbers (206 vs. 147) than fruits produced by a single visit. Finally, the progeny produced by multiple pollinator visits were more vigorous than those produced by single visits with respect to five measures of vegetative growth (MANCOVA, Wilks' lambda = 0.96, F(6,370) = 2.54, P < 0.02. These data demonstrate that conditions for pollen competition exist in nature and support the prediction that pollen competition enhances offspring vigor.     

Reproductive timetable for the tropical Vireya rhododendron,R. Macgregoriae

Summary The reproductive development of Rhododendron macgregoriae F.v.M., Section Vireya (Ericaceae) has been followed from 10 days before anthesis to the production of mature germinable seeds about 145 days after anthesis. The species is self-compatible but shows both protandry and physical separation of pollen from the receptive stigma. Pollen is mature and viable from shortly before anthesis until the corolla and attached anthers abscise some 9–12 days after the flowers open. Spontaneous dehiscence, however, occurs mostly in the first few days after opening. The stigma becomes receptive 6–7 days after anthesis, and nectar is produced at this time. Female gametophytes are not mature until about 10 days after anthesis. Fertilization occurs about 6–7 days after pollination, and although the endosperm commences development immediately, development of the embryo proper does not begin until some 40–45 days later. Pollinations made with fresh pollen between 5 and 14 days after anthesis were successful, but those made on dry stigmas in the first 4 days after anthesis, or on senescing pistils 21 days after anthesis, gave little or no seed set. In nature, autogamy is probably uncommon, the majority of pollinations are likely to be geitonogamous, but there is considerable potential for outcrossing.     

The timetable for development of maternal tissues sets the stage for male genomic selection in Betula pendula (Betilaceae)

The influence of the sequence of maternal tissue development in Betula pendula upon the potential for male gamete selection was investigated, and the timing of the fixed abortion of one of the two ovules was determined. We used scanning electronic microscopy, confocal laser scanning microscopy, and blue light microscopy. The stigmas remain fresh throughout male anthesis, and may also last after its end, depending on ambient temperatures. The presence of germinated pollen does not induce stigmatic necrotization, and grains may arrive at different times. The pollen tube tips remain within the stigma base until the end of female anthesis. The ovules will not develop until after necrotization of the stigmas. The pollen tubes thus have a fair start to the ovules, regardless of their different arrival times and of the original positions of the pollen grains at the stigma surface. Therefore, competition among different microgametophytes is possible, in spite of low pollination intensity. Our results indicate that when the first pollen tube penetrates an ovule, this ovule starts to outgrow the other one, and even if the other is also penetrated, its vascular support soon atrophies and the megagametophyte will shrivel. Fertilization of both ovules was never seen in this study.     

铁皮石斛花粉块结构及发育过程的解剖学特征研究北大核心CSCD

兰科植物的有性生殖特殊,每朵花只有1个花药,且花粉有聚集成块发育的特征。为了揭示铁皮石斛花粉块的发育特征,该研究以野生铁皮石斛不同时期的花药为材料,采用半薄切片和植物组织化学方法对其发育过程进行解剖学观察分析,并对成熟花粉块进行离体培养,观察花粉管的萌发状况。结果表明:(1)铁皮石斛花药壁由1层表皮细胞,2层药室内壁细胞,1层中层细胞和1层绒毡层细胞组成。开花时,绒毡层细胞退化,中层细胞没有退化,药室内壁细胞则形成纤维状细胞壁;药室中的小孢子母细胞没有明显的胼胝质壁结构。(2)小孢子发生属同时型,减数分裂后四分体小孢子不分散,以四合花粉状态发育,并进一步连接形成花粉块。(3)在小孢子发育中,孢粉素覆盖在整个花粉块表面形成花粉外壁,但花粉块内部的花粉没有花粉外壁结构;在花粉块表面的花粉外壁上未见花粉萌发孔。(4)在花粉离体萌发实验中,具有花粉外壁的花粉块表面花粉未见萌发,仅由花粉块内部的花粉萌发出花粉管。     

Variation of pollination and resource limitation in a low seed-set tree, Liriodendron chinense (Magnoliaceae)

Pollen limitation and resource limitation have been documented as the major factors responsible for plants commonly producing more ovules than seeds, but few studies have examined pollen deposition directly in natural populations at different sites and times. We investigated the causes of low seed set in four populations of Liriodendron chinense (Magnoliaceae), an insect‐pollinated endangered tree endemic to southern China, over 2–3 years. One pistil potentially produces two ovules. The number of pistils per flower varies among populations, but in three of the four populations the variation in a given population was not significantly different among years. Overall, populations with higher pistil numbers tend to set more seeds per flower, but a positive correlation between pistil numbers and seed production per flower was observed in only one of the four populations. The numbers of pollen grains deposited per stigma varied from 0 to 60. The proportion of pollinated stigmas per flower ranged from 44% to 88% among populations and years. The numbers of pollen grains deposited per stigma and the percentages of pollinated stigmas were significantly different between populations, and two populations showed significant differences between years. A positive correlation between stigmatic pollen load and seed set was sought in ten population‐by‐year combinations but, in a given population, high stigmatic pollen loads did not always result in high seed set. Examination of pollen deposition, pistil and seed production over several sites and years showed that in addition to pollination, other factors such as resource or genetic loads were likely to limit the (lower than 10%) seed set in L. chinense. It appears that small, isolated populations experience severe pollination limitation; one population studied had seed/ovule ratios of 0.84% and 1.88% in 1995 and 1996. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 140 , 31–38.     

Seed abortion in the sexual counterpart of <Emphasis Type="Italic">Brachiaria brizantha</Emphasis> apomicts (Poaceae)

Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three consecutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self- and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pollen viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self- and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression.     

Influences of Cross Pollination on Pollen Tube Growth and Fruit Set in Zuili Plums （Prunus salicina）

Zuili plum （Prunus salicina L.） trees usually set fruit poorly, although they produce high quality fruit. To elucidate the causes of the poor fruit set, pollen tube growth into pistils and fruit set percentage were investigated after cross-, self- and open-pollination. Ovule development in Zuili pistils was also investigated. Pollen tube penetration into the ovules via the obturator and micropyle was best when Zuili pistils were pollinated by cv. Black Amber （P. domestica） pollen grains, although cross-pollinations with Hongxinli and Miili （P. salicina） pollen were more effective than self- and open-pollination. The fruit set percentage was also highest in pistils pollinated with Black Amber pollen grains. Morphological observation of Zuili pistils revealed that the trees produce ＂double pistils＂, developing two ovaries from a basal pistil, at a rate as high as 28%. In such abnormal pistils, most ovules were lacking an embryo sac or were entirely degenerated. The percentage of normally developed ovules was 24.3% and 8.9% in normal and double pistils, respectively. From these results, we conclude that the main causes of poor fruit set of Zuili plums are a lack of effective cross-pollination and the production of high percentages of double pistils in which normally developed ovules are scarcely formed.     

Pollination Rates and Pollen Tube Growth in a Vulnerable Plant,Liriodendron chinense (Hemsl.) Sarg. (Magnoliaceae)

The limited seed production of insect-depended plant, Liriodendron chinense was once considered to be pollen-limited and insufficient cross pollination. In this study, we counted pollen grains deposited on stigmas in three populations in Guizhou, Hunan and Zhejiang provinces of China respectively. Over 61.9% stigmas were pollinated. From 1994 to 1996, the mean number of pollen grains on each stigma ranged from 4.4 to 42.6, much more than ovules(2) in each pistil. Based on observations of three years, both the pollination rate and pollen quantity on stigmas significantly affected seed set. When flowers opened without stamens dehiscencing at the early stage of anthesis, stigmas received considerable quantity of pollen grains. Pollen grains from different sources were able to germinate and pollen tube growth rates were not greatly variable. It is very likely that pollen grains arrived first would fertilize eggs. Since only several pollen tubes went through the stylar canal, the potential pollen competition may exist. In this case, there would be strong selection on floral syndrome which benefit early insect visits. Pollen grains from the early visits would have a greater chance to fertilize ovules than those from later visits, which implies that cross pollination is the predominant breeding system of this plant. The conclusion was also confirmed by following four artificial experiments. Three treatments, including flowers bagged, netted or with the perianth removed, all reduced seed set clearly, but flowers with the stamens removed (emasculation)had a higher seed production by open pollination. As the rates of deposited stigmas in three populations were 6～8 times more than full seed set, we considerthat lower seed production in this plant may not mainly be due to pollen limitation.     

Callose synthase (CalS5) is required for exine formation during microgametogenesis and for pollen viability in Arabidopsis

Callose (beta-1,3-glucan) is produced at different locations in response to biotic and abiotic cues. Arabidopsis contains 12 genes encoding callose synthase (CalS). We demonstrate that one of these genes, CalS5, encodes a callose synthase which is responsible for the synthesis of callose deposited at the primary cell wall of meiocytes, tetrads and microspores, and the expression of this gene is essential for exine formation in pollen wall. CalS5 encodes a transmembrane protein of 1923 amino acid residues with a molecular mass of 220 kDa. Knockout mutations of the CalS5 gene by T-DNA insertion resulted in a severe reduction in fertility. The reduced fertility in the cals5 mutants is attributed to the degeneration of microspores. However, megagametogenesis is not affected and the female gametes are completely fertile in cals5 mutants. The CalS5 gene is also expressed in other organs with the highest expression in meiocytes, tetrads, microspores and mature pollen. Callose deposition in the cals5 mutant was nearly completely lacking, suggesting that this gene is essential for the synthesis of callose in these tissues. As a result, the pollen exine wall was not formed properly, affecting the baculae and tectum structure and tryphine was deposited randomly as globular structures. These data suggest that callose synthesis has a vital function in building a properly sculpted exine, the integrity of which is essential for pollen viability.     

Microsporogenesis and pollen development in Leymus chinensis with emphasis on dynamic changes in callose deposition

Leymus chinensis is an economically and ecologically important grass that exhibits low seed production. To better understand the causes of its low sexual reproductivity, the microsporogenesis and pollen development of this species were investigated, with emphasis on dynamic changes in callose deposition. A variety of histochemical stains were employed, including Heidenhain's hematoxylin, decolorized aniline blue, DAPI, and acetocarmine, along with a temporary mount method. Microsporogenesis and pollen development generally took place from June 12 to 26. The meiosis of microspore mother cells (MMCs) was of the successive type and the tetrad was isobilateral in shape. Mature pollen grains comprised two sperms and a vegetative nucleus. Callose initially appeared in the center of the anther locule at the premeiotic phase, and then gradually and unevenly deposited around the MMC before the commencement of meiosis. At the onset of meiosis, the accumulation of callose enclosing the MMC peaked, accompanied by the disappearance of callose in the center of the locule. At the dyad and tetrad stages, the dyads and tetrads were surrounded by callose wall and the microspores in the tetrads were isolated by a crossed cell plate composed of callose. Microspores just released from tetrads were still enclosed in callose wall, and then callose gradually disappeared in the pollen wall. Ultimately, callose almost completely disappeared from the walls of mature pollen grains. In the large numbers of sections observed, most of the cases of meiosis of the MMCs, pollen development, and callose dynamics were normal, with only a few abnormities observed. The results suggest that microsporogenesis, male gametogenesis, and callose dynamics during these processes are generally normal in this species, and that the callose wall plays an important role in the production of functional pollen grains. The small numbers of abnormities of these processes that occurred likely do not adversely affect the production of viable pollen grains. Therefore, microsporogenesis and pollen development may not be factors in the low seed production of L. chinensis.     

Heterostylous morph differences in pollen transfer and deposition patterns in Primula sieboldii on a visitation by a queen bumblebee, measured with a semi-natural experimental system

Abstract Under a semi-natural setting the between-morph pollen exchange patterns were studied in distylous Primula sieboldii flowers by measuring pollen removal from the anthers on a single visit by a Bombus diversus tersatus queen, and stigmatic pollen deposition along the sequence of the visitation of the opposite-morph flowers by the bee. Despite the twofold larger number of pollen grains produced in a single flower of the long-styled morph compared to that of the short-styled morph, no significant difference in pollen removal from a flower was found between the morphs. The stigmas of the long-styled morph received significantly more opposite-morph pollen grains than those of the short-styled morph on a single visit by the bee. Sufficient legitimate pollen grains, surpassing the ovule number, were loaded on the stigmas of 27% and 17% of visited flowers of the long- and short-styled morphs, respectively. The short-styled morph could more efficiently donate pollen to the opposite morph stigmas than the long-styled morph.     

Precocious pollen germination in Arabidopsis plants with altered callose deposition during microsporogenesis

Pollination is essential for seed reproduction and for exchanges of genetic information between individual plants. In angiosperms, mature pollen grains released from dehisced anthers are transferred to the stigma where they become hydrated and begin to germinate. Pollen grains of wild-type Arabidopsis thaliana do not germinate inside the anther under normal growth conditions. We report two Arabidopsis lines that produced pollen grains able to in situ precociously germinate inside the anther. One of them was a callose synthase 9 (cs9) knockout mutant with a T-DNA insertion in the Callose Synthase 9 gene (CalS9). Male gametophytes carrying a cs9 mutant allele were defective and no homozygous progeny could be produced. Heterozygous mutant plants (cs9/+) produced approximately 50% defective pollen grains with an altered male germ unit (MGU) and aberrant callose deposition in bicellular pollen. Bicellular pollen grains germinated precociously inside the anther. Another line, a transgenic plant expressing callose synthase 5 (CalS5) under the CaMV 35S promoter, also contained abnormal callose deposition during microsporogenesis and displaced MGUs in pollen grains. We also observed that precocious pollen germination could be induced in wild-type plants by incubation with medium containing sucrose and calcium ion and by wounding in the anther. These results demonstrate that precocious pollen germination in Arabidopsis could be triggered by a genetic alteration and a physiological condition.     

The effect of cherry leaf roll virus infection on the performance of birch pollen and studies on virus replication in germinating pollen

The rates of germ tube elongation in vitro of pollen from cherry leaf roll virus (CLRV)-infected birch (Betula pendula) did not differ significantly from those of pollen from virus-free trees. Differences in percentage germination of pollen collected from trees at different sites were significant but percentages of germination of pollen from virus-infected and virus-free trees did not differ greatly from one another. In vivo, pollen from infected trees germinated on healthy and CLRV-infected stigmas and callose plugs formed in both types of tissues. However, the extent of callose plug formation was greater in the pollen tubes of virus-free grains germinating in infected stigmas than in reciprocal crosses. CLRV coat antigen was detected by ELISA in stigmatic tissue, from healthy trees, on which virus-carrying pollen grains had germinated. Using fluorescein isothiocyanate conjugated to CLRV-specific γ-globulin, viral coat antigen was detected throughout germ tubes from virus-carrying but not virus-free pollen germinating in vitro. Protoplasts released following Driselase digestion of pollen germ tubes from virus-infected trees contained CLRV antigen detectable by ELISA. During germination of virus-infected pollen there was little synthesis of viral coat protein or nucleic acid but, following inoculation with purified virus particles, protoplasts made from healthy germinating pollen produced increasing amounts of CLRV-specific antigen implying that CLRV replicated in this system.     

Callose Deposition During Megagametogenesis in Two Species of Oenothera

Callose deposits are present both in degenerating megasporesof the heteropolar tetrad in Oenothera hookeri and in degeneratingembryo sacs of the homopolar developing tetrad in O biennis.They are partially continuous with the cell wall and partiallyenclosed in the degenerating cytoplasm and show electron opaquebands within a less electron opaque material Vesicles calledcallose grains are present in the degenerating cytoplasm ofthe embryo sac in O biennis These show an electron opaque fibnllaror granular core surrounded by a halo of low electron opacity Similarities in fine structure between callose deposits of femalegametophytes which follow the degenerating pathway of development,and callose plugs present in pollen tubes during their growth,are discussed. Oenothera, evening primrose, megagametogenesis, megasporogenesis, callose, ultrastructure