Stigma development and receptivity in almond (Prunus dulcis)

BACKGROUND AND AIMS: Fertilization is essential in almond production, and pollination can be limiting in production areas. This study investigated stigma receptivity under defined developmental stages to clarify the relationship between stigma morphology, pollen germination, tube growth and fruit set. METHODS: Light and scanning electron microscopy were employed to examine stigma development at seven stages of flower development ranging from buds that were swollen to flowers in which petals were abscising. Flowers at different stages were hand pollinated and pollen germination and tube growth assessed. Artificial pollinations in the field were conducted to determine the effect of flower age on fruit set. KEY RESULTS: Later stages of flower development exhibited greater stigma receptivity, i.e. higher percentages of pollen germination and more extensive tube growth occurred in older (those opened to the flat petal stage or exhibiting petal fall) than younger flowers. Enhanced stigma receptivity was associated with elongation of stigmatic papillae and increased amounts of stigmatic exudate that inundated papillae at later developmental stages. Field pollinations indicated that the stigma was still receptive and nut set was maintained in older flowers. CONCLUSIONS: Stigma receptivity in almond does not become optimal until flowers are past the fully open stage. The stigma is still receptive and fruit set is maintained in flowers even at the stage when petals are abscising. Strategies to enhance pollination and crop yield, including the timing and placement of honey bees, should consider the effectiveness of developmentally advanced flowers.     

Papillar integrity as an indicator of stigmatic receptivity in kiwifruit (Actinidia deliciosa)

Stigmatic receptivity is a major factor limiting fruit set inkiwifruit (Actinidia deliciosa). The aim of this work was toknow what determines the cessation of stigmatic receptivityin this species. Stigmatic receptivity has been analysed inkiwifruit through the capacity of the stigma to sustain pollengermination and has been related to stigmatic development anddegeneration.The stigma of kiwifruit flowers has a papillatesurface which from anthesis is covered with an abundant exudate.Papillae are unicellular and contain a number of phenolic deposits.During the lifetime of the flower these papillae gradually loseturgidity, while stigmatic secretion increases until 5 d afteranthesis when papillae start rupturing and the papillar contentis liberated into the germination medium. Stigmatic receptivityis high at anthesis and lasts for the next 4 d. However, itdrastically decreases 5 d after anthesis and it is nil 2 d later.The pattern of stigmatic receptivity closely fits that of papillarintegrity, indicating that stigmatic receptivity relies on thisintegrity. Since papillar integrity can easily be evaluatedin stigmatic arms, this criterion can be used as a quick methodto estimate stigmatic receptivity. Key words: Actinidia deliciosa, kiwifruit, stigma, receptivity, pollen     

Morphological structure of the stigma and style of several genotypes of Prunus armeniaca L.

ABSTRACT

The morphology of the stigmatic surface and style of apricot (Prunus armeniaca L.) is described, in order to extend current knowledge of stylar and stigmatic structures. Eight apricot cultivars of different geographical origin and characterised by a different self-compatibility behaviour were analysed. Whole pistils were removed from flowers at different phenological stages. Analyses were performed on frozen-hydrated specimens by Low-Temperature SEM. Morphological observations were conducted on: stigma (shape, size and presence of exudate), stigmatic papillae (shape, size, density and distribution) and style (inner structures). A comparative study of the different apricot cultivars showed several morphological differences related to the genotype.     

Structural analysis of stigma development in relation with pollen–stigma interaction in sunflower

Structural analysis of stigma development in sunflower highlights the secretory role of papillae due to its semi-dry nature. Production of lipid-rich secretions is initiated at the staminate stage of the flowers in stigma development and increases at the receptive stage, coinciding with an extensive development of elaioplasts and endoplasmic reticulum network in the basal region of the papillae. Transfer cells, earlier identified only in the wet type of stigma, are also present in the transmitting tissue of the sunflower stigma. Attainment of physiological maturity by the stigmatic tissue, accompanying development from bud to pistillate stage, appears to affect the initial steps of pollen–stigma interaction. The nature of self-incompatibility in Helianthus has also been investigated in relation with pollen adhesion, hydration and germination. Pollen adhesion to the stigma is a rapid process in sunflower and stigma papillae exhibit greater affinity for pollen during cross pollination as compared to self-pollination. Components of the pollen coat and the pellicle on the surface of stigmatic papillae are critical for the initial phase of pollen–stigma interaction (adhesion and hydration). The lipidic components of pollen coat and the proteinaceous and lipidic components from the surface of the papillae coalesce during adhesion, leading to the movement of water from stigma to the pollen, thereby causing pollen hydration and its subsequent germination. Pollen germination (both in self-and cross-pollen) on the stigma surface and the growth of the pollen tube characterize the flexibility of self-incompatibility in sunflower. Compatible pollen grains germinate and the pollen tube penetrates the stigma surface to enter the nutrient-rich transmitting tissue. The pollen tube from incompatible pollen germination, however, fails to penetrate the stigmatic tissue and it grows parallel to the papillae. Present findings provide new insights into structural and functional relationships during stigma development and pollen–stigma interaction.     

Pistil anatomy and pollen tube development in Polygala vayredae Costa (Polygalaceae)

Low seed ovule ratios have been observed in natural populations of Polygala vayredae Costa, a narrowly endemic species from the oriental pre-Pyrenees. To evaluate physical and nutritional constraints and pollen tube attrition in this endemic species, stigma and style anatomy, as well as pollen tube development along the pistil were investigated using light and fluorescence microscopy. The structural morphology of the stigmatic region was also examined with scanning electron microscopy. Pollen grains that reached the stigmatic papillae came into contact with a lipid-rich exudate and germinated easily. Although a large number of pollen grains reach the stigmatic papillae, few pollen tubes were able to grow into the style towards the ovary. The style was hollow, with the stylar channel beginning a few cells below the stigmatic papillae. Initially, the stylar channel area was small compared to other levels of the style, and was surrounded by lipid-rich, highly metabolic active cells. Furthermore, lipid-rich mucilage was detected inside the stylar channel. At subsequent style levels towards the ovary, no major reserves were detected histochemically. The reduced intercellular spaces below the stigmatic papillae and the reduced area of the stylar channel at its commencement are suggested to physically constrain the number of pollen tubes that can develop. In subsequent levels of the style, the stylar channel could physically support a larger number of pollen tubes, but the lack of nutritional reserves cannot be disregarded as a cause of pollen tube attrition. Finally, the number of pollen tubes entering the ovary was greater than the number of ovules, suggesting that interactions occurring at this level play a major role in the final reproductive outcome in this species.     

Quantitative evaluation of stigma polymorphism in a tristylous weed,Lythrum salicaria (Lythraceae)

Tristyly involves three different forms of flowers that differ reciprocally in the heights of stigmas and anthers within flowers. Apart from the style and stamen lengths, heterostylous species also demonstrate pollen and stigma polymorphisms. We quantified stigma polymorphism in tristylous Lythrum salicaria by measuring the stigma diameters, structure of papillae, and density and distribution of papillae on the stigma from flower samples of 201 individuals belonging to three morphs. The diameter of the stigma and the distribution of papillae were quantified using a scanning electron microscope, and the structure of papillae was determined using a light microscope. The stigma diameter in the long morph was significantly greater than in the mid and short morphs. While the density of stigmatic papillae was significantly greater in the mid and short morphs than in the long morph, the total number of papillae per stigma did not differ across morphs. The length and diameter of papillae at the apex, neck, and base were significantly greater in the long morph followed by the mid and short morphs. A discriminant function analysis separated the long morph from the mid and short morphs based on the canonical scores of measurements of papillae structure. The stigma polymorphism coupled with those of pollen may play a functional role in self-incompatibility mechanisms.     

Control of smoulder (Botrytis narcissicold) in narcissus with fungicides

Smoulder, caused by Botrytis narcissicola, is the most widespread foliar disease of narcissus (Narcissus cultivars) in the UK and causes significant losses in bulb and flower yield. A range of current and novel fungicide treatments was examined to determine if control of the disease could be improved and to measure the effect of treatments on bulb yield. In assays on pot-grown plants, carbendazim+flusilazole, epoxiconazole, iprodione + thiophanate-methyl, tebuconazole and vinclozolin reduced lesion size by more than 90%, compared with untreated plants, when applied 1 day before inoculation with the fungus; vinclozolin and tebuconazole were also effective when applied 2 days after inoculation. In field experiments in Cambridgeshire and Lincolnshire, significant reductions were observed in secondary smoulder symptoms (leaf lesions and stem rot) following fungicide sprays. Spray programmes of the novel fungicides azoxystrobin, cyprodinil, kresoxim-methyl, tebuconazole and pyrimethanil were as effective, or more so, than the standard treatments (chlorothalonil, vinclozolin) used by growers. Mixtures of carbendazim + tebuconazole and azoxystrobin + tebuconazole also gave effective control of smoulder. Programmes of four to six sprays, using two or three fungicides with different modes of action, applied alternately, reduced smoulder by 35–69% and increased bulb yields by 7–59%. Although treatment around and after flowering resulted in the greatest control of secondary smoulder, treatment before flowering was required for the highest bulb yields. The choice of fungicides for use in spray programmes to provide effective and reliable control of smoulder, and the possibility of control using fewer sprays, are discussed.     

Reproductive biology of Nymphaea capensis Thunb. var. zanxibariensis (Casp.) Verdc. (Nymphaeaceae)

Anthesis in Nymphaea capensis var. zanzibariensis is diurnal with flowers opening and closing for three consecutive days. On the first day of anthesis, the stigmatic papillae secrete fluid and the outermost anthers are dehiscent. On the second day of anthesis the stamens form a cone above the dry stigmatic cup. The middle stamens open and turn outward. On the third day of flowering, all the stamens open and the dry stigmatic cup is exposed. The flowers are homogamous and not protogynous as the other Nymphaea. The gynoecium of the self-compatible N. capensis var. zanzibariensis , is characterized by a wet papillate stigma, a short hollow style, and secretory cells on the ventral surface of the ovary. The pollen is released on the receptive stigma. Following initial growth in intercellular spaces in the transmitting tract of the stigma, pollen tubes travel through the stylar canal and into the ovary.     

Developmental Aspects of Ultrastructure, Histochemistry and Receptivity of the Stigma of Nicotiana sylvestris

The bilobed papillate stigma of Nicotiana sylvestris Speg. andComes, is covered at maturity with a copious exudate containinglipid, protein and carbohydrate. The stigma is receptive fromthe very early stage of development and it also stains positivelyfor esterase activity. The stigma has three distinct zones:an epidermis with papillae; a subepidermal secretory zone; anda parenchymatous ground tissue. The behaviour of the cells ofthese three zones has been followed from 6 d before anthesisto one day after anthesis and pollination. The cells of theepidermis and the secretory zone stain intensively for lipids,proteins and carbohydrates in the initial stages. The secretoryzone develops large intercellular spaces containing heterogenoussecretory products which also stain positively for the aforesaidthree compounds. At maturity the secretory products are releasedto the surface through gaps formed in the epidermis by cellseparation. The main secretion of the stigma is produced bythe cells of the secretory zone. Less secretion is derived fromthe stigmatic papillae. Some amount of secretion is also releasedfrom the stylar transmitting tissue adjoining the stigma. Theglandular cells of the stigma contain numerous plastids, mitochondria,ribosomes, ER, cytoplasmic lipid droplets and some dictyosomes.The plastids and the vacuoles in the secretory cells of thestigma have a lot of electron dense (osmiophilic) inclusionsrespectively in the initial and later stages of development.The former are probably involved in the production of thesematerials. It is suggested that the proteins are directly secretedby rough ER compartments whereas smooth ER is involved in thesynthesis of lipidic materials. The carbohydrate moiety of theexudate is released by the eccrine mode (sugar mono- and dimers)with some addition of polymers by disintegration of the middlelamellae. The means by which the lipidic and osmiophilic materialis extruded remains unclear. Nicotiana sylvestris, stigma receptivity, organization, stigmatic secretory system, stigmatic exudate     

Pollination and Pollen-pistil Interaction in Oil Palm, Elaeis guineensis

Structural and cytochemical aspects of the pistil and detailsof pollination and pollen-pistil interaction were investigatedin the African oil palm (Elaeis guineensis Jacq.), an importantperennial oil crop. The stigma is trilobed, wet and papillate.The branched papillae are confined to a narrow linear zone oneach stigmatic lobe. Each stigmatic lobe harbours a deep stigmaticgroove, which runs adaxially along the surface. The stigmaticgroove is bordered by a well-defined layer of glandular cells,each of which has a pectinaceous cap on the inner tangentialwall. The style is hollow. The canal cells show thickeningson the inner tangential wall. The stigmatic groove and stylarcanal contain an extracellular matrix secreted by the canalcells which is rich in proteins, acidic polysaccharides andpectins. The canal cells at the base of the style are papillateand loosely fill the stylar canal. The stigma becomes receptivewhen the stigmatic lobes separate, and remains so for 24 h.Pollination is mediated by weevils as well as by the wind. Undernatural conditions the pollination efficiency was 100%. Pollinationinduces additional secretion in the stigmatic groove and stylarcanal. During post-pollination secretion, the pectinaceous capsof the cells lining the stigmatic groove are degraded. Pollengrains germinate on the stigmatic papillae and tubes grow onthe surface of the papillae, entering the stigmatic groove andadvancing along it into the stylar canal to eventually gainaccess to the locules. Pollen tubes are seen in the ovules 18–20h after pollination. Copyright 2001 Annals of Botany Company Arecaceae, Elaeis guineensis, African oil palm, pollination, stigmatic grove, stylar canal, Tenera hybrid, weevil     

黄瓜雌花发育过程中柱头的腺特征

利用透射电镜技术研究了黄瓜(Cucumis sativus L.)雌花柱头发育过程中传递组织、分泌组织和乳突细胞的超微结构.在整个发育过程中,乳突细胞和分泌组织细胞的细胞质内密布很多管状及槽库膨大的内质网,产生很多分泌囊泡;在成熟柱头的传递组织和分泌组织细胞间观察到大量的胞间连丝;乳突细胞和分泌细胞高度液泡化,质膜内折;在柱头发育过程中分泌组织细胞的核周腔扩大形成裂瓣状核,到柱头成熟阶段裂瓣状核更加明显.进一步的研究显示,在成熟柱头的不同组织细胞中,ATPase的活性呈现在质膜和液泡膜上,随着柱头的发育,PM-H -ATPase的比活性明显增强.结果表明,黄瓜雌花柱头的腺特征随发育进程而趋于显著.     

Cockroach pollination and breeding system of Uvaria elmeri (Annonaceae) in a lowland mixed-dipterocarp forest in Sarawak

Tropical forest plants are known to be pollinated by a diverse array of animals. Here we report on the pollination of a woody climber species, Uvaria elmeri (Annonaceae), by cockroaches in a lowland mixed-dipterocarp forest in Sarawak, Malaysia. To the best of our knowledge, this is the first report of pollination by cockroaches. The cauliflorous flowers in the understory are protogynous and bloomed for 50 h. An odor similar to decayed wood or a mushroom was secreted by flowers and was stronger during the male stage. Pollinators were cockroaches (Blattellidae) and drosophilid flies (Drosophilidae). Cockroaches, the main pollinators, visited flowers during both female and male stages at night, feeding on stigmatic exudate and pollen. Drosophilids, the secondary pollinators. mainly visited female-stage flowers during daytime, fed on stigmatic exudate. and laid eggs on stigmas. Neither autogamy nor self-compatibility was observed. Fruit production appeared to be pollen-limited. The fruit set, which was 2% of flowers in natural condition, was significantly lower than the 30% fruit set obtained by artificial cross-pollination. We discuss the traits of cockroaches as pollinators and the breeding system of U. elmeri.     

SEX POLYMORPHISM IN SILENE ACAULIS (CARYOPHYLLACEAE) AND THE POSSIBLE ROLE OF SEXUAL SELECTION IN MAINTAINING FEMALES

Pollen accumulation rates and their relationship to stigma morphology and pollinator visitation behavior were compared between female and hermaphrodite sex morphs of the alpine plant Silene acaulis var. subacaulescens (Caryophyllaceae). Although stigmas of female and perfect flowers collected comparable pollen loads, stigmas of female flowers became receptive earlier in anthesis and therefore recruited a larger number of pollen tubes. Together with early receptivity young female flowers had a larger stigmatic area with longer papillae than did perfect flowers. Pollinator behavior also differed between morphs, with bumble bees spending more time probing female than perfect flowers. Differences in stigma receptivity schedules of female and perfect flowers have consequences for different opportunities for sexual selection in the two sex morphs. Female flowers, by providing a more effective gametophytic screen, have the potential to produce higher quality offspring. This is proposed as a further compensatory advantage maintaining females, with a single fitness function, in populations containing hermaphrodites which have both male and female fitness functions.     

The dry and wet stigmas ofPrimula obconica: Ultrastructural and cytochemical dimorphisms

Summary The stigmatic papillae of the distylous speciesPrimula obconica are studied by means of cytochemical, light and electron microscopic techniques. The papillae on thrum stigmas are smaller than those on pin stigmas. At the bud stage, secretory vesicles are not a conspicious part of the cytoplasm, although certain signs of secretory activity are present. The young papillae bear the thin, superficial pellicle typical to dry stigmas. Small vesicles are numerous in mature papillae of both morphs, and seem to originate from the ER. A layer of closely packed, osmiophilic globuli is present in the outermost part of mature walls of pin papillae. At sites with cuticle disruption, the globuli seem to migrate outwards to be incorporated with the copious, blistery exudate. Due to this exudate the pin stigma is characterized as wet. Cytochemical tests suggest that the exudate contains mainly lipids, and different carbohydrates and protein are detected. It reacts positively in tests for peroxidase, acid phosphatase, and non-specific esterases. The thrum stigma remains dry at maturity, with a distinct pellicle also reacting positively to the enzyme tests. Only a few, scattered osmiophilic globuli, sized and situated as those in the pin papillae walls, are found in the thrum walls, and they do not form a proper layer.Thus the generally accepted correlation between dry stigmas and the sporophytic kind of self-incompatibility system is not substantiated withinP. obconica, and the possible influence of the dimorphisms to the pollen/stigma interaction is discussed.     

STIGMA OF NICOTIANA: ULTRASTRUCTURAL AND BIOCHEMICAL STUDIES

The aim of the present study has been to elucidate further the cytology of the glandular stigma of Nicotiana tabacum, the mechanism of the secretory process during stigma development and the biochemical composition of the exudate. The stigma consists of two distinct zones: a glandular zone formed by the papillae and 2–3 layers of cells (basal cells) immediately below them, and a non-glandular region formed by vacuolated cells which are in continuity with the transmitting tissue. The stigmatic exudate is a complex mixture of different chemical compounds such as proteins, saccharides, fatty acids and phenols. The role of stigma secretion is discussed in relation to pollen activation, recognition and pollen tube growth.     

The Structure and Cytochemistry of the Pistil of Sternbergia lutea (Amaryllidaceae)

Studies were carried out on structural and cytochemical aspectsof the pistil of Sternbergia lutea (L.) KerGawl. The stigmais of the wet papillate type; the papillae are unicellular andare arranged densely around the rim of a funnel-shaped stigma.The stigma exudate is limited and is confined to the bases ofthe papillae and the inner lining of the stigma. The papillaeare smooth in the distal part and are covered with intact cuticle-pelliclelining. The cuticle is disrupted at places towards the baseof the papillae releasing the exudate. The exudate is rich inpectins and other polysaccharides but poor in proteins and lipids.The papillae show dense cytoplasmic profiles with extensiveendoplasmic reticulum (ER), abundant mitochondria, polyribosomesand active dictyosomes. The style is hollow. The stylar cavityis surrounded by two to four layers of glandular cells. In theyoung pistil the canal is lined with a continuous cuticle, butin the mature pistil the cuticle becomes disrupted and the canalis filled with the secretion produced by the cells of the surroundingglandular tissue. Ultrastructurally, the cells of the glandulartissue are very similar to the stigmatic papillae. The innertangential wall of the cells bordering the canal is uniformlythicker than other walls. The secretion in the stylar canal,as well as the intercellular spaces of the glandular tissue,stain intensely for pectins and polysaccharides but poorly forproteins and lipids. Pollen tubes grow through the stylar canal.Structural and cytochemical details of the pistil of Sternbergiaare compared with other hollow-styled systems. Pistil, Sternbergia lutea (L.) Ker-Gawl., stigma and style, structure and cytochemistry     

POLLINATION IN LUPINUS NANUS SUBSP. LATIFOLIUS (LEGUMINOSAE)

Floral morphology and phenology and the timing of stigmatic receptivity and pollen viability were studied to elucidate the mechanisms by which self-pollination in Lupinus nanus subsp. latifolius is minimized under natural conditions. Pollen germination and pollen tube growth suggest that a physiological self-incompatibility system does not exist. Instead, self-pollination is minimized by protandry and by a collar of peristigmatic hairs, which initially inhibit access of autologous pollen to the stigma. These hairs subsequently wilt, permitting self-pollination of unvisited flowers. Pollen germinates in vivo one day before substantial metabolic enzyme activity can be detected. Citric acid cycle enzymes are not detectable in pollen, but those of anaerobic metabolism are. Beginning on the second day postanthesis, stigmatic secretions exude from weak areas on lateral walls of the elongate epidermal papillae, welling up in the interstices between these cells. This contrasts with the stigmas of other papilionoid legumes, in which secretions accumulate beneath the cuticle covering the stigmatic cells, and the often-thin cuticle must be ruptured before pollen and exudate can come into contact.     

A comparative analysis of the distribution and composition of lipidic constituents and associated enzymes in pollen and stigma of sunflower

Spatial distribution and compositional analyses of the lipidic constituents in pollen and stigma of sunflower (Helianthus annuus L. cv. Morden) were conducted using ultrastructural, histochemical, and biochemical analysis. Detection of secretions at the base of stigmatic papillae and neutral lipid accumulations on the surface of stigmatic papillae and between adjacent pseudopapillae demonstrates the semidry nature of stigma surface in sunflower. Pollen coat is richer in lipids (8%) than stigma (2.2%) on fresh weight basis. Nile Red-fluorescing neutral lipids are preferentially localized in the pollen coat. Neutral esters and triacylglycerols (TAGs) are the major lipidic constituents in pollen grains and stigma, respectively. Lignoceric acid (24:0) and cis-11-eicosenoic acid (20:1) are specifically expressed only in the pollen coat. Similar long-chain fatty acids have earlier been demonstrated to play a significant role during the initial signaling mechanism leading to hydration of pollen grains on the stigma surface. Lipase (EC 3.1.1.3) activity is expressed both in pollen grains and stigma. Stigma exhibits a better expression of acyl-ester hydrolase (EC 3.1.1.1) activity than that of observed in both the pollen fractions. Expression of two acyl-ester hydrolases (41 and 38 kDa) has been found to be specific to pollen coat. Specific expression of lignoceric acid (24:0) in pollen coat and localization of lipase in pollen and stigma have been discussed to assign possible roles that they might play during pollen–stigma interaction.     

烟台甜樱桃柱头的可授性、形态特征与坐果率

采用数码照相、电镜扫描、联苯胺-过氧化氢测试及去雄套袋等技术手段, 对烟台甜樱桃(Cerasus avium)花期不同发育阶段柱头的可授性、形态特征和坐果状况进行了观察。结果表明, 烟台甜樱桃在套袋状态下, 柱头可授期从开花前1天开始可持续5–7天。从杯状花期到花瓣平展期, 柱头逐渐有乳突细胞破裂并呈现分泌液, 出现渐强的可授性; 从花瓣平展期到花瓣脱落期, 柱头由暗黄渐变至暗黑, 逐渐萎缩并丧失可授性。去雄套袋及人工授粉实验结果显示, 在大蕾期、杯状花期、花瓣展放期、花瓣平展期和花瓣脱落期进行人工授粉, 烟台甜樱桃的坐果率分别为60.50%、58.33%、62.08%、57.14%和39.13%。在自然条件下烟台甜樱桃的坐果率一般为30%–42%, 传粉成功的最佳期主要发生在杯状花期至花瓣平展期。