The Pollen-stigma Interaction: Pollen-tube Penetration in Crocus

In a compatible pollination in Crocus, pollen tube tips enterthe stigma papillae after the enzymic erosion of the cuticle,and the tubes continue downward growth towards the ovary betweenthe cuticle and the underlying pectocellulosic wall. The cuticleof the receptive zone of the stigma papilla is chambered, thechambers containing a secretion accumulated during the maturationof the stigma. Pollen exudates contain various acid hydrolases,but are incapable alone of eroding stigma cutin. Furthermore,there is no penetration when the proteins of the wall-held stigmasecretions are degraded enzymically. These facts are taken toindicate that the pollen contributes a ‘cutinase’precursor which is activated by a factor or factors held inthe stigma secretion. Pollens of certain Cruciferae producetubes capable of penetrating the Crocus stigma cuticle, suggestingthat notwithstanding the taxonomic remoteness of Cruciferaeand Iridaceae the enzyme activation systems are quite similar.     

Development of the Anther of Annona squamosa L.

In the stamen of Annona squamosa the initial hypodermal archesporiumcomprises a uniseriate row of cells, of which subsequently alternatecells become septal initials and sporogenous cells respectively.Morphologically the tapetum has a triple origin from the parietalcell derivatives, the connective cells and the septal initials;the tapetum derived from the former two is secretory while thelast forms a periplasmodium. Cytokinesis of microspore mothercell is by successive cytoplasmic constriction. The pollen grainsremain loosely attached as tetrads. Annona squamosa L., anther development     

Reproduction in Seagrasses: Nature of the Pollen and Receptive Surface of the Stigma in the Hydrocharitaceae

This paper describes the characteristics of the pollen and thereceptive surfaces of the stigmas in the three marine angiospermsincluded in the Hydrocharitaceae. The pollen in Enhalus acoroidesand Thalassia hemprichii is spherical and has an ornamentedexine. An exine layer is not found in Halophila stipulacea wherereniform pollen grains are contained within transparent moniliformtubes. Cytochemical tests show that the pollen wall in the threespecies contains acidic and neutral polysaccharides and acidhydrolase acitivity is detected in the intine of H. stipulaceaand T. hemprichii. In Thalassia, one of the intine enzymes,acid phosphatase, is unambiguously associated with cytoplasmicinclusions. Flowering in Thalassia is coincident with the spring tides andthe pollen is released as a mass suspended in a thecal slimewhich contains approximately 5 per cent by weight carbohydrate,the principal mono-saccharide being mannose. Electrophoreticanalysis of the pollen-free slime shows a single glycoproteincomponent. The stigmas of the three seagrasses are papillate and of the‘dry’ type possessing a continuous protein-aceouspellicle subtended by a cuticle. The stigma pellicle exhibitscytochemically detectable esterase activity and binds the lectinconcanavalin A. Acid phosphatase activity is localized beneaththe cuticle at the tips of the stigma papillae. The discoveries show that the characteristics of the pollenand stigmas in the seagrasses are comparable with those foundin terrestrial flowering plants. The similarity in enzymaticproperites of the pollen wall and stigma pellicle suggests that,intriguingly, a similar mechanism of cuticle erosion might wellfollow compatible pollination both on land and in the sea. Enhalus acoroides, Thalassia hemprichii, Halophila stipulacea, Halophila decipiens, seagrasses pollen wall, stigma surface, hydrolytic enzymes     

The Structure and Cytochemistry of the Pistil of Hypericum calycinum: The Stigma

The pistil of Hypericum calycinum has a pentacarpellary, syncarpousovary with five slender styles, each terminating in a smallstigma. The stigma is dry and papillate with a thin lining ofpellicle. The cuticle is thin and continuous around the papillae.A large vacuole filled with tannins occupies the major partof the papillae and the cytoplasm forms a thin lining aroundthe vacuole. The cell wall of the mature papillae show two distinctlayers - an outer layer of loosely woven fibrils and an innerdenser layer with compact fibrils. A large number of small lipoidalbodies accumulate just below the cuticle. The papillae havefewer organelles than those typical of glandular cells. Dictyosomesobserved occasionally are without associated vesicles. The cytoplasmis rich in ribosomes. The basal portions of the papillae mergeinto the transmitting tissue made up of loosely arranged cells.The intercellular matrix of the transmitting tissue is richin lipids. Pollen grains are deposited between the papillae.Upon pollen germination, pollen tubes enter the stigma throughthe interstices between the papillae Hypericum calycinum, cytochemistry, pistil, pollen-pistil interaction, stigma, ultrastructure     

Structural and Cytochemical Characteristics of the Stigma and Style in Vitis vinifera L. var. Sangiovese (Vitaceae)

Studies were carried out on structural and cytochemical aspectsof the stigma and style ofVitis vinifera . The stigma is ofthe wet papillate type with a continuous cuticle and pellicle.During the development of the papillae, the cell walls increasein thickness and produce a secretion product constituted oflipids that pass through the wall forming the exudate. The styleis solid with a central core of transmitting tissue which hasconspicuous intercellular spaces that increase remarkably fromthe periphery to the centre where the cuticle is present. Theintercellular spaces, where the pollen tubes grow, contain amatrix that includes polysaccharides, pectic substances andscattered areas of lipidic nature. Cytochemistry; stigma; style; ultrastructure; Vitis vinifera     

Structure and Cytochemistry of the Stigma and Pollen--Pistil Interaction in Zephyranthes

The stigma in Zephyranthes candida and Z. citrina is of thedry type with a continuous cuticle—pellicle. In some papillae,however, the terminal portion of the cuticle—pellicleis lifted upwards and occasionally even disrupted by the accumulationof a secretion product below it. Both non-specific esterasesand acid phosphatases are present on the stigma surface. Thestyle is solid with a central core of transmitting tissue whichhas conspicuous intercellular spaces containing a matrix thatincludes proteins, polysaccharides and pectic substances. Zephyranthes citrina is self-compatible while Z. candida isself-incompatible. Followng incompatible pollination in Z. candida,pollen germination is normal but pollen tube growth is inhibitedat the junction of the stigma and style. Self-incompatibilitycan be overcome by bud pollination. Protein synthesis is necessaryfor pollen germination in both species. Concanavalin A bindsto the stigma surface of both species, but does not affect pollentube penetration in Z. candida. In crosses between the two speciestypical unilateral incompatibility is observed when Z. candidais used as the pistillate parent. Zephyranthes, stigma-surface enzymes, dry stigma, pollen-pistil interaction, self-incompatibility, unilateral-incompatibility     

Fine Structure and Cytochemistry of the Stigma Surface and Incompatibility in some Distylous Linum Species

The receptive surface of the stigma in distylous Linum grandiflorumand L. pubescens was studied by electron microscopy and cytochemicaltechniques. In both floral morphs a proteinaceous-lipoidal coatingis present on the papilla surface. In thrum stigmas the cuticleis highly irregular and pitted at the papilla tip. The cuticleis dislodged and torn at anthesis and an osmiophilic secretionproduct is released within a pectinaceous matrix. The secretionproduct stains for proteins and lipids and contributes to adhesionof pollen. In the larger pin papillae the cuticle is wavy, continuous,thicker than in thrum papillae and adjoins the cell wall. Inboth species the surface of the two types of pollen grains iscoated with lipids and protein. A similar behaviour of the male gametophyte is observed in incompatiblepollinations of L. pubescens, L. mucronatum and L. grandiflorum.In intramorph thrum pollinations pollen tubes are arrested withinthe stigma. In intramorph pin pollinations the majority of pollengrains fail to adhere to the stigma. Low permeability to waterin pin papillae, as determined by the neutral red test, maybe a factor preventing imbibition of the few adhering grains.Tubes of the few germinated grains are inhibited inside thestigma. On the part of the stigma, the difference in the major siteof inhibition in the two intramorph incompatible combinationsmay be accounted for by the dissimilar properties of the papillae,i.e. the occurrence of wet thrum stigmas and dry pin stigmas.Functionally, the unusual association of sporophytic incompatibilitywith wet thrum stigmas is attributed to retention of the secretorymaterial on individual papillae. Stigmatic papillae, cuticle, pollen coat, distyly, incompatibility, Linum grandiflorum, L. pubescens, L. mucronatum     

Esterases in pollen and stigma of Brassica

The dry type stigma of Brassica is covered with a continuous layer of cuticle. Cutinase and non-specific esterases may be involved in breakdown of this cuticle barrier during pollen-stigma interaction, but only a little is known about their nature and characteristics. We report here the presence of two distinct esterases from stigma and pollen of Brassica. A 33 kD esterase assayed using MU-butyrate substrate shows high activity in stigma papillae. A similar esterase from Tropaeolum pollen has been shown to possess active cutinase activity. The esterase activity in anther tissue is due to a 24 kD enzyme with substrate specificity toward acetate esters. Both enzymes require sulfhydryl groups for their catalytic activity. Immunogold labelling of antibodies raised against these esterases localised the proteins at the subcellular level. Antibodies for MU-butyrate hydrolase gave a positive signal in the cell walls of mature stigma papillae and in the tapetum and microspores during early stages of anther development. In the mature anther, a positive signal in the cytoplasm of pollen grains with some detectable localisation in the exine layer of the pollen wall was obtained. Similar results were obtained with acetate hydrolase antibodies. These esterases are thus spatially and temporally regulated in stigma and anther tissues.Abbreviations MU methyl umbelliferyl - pAbC anti-butyrate hydrolase polyclonal antibodies - pAbE anti-acetate hydrolase polyclonal antibodies     

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     

Pollen-Stigma Interaction in the Leguminosae: Stigma Organization and the Breeding System in Vicia faba L.

As in other papilionoid Leguminosae, the receptive surface ofthe stigma of Vicia faba L. is invested by a detached cuticle.This cuticle, the so-called ‘stigma membrane’ ofplant breeding literature, is lifted away from the epidermisduring development by the accumulation of a lipid-rich secretionreleased into the intercellular spaces of the stigma head bythe epidermal cells and the underlying three to four cell layers.The cuticle is thickened over the prominences left by the epidermalpapillae, thinning out between. Pollen, whether self or cross,cannot hydrate and germinate in contact with the intact stigmasurface, but must await the disruption of the cuticle and therelease of the retained secretion. In most genotypes this takesplace only when the flower is tripped by visiting pollinatorsor in consequence of severe agitation by wind. A comparison of lines differing in their degree of autofertilityin field conditions has revealed various differences in stigmastructure. A highly autofertile line had low papillae at thereceptive tip, with a relatively thin intervening cuticle, whilean autosterile line had longer papillae and a thicker cuticle.A line with partial autofertility was intermediate in thesecharacteristics. These properties of the stigma surface, togetherwith other differences in flower structure, are probably adequateto account for the variation in the degree of autosterility,since no evidence was obtained suggesting the presence of aneffective physiological self-incompatibility system in any ofthe lines studied. Since the rupturing of the stigma cuticle is affected by theturgor pressure of the cells of the stigma and style, some degreeof environmental interaction is to be expected: autofertilityshould be at the highest in conditions of adequate water supply,and lowest where there is water deprivation. Pollen-stigma interaction, Vicia faba L., Leguminosae, breeding system, self-incompatibility     

Floral Structure, Stigma Receptivity and Pollen Viability in Relation to Protandry and Self-incompatibility in Silky Oak (Grevillea robusta A. Cunn.)

The reproductive biology of Grevillea robusta growing underexotic conditions in Kenya and Australia is reported. The speciesshowed both protandry and a self-incompatibility mechanism.The stigma was wet and papillate with a distinct groove in themiddle. The anthers dehisced prior to anthesis, when the perianthopened. Stigmatic receptivity began 1 d after anthesis, withthe greatest pollen germination rates and longest pollen tubesobtained 2 d after anthesis. Nectar secretion commenced withpollen dehiscence and was abundant at anthesis. Most stigmaticgrooves opened widely 1–2 d after anthesis and stigmasshowed taller papillae and abundant secretion. Controlled pollinationsgave a greater fruit set from cross-pollination (5.9% in Apriland 17.5% in July) than open-pollination (0.1% in April and3.3% in July). No fruit set from self-pollination was obtainedin April, and very few fruit set for geitonogamous (two outof 1622; 0.1%) or for autogamous (one out of 2707 flowers; 0.04%)pollination treatments in July. Following self-pollination,growth of pollen tubes was poorer than in other treatments,and was generally arrested in the upper style. Cross-pollinatedflowers produced normal and straight pollen tubes, while self-pollentubes had growth abnormalities. Most of the open-pollinatedflowers were found without pollen or with only self-pollen ontheir stigmas indicating that the amount of cross-pollen reachingthe stigma under open-pollination may be a factor limiting seedproduction. Flowers shed soon after the fertilization phasewere those with ungerminated pollen or no pollen. Although avery low rate of selfing may occur, G. robusta presents a self-incompatibilitysystem and allogamy is its primary breeding behaviour.Copyright2000 Annals of Botany Company Grevillea robusta, silky oak, Proteaceae, protandry, controlled pollinations, receptivity, pollen-tube growth, self-incompatibility, pollination     

Papillate Tepal Hoods and Delayed Pollen Germination inAlbucaL. (Liliaceae)

In the genusAlbuca, pollen germination is delayed until thetepals close around the stigma immediately before the onsetof floral senescence. At this time, papillae on the tepal apicesand the stigma swell and produce an exudate in which pollenrapidly germinates. No pollen germination occurs when the tepalsare artificially prevented from closing around the stigma.Copyright1998 Annals of Botany Company Albuca, Liliaceae, delayed pollen germination, gametophytic self-incompatibility, perianth contributing to pollen germination, pollen tube inhibition in ovary.     

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     

Stigma-surface Esterase Activity and Stigma Receptivity in Some Taxa Characterized by Wet Stigmas

Stigma-surface esterase activity and stigma receptivity througha sequence of developmental stages of the pistil have been studiedin four taxa characterized by having wet stigmas — Petuniahybrida, Nicotiana tabacum, Crinum defixum and Amaryllis vittata.The style is solid in the first two and hollow in the lattertwo taxa. In all the taxa, stigma—surface esterase couldbe detected in a thin surface layer (pellicle) from a very earlystage of pistil development, irrespective of the presence orabsence of the exudate. However, the taxa showed variation instigma receptivity. In Petunia and Nicotiana, stigmas from pistilsof all the stages supported pollen germination and tube growth.In Amaryllis and Crinum, stigmas of only the mature pistils,when the exudate is present on the stigma, supported normalpollen germination and tube growth. It is inferred that in taxacharacterized by a wet stigma and solid style, the factors requiredfor pollen germination are present from an early stage of pistildevelopment and the exudate per se is not involved in pollengermination. In taxa characterized by a wet stigma and hollowstyle, however, the pellicle does not carry the factors requiredfor pollen germination and tube growth; they appear to be presentin the exudate. Petunia hybrida Vilm, Nicotiana tabacum L., Crinum defixum, Ker-Gawl, Amaryllis vittata Ait., tobacco, pollination, pollen germination, stigmatic exudate, stigma receptivity, stigma-surface esterase, esterase activity     

THE TRANSMITTING TRACT IN GLADIOLUS. I. THE STIGMA AND THE POLLEN-STIGMA INTERACTION

The stigma papillae in Gladiolus are of the “dry” type and are highly vacuolated cells with an organelle-rich peripheral cytoplasm. The cell wall of each papilla is overlain by a distinctive cuticle possessing an irregularly scalloped inner margin. Between the cell wall and cuticle is a layer of amorphous sub-cuticular material. Lipids are detected on the papilla surface. A pollen grain will hydrate and germinate only on a papilla and not on any other (non-papillate) portion of the stigma. The pollen tube penetrates the papilla cuticle, which is forced away from the papilla cell wall by sub-cuticular pollen tube growth. As the cuticle lifts away, the sub-cuticular material disperses. At the base of the papilla, the pollen tube grows onto the adaxial non-papillate surface of the stigma lobe. At this site, the cuticle has been lifted away from the underlying cells by release of a mucilaginous substance from the latter, and the pollen tube grows within this substance beneath the detached cuticle. The cytological features of Gladiolus papillae are compared with other stigma papillae described in the literature. Also, a review of the literature, as well as some of the findings of the present study, suggest that certain prevalent interpretations of dry stigma structure and function may be open to question.     

Mechanism of Pollination in Gladiolus: Roles of the Stigma and Pollen-tube Guide

Gladiolus has a dry type of stigma. Compatible pollen grainsalight and germinate on the receptive surface of the papillae,penetrate the cuticle and grow towards the style through a sub-cuticularpollen-tube guide of mucilage. This is secreted from epidermalcells of the stylodium and style canal. The cuticle, which coversthe pollen tube guide mucilage, is continuous through the stylecanal to the ovary. The wet stigma of Lilium also has cuticulartissue running through the style canal, covering the mucilage.     

Evidence for Stigmatic Self-incompatibility, Pollination Induced Ovule Enlargement and Transmitting Tissue Exudates in the Paleoherb, Saururus cernuus L. (Saururaceae)

Saururus cernuus, a species belonging to the primitive herbaceousangiosperm family Saururaceae, exhibits high rates of self-sterility.We investigated the structural and functional aspects of pollen-carpelinteractions following cross and self pollination to assessthe tissue specific site and timing of self-sterility and factorsimportant for successful cross pollen tube growth. Self-sterilitywas due to inhibition of self pollen germination at a dry stigma.Self pollination was associated with anomalous foot formation,reduced cell wall expansion and secretory activity of stigmaticpapillae, and callose production in stigmatic papillae. Followinggermination, cross compatible pollen tubes entered a solid coreof transmitting tissue and grew to the base of a short style.Entry of cross pollen tubes into the ovary was coincident withovule enlargement which placed the micropyle in the proximityof cross pollen tube tips. Ovule enlargement also occurred followingself pollination. Cross pollen tubes either entered an exudate-filledmicropyle directly from the style, or growth in the ovary waslocalized to the epidermis of the locule and outer integumentprior to entry into the micropyle. Prior to pollination, thetransmitting tract was void of secretions except for exudatein the micropyle. Growth of pollen tubes on the locule and integumentwas associated with exudate apparently arising from transmittingcells adjacent to growing pollen tubes. The present study providesthe first evidence in a primitive herbaceous species of stigmaticself-incompatibility (SI) in association with a dry stigma,pollination-induced signalling events affecting developmentof carpellary tissues, and micropylar exudates. Copyright 1999Annals of Botany Company SI evolution, dry stigma, exudates, pollen-carpel signalling, Saururaceae.     

The Proteaceous Pistil: Morphological and Anatomical Aspects of the Pollen Presenter and Style of Eight Species Across Five Genera

The morphology and anatomy of pollen presenters, styles andpollen of species ofBanksia, Dryandra, Hakea, IsopogonandMacadamiawerestudied. Serial sections of pistils and SEM images of pollenwere quantified to determine whether the low fertility observedin the Proteaceae has a structural basis. Pollen access to thestigma was investigated. There were three types of stigmaticcavity. A groove in which the stigmatic papillae were enclosedwas present inDryandra, BanksiaandHakea. Macadamiahad a groovewith protruding papillae, andIsopogonhad a tube which enclosedthe papillae. Anatomical studies showed the pollen presenterto be structurally complex but overall to have similar internalanatomy across the species studied. The species could be groupedaccording to presence or absence of transfer tissue and presenceor absence of sclerenchyma, but these groups were not mutuallyexclusive. In the pistil there were three structural filtersto pollen tube passage. The first was at the stigma, where pollengrain access could be limited by the diameter or length of thestigmatic groove or the capacity of the pollination chamberin relation to pollen volume. The second and third related toa narrowing of the transmitting tissue tract within the pollenpresenter and in the lower style which could influence pollentube passage to the ovule.Copyright 1999 Annals of Botany Company Proteaceae,Banksia coccinea, Banksia ericifolia, Dryandra formosa, Dryandra nana, Dryandra quercifolia, Hakea bucculenta, Isopogon cuneatus, Macadamia integrifolia,stigmatic cavity, fertility, pollen presenter, structural limitation, stigma, pollen grain, image analysis, transfer tissue.     

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     

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.