Floral Nectar Secretion and Ploidy in Brassica rapa and B. napus (Brassicaceae). II. Quantified Variability of Nectary Structure and Function in Rapid-cycling Lines

Haploid, diploid and tetraploid lines ofBrassica rapaL. (syn.campestris),and allotetraploidB. napusL., were examined to determine theinfluence of ploidy on floral features, particularly nectarymorphology and anatomy, and to relate nectary structure to nectarproduction capacity. Except for haploids, all lines were rapid-cycling.Average flower dry weight, and petal length and width, werein the descending orderB. napus>B. rapa (4n) >2n>n.Pollen grains of 4nplants were larger than those of 2nplants;haploids lacked pollen. All lines developed nectaries. Typically, each flower producedtwo pairs of nectaries, of different types and nectar productioncapacity. Normally, each lateral gland was located above thebase of a short stamen, and together this pair yielded mostof a flower 's nectar carbohydrate. Each median nectary aroseat the outer junction of the bases of two adjacent long stamens.All lateral nectaries received a vascular supply of phloem alone,but median glands received reduced amounts of phloem, or lackedvasculature altogether. Most nectaries were solitary, but 14%of all flowers, and especially those of 2n B. rapa,had at leastone median and lateral gland connected. Obvious variation existed in nectary morphology between ploidylevels, between flowers of the same plant, and even within flowers.Ten forms of each nectary type were recognized. Plants producingthe most nectar carbohydrate had high frequencies of lateralnectaries which were symmetrical, unfurrowed swellings. TetraploidsofB. rapahad both the highest frequencies of furrowed lateralglands, and of isolated segments of nectarial tissue at thatposition. Even these separated nectarial outgrowths receivedphloem and produced a nectar droplet. At the median location,nectaries were commonly of two forms: peg- or fan-shaped. Lobeson median nectaries, up to four per nectary, were detected inalmost half of glands of 4nflowers examined; lobes were absentin haploids. Brassica rapa; Brassica napus; flower size; nectar production; nectary variability; petal size; ploidyphloem; pollen; rapeseed     

Influence of elevated CO2 and ultraviolet-B radiation levels on floral nectar production: a nectary-morphological perspective

 Investigations of the effects of two global events – elevated CO₂ levels and enhanced ultraviolet-B (UV-B) radiation – on floral nectar production are reviewed from twelve dicotyledonous families. Furthermore, to allow comparisons between nectary morphology and nectar production in treated plants of these fifteen species, new data on floral nectary structure are provided for Malcolmia maritima (L.) R. Br. (Brassicaceae) and Scabiosa columbaria L. (Dipsacaceae). All but the last taxon possessed mesenchymatic floral nectaries with surface stomata. Few clear relationships existed between nectary morphology and various physiological responses to CO₂ or UV-B enrichment, indicating that species responded notwithstanding nectary structure itself. Overall, nectar-solute concentration was least affected by elevated CO₂ or UV-B radiation; consequently, changes in nectar volume were responsible for differences in nectar-sugar production per flower. Three species of Fabaceae experienced no change in floral nectar production upon exposure to elevated CO₂. To date, no study of enhanced UV-B radiation reported a consistent reduction in floral nectar production; three species of Brassicaceae responded differently, but various levels of ozone depletion were simulated. Experimentation with more taxa – including those possessing nectary types such as septal (gynopleural) nectaries (e.g. many monocotyledons) or aggregations of glandular trichomes – and expanding such physiological studies to species possessing extrafloral nectaries, are recommended. Received August 8, 2002; accepted November 23, 2002 Published online: June 2, 2003     

芝麻菜花蜜腺的结构和发育研究

通过解剖镜观察、石蜡切片和薄切片等方法,对芝麻菜的花蜜腺的位置、形态、结构、发育过程及泌蜜前后组织化学变化进行了研究。芝麻菜花蜜腺4枚,分成两对,其中一对侧蜜腺较大,棱柱状,分别着生在外轮2个短雄蕊基部内侧的花托上,结构上由表皮、产蜜组织和维管组织构成;另一对中蜜腺较小,近棒状,分别着生在内轮4个长雄蕊外侧的花托上,结构上仅由表皮和产蜜组织构成。二者表皮细胞外都具角质层,且蜜腺产蜜组织细胞中只含少量的多糖物质。两类蜜腺的蜜汁均由变态气孔泌出体外。无论侧蜜腺还是中蜜腺,蜜腺原基皆是在雌、雄蕊已分化后,由花托相应位置表皮下的1～2层细胞分裂形成的。在蜜腺发育中,产蜜组织细胞在泌蜜前后不具明显的液泡变化。     

Nectar, nectaries, flower visitors, and breeding system in five terrestrial Orchidaceae from central Argentina

Floral nectar sugar composition, nectary anatomy, and visitors are studied in five Argentine Orchidaceae, from 18 populations. Hand-pollinations were performed to evaluate their breeding system. We found two different types of perigonal nectaries located either in the spur (Habenaria gouriieana, H. hieronymi, Habenariinae), or in the basal lateral parts of the labellum (Beadlea dutraei, Pelexia bonariensis, Stenorrhynchos orchioides, Spiranthinae). The spur ofHabenaria is a nonvascularised and nonstructural nectary. The inner epidermis bears one-celled long papillae. In bud stage, the papillae are filled with starch grains, but when the flower opens and nectar secretion starts, they show no starch grains. This fact may indicate that starch is a source for some of the secreted nectar. In the remainder genera, the lateral basal parts of the labellum are secretory. The two glands are located in the adaxial basal lateral faces of the labellum. These nectaries are structural and nonvascularised.Stenorrhynchos produces abundant, concentrated nectar (40–50%).Habenaria gourlieana accumulates copious nectar in a lower concentration (<20%), whereas the other species produce small quantities of concentrated nectar (ca. 50%). Three of the studied species have sucrose predominant nectar (Beadlea dutrael, Habenaria gourlieana, andPelexia bonariensis) whileH. hieronymi, Stenorrhynchos orchioides have hexose predominant ones. Nectar removal and/or pollination induce flower senescence.H. gouriieana is visited by sphingids,S. orchioides by hummingbirds, andB. dutrael by bees. For the two other species we did not record flower visitors.Pelexia bonariensis, B. dutrael, andS. orchiodes are self-compatible species but a pollinator is needed.     

Floral nectar production and carbohydrate composition and the structure of receptacular nectaries in the invasive plant <Emphasis Type="Italic">Bunias orientalis</Emphasis> L. (Brassicaceae)

The data relating to the nectaries and nectar secretion in invasive Brassicacean taxa are scarce. In the present paper, the nectar production and nectar carbohydrate composition as well as the morphology, anatomy and ultrastructure of the floral nectaries in Bunias orientalis were investigated. Nectary glands were examined using light, fluorescence, scanning electron and transmission electron microscopy. The quantities of nectar produced by flowers and total sugar mass in nectar were relatively low. Total nectar carbohydrate production per 10 flowers averaged 0.3 mg. Nectar contained exclusively glucose (G) and fructose (F) with overall G/F ratio greater than 1. The flowers of B. orientalis have four nectaries placed at the base of the ovary. The nectarium is intermediate between two nectary types: the lateral and median nectary type (lateral and median glands stay separated) and the annular nectary type (both nectaries are united into one). Both pairs of glands represent photosynthetic type and consist of epidermis and glandular tissue. However, they differ in their shape, size, secretory activity, dimensions of epidermal and parenchyma cells, thickness of secretory parenchyma, phloem supply, presence of modified stomata and cuticle ornamentation. The cells of nectaries contain dense cytoplasm, plastids with starch grains and numerous mitochondria. Companion cells of phloem lack cell wall ingrowths. The ultrastructure of secretory cells indicates an eccrine mechanism of secretion. Nectar is exuded throughout modified stomata.     

高压冷冻及低温替代技术制备的拟南芥花蜜腺超微结构的研究

采用高压冷冻和低温替代技术对不同时期泌蜜前、泌蜜早期和泌蜜晚期的拟南齐(Arabidopsisthaliana L.)成熟花蜜腺的超微结构进行了研究。着重对小泡运输过程中是否与细胞质膜发生融合以及蜜腺组织中深色细胞与伴胞的区别等问题进行了讨论。拟南芥花中有一对较大的侧蜜腺以及2～4枚中蜜腺。中蜜腺位于2枚长雄蕊基部或它们之间,而侧蜜腺则位于两花瓣之间的短雄蕊附近。泌蜜前和泌蜜期,液泡的大小、高尔基体及内质网的数量、线粒体的分布以及质体内淀粉粒的大小都会发生一定的变化。当高尔基小泡从细胞内运输至细胞外时,并没有发生与细胞质膜融合的过程,与经典的“胞吐”假说不同。深色细胞在泌蜜期大量出现与筛分子旁的伴胞明显不同,前者与蜜腺顶端的气孔器相连,形成“通道”从而使蜜汁从蜜腺排出。     

A macro- and micromorphological survey of floral and extrafloral nectaries in the epiphytic cactus Rhipsalis teres (Cactoideae: Rhipsalideae)

Floral and extrafloral nectaries in plants favor pollination and defense against herbivory. Despite their wide distribution in plants and differences in position, structure, and topography, their biological and systematic significance has been underutilized. This study investigated the macro- and micromorphology of floral and extrafloral nectaries in the epiphytic cactus Rhipsalis teres and reports unusual bristle-like structures (bracteoles) functioning as extrafloral nectaries in the cactus family. The floral nectary is disc-shaped embedded in the hypanthial floral cup with anomocytic stomata as secreting structures present on the epidermal nectarial tissue. Small multicellular bristle-like extrafloral nectar-secreting structures, homologues to bracts, were observed on the plants’ stems and function as bracteolar nectaries having a relatively long and continuous secretory activity throughout several stages of the reproductive structures. Both the floral and bracteolar nectaries are functional. It is possible that in the latter nectar discharge occurs though epidermal cells, which build up pressure inside as nectar accumulates, thereby ending with rupture of the cuticle to release the liquid. The nectar in both secreting structures is scentless and colorless, and the concentration from floral nectaries is slightly lower than that of the bracteolar nectaries, 70.6% and 76.4%, respectively. The relatively higher concentration in the latter might be correlated with exposure, relative humidity and water evaporation, leading to crystallization of sugars on the stem surface in a short period of time.     

群心菜花蜜腺的发育解剖学研究

群心菜（Ｃａｒｄａｒｉａｄｒａｂａ（Ｌ．）Ｄｅｓｖ）花蜜腺６枚,包括４枚侧蜜腺的和２枚中蜜腺,属十字花科侧中蜜腺类型中的侧分离中间亚型,侧中蜜腺结构相同,都由分泌表皮,产蜜组织组成,分泌表皮顶部分布的有变态气孔器,产蜜组织中无维管束分布,属较原始的十字花科花蜜腺亚型类型,在花的各部分基本分化完成后,由花托表层细胞恢复分裂能力形成蜜腺原基,蜜腺原基经分裂,分化和形态建成,发育形成成熟蜜腺,侧中蜜腺发     

Developmental and Anatomical Studies on the Floral Nectaries of Capsella bursa-pastoris

The development and structure of the floral nectaries of Capsella bursa-pastoris (L.) Medic. were examined. The nectaries consisted of four separated parts which were semiorbicular and were morphologically and anatomically similar to one another. They were located at the receptacle between stamens, and each part was composed of secretory epidermis, nectariferous tissue and vascular bundles, belonging to structural nectary. When the various floral organs were developed, 2--3 superficial layer cells of the receptacle between stamens became meristemoid and contributed to primordia the formation of nectary. By intercalary meristematic activity, the four nectaries formed synchronously. During the different stages of nectary differentiation, the content of starch gra ins and vacuolation in the cells of epidermis and nectariferous tissue changed regularly. According to the structural and histochemical changes the pre-nectar might be supplied by phloem. The nectar formed in nectariferous tissue was then secreted to the sub-stomatal chamber and where it was finally excreted from the stoma.     

Floral nectary morphology and evolution in Pedicularis (Orobanchaceae)

Intricate associations between floral morphology and pollinator foraging behaviour are common. In this context, the presence and form of floral nectaries can play a crucial role in driving floral evolution and diversity in flowering plants. However, the reconstruction of the ancestral state of nectary form is often hampered by a lack of anatomical studies and well‐resolved phylogenetic trees. Here, we studied 39 differentially pollinated Pedicularis spp., a genus with pronounced interspecific variation in colour, shape and size of the corolla. Anatomical and scanning electron microscopy observations revealed two nectary forms [bulged (N = 27) or elongated (N = 5)] or the absence of nectaries (N = 7). In a phylogenetic context, our data suggest that: (1) the bulged nectary should be the ancestral state; (2) nectaries were independently lost in some beaked species; and (3) elongated nectaries evolved independently in some clades of beakless species. Phylogenetic path analysis showed that nectary presence is indirectly correlated with beak length/pollinator behaviour through an intermediate factor, nectar production. No significant correlation was found between nectary type and nectar production, beak length or pollinator behaviour. Some beaked species had nectary structures, although they did not produce nectar. The nectary in beaked species may be a vestigial structure retained during a recent rapid radiation of Pedicularis, especially in the Himalaya–Hengduan Mountains of south‐western China. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 592–607.     

The systematic significance of floral morphology,nectaries, and nectar concentration in epiphytic cacti of tribes Hylocereeae and Rhipsalideae (Cactaceae)

A long-standing interest in cactus taxonomy has existed since the Linnaean generation, but an appreciation of the reproductive biology of cacti started early in the 1900s. Numerous studies indicate that plant reproductive traits provide valuable systematic information. Despite the extensive reproductive versatility and specializations in breeding systems coupled with the striking floral shapes, the reproductive biology of the Cactaceae has been investigated in approximately 10% of its species. Hence, the systematic value of architectural design and organization of internal floral parts has remained virtually unexplored in the family. This study represents the most extensive survey of flower and nectary morphology in the Cactaceae focusing on tribes Hylocereeae and Rhipsalideae (subfamily Cactoideae). Our objectives were (1) to conduct comparative morphological analyses of flowers and floral nectaries and (2) to compare nectar solute concentration in these two tribes consisting of holo- and semi-epiphytic species. Flower morphology, nectary types, and sugar concentration of nectar have strong taxonomic implications at the tribal, generic and specific levels. Foremost, three types of nectaries were found, namely chamber nectary (with the open and diffuse subtypes), furrow nectary (including the holder nectary subtype), and annular nectary. All Hylocereeae species possess chamber nectaries, in which the nectarial tissue has both trichomes and stomata. The Rhipsalideae are distinguished by two kinds of floral nectaries: furrow and annular, both nectary types with stomata only. The annular nectary type characterizes the genus Rhipsalis. Nectar concentration is another significant taxonomic indicator separating the Hylocereeae and Rhipsalideae and establishing trends linked to nectar sugar concentration and amount of nectar production in relation to flower size. There is an inverse relationship between flower size and amount of nectar production in the smaller Rhipsalideae flowers, in which nectar concentration is more than two-fold higher despite the smaller volume of nectar produced when compared to the large Hylocereeae flowers. Variability of nectary morphology and nectar concentration was also evaluated as potential synapomorphic characters in recent phylogenies of these tribes. In conclusion, our data provide strong evidence of the systematic value of floral nectaries and nectar sugar concentration in the Cactaceae, particularly at different taxonomic levels in the Hylocereeae and Rhipsalideae.     

Is nectar reabsorption restricted by the stalk cells of floral and extrafloral nectary trichomes?

Reabsorption is a phase of nectar dynamics that occurs concurrently with secretion; it has been described in floral nectaries that exude nectar through stomata or unicellular trichomes, but has not yet been recorded in extrafloral glands. Apparently, nectar reabsorption does not occur in multicellular secretory trichomes (MST) due to the presence of lipophilic impregnations – which resemble Casparian strips – in the anticlinal walls of the stalk cells. It has been assumed that these impregnations restrict solute movement within MST to occur unidirectionally and exclusively by the symplast, thereby preventing nectar reflux toward the underlying nectary tissues. We hypothesised that reabsorption is absent in nectaries possessing MST. The fluorochrome lucifer yellow (LYCH) was applied to standing nectar of two floral and extrafloral glands of distantly related species, and then emission spectra from nectary sections were systematically analysed using confocal microscopy. Passive uptake of LYCH via the stalk cells to the nectary tissues occurred in all MST examined. Moreover, we present evidence of nectar reabsorption in extrafloral nectaries, demonstrating that LYCH passed the stalk cells of MST, although it did not reach the deepest nectary tissues. Identical (control) experiments performed with neutral red (NR) demonstrated no uptake of this stain by actively secreting MST, whereas diffusion of NR did occur in plasmolysed MST of floral nectaries at the post‐secretory phase, indicating that nectar reabsorption by MST is governed by stalk cell physiology. Interestingly, non‐secretory trichomes failed to reabsorb nectar. The role of various nectary components is discussed in relation to the control of nectar reabsorption by secretory trichomes.     

Characteristics of floral nectaries and nectar in two species of Crataegus (Rosaceae)

 The structure of floral nectaries of Crataegus coccinea and C. crus-galli was examined using light and scanning electron microscopy. The radial length of the floral nectary, measured from longitudinal sections of flowers, was 30% larger in C. crus-galli than in C. coccinea. For both Crataegus species the glandular tissue thickness was similar – approx. 400 μm. Also, the number of stomata per mm² of nectary surface in C. crus-galli was much higher (by 43%) than for C. coccinea. Stomata were situated in deep hollows. For both taxa the period of nectar secretion was 4 days. The mean quantity of total sugar in nectar per 10 flowers of C. crus-galli and C. coccinea was 3.87 mg and 0.33 mg, respectively. Received August 28, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Comparative account of nectary structure in Hexisea imbricata (Lindl.) Rchb.f. (Orchidaceae)

• Background and Aims Despite the number of orchid speciesthat are thought to be pollinated by hummingbirds, our knowledgeof the nectaries of these orchids is based solely on a singlespecies, Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge.Nevertheless, it is predicted that such nectaries are likelyto be very diverse and the purpose of this paper is to comparethe nectary and the process of nectar secretion in Hexisea imbricata(Lindl.) Rchb.f. with that of Maxillaria coccinea so as to beginto characterize the nectaries of presumed ornithophilous Neotropicalorchids. • Methods Light microscopy, transmission electronmicroscopyand histochemistry were used to examine the histology and chemicalcomposition of nectary tissue and the process of nectar secretionin H. imbricata. • Key Results and Conclusions The nectary of H. imbricatahas a vascular supply, is bound by a single-layered epidermiswith few stomata and comprises two or three layers of subepidermalsecretory cells beneath which lie several layers of palisade-likeparenchymatous cells, some of which contain raphides or mucilage.The secretory cells are collenchymatous and their walls havenumerous pits with associated plasmodesmata. They contain thefull complement of organelles characteristic of secretory cellsas well as intravacuolar protein bodies but some of the secretoryepidermal cells, following secretion, collapse and their anticlinalwalls seem to fold. Nectar secretion is thought to be granulocrineand, following starch depletion, lipid droplets collect withinthe plastids. The nectar accumulates beneath the cuticle whichsubsequently forms swellings. Finally, nectar collects in thesaccate nectary spur formed by the fusion of the margins ofthe labellum and the base of the column-foot. Thus, althoughthe nectary of H. imbricata and M. coccinea have many featuresin common, they nevertheless display a number of important differences.     

Comparative anatomy and morphology of nectar-producing Melastomataceae

Background and Aims

Most neotropical Melastomataceae have bee-pollinated flowers with poricidal anthers. However, nectar rewards are known to be produced in about 80 species in eight genera from four different tribes. These nectar-producing species are pollinated by both vertebrates and invertebrates.

Methods

The floral morphology and anatomy of 14 species was studied in six genera of nectar-producing Melastomataceae (Blakea, Brachyotum, Charianthus, Huilaea, Meriania and Miconia). Anatomical methods included scanning electron microscopy, and serial sections of paraffin-embedded flowers.

Key Results

All vertebrate-pollinated melastome flowers have petals that do not open completely at anthesis, thus forming a pseudo-tubular corolla, while closely related species that are bee pollinated have rotate or reflexed corollas. In most species, nectar secretion is related to stomatal or epidermal nectaries and not filament slits as previously reported. Moreover, the nectar is probably supplied by large vascular bundles near the release area. Blakea and Huilaea have nectary stomata located upon the dorsal anther connective appendages. Brachyotum also has nectary stomata on the anther connectives, but these are distributed lengthwise along most of the connective. Meriania may release nectar through the anther connective, but has additional nectary stomata on the inner walls of the hypanthium. Miconia has nectary stomata on the ovary apex. Charianthus nectaries were not found, but there is circumstantial evidence that nectar release occurs through the epidermis at the apex of the ovary and the lower portions of the inner wall of the hypanthium.

Conclusions

Nectar release in Melastomataceae is apparently related to nectary stomata and not filament slits. The presence of nectary stomata on stamens and on ovary apices in different lineages suggests that the acquisition of nectaries is a derived condition. Nectary location also supports a derived condition, because location is strongly consistent within each genus, but differs between genera.Key words: Blakea, Brachyotum, Charianthus, Huilaea, Meriania, Melastomataceae, Miconia, nectaries, nectary stomata, pollination     

The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp.) II. Nectar composition,origin of nectar solutes,and nectary functioning

Nectar was collected from the extrafloral nectaries of leaf stipels and inflorescence stalks, and phloem sap from cryopunctured fruits of cowpea plants. Daily sugar losses as nectar were equivalent to only 0.1–2% of the plant's current net photosynthate, and were maximal in the fourth week after anthesis. Sucrose:glucose:fructose weight ratios of nectar varied from 1.5:1:1 to 0.5:1:1, whereas over 95% of phloem-sap sugar was sucrose. [¹⁴C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [¹⁴C]glucose and [¹⁴C]fructose prior to or during nectar secretion. Invertase (EC 3.2.1.26) activity was demonstrated for inflorescence-stalk nectar but not stipel nectar. The nectar invertase was largely associated with secretory cells that are extruded into the nectar during nectary functioning, and was active only after osmotic disruption of these cells upon dilution of the nectar. The nectar invertase functioned optimally (phloem-sap sucrose as substrate) at pH 5.5, with a starting sucrose concentration of 15% (w/v). Stipel nectar was much lower in amino compounds relative to sugars (0.08–0.17 mg g^-1 total sugar) than inflorescence nectar (22–30 mg g^-1) or phloem sap (81–162 mg g^-1). The two classes of nectar and phloem sap also differed noticeably in their complements of organic acids. Xylem feeding to leaves of a range of ¹⁴C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. ¹⁴C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. ¹⁴C-labelled glycine, serine, arginine and aspartic acid showed limited direct access to phloem and nectar, although labelled metabolic products were transferred and secreted. The ureide allantoin was present in phloem, but absent from both types of nectar. Models of nectary functioning are proposed.     

Ultrastructure of the Floral Nectary of Arabidopsis thaliana L. Prepared from High Pressure Freezing and Freeze Substitution

High pressure freezing and freeze substitution techniques were used to study the floral nectary of Arabidopsis thaliana L. focused on its ultrastmcture of mature nectary at pre-secretory, early secretory and heavy secretory stages. Questions of whether or not the transported vesicles fuse with the plasma]emma, and whether or not the "densely-stained cells" were comparable to companion cells during the secretory process were especially emphasized. The flowers of A. thaliana contained a pair of rather large lateral nectaries and 2 to 4 median ones. The median nectaries were situated at the bases of the two long stamens or between them while each of the. lateral ones was situated nearby each short stamen and between two petals. Before and during the secretion changes in the size of vacuoles and starch grains in the chloroplasts, the number of dictyosomes and endoplasmic mticulum and the distribution of mitechondria occurred before and during the stage of secretion. It appears that transported vesicles from the dictyosomes were transferred directly from one nectarfferous cell to another or to the exterior. They did not fuse with the plasmalemma when they left the cells. A great number of special "densely-stained cells" were different from the companion cells nearby the sieve elements. It was suggested that a tunnel be formed by these "densely-stained cells" connecting the stomata at the top of nectary leading the out-flow of nectar.