Structure of the receptacular nectary and circadian metabolism of starch in the ant‐guarded plant Ipomoea cairica (Convolvulaceae)

Nectaries occur widely in Convolvulaceae. These structures remain little studied despite their possible importance in plant–animal interactions. In this paper, we sought to describe the structure and ultrastructure of the receptacular nectaries (RNs) of Ipomoea cairica, together with the dynamics of nectar secretion. Samples of floral buds, flowers at anthesis and immature fruits were collected, fixed and processed using routine methods for light, scanning and transmission electron microscopy. Circadian starch dynamics were determined through starch measurements on nectary sections. The secretion samples were subjected to thin layer chromatography. RNs of I. cairica were cryptic, having patches of nectar‐secreting trichomes, subglandular parenchyma cells and thick‐walled cells delimiting the nectary aperture. The glandular trichomes were peltate type and had typical ultrastructural features related to nectar secretion. The nectar is composed of sucrose, fructose and glucose. Nectar secretion was observed in young floral buds and continued as the flower developed, lasting until the fruit matured. The starch content of the subglandular tissue showed circadian variation, increasing during the day and decreasing at night. The plastids were distinct in different portions of the nectary. The continuous day–night secretory pattern of the RNs of I. cairica is associated with pre‐nectar source circadian changes in which the starch acts as a buffer, ensuring uninterrupted nectar secretion. This circadian variation may be present in other extrafloral nectaries and be responsible for full daytime secretion. We conclude that sampling time is relevant in ultrastructural studies of dynamic extranuptial nectaries that undergo various changes throughout the day.     

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.     

Four o'clock pollination biology: nectaries,nectar and flower visitors in Nyctaginaceae from southern South America

Floral nectary structure and nectar sugar composition were investigated in relation to other floral traits and flower visitors in contrasting species of Nyctaginaceae from southern South America, representing four tribes (Bougainvilleeae, Colignonieae, Nyctagineae, Pisoneae). Our comparative data will aid in the understanding of plant–pollinator interactions and in the development of hypotheses on the origin of floral and reproductive characters in this family. The nectaries are located on the inner side of the staminal tube. The nectariferous tissue is composed of an epidermis and three to ten layers of secretory parenchymal cells, supplied indirectly by the filament vascular bundles. Stomata appear to be associated with nectar secretion. For the first time in Nyctaginaceae, nectary ultrastructure is described in Boerhavia diffusa var. leiocarpa. Nectary parenchyma cells are densely cytoplasmic and contain numerous starch grains. Plasmodesmata connect the nectariferous cells. Flowers of Nyctaginaceae secrete a small volume of nectar of variable concentration (10–47%). Nectar is dominated by hexoses, but Mirabilis jalapa showed a balanced proportion of sucrose and hexoses. Hymenoptera are the most common visitors for most species; nocturnal Lepidoptera are the most common visitors for M. jalapa and Bougainvillea stipitata. We found relatively low variation in the nectary characteristics of Nyctaginaceae compared with broad variation in flower structure, shape, colour and nectar traits. © 2013 The Linnean Society of London     

Secretion and composition of nectar and the structure of perigonal nectaries in <Emphasis Type="Italic">Fritillaria meleagris</Emphasis> L. (Liliaceae)

The structure of perigonal nectaries, nectar production and carbohydrate composition were compared at various stages in the lifespan of the flower of Fritillaria meleagris L. The six nectaries each occupied a groove that is located 2–4 mm above the tepal base. The average nectary measured 11.0 mm long and 1.0–1.2 mm wide. The structure of nectaries situated on both inner and outer tepal whorls was identical, and at anthesis they were equally accessible to potential pollinators. However, secretion from nectaries associated with inner tepals tended to exceed that produced by nectaries located on the outer tepals. On average, regardless of flower stage, one flower secreted 10.87 ± 12.98 mg of nectar (mean and SD; N = 182). The nectar concentration ranged between 3 and 75%, with average concentration of sugars exceeding 50%. Both nectar production and concentration were dependent on the stage of anthesis, with the highest scores being recorded during full anthesis (21.75 ± 16.08 mg; 70.5%, mass and concentration, respectively) and the lowest at the end of anthesis (1.32 ± 2.69 mg; 16.9%, mass and concentration, respectively). A decline in both mass of nectar secreted and nectar concentration during the final stage of anthesis indicates nectar resorption. Nectar was composed of sucrose, glucose and fructose in approx. equal quantities, and its composition did not change significantly during subsequent stages of flowering. The nectaries comprised a single-layered secretory epidermis and several layers of subepidermal parenchyma. The nectariferous cells did not accumulate starch during any of the investigated stages. The nectary was supplied with one large and several smaller vascular bundles comprising xylem and phloem. Transport of assimilates and nectar secretion by protoplasts of secretory cells (and probably also nectar resorption) were facilitated by cell wall ingrowths present on the tangential walls of epidermal cells and subepidermal parenchyma. Epidermal cells lacked stomata. Nectar passed across the cell wall and through the cuticle which was clearly perforated with pores.     

Occurrence,structure, ontogeny and biology of nectaries inKigelia pinnata DC

The occurrence, morphology, ontogeny, structure and preliminary nectar analysis of floral and extrafloral nectaries are studied inKigelia pinnata of the Bignoniaceae. The extrafloral nectaries occur on foliage leaves, sepals and outer wall of the ovary, while the floral nectary is situated around the ovary base as an annular, massive, yellowish ring on the torus. The extrafloral nectaries originate from a single nectary initial. The floral nectary develops from a group of parenchymatous cells on the torus. The extrafloral nectaries are differentiated into multicellular foot, stalk and cupular or patelliform head. The floral nectary consists of parenchymatous tissue. The floral nectaries are supplied with phloem tissue. The secretion is copious in floral nectary. Function of the nectary, preliminary nectar analysis, and symbiotic relation between nectaries and animal visitors are discussed.     

Nectar-carbohydrate production and composition vary in relation to nectary anatomy and location within individual flowers of several species of Brassicaceae

Nectar-carbohydrate production and composition were investigated by high-performance liquid chromatography and enzymology in nine species from five tribes of the Brassicaceae. In six species (Arabidopsis thaliana (L.) Heynh., Brassica napus L., B. rapa L., Lobularia maritima (L.) Desv., Raphanus sativus L., Sinapis arvensis L.) that produced nectar from both lateral nectaries (associated with the short stamens) and median nectaries (outside the long stamens), on average 95% of the total nectar carbohydrate was collected from the lateral ones. Nectar from these glands possessed a higher glucose/fructose ratio (usually 1.0–1.2) than that from the median nectaries (0.2–0.9) within the same flower. Comparatively little sucrose was detected in any nectar samples except from Matthiola bicornus (Sibth. et Sm.) DC., which possessed lateral nectaries only and produced a sucrose-dominant exudate. The anatomy of the nectarial tissue in nectar-secreting flowers of six species, Hesperis matronalis L., L. maritima, M. bicornus, R. sativus, S. arvensis, and Sisymbrium loeselii L., was studied by light and scanning-electron microscopy. Phloem alone supplied the nectaries. However, in accordance with their overall nectar-carbohydrate production, the lateral glands received relatively rich quantities of phloem that penetrated far into the glandular tissue, whereas median glands were supplied with phloem that often barely innervated them. All nectarial tissue possessed modified stomata (with the exception of the median glands of S. loeselii, which did not produce nectar); further evidence was gathered to indicate that these structures do not regulate nectar flow by guard-cell movements. The numbers of modified stomata per gland showed no relation to nectar-carbohydrate production. Taken together, the data on nectar biochemistry and nectary anatomy indicate the existence of two distinct nectary types in those Brassicacean species that possess both lateral and median nectaries, regardless of whether nectarial tissue is united around the entire receptacle or not. It is proposed that the term “nectarium” be used to represent collectively the multiple nectaries that can be found in individual flowers. Received: 21 July 1997 / Accepted: 19 September 1997     

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     

Nectaries and male-biased nectar production in protandrous flowers of a perennial umbellifer <Emphasis Type="Italic">Angelica sylvestris</Emphasis> L. (Apiaceae)

Nectar is the most common floral pollinator reward. In dichogamous species, floral nectar production rates can differ between sexual phases. We studied the structure of nectaries located on the stylopodium and nectar production in protandrous umbellifer Angelica sylvestris. Our study species produced nectar in both floral sexual phases. Nectar sugar concentration was low (on average 22 ± 11 %, mean ± SD) and the nectar hexose rich and composed of sucrose, glucose, fructose and a small amount of amino acids, including β-alanine, a non-protein amino acid. Although nectar composition and sugar concentration varied little between floral sexual phases, nectar production showed a threefold reduction during the stigma receptive period. This is in contrast to other studies of Apiaceae that have reported female-biased nectar production, but in the direction predicted by plant sexual selection theory, suggesting that in pollen-unlimited species, floral rewards mainly enhance male reproductive success. The structure of the nectary was similar at the two sexual stages investigated, and composed of a secretory epidermis and several layers of nectariferous and subsecretory parenchyma. The nectary cells were small, had large nuclei, numerous small vacuoles and dense, intensely staining cytoplasm with abundant endoplasmic reticulum, mitochondria and secretory vesicles. They contained abundant resin-like material that may potentially act as defence against microbes. Starch was rarely observed in the nectary cells, occurring predominantly at the female stage and mainly in guard and parenchyma cells in close proximity to stomata, and in subsecretory parenchyma. The main route of nectar release in A. sylvestris seems to be via modified stomata.     

鹅掌柴花蜜腺的发育解剖学研究

对鹅掌柴(Scheffler octophylla Harms．)花蜜腺的发育进行解剖结构观察。鹅掌柴花盘蜜腺位于下位子房上方环绕花柱基部。蜜腺由分泌表皮、产蜜组织组成,心皮维管束与其相邻并发出一些伸入蜜腺基部的短分枝。蜜腺起源于心皮原基基部外侧的几层细胞。鹅掌柴花蜜腺为淀粉型蜜腺,淀粉粒为许多微小颗粒聚集成的复粒。原蜜汁由蜜腺基部维管束的筛管提供,达产蜜组织细胞和表皮细胞后以淀粉粒的形式贮藏。泌出的蜜汁一部分来自淀粉粒的降解,一部分来自泌蜜期输入的原蜜汁。表皮和产蜜组织细胞均具泌蜜功能。泌出的蜜汁大部分通过气孔排出,还有部分由角质层渗出。     

Flower nectary structure in Cornus alba L.

The structure of the floral nectaries of Cornus alba was studied using light microscopy as well as scanning and transmission electron microscopy. It was found that the nectary gland of white dogwood had the shape of a fleshy ring surrounding the base of the style of the inferior ovary. Nectar secretion occurs through slightly depressed stomata, evenly distributed in the epidermis of the nectary. The nectariferous tissue is composed of over a dozen layers of heterogeneously structured cells. Between groups of cells with a typical structure, characteristic for the secretory tissue, cells occur with degenerated content and a high degree of vacuolization. In the area of the nectary gland cells, no vascular tissue elements were observed. The nectary was irrigated by the vasculature of the flower receptacle.     

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.     

垂柳花蜜腺的发育解剖学研究

垂柳雌花蜜腺一枚,位于于房与花序轴之间,多呈扁平广卵形,由分泌表皮、泌蜜组织和维管束组成。雄花蜜腺呈基部相连的两枚突起,一枚位于花丝与花序轴之间,基部宽扁,上部棒状;另一枚位于花丝与苞片之间,棒状,仅由分泌表皮和泌蜜组织组成。雌、雄花蜜腺均起源于花托表面2—3层细胞。在蜜腺发育过程中,雌、雄花蜜腺泌蜜组织细胞的液泡发生规律性变化．雌花蜜腺为淀粉型蜜腺,而雄花蜜腺为非淀粉型蜜腺。雌、雄花蜜腺的原宜汁分别由蜜腺维管束韧应部或花丝维管束韧皮部提供,其蜜计最后均由分泌表皮细胞和变态气孔排出。     

The role of alanine synthesis and nitrate-induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre-stored and direct phloem-derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N-containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia-related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.     

Nectaries and reproductive biology of Croton sarcopetalus (Euphorbiaceae)

Flower morphology, nectary structure, nectar chemical composition, breeding system, floral visitors and pollination were analysed in Croton sarcopetalus , a diclinous-monoecious shrub from Argentina. Male flowers have five receptacular nectaries, with no special vascular bundles, that consist of a uniserial epidermis with stomata subtended by a secretory parenchyma. Female flowers bear two different types of nectaries: inner (IN) and outer (ON) floral nectaries. IN, five in all, are structurally similar to the nectaries of male flowers. The five ON are vascularized, stalked, and composed of secretory, column-shaped epidermal cells without stomata subtended by secretory and ground parenchyma. In addition, ON act as post-floral nectaries secreting nectar during fruit ripening. Extrafloral nectaries (EFN) are located on petioles, stipules and leaf margins. Petiolar EFN are patelliform, stalked and anatomically similar to the ON of the female flower. Nectar sampled from all nectary types is hexose dominant, except for the ON of the female flower at the post-floral stage that is sucrose dominant. The species is self-compatible, but geitonogamous fertilization is rarely possible because male and female flowers are not usually open at the same time in the same individual, i.e. there is temporal dioecism. Flowers are visited by 22 insect species, wasps being the most important group of pollinators. No significant differences were found in fruit and seed set between natural and hand pollinated flowers. This pattern indicates that fruit production in this species is not pollen/pollinator limited and is mediated by a wide array of pollinators.     

Nectar formation and floral nectary anatomy of Anigozanthos flavidus: a combined magnetic resonance imaging and spectroscopy study

Metabolic processes underlying the formation of floral nectar carbohydrates, especially the generation of the proportions of fructose, glucose, and sucrose, are important for understanding ecological plant-pollinator interactions. The ratio of sucrose-derived hexoses, fructose and glucose, in the floral nectar of Anigozanthos flavidus (Haemodoraceae) was observed to be different from 1:1, which cannot be explained by the simple action of invertases. Various NMR techniques were used to investigate how such an unbalanced ratio of the two nectar hexoses can be formed. High-resolution (13)C NMR spectroscopy in solution was used to determine the proportion of carbohydrates in vascular bundles of excised inflorescences fed with (13)C-labelled carbohydrates. These experiments verified that feeding did not affect the metabolic processes involved in nectar formation. In vivo magnetic resonance imaging (e.g. cyclic J cross-polarization) was used to detect carbohydrates in vascular bundles and (1)H spin echo imaging non-invasively displayed the architecture of tepal nectaries and showed how they are connected to the vascular bundles. A model of the carbohydrate metabolism involved in forming A. flavidus floral nectar was established. Sucrose from the vascular bundles is not directly secreted into the lumen of the nectary but, either before or after invertase-catalysed hydrolyses, taken up by nectary cells and cycled at least partly through glycolysis, gluconeogenesis, and the pentose phosphate pathway. Secretion of the two hexoses in the cytosolic proportion could elegantly explain the observed fructose:glucose ratio of the nectar.     

Anatomy and histochemistry of the nectaries of Rodriguezia venusta (Lindl.) Rchb. f. (Orchidaceae)

Orchidaceae is one of the largest angiosperm families. Although extensively studied, reports of anatomy of secretory structures of orchids are relatively scarce. Rodriguezia venusta is an epiphytic orchid occurring in Brazil and Peru that has floral and extrafloral nectaries. This study describes the structure and the histochemistry of these secretory structures. Floral and extrafloral nectary samples were obtained from R. venusta plants that were collected in a gallery forest in the State of Bahia, Brazil, and grown in a greenhouse. Theses samples were fixed and processed according to routine procedures in plant anatomy and histochemistry or for scanning electron microscopy. The extrafloral nectaries occur on the edge and sub-edge of young leaves and at the basal portion of bracts that subtend the floral buds. They are structurally very similar, being formed by a nectary parenchyma and a simple epidermis with stomata (“non-structured nectaries”). The floral nectary is inserted at the floral receptacle fused with the labellum base, between this structure and the two inferior connate sepals. This nectary consists of an epidermis with numerous specific nectar secreting trichomes, a subnectary and a nectary parenchyma abundantly supplied by vascular terminations. Its structure is complex and distinct from other floral nectaries described for Orchidaceae.     

Structure,ultrastructure and evolution of floral nectaries in the twinflower tribe Linnaeeae and related taxa (Caprifoliaceae)

Linnaeeae is a small tribe of Caprifoliaceae consisting of six genera and c. 20 species. In Linnaeeae, floral nectaries are located on the corolla‐filament‐tube and nectar is produced from unicellular glandular hairs. We studied 23 taxa using scanning electron microscopy (SEM), light microscopy (LM) and transmission electron microscopy (TEM) and found two distinct nectary morphologies, zonate and gibbous types, and two distinct types of glandular hair, clavate and smooth base types. Plesiomorphic characters associated with the nectary and identified in the tribe include hypocrateriform corollas, dichogamous flowers, zonate nectaries, wet papillate stigmas, vestigial nectary disc and smooth pollen grains. Apomorphic characters include bilabiate corollas, homogamous flowers, bulging nectaries, dry papillate stigmas and echinulate pollen grains. The nectary structure is similar in Vesalea and Linnaea and differs from the rest of the tribe, in accordance with recent phylogenetic results. Nectar secretion is typically granulocrine with subcuticular accumulation of nectar, which we compared with the secretion in multicellular hairs of Adoxa moschatellina. The cuticle on the hair becomes detached from the cell wall and large subcuticular spaces filled with nectar are formed. Nectar is probably released in areas with a thin cuticle. In Zabelia, the smooth basal part of the hair could help to build up the hydrostatic pressure.     

EXTRAFLORAL NECTAR OF FEROCACTUS ACANTHODES (CACTACEAE): COMPOSITION AND ITS IMPORTANCE TO ANTS

Ants are attracted to extrafloral nectaries subtending reproductive organs of Ferocactus acanthodes var. lecontei (Cactaceae) in central Arizona. Extrafloral nectar produced by these glands contained amino acids, sugars, and water. Nectar quality and composition varied temporally in relation to plant reproductive phenology. The number of nectar glands on a barrel cactus did not change significantly, however; the mass of nectar produced per gland increased significantly with immature fruit production. Of the three sugars present in extrafloral nectar (fructose, glucose, and sucrose), only glucose occurred at a higher concentration in June, when immature fruits first appeared on barrel cactus. Amino acid concentration and composition in extrafloral nectar of barrel cactus did not change significantly over time. Ant density on barrel cactus increased significantly from mid-May to mid-June at two field sites. Water availability per nectar gland increased 158% from May to June. Water plays an important role in attracting ants to barrel cacti.     

Foliar nectar production and ant activity on a neotropical tree,Ochroma pyramidale

Summary In second growth forest in lowland Costa Rica, ants forage at the foliar nectaries of juvenile Ochroma pyramidale. The relationship between leaf development, foliar nectar production and ant visitation indicates that nectar secretion and ant maintenance are greatest following rapid leaf expansion. Nectar measurements in the glasshouse corroborate field measurements showing that nectar production on a sapling is continuous through time and correlated with distribution and abundance of ants within a sapling. The presence of two nectary types, leaf vein and petiolar, on the leaves of O. pyramidale results in the continual maintenance of ants on the leaf undersurface. Nectar production of a sapling increases with increasing leaf area resulting in greater number of ants per sapling. Energetic costs of nectar production and ant maintenance appear low, representing about one per cent of the total energy invested in leaves.Spatial and diurnal patterns of ant activity changed very little over the study period. Removal and exclusion of ants from saplings results in the utilization of foliar nectar by trigonid bees. A significant difference in leaf damage between ant-visited and unvisited saplings, coupled with ant behavioral characteristics, is consistent with the hypothesis that ants act as antiherbivore agents on Ochroma.     

A cell wall invertase controls nectar volume and sugar composition

Nectar volume and sugar composition are key determinants of the strength of plant–pollinator mutualisms. The main nectar sugars are sucrose, glucose and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose-dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa. Moreover, a brcwinv4a null mutant: (i) has greatly reduced CWINV activity in the nectaries; (ii) produces a sucrose-rich nectar; but (iii) with significantly less volume. These results definitively demonstrate that CWINV activity is not only essential for the production of a hexose-rich nectar, but also support a hypothetical model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular-to-extracellular sucrose ratio that facilitates the continuous export of sucrose into the nectary apoplast. The extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. These results cumulatively indicate that modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition. Finally, honeybees prefer nectars with some sucrose, but wild-type B. rapa flowers were much more heavily visited than flowers of brcwinv4a, suggesting that the potentially attractive sucrose-rich nectar of brcwinv4a could not compensate for its low volume.