Extreme intraplant variation in nectar sugar composition in an insect-pollinated perennial herb

Variation in nectar chemistry among plants, flowers, or individual nectaries of a given species has been only rarely explored, yet it is an essential aspect to our understanding of how pollinator-mediated selection might act on nectar traits. This paper describes variation in nectar sugar composition in a population of the perennial herb Helleborus foetidus (Ranunculaceae) and dissects it into components due to variation among plants, flowers of the same plant, and nectaries of the same flower. The proportions of sucrose, glucose, and fructose in single-nectary nectar samples collected at two times in the flowering season were determined using high performance liquid chromatography (HPLC). Sugar composition varied extensively among nectaries, and nearly all combinations of individual sugars were recorded. Population-wide variance was mainly accounted for by variation among flowers of the same plant (56% of total), nectaries of the same flower (30%), and only minimally by differences among plants (14%). In absolute terms, intraplant variation was similar to or greater than that ordinarily reported in interspecific comparisons. Results suggest that the prevailing notion of intraspecific constancy in nectar sugar composition may be unwarranted for some species and that more elaborate nectar sampling designs are required to detect and appropriately account for extensive within-plant variance. Within-plant variation in nectar sugar composition will limit the ability of pollinators to exert selection on nectar chemistry in H. foetidus and may be advantageous to plants by reducing the number of flowers visited per foraging bout by variance-sensitive, risk-averse pollinators.     

Interpopulation variation in nectar production in Aconitum columbianum (Ranunculaceae)

Summary In Aconitum columbianum there are extreme interpopulation differences in rates of nectar secretion per flower. Since nectar sugar concentration varies little among populations, increased nectar secretion results in a greater mass of sugar per flower for pollinator attraction. These differences in the amount of reward offered per flower account at least in part for observed higher levels of pollinator activity in populations with high nectar production. Nectar production is correlated also with nectary depth, i.e., flowers in populations with deep nectaries have higher rates of nectar secretion than those with shallow nectaries. Nectary depth differences adapt populations to different pollinator-types. Populations with deeper nectaries are adapted to pollination by bumblebees with longer tongues and more specialized foraging behaviors. In conclusion, there are basic differences in pollination ecology among geographical races of a. columbianum, which are indicated by correlated interpopulution differences in (1) nectar production, (2) level of pollinator activity, (3) nectar depth, and (4) pollinator-type.     

Intra-plant variation in nectar sugar composition in two Aquilegia species (Ranunculaceae): contrasting patterns under field and glasshouse conditions

BACKGROUND AND AIMS: Intra-specific variation in nectar chemistry under natural conditions has been only rarely explored, yet it is an essential aspect of our understanding of how pollinator-mediated selection might act on nectar traits. This paper examines intra-specific variation in nectar sugar composition in field and glasshouse plants of the bumblebee-pollinated perennial herbs Aquilegia vulgaris subsp. vulgaris and Aquilegia pyrenaica subsp. cazorlensis (Ranunculaceae). The aims of the study are to assess the generality of extreme intra-plant variation in nectar sugar composition recently reported for other species in the field, and gaining insight on the possible mechanisms involved. METHODS: The proportions of glucose, fructose and sucrose in single-nectary nectar samples collected from field and glasshouse plants were determined using high performance liquid chromatography. A hierarchical variance partition was used to dissect total variance into components due to variation among plants, flowers within plants, and nectaries within flowers. KEY RESULTS: Nectar of the two species was mostly sucrose-dominated, but composition varied widely in the field, ranging from sucrose-only to fructose-dominated. Most intra-specific variance was due to differences among nectaries of the same flower, and flowers of the same plant. The high intra-plant variation in sugar composition exhibited by field plants vanished in the glasshouse, where nectar composition emerged as a remarkably constant feature across plants, flowers and nectaries. CONCLUSIONS: In addition to corroborating the results of previous studies documenting extreme intra-plant variation in nectar sugar composition in the field, this study suggests that such variation may ultimately be caused by biotic factors operating on the nectar in the field but not in the glasshouse. Pollinator visitation and pollinator-borne yeasts are suggested as likely causal agents.     

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     

Catching ants with honey: an experimental test of distraction and satiation as alternative modes of escape from flower-damaging ants

According to the distraction hypothesis, extrafloral nectaries (EFN) evolved under selection to entice ants away from floral nectaries, reducing ant-mediated damage to flowers and/or interference with pollinators. Predator-satiation, through production of nectar in either surplus flowers or EFN, provides an alternative mechanism for reducing the impact of ants as flower visitors. I tested these two hypotheses by experimentally adding EFN to flowering plants of the alpine wildflower, Polemonium viscosum, and by surveying the relationship between ant visitation and nectary number in nature. Plants of P. viscosum lack EFN and experience flower damage by ants of Formica neorufibarbus gelida. Ant behavior was compared on plants with five flowers and three experimental EFN and on controls with equal floral display, but no EFN. Addition of EFN increased flower visitation by ants. The effect of EFN on flower visitation did not depend on proximity of EFN to flowers or attractiveness of EFN to ants. Findings suggest that ants perceived patch quality on a whole plant basis, rather than responding to EFN and flowers as distinct nectar patches. Ant visitation did not keep pace with nectary number in nature. The relationship between ant visitation and nectary number per plant was weak and shallow as predicted under satiation. Ant foraging choices on experimental inflorescences showed that ants bypass flowers avoided by earlier ants, enhancing probability of escape via satiation. Results do not support the idea that EFN evolve to reduce flower visitation by ants, but show instead that nectar in surplus flowers can satiate ants and reduce their negative impacts on flower function and integrity.     

Behavioural differences between male and female carpenter bees in nectar robbing and its effect on reproductive success in Glechoma longituba (Lamiaceae)

Male and female nectar robbers may show significantly different behaviour on host plants and thus have different impacts on reproductive fitness of the plants. A 4-year study in natural populations of Glechoma longituba has shown that male carpenter bees (Xylocopa sinensis) are responsible for most of the nectar robbing from these flowers, while female bees account for little nectar robbing, demonstrating distinct behavioural differentiation between male and female bees in visiting flowers. The smaller male bee spends less time visiting a single flower than the larger female bee, consequently, the male bee is capable of visiting more flowers per unit time and has a higher foraging efficiency. Moreover, the robbing behaviour of female carpenter bees is more destructive and affects flower structures (ovules and nectaries) and floral life-span more than that of the male bee. According to the energy trade-off hypothesis, the net energy gain for male bees during nectar robbing greatly surpasses energy payout (17.72 versus 2.43 J), while the female bee net energy gain is barely adequate to meet energy payout per unit time (3.78 versus 2.39 J). The differences in net energy gain for male and female bees per unit time in nectar robbing are the likely cause of observed behavioural differences between the sexes. The differences in food resource preference between male and female bees constitute an optimal resource allocation pattern that enables the visitors to utilise floral resources more efficiently.     

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.     

Within-plant patchiness in nectar standing crop in Anchusa strigosa

Patterns of variation in nectar standing crop were investigated in Anchusa strigosa. Analysis of the data indicates that nectar volumes of neighboring flowers were positively correlated, and that volume of nectar per flower varied significantly between different hours of the same day, between individual plants, and between pairs of neighboring flowers on the same plant. Most of the explained variation was accounted for by differences between pairs of neighboring flowers on the same plant, indicating that nectar was patchily distributed within individual plants. The implications of these results to pollinator foraging behavior are discussed.     

Nectar robbing improves male reproductive success of the endangered Aconitum napellus ssp. lusitanicum

Nectar robbery is usually thought to impact negatively on the reproductive success of plants, but also neutral or even positive effects have been reported. Very few studies have investigated the effects of nectar robbing on the behaviour of legitimate pollinators so far. Such behavioural changes may lead to the reduction of geitonogamy or to increased pollen movement. We simulated nectar robbing in experimental sites as well as in natural populations of Aconitum napellus ssp. lusitanicum, a rare plant pollinated by long-tongued bumblebees. In an experimental setup, we removed the nectaries of 40 % of the flowers, which is similar to rates of robbing observed in wild populations. Patches of plants with experimentally robbed flowers were compared with control patches containing plants with untreated flowers. We observed pollinator behaviour, mimicked male reproductive success (pollen dispersal) using fluorescent dye, and measured female reproductive success (seed set). The main legitimate visitors were bumblebees while honeybees were often observed robbing nectar. They did so by “base working”, i.e. sliding between tepals. Bumblebees tended to visit fewer flowers per plant and spent less time per single flower when these had been experimentally robbed. This change in behaviour consequently increased the proportion of flowers visited by bumblebees in patches with robbed flowers. Fluorescent dye mimicking pollen flow was dispersed larger distances after pollinators had visited patches with robbed flowers compared to control patches. Average seed set per plant was not affected by nectar robbing. Our results demonstrated that A. napellus does not suffer from nectar robbery but may rather benefit via improved pollen dispersal and thus, male reproductive success. Knowledge on such combined effects of behavioural changes of pollinators due to nectar robbery is important to understand the evolutionary significance of exploiters of such mutualistic relationships between plants and their pollinators.     

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare. Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.     

Variation of nectar production in relation to plant characteristics in protandrous Aconitum gymnandrum

Aims Floral nectar plays a vital role in plant reproductive success by attracting pollinators. Nectar traits of a flower can depend directly on plant characteristics other than environmental factors and exhibit extensive flower- and plant-level variations. Studies on nectar traits frequently focused on intraplant variation for dichogamous plants, but few have paid attention to both intra- and interplant nectar variations in relation to plant characteristics. Revealing within- and among-plant variation and its relative magnitude is important for our understanding of how pollinator-mediated selection can act on nectar traits and evolution of nectar traits.Methods Through investigating protandrous Aconitum gymnandrum populations at the Alpine Meadows and Wetland Ecosystems Research Station of Lanzhou University, we examined the relationships between nectar production per flower and plant characteristics (e.g. flower position within inflorescences, floral sexual phases, flowering time, inflorescence size and floral attractive traits).Important findings A. gymnandrum exhibited a declining gradient in the nectar volume along inflorescences, with more nectar in basal flowers than distal ones. Protandrous flowers of A. gymnandrum did not show gender-biased nectar production while the nectar volume varied with different stages of floral sexual phases. The significant correlation between the first flowering date of individuals and the mean nectar volume per flower was positive in 2013, but became negative in 2014, suggesting complex effects of biotic and abiotic factors. The mean nectar volume per flower was not related to inflorescence size (the number of total flowers per plant). Furthermore, nectar production was weakly associated with floral attractive traits (the petal width and the galea height), even if the effect of flowering time of individuals was removed, suggesting that the honesty of floral traits as signals of nectar reward for pollinators is not stable in this species.     

The effect of hummingbird flower mites on nectar availability of two sympatric Heliconia species in a Brazilian Atlantic forest

BACKGROUND AND AIMS: Hummingbird flower mites feed and reproduce in flowers of host plants pollinated by hummingbirds, and use the nostrils and bill of the hummingbird to move from plant to plant. These mites compete with the pollinator for the nectar produced by flowers. An investigation was made of the relationship between the pattern of nectar production and the effects of hummingbird flower mites in the flowers of two sympatric species of Heliconia (Heliconiaceae). METHODS: Nectar production was sampled by carrying out two experiments: 2-hour intervals and accumulated nectar. Flowers with and without mites were used in both experiments. KEY RESULTS: Exclusion of mites increased nectar production, especially in accumulated daily production (a maximum of 49 % more nectar). Both Heliconia species had the same pattern of nectar production: a high concentration in the morning, which was progressively reduced as the day passed. This pattern of nectar production coincides with the behaviour of the pollinator, which makes more frequent visits in the morning, as observed in a previous study. CONCLUSIONS: The results suggest that the impact of mites on nectar availability of Heliconia is more important with regard to total volume of nectar produced irrespective of flower longevity. A high variation among individuals in nectar produced in the populations was also observed. Hummingbird flower mites strongly affect availability of nectar for hummingbirds.     

Induced Floral and Extrafloral Nectar Production Affect Ant‐pollinator Interactions and Plant Fitness

Thousands of plant species throughout tropical and temperate zones secrete extrafloral nectar to attract ants, whose presence provides an indirect defense against herbivores. Extrafloral nectaries are located close to flowers and may modify competition between ants and pollinators. Here, we used Lima bean (Phaseolus lunatus L.) to study the plants interaction between ants and flower visitors and its consequences for plant fitness. To test these objectives, we carried out two field experiments in which we manipulated the presence of ants and nectar production via induction with jasmonic acid (JA). We then measured floral and extrafloral nectar production, the number of patrolling ants and flower visitors as well as specific plant fitness traits. Lima bean plants under JA induction produced more nectar in both extrafloral nectaries and flowers, attracted more ants and produced more flowers and seeds than non‐induced plants. Despite an increase in floral nectar in JA plants, application of this hormone had no significant effects on flower visitor attraction. Finally, ant presence did not result in a decrease in the number of visits, but our results suggest that ants could negatively affect pollination efficiency. In particular, JA‐induced plants without ants produced a greater number of seeds compared with the JA‐treated plants with ants.     

VARIATION IN NECTAR SUGAR COMPOSITION AT THE INTRAPLANT LEVEL IN IPOMOPSIS LONGIFLORA (POLEMONIACEAE)

The nectar sugar composition of individual flowers from single plants in both greenhouse and field populations was determined by High-Performance Liquid Chromatography (HPLC). Intraplant variability in nectar sugar composition of greenhouse-grown plants was significantly greater than can be attributed to the analytical methodology. Individual greenhouse-grown plants had significantly different sugar compositions, suggesting genetic differences. Under field conditions the situation was complex. Variation among flowers from single plants in field populations was significantly greater than that of plants under greenhouse conditions. Unprotected flowers of some field populations were more variable at the intraplant level than others. Field experiments showed that, in untreated (open-visited) flowers, the percent sucrose relative to other sugars declined significantly and coefficients of variability increased significantly with flower age. However, nectars never exposed to the natural environment did not change significantly with flower age. Further, within single plants it was found that percent sucrose in nectars varied in a small but significant way during the growing season. The data suggest that, in this species, both intrinsic and extrinsic factors affect nectar sugar composition and it is desirable to obtain samples from numerous flowers to accurately characterize the nectar sugar composition of an individual under field conditions.     

Nectary tracks as pollinator manipulators: The pollination ecology of Swertia bimaculata (Gentianaceae)

Floral nectaries are closely associated with biotic pollination, and the nectar produced by corolla nectaries is generally enclosed in floral structures. Although some Swertia spp. (Gentianaceae), including S. bimaculata, evolved a peculiar form of corolla nectaries (known as “gland patches”) arranged in a conspicuous ring on the rotate corolla and that completely expose their nectar, little is known about the pollination of these plants. Two hypotheses were made concerning the possible effects of gland patches: visual attraction and visitor manipulation. The floral traits, mating system, and insect pollination of S. bimaculata were examined, and the pollination effects of gland patches were evaluated. A comparative study was made using Swertia kouitchensis, a species with fimbriate nectaries. Swertia bimaculata flowers were protandrous, with obvious stamen movement leading to herkogamy in the female phase and to a significant reduction in nectary–anther distance. The species is strongly entomophilous and facultatively xenogamous. The daily reward provided per flower decreased significantly after the male phase. The most effective pollinators were large dipterans, and the visiting proportion of Diptera was significantly higher in S. bimaculata than in S. kouitchensis. Most visitors performed “circling behavior” in S. bimaculata flowers. Removing or blocking the nectaries caused no reduction in visiting frequency but a significant reduction in visit duration, interrupting the circling behavior. The circling behavior was encouraged by nectar abundance and promoted pollen dispersal. Visitor species with small body size had little chance to contact the anthers or stigma, revealing a filtration effect exerted by the floral design. These results rejected the “visual attraction” hypothesis and supported the “visitor manipulation” hypothesis. The nectary whorl within a flower acted like a ring‐shaped track that urged nectar foragers to circle on the corolla, making pollination in S. bimaculata flowers more orderly and selective than that in classically generalist flowers.     

Causes and consequences of floral damage in Aconitum lycoctonum at high and low elevations in Switzerland

The outcomes of interactions among plants and the insects that use their flowers are likely to vary among the physical environments and the communities in which they grow. In this study we quantified floral damage of Aconitum lycoctonum in high (>2000 m) and low (c. 500 m) elevation populations in Switzerland. At high elevation, floral damage was frequent and was caused by nectar-robbing short-tongued bumblebees. Nectar robbers make a hole in the flower when they collect nectar. A nectar robber exclusion experiment showed that nectar robbery by short-tongued bumblebees had no effect on the female reproductive success of plants; robbing bees rarely damaged the nectaries, and damage to the petals probably does not decrease flower longevity. In addition, nectar robbers tended to collect pollen during about 10% of their visits. Thus, these bees may act as low-efficiency pollinators and may, at times, be mutualistic associates. At low elevation, the holes in the flowers were caused by beetles (Meligethes viridescens) and not by short-tongued bumblebees. The beetles eat pollen and might also consume nectar. Since the beetles gain access to pollen and nectar by entering the flower through its opening, and later chew holes while foraging on floral tissue, the beetles are pollen eaters rather than nectar robbers. Overall, our results show that not all floral damage is caused by nectar robbers and that there can be strong altitudinal variation in the causes and consequences of floral damage.     

COMPARATIVE REPRODUCTIVE BIOLOGY OF HERMAPHRODITIC AND MALE-STERILE IRIS DOUGLASIANA HERB. (IRIDACEAE)

A comparative study of the reproductive biology of male-sterile and hermaphroditic plants in a gynodioecious population of Iris douglasiana Herb. (Iridaceae) was conducted at the University of California's Marine Laboratory at Bodega Bay, California, between 1976–1979. Each year of the study, there were 11.1% male-sterile plants in the population, some of which began blooming at the same time as the earliest blooming hermaphrodites. Male-sterile flowers made up between 7–21% of the flowers produced during the male-sterile flowering period. Male-sterile flowers had smaller sepals and petals than hermaphrodites, there were fewer of them per square meter, and they had fewer pollinated stigmas than did hermaphroditic flowers. In a test to determine pollinator preference, intact hermaphroditic flowers tended to have more pollinated stigmas than did hermaphrodites with their stamens removed or those flowers with shortened sepals made to resemble the smaller male-sterile flowers. Floral phenology and nectar-flow patterns were similar in both types of flowers as were the kinds of amino acids and sugar rewards in the nectar. Male-sterile flowers, however, produced much less nectar per flower. There were no significant differences in the number of ovules per flower or the number of seeds produced per capsule between the two flower types, but the loss of seeds through larval predation was much greater in capsules from hermaphroditic flowers. Early flowering and setting of seed by plants with male-sterile flowers could give them a reproductive advantage over plants with hermaphroditic flowers which experience higher levels of larval predation later in the growing season.     

Choice of flowers by foraging honey bees (Apis mellifera): possible morphological cues

Abstract.

• 1 Honey bees foraging for nectar on lavender (Lavandula stoechas) chose inflorescences with more of their flowers open. The number of open flowers predicted whether an inflorescence was visited by bees, inspected but rejected, or ignored. Inflorescences chosen arbitrarily by observers had numbers of open flowers intermediate between those of visited and ignored inflorescences.
• 2 Differences in morphological characters between types of inflorescence correlated with nectar volume and sugar weight per flower so that visited inflorescences had a disproportionately greater volume of nectar and weight of sugar per flower and greater variance in nectar volume.
• 3 Although there were significant associations between nectar content and the morphological characters of inflorescences, discriminant function analysis revealed discrimination on the basis of morphology rather than nectar content.
• 4 Visited inflorescences tended to have smaller than average flowers but bees tended to probe the largest flowers on visited inflorescences.
• 5 Choice of flowers within inflorescences is explicable in terms of the relationship between flower size and nectar content.

     

芭蕉科花蜜腺形态比较: 兼论其与传粉者的关系

对狭义芭蕉科3个属的代表性种芭蕉(Musa basjoo)、象腿蕉(Ensete glaucum)和地涌金莲(Musella lasiocarpa)的花蜜腺形态进行了比较研究。结果表明它们的蜜腺属于隔膜蜜腺。雌花的蜜腺着生于子房的上部, 胚珠的上方; 雄花蜜腺占据了整个败育子房的位置。蜜腺结构由许多腔道组成, 这些腔道在横切面上呈现出复杂的发散式迷宫状结构。这3种植物花蜜腺的栅栏状表皮细胞、维管束和蜜腺开口方式相似, 而从纵切面和横切面上观察其结构存在一些差异。PAS反应显示象腿蕉泌蜜组织中淀粉粒含量高于其他两个种; 芭蕉和象腿蕉的蜜腺腔里有许多纤维状物质存在。3种植物的传粉综合征多样化, 花序和花的特征(如花序下垂或直立、苞片的颜色、泌蜜量和泌蜜时间等)和传粉样式之间有密切关系。它们的蜜腺结构和传粉者行为之间没有明显的相关性, 但是胶质或水质的花蜜对传粉者的取食方式有一定影响。     

Nectar secretion pattern of the dish-shaped flower,Cayratia japonica (Vitaceae), and nectar utilization patterns by insect visitors

We studied the relationship between the diurnal nectar secretion pattern of flowers of Cayratia japonica and insect visiting patterns to these flowers. Flower morphology of C. japonica changed greatly for about 12 hours after flower-opening and the maximum duration of nectar secretion was 2 days. The nectar volume peaked at 11∶00 and 15∶00, and declined at night and at 13∶00 regardless of time elapsed after flower-opening. The nectar volume at the two peaks was, on average, 0.25 μl on bagged inflorescences and 0.1μl on unbagged inflorescences (both, sugar concentration=60%). The flower secreted nectar compensatory when the nectar was removed. This means that insects consume more nectar than the difference of nectar volume between bagged and unbagged flowers. Apis cerana is a primary visitor of this flower, and was the only species for which we confirmed pollen on the body, among many species of flower visiting insects to this flower. Apis cerana visited intensively at the two peaks of nectar secretion. Visits of the other insects were rather constant or intensive only when there was no nectar secretion. Thus flowers of C. japonica with morphologically unprotected nectaries may increase likelihood that their nectar is used by certain pollinators, by controlling the nectar secretion time in day. In this study the pattern of nectar secretion allowed A. cerana maximum harvest of nectar.