About this Research Topic Nectar is one of the key developments in the evolution of plant-animal mutualisms. Keywords : Nectaries, Nectars, Nectar, Floral nectar, Extrafloral nectar Important Note : All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements.
Topic Editors. They are derived from sucrose transported in phloem sap or are produced directly in the nectary cells Pacini and Nepi ; Nepi et al. Baker and Baker a, b recognized four classes of nectar sucrose dominant, sucrose rich, hexose rich and hexose dominant according to the ratio by weight of sucrose to the combined hexoses: glucose and fructose. They also suggested that there are co-evolutionary relationships between the sugar proportions in nectar and the types of floral visitors, and high sucrose content in nectar was correlated with pollination by hummingbirds, Lepidoptera and long-tongued bees, whereas high proportions of glucose and fructose were typical of flowers pollinated by passerine birds, flies and short-tongued bees.
Nectar also can contain other sugars that are present in trace amounts, e. Additionally, other less abundant constituents such as amino acids, proteins, organic acids, and some other secondary metabolites such as phenolic compounds, alkaloids or terpenoids were identified in nectar Baker and Baker b ; Nicolson and Thornburg ; Nepi It could not be excluded that pollinators are able to detect differences in the chemical composition of nectar produced by male and female flowers, or in that of flowers at male and female stages.
In Cucurbita pepo , female flowers produce significantly more nectar sugar than do male flowers, but both nectars are sucrose rich Nepi et al. However, Langenberger and Davis showed that in protandrous flowers of Carum carvi , the nectar sugar profile differs between sexual stages.
Indeed, in this species, nectar was hexose rich in the male stage but hexose dominant during the female phase. Changes in nectar composition may result from nectar resorption. In some umbelliferous plant species, the overlap of sexual phases in a flower may be substantial, whereas they may be completely separate in others Bell ; Webb Recently, some authors have demonstrated that, even in some Apiaceae species, pollinators are able to distinguish between floral sexual phases Schlessman et al.
In Apiaceae, nectar is secreted by a stylopodium formed from the expanded base of the style. Morphologically, the stylopodium is the swollen ovary roof of an inferior ovary.
The gynoecium comprises the two separate style branches. The nectary is also divided into two distinct halves. Erbar and Leins undertook a comprehensive study of the anatomy and morphology of the stylopodium in Apiales and assessed the taxonomic value of nectary characters for several euasterid II families. Despite the availability of some morphological and anatomical data, studies of nectar secretion in dichogamous species of Apiaceae are uncommon.
Our objectives were to compare the structure of the nectary of Angelica sylvestris L. Since they may differ in secretory activity and, consequently, have the potential to affect nectar both quantitatively and qualitatively, they may influence pollinator behaviour. Thus, we also compared the mass of secreted nectar and its chemical composition during both male and female stages.
This, to the best of our knowledge, is the first time for nectary structure and nectar secretion to be quantitatively studied for a member of the Apiaceae at two consecutive sexual floral stages. Angelica sylvestris is a member of a large genus of ca.
This species is a herbaceous perennial producing cauline leaves arranged in a rosette, and erect flower stems up to over 2 m tall Cannon The flowers are open and arranged in large multi-layered inflorescences termed compound umbels Fig. Petals are greenish white to pinkish, flower symmetry is mostly actinomorphic Fig. The flowers of A. Inflorescence and flowers of Angelica sylvestris.
Plants used in this study came from natural populations represented in the collection of the University of Warsaw Botanic Garden. Approximately young plant rosettes were removed and transplanted to individual pots of standard garden soil.
The study took place in and , and the same plants were used over the 2-year study period. Each season, before flowering commenced, experimental plants were transferred to a closed greenhouse chamber to prevent insect visits. Following flowering, they were again transferred to the garden.
Throughout the whole growing season the plants were watered fortnightly with no fertilizer applied. To determine length of gender stages and the period of secretory activity, umbels 10—15 in bloom were monitored twice a day using a hand lens 8—10 a.
Male stage umbellets from 10 plants were randomly selected approx. Similar procedure was applied to umbellets bearing female stage flowers. The structure of nectaries and the process of nectar secretion were investigated for both male and female sexual stages. During the male stage, anther dehiscence had occurred, but the style and stigma had not expanded style projected no more than 1—2 mm above stylopodium. This stage, depending on weather, lasted 3—5 days.
Samples for microscopic investigations and for nectar analysis were collected from flowers with all five anthers dehisced. Female stage followed the male stage, and lasted 1—2 days. During the female stage, the two styles and stigmata had completely expanded, but the stamens had completely abscised Fig.
Since the nectar production of female-phase flowers was very low, to collect a sufficiently large and measurable volume of nectar, samples for microscopic investigations and for nectar analysis were collected from flowers on the second day of the female stage.
With the exception of field observations, the presence of nectar on the surface of the stylopodium was also checked for excised flowers using a Nikon SMZ stereomicroscope Nikon Corp. For this purpose, 15 whole gynoecia with stylopodia of flowers in the male stage and 15 whole gynoecia of flowers in the female stage, each sampled from five plants, were fixed in 2. The fixed material was then dehydrated using a graded ethanol series, and infiltrated and embedded in Spurr low viscosity resin Sigma.
Semi-thin sections 0. Histochemical tests were applied in and to detect the presence of lipids starch and phenolic compounds in nectary tissue. Approximately, 20 flowers at each stage investigated sampled from 10 plants were used for each treatment. The sections were examined by means of a Nikon E light microscope equipped with a Canon D digital camera. They were then sputter coated with gold and examined by means of a Tescan Vega II LS scanning electron microscope at an accelerating voltage of 30 kV.
Nectar was sampled for mass and sugar concentration in and The samples were collected from flowers in both male and female stages. For this purpose, the entire inflorescences of 10 plants were protected against insect visits by means of nylon mesh mesh 0. When the flowers reached the appropriate sexual stage, they were excised and immediately transferred to the lab.
Using a stereoscopic microscope, the nectar was subsequently sampled using micro-capillary pipettes of known mass. A single sample contained nectar collected from 8—10 flowers of the same plant. Thirty samples from each of the two floral sexual phases were collected in and in total samples. Nectar concentration was determined as follows. Each sample of nectar was collected from 15—25 flowers at the male stage and, owing to the smaller volume of available nectar, from 30—40 flowers at the female stage.
The small volume of nectar produced meant that we were only able to collect 23 samples in 10 and 13, respectively from male and female flower phases and 39 in 30 and 9, respectively from male and female flower phases. Nectar destined for chemical analysis was collected in and fixed in 1 ml dehydrated ethanol To determine the composition of nectar sugars during both floral sexual stages, nectar from 50 flowers for each stage of development was collected using micro-pipettes and analysed by isocratic HPLC utilizing LC1 Waters system.
Water MilliQ, pH 7 , with a flow rate of 0. Sugars were separated in a Waters Sugar-Pack I column 6. The content of fructose, glucose and sucrose were determined and expressed as the percentage of total sugars. A solvent composed of TEA-phosphate buffer pH 5. According to AccQtag protocol Waters Corp. Statistica 7. Two-way mixed model ANOVA was employed for nectar production and concentration data with study year as random factor and floral sexual phase as fixed factor, with post hoc tests applied where appropriate.
To obtain normal distribution, the data were square root transformed prior to analysis. Stylopodia gynoecial nectaries of A. The nectary at the male stage measured, on average, 0. Nectar appeared upon the stylopodium concomitantly with the dehiscence of the anthers and was generally present throughout the lifespan of the flower Fig. At the male stage, large droplets of nectar accumulated close to secretory stomata Fig. In both investigated stages, the surface of the stylopodium was undulate and slightly raised centrally at the base of the styles.
The stomata were sunken in epidermal depressions distributed uniformly over the surface of the gland Fig. The guard cells had a smooth cuticle, in contrast to epidermal cells, which had a striate cuticle, but the cuticular ridges did not follow any regular pattern Fig.
Between the ridges, small pores were visible in the cuticle Fig. Number of flowers in anthesis, nectar volume and organ biomass [roots, axes main stem and petioles , inflorescences and laminas] as a function of flower morph of buckwheat plants, on the day when flowering reached the last inflorescence at the top of the main stem. Plants were grown in a hydroponic system under controlled conditions. Nectar was collected on all flowers in anthesis.
Nectar volumes per flower on the 4th inflorescence and total sugars in the phloem sap collected at the tip of the peduncle after removal of the inflorescence and an 8—9-h period of exudation, as a function of floral morph of buckwheat.
Plants were grown in peat compost under controlled conditions. Sugar content of phloem sap is reported to 4th inflorescence dry weight. Nectar production varied among plants of the same morph. One month after sowing, nectar production by the plant began with the first open flowers which appeared on the first inflorescence acropetally numbering Fig. Flowering progressed acropetally from raceme to raceme, up to the terminal cluster. The flowering peak, i. Thereafter, the number of flowers at anthesis slowly decreased.
After 3 months of cultivation, flowering stopped in some of the first inflorescences. Most plants stopped flowering after 4—5 months of cultivation; the four inflorescences produced between and flowers per plant. Weekly production of nectar and flowers by thrum buckwheat plants.
Inflorescences are numbered acropetally. A Nectar volume per flower of 1st, 4th, 7th or 10th inflorescence. B Number of flowers reaching anthesis per week on 1st, 4th, 7th and 10th inflorescence and sum of nectar production per flower of 1st, 4th, 7th and 10th inflorescence.
At flowering peak, flowers in the upper inflorescences were the most productive Fig. Transferring plants from light to darkness significantly decreased the number of flowers at anthesis as well as nectar secretion by the few flowers that opened Table 3.
On following days, there was no further anthesis and nectar was not produced by the flowers that remained closed. After 1 week without light, inflorescences became senescent and leaves turned yellow. Effects of a dark treatment applied to whole thrum buckwheat plants on the number of newly open flowers and nectar volumes per flower on the 4th inflorescence. Comparing control inflorescences either kept in free atmosphere or enclosed in a transparent colourless plastic bag with inflorescences wrapped in an opaque plastic bag demonstrated that darkness rapidly disturbed inflorescence functioning.
The number of open flowers per inflorescence was significantly reduced Table 4. Corolla unfolding of the few flowers that opened in darkness was often incomplete but nectar secretion by these flowers was not affected; however, as illustrated by control inflorescences wrapped in a transparent bag, nectar secretion increased in a confined atmosphere Table 4. On the following days, no further flowers opened on light-protected inflorescences and the experiment was stopped after 7 d.
Effects of a dark treatment applied to the 4th inflorescence of thrum buckwheat plants on the number of newly open flowers and nectar volumes per flower on this inflorescence. The number of open flowers per day decreased significantly in defoliated plants after day 25 Fig. Following defoliation, nectar secretion by flowers slowly decreased and stabilized 19 d later Fig. Similarly, sugar concentration in nectar of defoliated plants decreased and stabilized 14 d after defoliation Fig.
When the defoliation experiment was discontinued, 40 d after leaf excisions, there was usually no newly open flower on the defoliated plants. Number of newly open flowers, nectar volume and sugar concentration as a function of time from defoliation of the whole thrum buckwheat plants; defoliated and control plants as indicated. Significant differences between treatments, at a given time, are indicated by asterisks one-way ANOVA, statistical significance: n. In buckwheat, nectar secretion of the receptacular nectaries started after flower opening.
Nectar drops accumulated all around the nectaries, which appeared as eight hook-shaped protrusions located on the receptacle between the stamens. These protusions were called globular stalked nectaries by Da Craene and Akeroyds The nectary epidermis mediates nectar release in a majority of plant species Fahn, ; Pacini et al. In buckwheat, it was composed of suberized cells, except at the ventral face of the hook, which consisted of alive unicellular hairs.
The location of nectar in the flowers of buckwheat, the nature of the epidermis cells, and the absence of modified stomata and lysigenous cavities to eliminate nectar support the view that the nectar is secreted through the trichomes Fahn, ; De Craene and Smets, Such a mode of secretion with one-celled secretory hairs has been observed in Dipsacales, Orchidaceae and Tropaeolaceae Bernardello, Epidermis that includes nectar-secreting structures can be related to epidermal nectaries in general but in some groups, such as Polygonaceae, trichomes or papillae are related to mesenchymatic nectaries and are located in the epidermis of the nectaries De Craene and Smets, ; Bernardello, The techniques used in the present study did not allow identification of the route by which the nectar passes through the cuticle of the cells, a barrier that may be crossed either through pores, by permeation or after cuticle rupture Fahn, In buckwheat, the multilayered nectary parenchyma, located beneath the epidermis, was supplied with water and nutrients by vascular bundles made up of phloem and xylem and connected to the vascular system of the other floral organs.
Nectar assimilates originating from tissues situated outside the nectary are undoubtedly unloaded from the phloem Fahn, ; Pacini and Nepi, The source of nectar sugars may be immediate photosynthesis or may require temporary starch storage in nectary amyloplasts before nectar secretion Pacini et al.
No plasts were observed in the nectary parenchyma or in the epidermis whereas the sub-nectary parenchyma contained chloroplasts and vascular bundles. Therefore, the pre-nectar probably proceeded, at least partially, directly from the phloem sap and enzymes present in the nectary parenchyma partially hydrolysed the pre-nectar sucrose into glucose and fructose prior to secretion Fahn, ; Pacini and Nepi, This idea is supported by the fact that buckwheat nectar consists of the three sugars mentioned above Cawoy et al.
Sub-nectary parenchyma might also contribute to nectar sucrose production. Further observations under UV light to identify the presence of chloroplasts and with IKI iodine—potassium—iodine , which stains starch grains in amyloplasts, are required Nepi, These should help to validate our hypothesis that there are neither chloroplasts nor amyloplasts in the nectary parenchyma of buckwheat.
Moreover, it would be of interest to look for amyloplasts in younger flower buds as these can disappear before anthesis Peng et al. Previous studies have clearly demonstrated that the two morphs of buckwheat do not differ in the number of racemes, cymes and flowers they produce Quinet et al. The present study showed that plants of both floral morphs were similar as regards root, axis main stem and petioles , lamina and inflorescence weights.
However, as reported by Cawoy et al. Nevertheless, the sugar composition of nectar was identical in the two morphs. According to Pacini et al. Although no precise measurements were performed here, microscopic observations of the nectary parenchyma suggested that there is no difference between the two morphs.
Similarly, no difference in sugar supply by phloem sap to inflorescences of the two morphs was detected. However, as phloem sap at the tip of the peduncle of an inflorescence brought sugars not only for nectar but also for morphogenesis and development of the multiple reproductive structures that coexist in a raceme, more precise measurements, on a flower scale, would be required to invalidate the hypothesis of a higher sugar supply to thrum flowers.
Nectar production of flowers was positively correlated with the number of newly open flowers per plant, which fluctuated with plant age. The amount of nectar produced per flower and per plant was highest during the flowering peak, which occurred 1 month after the first anthesis.
According to Alekseyeva and Bureyko , the flowering peak in the field corresponds to the period when the highest bee visitation rate is observed. Inflorescences at the top of the main stem, which are a priori more accessible for pollinators than lower inflorescences owing to a lack of leaf cover, had the greatest nectar production per flower. Within a floral morph, nectar volume per flower varied among individuals. Thus, under the conditions used here, a difference in organ biomass was not the cause of the variation inm nectar production by plants between individuals from the same morph.
As in many angiosperms van Doorn and van Meeteren, , transferring buckwheat plants to dark suppressed flower anthesis. Nectar secretion was also drastically reduced in the few flowers that opened following transfer to darkness. Interestingly, darkening the inflorescence only inhibited flower opening but did not prevent nectar accumulation in the few flowers that succeeded in reaching anthesis. Massimo Nepi was awarded a PhD in agricultural biology in He is currently employed as researcher at the Department of Environmental Sciences of the University of Siena, where he carries out studies concerning the reproductive biology of angiosperms.
In recent years his main research interest has been nectar and nectary biology. Ettore Pacini graduated in botany in at the University of Siena, where he is still engaged as full professor of Botany.
His main research interest has been higher plant reproduction, first from a cytological point of view and also from an ecological point of view during the last two decades.
Recently he became a member of the prestigious Accademia dei Lincei, the first Scientific Academy, founded in Adler, Ecology, Vol. Skip to main content Skip to table of contents. Advertisement Hide.
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