Labellar anatomy and secretion in Bulbophyllum Thouars (Orchidaceae: Bulbophyllinae) sect. Racemosae Benth. & Hook. f

Labellum Features and Chemical Composition of Floral Scent in Bulbophyllum carunculatum Garay, Hamer & Siegrist (Section Lepidorhiza Schltr., Bulbophyllinae Schltr., Orchidaceae Juss.)

The vast majority of fly-pollinated Bulbophyllum species use a combination of visual and olfactory clues to mimic food sources and brood/oviposition sites of pollinators. The aims of the present work were to characterize the floral secretory tissue and the floral scent and compare them with those previously described in B. echinolabium. Based on the histochemical results, the labellar secretion in B. carunculatum is the protein-rich mucilage. The adaxial epidermal cells of the labellum showed typical features of secretory activity. Plastids contained plastoglobuli, which are thought to be the places for scent production in osmophores. Juxtaposed with FeCl₃ staining, the presence of dihydroxyphenolic globules in the cytoplasm of the epidermis and sub-epidermis was confirmed. Phenolic derivatives were also described with GC/MS analysis of the floral scent. The number of aromatic compounds and hydrocarbons was indicated in the floral scent of B. carunculatum. Moreover, pregnane-3,20-dione, occurring in the highest percentage in the floral fragrance of B. carunculatum, is a biologically active, 5-alpha-reduced metabolite of plasma progesterone. Progesterone is a mammalian gonadal hormone, but, like other steroid hormones, has been found in plants as intermediates in different biosynthetic pathways. The research on biosynthesis and functions of progesterone and its derivatives in flowers is still lacking.

Floral glands in myophilous and sapromyophilous species of Pleurothallidinae (Epidendroideae, Orchidaceae)-osmophores, nectaries, and a unique sticky gland

Pleurothallidinae orchids have been the focus of many multidisciplinary studies due to their challenging systematics and taxonomy. The synapomorphies already recognized in the group are mostly related to floral characters, the last proposed being the occurrence of alkanes in the floral fragrance. The composition of the floral bouquet varied significantly among the studied species, leading us to hypothesize that the variations in volatiles emitted could be linked to the structure of osmophores, especially when comparing the myophilous and sapromyophilous pollination syndromes. Sepals and labellum at different developmental stages of seven Brazilian Pleurothallidinae species were examined using light, scanning, and transmission electron microscopy. Nectar reabsorption was assessed by Lucifer Yellow CH tracer and imaged under confocal microscopy. Nectaries were restricted to the labellum of the myophilous species, whereas osmophores occurred in the dorsal and/or lateral sepals, varying according to species. In the sapromyophilous species, floral nectaries were not detected and osmophores were restricted to the labellum. Osmophore structure was correlated with the volatiles emitted, being the trichome osmophores notably present on the sepals of both myophilous species that possess nectaries. For the first time, we demonstrated reabsorption of the released nectar in Pleurothallidinae and the occurrence of a unique gland named sticky-exudate glands, which occurred in the lateral sepals and labellum of Echinosepala aspasicensis, a sapromyophilous species, that released a heterogeneous exudate composed of polysaccharides and lipids. Similar glands have been reported in Bulbophyllum, highlighting the convergence between both groups.

Lip morphology and ultrastructure of osmophores in Cyclopogon (Orchidaceae) reveal a degree of morphological differentiation among species

Floral fragrances play an important role in pollinator attraction; they serve as signals for primary rewards such as nectar. The presence of osmophores (a specialized glandular tissue that produces fragrance) in the terrestrial orchid Cyclopogon has only been described in a single species. There are fragrant and odorless species within this genus, but no data on the similarities or differences between them has been published. Here, we present new data on the lip morphology and ultrastructure from eight Cyclopogon species to better understand the mechanisms of pollinator attraction of this group. To achieve this goal, we used light, scanning, and transmission electron microscopy. Five out of eight species presented globular trichomes associated with fragrance production on the abaxial side of the labellum. Together with subepidermal cells, they compose the secretory tissue. Despite the absence of globular trichomes, C. apricus also presented cells associated with fragrance production. Lipid droplets, plastids containing plastoglobuli, and starch were indicators of secretory activity present in these species. This same pattern of fragrance production and release, associated with flower morphology, may result in the restriction of pollination by halictid bees.

Labellum structure of Bulbophyllum echinolabium J.J. Sm. (section Lepidorhiza Schltr., Bulbophyllinae Schltr., Orchidaceae Juss.)

This micromorphological, chemical and ultrastructural study is a continuation of research conducted on the section Lepidorhiza. The Bulbophyllum echinolabium flowers comprised features that characterize a sapromyophilous syndrome, having large, glistening parts that emit an intense scent of rotten meat. The secretory activity was described in the hypochile (nectary in longitudinal groove and in the prickles) and the epichile (putative osmophore). The ultrastructural studies revealed a dense cytoplasm in the epidermis and subepidermal tissue with large nuclei and numerous mitochondria, the profiles of SER and RER, and dictyosomes. Large amounts of heterogeneous residues of secreted material (possibly phenolic) were present on the cuticle surface, similar to the unusual prominent periplasmic space with flocculent secretory material. The chemical analysis (GC/MS) of the scent profile of lips comprised carbohydrates and their derivatives (29.55% of all compounds), amino acids (1.66%), lipids (8.04%) and other organic compounds (60.73%). A great number of identified compounds are Diptera attractants (mainly Milichiidae, Tephritidae, Drosophilidae, Muscidae, Sarcophagidae, Tachinidae). The examination of visual and olfactory features indicates correlation between colour of flowers and the type of olfactory mimicry, where a dark colour labellum emits strong smell of rotten waste.

Nectar Replaced by Volatile Secretion: A Potential New Role for Nectarless Flowers in a Bee-Pollinated Plant Species

The presence of nectarless flowers in nectariferous plants is a widespread phenomenon in angiosperms. However, the frequency and distribution of nectarless flowers in natural populations, and the transition from nectariferous to nectarless flowers are poorly known. Variation in nectar production may affect mutualism stability, since energetic resource availability influences pollinators' foraging behavior. Here, we described the spatial and temporal nectar production patterns of Jacaranda oxyphylla, a bee-pollinated species that naturally presents nectarless flowers. Additionally, we compared nectariferous and nectarless floral disks in order to identify histological, subcellular and chemical changes that accompanied the loss of nectar production ability. For that we used standard methods for light and transmission electron microscopy, and gas chromatography coupled to mass spectrometry for chemical analyses. We verified that 47% of flowers did not produce nectar during the whole flower lifespan (nectarless flowers). We also observed remarkable inter-plant variation, with individuals having only nectarless flowers, others only nectariferous ones and most of them showing different proportions of both flower types, with variable nectar volumes (3-21 μl). Additionally, among nectariferous flowers, we registered two distinct rhythms of nectar production. 'Early' flowers produced nectar from 0 to 24 h, and 'late' flowers produced nectar from 24 to 48 h of anthesis. Although disks from nectariferous and nectarless flowers displayed similar histological organization, they differed strongly at subcellular level. Nectariferous ('early' and 'late') flowers exhibited a cellular apparatus typical of nectar secretion, while nectarless flowers exhibited osmophoric features. We found three aliphatic and one aromatic compound(s) that were detected in both the headspace of flowers and the disks of nectarless flowers, but not the disks of nectariferous flowers Although the remarkable variation in nectar availability may discourage pollinator visits, nectarless flowers might compensate it by producing volatile compounds that can be part of floral scent, acting as chemical attractants. Thus, nectarless flowers may be helping to maintain pollination in this scenario of trophic resource supply scarcity. We suggest that J. oxyphylla can be transitioning from a nectar-based pollination system to another resource-based or even to a deceit mechanism of pollination.

Floral features of two species of Bulbophyllum section Lepidorhiza Schltr.: B. levanae Ames and B. nymphopolitanum Kraenzl. (Bulbophyllinae Schltr., Orchidaceae)

Two representatives of section Lepidorhiza, previously sometimes considered conspecific, Bulbophyllum levanae and Bulbophyllum nymphopolitanum, demonstrated both similarities and differences in floral features. There were significant differences in the length of sepals and micromorphological features of the labellum. In both species, osmophores are located on the extended apices of sepals and possibly on petals. An abundance of proteins in tepals is probably associated with the unpleasant scent of the flowers, whereas the thin wax layers on the epidermis are probably involved in the maintenance of the brilliance of floral tepals, which strongly attracts flies. In all tepals of both species, we noted the presence of dihydroxyphenolic globules in the cytoplasm after staining with FeCl₃. Comparison with ultrastructure results revealed that they were associated with plastids containing plastoglobuli. The most remarkable feature was the presence of a prominent periplasmic space in the epidermal cells of both investigated species. Furthermore, in the labellum of B. levanae, the cuticle contained microchannels. The combination of periplasmic space and microchannels has not previously been recorded.

Nectar and oleiferous trichomes as floral attractants in Bulbophyllum saltatorium Lindl. (Orchidaceae)

Although many Orchidaceae have deceit flowers that produce no reward, the most common reward, when present, is nectar. Bulbophyllum, however, is unusual in that the labellar secretions of most species investigated to date lack sugars, and, therefore, cannot be considered true nectar. The African species Bulbophyllum saltatorium is an exception in that it produces not only nectar but also possesses specialized, capitate oleiferous trichomes. The nectary of B. saltatorium is borne on the labellum and is represented by a deep, narrow, median longitudinal groove, having a small aperture, and flanked by trichomes. Isodiametric epidermal cells lining this groove secrete nectar which collects both in the groove and on the surface of the labellum. As well as a nectary, the labellum of B. saltatorium also bears three types of unicellular trichomes: the longest trichomes are borne distally and abaxially; the marginal ones form a rim around the entire labellum, and finally, massive, capitate trichomes occur proximally and adaxially. These are oleiferous, containing large quantities of oil which might function as precursors of volatile components of fragrance or provide a food-reward. To the best of our knowledge, this is the first time for such oleiferous trichomes to be described for Bulbophyllum. Therefore, apart from their color and markings, flowers of this species are able to attract pollinators in at least two, possibly three ways: food-reward in the form of nectar; fragrance; and possibly food-rewards in the form of food-hairs.

Floral structure of two species of Bulbophyllum section Cirrhopetalum Lindl.: B. weberi Ames and B. cumingii (Lindl.) Rchb. f. (Bulbophyllinae Schltr., Orchidaceae)

Flowers of Bulbophyllum weberi and B. cumingii are characterized by fly-pollinated features. The secretory activity was described in dorsal sepals in both species (putative osmophores), petals in B. weberi (possible osmophores) and adaxial surface of lips in both species. In the cells of dorsal sepals and petals of B. weberi proteins, dihydroxyphenols, lipids and starch grains were detected, in lateral sepals-lipids. Whereas in dorsal sepal of B. cumingii only lipids and starch grains were noted, in lateral sepals-proteins and dihydroxyphenols and in petals-proteins and starch grains. The lips in both species differed histochemically and ultrastructurally. The epidermal cells of lip groove in B. weberi contained lipids, proteins, starch grains in cytoplasm, dihydroxyphenols in vacuoles and pectic acids/mucilage on surface. Whereas in B. cumingii-few lipids, starch grains, no proteins, no dihydroxyphenols and no mucilage were noted. Ultrastructurally, in B. weberi, the secretory material was present on surface and vesicles building into plasmalemma, while in B. cumingii-cell wall ingrowths and microchannels in cuticle. The osmiophilic irregular materials and globular, osmiophilic globules in B. weberi are probably tannin-like materials. For the first time, we described the cell wall ingrowths in Bulbophyllum species: in lip of B. cumingii and petals of B. weberi.

How do secretory products cross the plant cell wall to be released? A new hypothesis involving cyclic mechanical actions of the protoplast

BACKGROUND

In plants, the products of secretory activity leave the protoplast and cross the plasma membrane by means of transporters, fusion with membranous vesicles or, less commonly, as result of disintegration of the cell. These mechanisms do not address an intriguing question: How do secretory products cross the cell wall? Furthermore, how do these substances reach the external surface of the plant body? Such diverse substances as oils, polysaccharides or nectar are forced to cross the cell wall and, in fact, do so. How are chemical materials that are repelled by the cell wall or that are sufficiently viscous to not cross passively released from plant cells?

SCOPE AND CONCLUSIONS

I propose a cell-cycle model developed based on observations of different secreting systems, some unpublished results and an extensive literature review, aiming to understand the processes involved in both the secretory process and the release of secretion products. In the absence of facilitated diffusion, a mechanical action of the protoplast is necessary to ensure that some substances can cross the cell wall. The mechanical action of the protoplast, in the form of successive cycles of contraction and expansion, causes the material accumulated in the periplasmic space to cross the cell wall and the cuticle. This action is particularly relevant for the release of lipids, resins and highly viscous hydrophilic secretions. The proposed cell-cycle model and the statements regarding exudate release will also apply to secretory glands not elaborated upon here. Continuous secretion of several days, as observed in extrafloral nectaries, salt glands and some mucilage-producing glands, is only possible because the process is cyclical.