Secretory Patterns in Colleters of Apocynaceae

Colleters of Apocynaceae are glands related to different types of protection of vegetative and floral meristems through the production of mucilage or a mixture of many different compounds. Although several anatomical papers have shown histological and histochemical aspects of colleters of the family, almost nothing is known about their secretory process. In this study, we analyzed two types of colleters in Apocynaceae: one produces mucilage and lipophilic compounds, while the other produces an exclusively mucilaginous secretion. The secretory epidermis of the colleters of Allamanda schottii and Blepharodon bicuspidatum has a dense cytoplasm with organelles responsible for the production of mucilage and lipids. This heterogeneous secretion is released through granulocrine and eccrine mechanisms and is temporarily stored in a subcuticular space before crossing the cuticle. Conversely, colleters of Mandevilla splendens and Peplonia axillaris produce only mucilage and have a very different secretory apparatus. The mechanism of secretion is granulocrine, and the exudate is firstly accumulated in a large periplasmic space and later in an intramural space before crossing the cuticle. Notably, the structure of the cuticle varies according to the secretion composition. Although the colleters of the family are histologically similar, this study demonstrates a metabolic and subcellular variability previously unknown for Apocynaceae.

Glandular trichomes of Robinia viscosa Vent. var. hartwigii (Koehne) Ashe (Faboideae, Fabaceae)—morphology, histochemistry and ultrastructure

Main Conclusion Permanent glandular trichomes of Robinia viscosa var. hartwigii produce viscous secretion containing several secondary metabolites, as lipids, mucilage, flavonoids, proteins and alkaloids. Abstract Robinia viscosa var. hartwigii (Hartweg’s locust) is an ornamental tree with high apicultural value. It can be planted in urban greenery and in degraded areas. The shoots, leaves, and inflorescences of this plant are equipped with numerous persistent glandular trichomes producing sticky secretion. The distribution, origin, development, morphology, anatomy, and ultrastructure of glandular trichomes of Hartweg's locust flowers as well as the localisation and composition of their secretory products were investigated for the first time. To this end, light, scanning, and transmission electron microscopy combined with histochemical and fluorescence techniques were used. The massive glandular trichomes differing in the distribution, length, and stage of development were built of a multicellular and multiseriate stalk and a multicellular head. The secretory cells in the stalk and head had large nuclei with nucleoli, numerous chloroplasts with thylakoids and starch grains, mitochondria, endoplasmic reticulum profiles, Golgi apparatus, vesicles, and multivesicular bodies. Many vacuoles contained phenolic compounds dissolved or forming various condensed deposits. The secretion components were transported through symplast elements, and the granulocrine and eccrine modes of nectar secretion were observed. The secretion was accumulated in the subcuticular space at the trichome apex and released through a pore in the cuticle. Histochemical and fluorescence assays showed that the trichomes and secretion contained lipophilic and polyphenol compounds, polysaccharides, proteins, and alkaloids. We suggest that these metabolites may serve an important function in protection of plants against biotic stress conditions and may also be a source of phytopharmaceuticals in the future.

Micromorphological traits and essential oil of Micromeria longipedunculata Bräuchler (Lamiaceae)

Micromeria longipedunculata Bräuchler (syn. M. parviflora (Vis.) Rchb.) is an endemic Illyric-Balkan plant species from Bosnia and Herzegovina, Montenegro, and Albania. We investigated types and distribution of trichomes, pollen morphology, and chemical composition of essential oil (analysed by GC and GC-MS) in M. longipedunculata. Non-glandular trichomes, peltate trichomes, and two types of capitate trichomes (type 1 composed of one basal epidermal cell, and one head cell with subcuticular space; type 2 composed of one basal epidermal cell, two or three stalk cells, and one head cell with subcuticular space) were observed on leaves, bracteoles, the calyx, corolla, and the stem. The pollen grains had six apertures which were set in the equatorial pollen belt and showed medium reticulate ornamentation. A phytochemical analysis of essential oils from four different localities is characterized by similar chemical composition with spathulenol (23.7–39.5%), piperitone oxide (7.7–12.1%) and piperitone (7.3–8.9%) as the major compounds.

Structure and distribution of glandular and non-glandular trichomes on above-ground organs in Inula helenium L. (Asteraceae)

Micromorphology and distribution of glandular and non-glandular trichomes on the above-ground organs of <em>Inula helenium </em>L. were investigated using light and scanning electron microscopy (SEM). Two types of biseriate glandular trichomes, i.e. sessile and stalk hairs, and non-glandular trichomes were recorded. Sessile glandular trichomes were found on all examined <em>I. helenium </em>organs (with their highest density on the abaxial surface of leaves and disk florets, and on stems), whereas stalk glandular trichomes were found on leaves and stems. Sessile trichomes were characterised by a slightly lower height (58–103 μm) and width (32–35 μm) than the stalk trichomes (62–111 μm x 31–36 μm). Glandular hairs were composed of 5–7 (sessile trichomes) or 6–9 (stalk trichomes) cell tiers. Apical trichome cell tiers exhibited features of secretory cells. Secretion was accumulated in subcuticular space, which expanded and ruptured at the top, and released its content. Histochemical assays showed the presence of lipids and polyphenols, whereas no starch was detected. Non-glandular trichomes were seen on involucral bracts, leaves and stems (more frequently on involucral bracts). Their structure comprised 2–9 cells; basal cells (1–6) were smaller and linearly arranged, while apical cells had a prozenchymatous shape. The apical cell was the longest and sharply pointed. Applied histochemical tests revealed orange-red (presence of lipids) and brow colour (presence of polyphenols) in the apical cells of the trichomes. This may suggest that beside their protective role, the trichomes may participate in secretion of secondary metabolites.

Phytochemical and Micromorphological Traits of Geranium dalmaticum and G. macrorrhizum (Geraniaceae)

A study of types and distribution of trichomes, pollen morphology, chemical composition of essential oil, and the content of macroelements and trace elements was conducted on endemic and previously uninvestigated Geranium dalmaticum (Beck) Rech. f. and widely distributed G. macrorrhizum L. growing in Croatia. The essential oil of G. dalmaticum was characterized by a high concentration of monoterpenes (82.2%) and the oil of G. macrorrhizum by mainly sesquiterpenes (76.7%). Among the investigated macroelements, the content of Ca was the highest (19,125.6 mg/kg and 22,439.6 mg/kg in G. dalmaticum and G. macrorrhizum, respectively). The content of trace elements ranged from 0.16 (Cr) to 110.01 mg/kg (Al) in G. dalmaticum, and from 0.01 (Cd) to 101.8 mg/kg (Al) in G. macrorrhizum. Non-glandular trichomes and two (in G. dalmaticum three) types of capitate glandular trichomes (type 1 composed of one basal epidermal cell, and one head cell with subcuticular space; type 2 composed of one basal epidermal cell, one or two stalk cells, and one head cell with subcuticular space; type 3 composed of one basal epidermal cell, 2–6 stalk cells, and one head cell with subcuticular space) were observed on leaves, stems and calyxes.

Glandular Trichomes and Essential Oil ofThymus quinquecostatus

The distribution and types of glandular trichomes and essential oil chemistry ofThymus quinquecostatuswere studied. The glandular trichomes are distributed on the surface of stem, leaf, rachis, calyx and corolla, except petiole, pistil and stamen. Three morphologically distinct types of glandular trichomes are described. Peltate trichomes, consisting of a basal cell, a stalk cell and a 12-celled head, are distributed on the stem, leaf, corolla and outer side of calyx. Capitate trichomes, consisting of a unicellular base, a 1–2-celled stalk and a unicellular head, are distributed more diffusely than peltate ones, existing on stem, leaf, rachis and calyx. Digitiform trichomes are just distributed on the outer side of corolla, consisting of 1 basal cell, 3 stalk cells and 1 head cell. All three types of glandular trichomes can secrete essential oil, and in small capitate trichomes of rachis, all peltate trichomes and digitiform trichomes, essential oil is stored in a large subcuticular space, released by cuticle rupture, whereas, in other capitate trichomes, essential oil crosses the thin cuticle. The essential oil ofT. quinquecostatusis yellow, and its content is highest in the growth period. 68 constituents were identified in the essential oils. The main constituent is linalool.

Phytochemical and Micromorphological Traits of Endemic Micromeria pseudocroatica (Lamiaceae)

Micromeria pseudocroatica Şilic is an endemic species distributed in southern Croatia. Chemical composition of the essential oil (analysed by GC and GC-MS), and the content of macroelements and trace elements analysed by ICP-AES was conducted. Additionally, a study on the types and distribution of trichomes was done by scanning electron microscopy. The essential oil was characterized by a high concentration of oxygenated monoterpenes, whose major compounds are borneol (22.7% and 24.8%) and camphor (16.1% and 13.9%). The content of Ca was highest (13202.69 mg/kg and 13223.83 mg/kg) among the investigated macroelements, while Fe was the most represented element (100.68 and 326.48 mg/kg) among the micronutrients. The content of potentially toxic elements that is Se, Cu, and Hg was below the limit of quantification. Non-glandular trichomes, peltate trichomes, and two types of capitate trichomes (type 1: one basal epidermal cell, one head cell with subcuticular space; type 2: one basal epidermal cell, two stalk cells, and one head cell with subcuticular space) were observed on leaves, bracteoles, calyx, corolla and stem.

Changes in Structure and Histochemistry of Glandular Trichomes ofThymus quinquecostatusCelak

The types, morphology, distribution, structure, and development process of the glandular trichomes on the leaves ofThymus quinquecostatusCelak had been investigated in this study. Two different types of glandular trichomes were determined in detail, namely, capitate trichomes and peltate ones. Besides, there were distinct differences on morphology, distribution, structure, and development process between the two kinds of trichomes. As the peltate trichome stepping into senium stage, it caved in the epidermis integrally, which was different from the capitate one. The secretion of the capitate trichome contained essential oil, polyphenols, and flavonoids, while, in addition to these three components, the secretion of the peltate one also contained acid polysaccharides. A distinctive difference was also seen in the secretory pathway of the secretion between the two types of trichomes. The secretion of capitate one was extruded through the cuticle of the head cell, but the secretion of the peltate one kept accumulating in the subcuticular space of the head cells until it was released by cuticle rupture.

Micromorphology of flowers, anatomy and ultrastructure of Chamomilla recutita (L.) Rausch. (Asteraceae) nectary

Investigations of the micromorphology of flowers and the structure of nectaries in <i>Chamomilla recutita</i> L. (Rausch.) were carried out with the use of stereoscopic, light, scanning and transmission electron microscopy. Biseriate glandular trichomes consisting of 5-6 cell layers were found on the surface of the corollas of ray and disc florets. Accumulation of secretion within the subcuticular space was accompanied by degradation of trichome cells. Secretion release followed rupture of the cuticle in the apical part of the trichome. The ovary of the ray florets exhibited characteristic ribs covered with epidermis composed of radially elongated palisade cells. Nectariferous glands were present only in the disc florets. The ring-like nectary (93 × 163 µm; height × diameter) was located above the inferior ovary. The gland structure was formed by single-layer epidermis and 5-8 layers of specialised nectariferous parenchyma. Nectar was released via modified 15-20 µm wide stomata. The guard cells were slightly elevated above the surface of the other epidermal cells or were located slightly below them. The stomatal cells were characterised by small external and internal cuticular ledges. No vascular bundles were observed in the nectary. The gland was supplied by branches of vascular bundles reaching the style and ending at the nectary base. The nectariferous tissue was formed by isodiametric cells with a diameter of 11-20 µm. The cell interior was filled with electron dense cytoplasm containing a large nucleus, numerous pleomorphic plastids, mitochondria with a distinct system of cristae, Golgi bodies, ER profiles, and ribosomes. The plastid stroma was characterised by presence of pastoglobuli, intraplastid tubules, and lighter zones. Several small vacuoles were found in each cell. Plasmodesmata were visible in the walls of some cells. Lighter periplasmic space in which apoplastic transport of nectar might take place was observed between the plasmalemma and the cell wall. The presence of an osmiophilic substance in the intercellular spaces additionally corroborates this assumption.

Structure of trichomatous nectaries in flowers of Lonicera kamtschatica (Sevast.) Pojark.

The structure of the floral nectaries of <i>Lonicera kamtschatica</i> was examined using light microscopy, scanning electron microscopy and transmission electron microscopy. Nectariferous tissues are located in the lower portion of the corolla tube. It was found that the secretory tissue of the nectary was composed of two layers of epidermal formations: short papillae and about 3x longer unicellular trichomes. They cover the adaxial surface of a small spur. Nectar secretion takes place through the apical portion of the trichomes and papillae. The cell wall of the upper part of the trichome has protuberances participating in nectar transfer to the subcuticular space which reaches large dimensions. The lateral walls of the trichomes are saturated with cutin. The papillae have much thicker walls than the trichomes. In the papillae, there are no wall protuberances. Less secretion accumulates in the subcuticular cavities of the papillae than in the trichomes.