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Plants have evolved numerous secretory structures that fulfill diverse roles and shape their interactions with other organisms. Rivea ornata (Roxb.) Choisy (Convolvulaceae) is one species that possesses various external secretory organs hypothesized to be ecologically important. This study, therefore, aimed to investigate five secretory structures...
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... histochemical assays revealed that the secretory structures in R. ornata produce various groups of metabolites, as all examined compounds tested positive, with the exception of proteins ( Table 1 and Figures 5-9). Lipids appear to be restricted to structural layers such as cell walls, while phenolic compounds, terpenoids, flavonoids, and alkaloids are mainly found in substances that accumulate in cytoplasmic components. ...Context 2
... appear to be restricted to structural layers such as cell walls, while phenolic compounds, terpenoids, flavonoids, and alkaloids are mainly found in substances that accumulate in cytoplasmic components. Polysaccharides, in general, occur in both cell walls and cell contents (Table 1 and Figures 5-9). Additionally, starch grains were found only in the nectary disc, and only sparsely (Table 1 and Figure 5L,M), but they were also detected in abundance in parenchyma cells of the receptacle ( Figure 5L,M) and in cells near petiolar nectaries ( Figure 6H,I). ...Context 3
... in general, occur in both cell walls and cell contents (Table 1 and Figures 5-9). Additionally, starch grains were found only in the nectary disc, and only sparsely (Table 1 and Figure 5L,M), but they were also detected in abundance in parenchyma cells of the receptacle ( Figure 5L,M) and in cells near petiolar nectaries ( Figure 6H,I). ...Context 4
... was the only structure that produced starch in its tissues. The histochemical results from the nectary epidermis and the nectariferous tissue of the nectary disc were congruent (Table 1 and Figure 5). However, the positive results for most of the detected chemicals were more pronounced in the nectariferous parenchyma near the adaxial epidermis (e.g., Figure 5F,N,P,T,V,X). ...Context 5
... results were not different between the two morphotypes. Substances with positive histochemical results were principally stored in the head of the glandular trichome and in the elongated epidermal cells of the petiolar nectaries, but they were sometimes also found in the stalk and basal cells of the glandular trichomes, or even in nectariferous cells (Table 1 and Figure 6). Starch grains were not found in the glandular trichomes, but accumulated in a strand of parenchyma cells located next to the outer edge of vascular bundles in petiolar nectaries ( Figure 6H,I); some of the starch grains were located in the subnectariferous region, while others were found in the cortex, which was not a part of the secretory working unit in petiolar nectaries. ...Context 6
... and foliar glands not only shared homologous structures but also presented similar histochemical results (Table 1; Figures 7 and 8). Positive reactions were mainly found at the head and stalk cells of the trichomes. ...Context 7
... was the only structure that produced starch in its tissues. The histochemical results from the nectary epidermis and the nectariferous tissue of the nectary disc were congruent (Table 1 and Figure 5). However, the positive results for most of the detected chemicals were more pronounced in the nectariferous parenchyma near the adaxial epidermis (e.g., Figure 5F,N,P,T,V,X). ...Context 8
... the staminal hairs, positive histochemical results were found in both the head and stalk regions, but they generally showed different degrees of chromatic reaction (Table 1 and Figure 9). Only phenolic compounds were absent from the head of the hairs (Table 1 and Figure 9D,E). For lipids, while Sudan black B gave relatively indistinct positive reactions in both the head and stalk regions, neutral red fluorochrome results were noticeably evident at the head ( Figure 9B,C). ...Citations
... The histochemical and fluorescence assays showed the presence of lipid compounds (total lipids, acidic, and neutral lipids) and phenolic compounds (total phenols, phenolic acids, and flavonoids) in the tissues and glandular trichomes of the D. moldavica nectary. Various authors also reported the presence of similar groups of metabolites in nectaries of other plant species [57][58][59][60]. Similar to the present findings, these researchers showed that the metabolites were located in the nectary epidermis and/or parenchyma; next, the compounds penetrated into the secreted nectar. ...
Dracocephalum moldavica is an aromatic plant with a lemon scent and versatile use. Its flowers produce large amounts of nectar, which is collected by bees and bumblebees. The aim of the study was to investigate the structure of the floral nectary in this melliferous plant, which has not been analysed to date. The analyses were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopy, as well as histochemical techniques. The four-lobed nectary with a diameter of 0.9–1.2 mm and a maximum height of 1.2 mm is located at the ovary base; one of its lobes is larger than the others and bears 20–30 nectarostomata and 8–9 glandular trichomes. The histochemical assays revealed the presence of essential oil and phenolic compounds in the nectary tissues and in glandular trichomes. The nectary tissues are supplied by xylem- and phloem-containing vascular bundles. The nectariferous parenchyma cells have numerous mitochondria, plastids, ribosomes, dictyosomes, ER profiles, vesicles, thin cell walls, and plasmodesmata. Starch grains are present only in the tissues of nectaries in floral buds. The study showed high metabolic activity of D. moldavica nectary glands, i.e., production of not only nectar but also essential oil, which may increase the attractiveness of the flowers to pollinators, inhibit the growth of fungal and bacterial pathogens, and limit pest foraging.
