Bark anatomy, chemical composition and ethanol-water extract composition of Anadenanthera peregrina and Anadenanthera colubrina

The bark of Anadenanthera peregrina (L.) Speg and Anadenanthera colubrina (Vell.) Brenan were characterized in relation to anatomical and chemical features. The barks were similar and included a thin conducting phloem, a largely dilated and sclerified non-conducting phloem, and a rhyridome with periderms with thin phellem interspersed by cortical tissues. Only small differences between species were observed that cannot be used alone for taxonomic purposes. The summative chemical composition of A. peregrina and A. colubrina was respectively: 8.2% and 7.7% ash; 28.8% and 29.3% extractives; 2.4% and 2.6% suberin; and 18.9% lignin. The monosaccharide composition showed the predominance of glucose (on average 82% of total neutral sugars) and of xylose (9%). The ethanol-water extracts of A. peregrina and A. colubrina barks included a high content of phenolics, respectively: total phenolics 583 and 682 mg GAE/g extract; 148 and 445 mg CE/g extract; tannins 587 and 98 mg CE/g extract. The antioxidant activity was 238 and 269 mg Trolox/g extract. The barks of the Anadenanthera species are a potential source of polar extractives that will represent an important valorization and therefore contribute to improve the overall economic potential and sustainability of A. peregrina and A. colubrina


Introduction
Anadenanthera is a small neotropical genus from South America with only two species: Anadenanthera peregrina (L.) Speg and Anadenanthera colubrina (Vell.) Brenan [1]. They belong to the family Leguminosae, subfamily Mimosaceae, and can grow in areas from savannah to dry rainforest. In Brazil, where they are usually named "angico", they occur predominantly in seasonal forests and riparian galleries [2,3,4].
Both species have a strong morphological similarity and their differentiation usually requires the examination of flowers with buds, fruits and leaves [1,4]. The bark may provide taxonomic information when these bodies are not available either in herbaria or in the field. PLOS  Although the external macroscopic characteristics of barks is important and is often used for identification, in the case of similar species, as it is the case of A. peregrina and A. colubrina, it is necessary to have a microscopic knowledge of the internal structure [5]. This has been shown by e.g. [6] for several Eucalyptus species, [7] who distinguished six bark types within the Cassinoideae subfamily, and [8] to distinguish Lebeckia genera. Both A. peregrina and A. colubrina are used for timber, charcoal and as firewood, as well as in popular medicine [9]. The wood has high density, resistance and outdoor durability, as well as a smooth lustrous surface with high decorative value, and is used for flooring and in building and naval construction [10,3].
Very few studies have been published on the stem characterization of the species of this genus. The wood anatomy and some physical properties were described for A. peregrina [10,11] and the wood anatomy for A. colubrina [12]. As regards bark anatomy, [13] gave information of anatomy and hystochemistry of secretory ducts in the bark of A. peregrina, [14] and [15] gave general information of various genus of Leguminosae, and [16] examined 28 species of the subfamily Caesalpinioideae, Mimosaceae and Papilionoideae from Brazil, including the genus Adenanthera and A. peregrina.
More information exists on some chemical features of the barks of these species, since they have a high content of tannins that are used for the leather and dyeing industries (17,18,19,4]. They are also used in the popular medicine for treatment of respiratory and lung infections, and against diarrhea [20]. Some phytochemical, pharmacological and toxicological studies of bark extracts and tannin quantification have been made for A. peregrina [21,22] and A. colubrina [23,24,25,26,27].
Barks are a subject of recent research on their structural and chemical characterization targeting their potential as a feedstock for biorefineries. Barks from various species were studied e.g. Eucalyptus spp. [28,29,30], Pseudotsuga menziesii [31], Quercus cerris [32], Tectona grandis [33]. Overall it has been shown that knowledge on the anatomy and chemical composition of the barks is essential for designing their fractionation and valorization routes.
The barks of A. peregrina and A. colubrina are studied here for the first time with a comprehensive characterization of their structural and anatomical features as well as of the chemical composition, including polar extracts and their antioxidant properties.
The objective is dual: i) to see if the detailed information on the bark may be used as a taxonomical tool for species identification; and ii) to establish a background information for bark valorization to be used for targeting processing routes and products.

Material and methods
No specific permits were required for the collection of bark of the species studied, since these were collected in the experimental field of the Federal University of Lavras and the nondestructive method was used. The field studies did not involve endangered or protected species.

Site characterization and sampling
The barks from three trees of Anadenanthera peregrina (L.) Speg. and of Anadenanthera colubrina (Vell.) Brenan were collected. The trees were growing in a remaining seasonal semideciduous mountain forest belonging to the Mata Atlântica bioma, located in the campus fields of the Federal University of Lavras, in the southern part of Minas Gerais, Brazil (21˚14'S; 45˚00'W, mean altitude 900 m). The climate is mesothermic with mild summers and dry winters (Cwb following the Köppen classification) with a mean annual temperature of 19.4˚C and 1530 mm annual precipitation.
The trees were harvested and the bark removed by separation from the wood by making an incision cut and pulling out. The samples used for analysis were taken from the lower part of the stem until 1.3 m of height.

Anatomical characterization
The samples were impregnated with DP 1500 polyethylene glycol and transversal, tangential and radial microscopic sections were prepared from cambium to the outside [34,35]. The proportion of cell types was determined using a 48-point grid on three images per sample on transverse sections from cambium to the outside. Sieve tubes were only quantified in the noncollapsed phloem. The length, width and wall thickness of fibres and sieve tube elements were measured in macerated samples.
The microscopic observations were made with a Leica DMLA optical microscope, the photomicrographs were taken with a Nikon FXA camera and the measurements done by using the Leica Qwin V 3.5.0 software. Terminology followed [36].
The macroscopic observation was made on the cross-section of the samples after surface sanding using a magnifying glass Leica MZ6 and photographed with a digital Samsung 10 MP L201 camera.

Chemical composition
The bark samples were ground and the fraction 40-60 mesh used for chemical analysis.
A bark composite sample of the three trees per species was made and carefully homogenized before analysis. Determination of ash followed TAPPI standards (T 211 om-93), extractives were determined gravimetrically after successive Soxhlet extractions with dichloromethane, ethanol and water during 6 h, 16 h and 16 h, respectively (TAPPI Standard T204 om-88 and T207 om-93). The depolymerization and removal of suberin was made on the extractive-free samples using sodium methoxide in methanol [37].
The solid residue remaining after suberin depolymerization was used for determination of lignin and carbohydrates by acid hydrolysis. Acid insoluble (Klason) and acid soluble lignin were determined according to TAPPI T 222 om88 and TAPPI UM 250 standards, respectively. The composition of polysaccharides was evaluated after hydrolysis by determining the content in neutral monossacharides (rhamnose, arabinose, xylose, galactose, mannose and glucose) in the hydrolysate from the lignin analysis using high pressure ion-exchange chromatography with a pulsed amperometric detector (HPIC-PAD). The compounds were separated in a Dionex ICS-3000 system, with an Aminotrap plus Carbopac PA10 column (250 x 4 mm) using a linear gradient of NaOH and CH 3 COONa solution (0-20 min 18 mM NaOH; 20-25 min 50 mM NaOH+170 mM CH3COONa; 25-40 min 50 mM NaOH+170 mM CH3COONa) as eluent at a flow rate of 1mL/min; the column temperature was maintained at 30˚C. All the chemical determinations were made on duplicate samples.

Phenolic content of bark extracts
Extraction was carried out with ethanol-water (50%) for 60 min at 50˚C using an ultrasonic bath and the extraction yield was calculated as the percent mass loss of the starting material.
The total amount of soluble phenolics was estimated by the Folin-Ciocalteu method using gallic acid as standard [38]. Total flavonoids were quantified by an aluminium chloride colorimetric assay using catechin as a standard [39] and the proanthocyanidins content (condensed tannins) was determined by the vanillin-H 2 SO 4 method using catechin as a standard [40]. The experimental procedure is described in detail in [30].

Antioxidant activity of bark extracts
The antioxidant activity was assessed by the DPPH free radical assay [41] following a procedure described before [30]. The antioxidant activity was expressed as IC 50 values and also as in terms of Trolox equivalents (TEAC).

Bark anatomy
The bark anatomy of A. peregrina and A. colubrina was similar. The anatomical bark features of both species are represented in Figs 1, 2 and 3. The macroscopic outer aspect of A. peregrina and A. colubrina barks was similar. The barks are brown and thick (6.5-17 mm and 5-12.1 mm respectively), wrinkled and irregularly scaly with sharp hardened outgrowths. The bark of A. colubrina was more difficult to remove from the trunk than that of A. peregrina that could be easily pulled out.
The bark included the rhytidome and the phloem; the nonconducting phloem was perfectly distinct from the conducting phloem by a lighter colour. Annual growth increments were not detected. In both species, the rhytidome was composed by several periderms that developed sequentially straight to undulated and were interspersed with cortical parenchyma cells (Figs 1A and 2A). In each periderm the phellem consisted of tabular cells with thick tangential walls and the phelloderm included layers of rectangular to round cells, some with thin walls and others uniformly thickened and even sclerified into sclereids (Figs 1D and 2D). These sclereids may give a certain pattern to the phelloderm either forming irregular groups or a somewhat concentering ring, more evident in A. peregrina The cortical parenchyma cells, derived from the cortex i.e. the phellogen developed in the cortex, were arranged without intercellular spaces, particularly obvious in the outgrowths of A. colubrina (Fig 2A), and could be confused as phellem cells of the periderm. In fact the periderm of A. peregrina and A. colubrina included only a small proportion of suberized phellem cells.
Below the innermost periderm, there was a zone with cortical-like cells, here called "cortex", where cells were dilated by tangential expansion and also sclerified forming groups of sclereids ( Fig 2B); this area is inner bordered by a ring of sclerified cells that make the transition to the phloem (Figs 2B and 3B).
A. peregrina and A. colubrina bark structure was similar also concerning: 1) the demarcation between non-conducting and conducting phloem that was gradual and marked by collapsed sieve tube elements; 2) phloem stratification; 3) type and position of sieve plates; 4) dilatation tissue restrict to the rays; 5) arrangement and proportion of sclereids; and 6) presence of secretory cells.
The conducting phloem was narrow (about 20 cell rows) and included the sieve-tube elements with turgid companion cells, axial and radial parenchyma cells and fibers (Fig 1C). The sieve-tube elements (corresponding to approximately 2% in A. peregrina and A. colubrina) were solitary or in small groups of 2 or 3, and scattered among axial parenchyma cells. The sieve plates are compound scalariform and inclined, with 5-10 sieve areas ( Fig 1F); lateral sieve tube walls were observed with pores clustered in sieve areas (Fig 1F). The rays (13% in A. peregrina and 14% in A. colubrina) were non-storied; mostly multiseriate (2-4 cells), homocelular with procumbent ceIls (Fig 1F), and more or less straight until the beginning of the dilatation growth. The number of cells in the rays was the same in both species (7-28 in height and 6-12 cells/mm).
The non-conducting phloem is characterized by the collapsed sieve-tube elements (Figs 1B, 1C and 2C) and the dilatation growth. In A. peregrina and A. colubrina the dilatation is evident mainly in the outer phloem and results mostly through the anticlinal ray cell division, giving rise to a slightly to dilated rays (Figs 1B, 2B and 2C); in minor extent axial and radial parenchyma cells also undergo some tangential dilatation.
The dilatation tissue represented about 12% in A. peregrina and 29%.in A. colubrina. Clusters of sclereids occured resulting in conspicuous nodules of irregular or radial alignment (Figs 1E and 2B) with a characteristic pattern. Sclereids (4% in A. peregrina and 8% in A. colubrine) appeared isodiametric with a polylamellated and pitted wall. Sclereids constitute the mechanical support for bark radial growth in addition that given by the fibers (higher in A. peregrina).
In A. peregrina and A. colubrina, prismatic crystals occurred in chambered axial parenchyma cells (Fig 3A) and also adjacent to fibrous bands (Fig 3C). Single crystals were frequent in sclerified cells of the phelloderm, mostly in A. colubrina.
Starch grains were observed in parenchyma cells of A. colubrina (Fig 3B) Phenolic deposits of brown colour were particularly abundant through the bark, mainly in the rhytidome i.e. phellem, phelloderm and cortex, and in the non-conducting phloem, arranged in tangential rows or groups (Fig 3D and 3E). In both species, enlarged parenchyma cells (secretory cells) were observed in nonconducting tissue arranged in groups (Figs 1F and 2E). Table 1 summarizes the chemical composition and the polysaccharides composition of the barks of A. peregrina and A. colubrina. The chemical composition of the bark of both species was similar. The content in extractives was very high at 29%, corresponding mainly to polar extractives soluble in ethanol and water that represent 85% and 90%, respectively of the total extractives. The suberin content was low corresponding on average to 2.5% of the barks. Lignin content was on average 18.9% and polysaccharides 40%. The monosaccharide composition showed a predominance of glucose corresponding on average to 82% of the neutral monosaccharides.

Chemical composition
The hemicelluloses included mostly xylan i.e. xylose and arabinose accounted on average for 8.8% and 2.6% of the total neutral monomeric sugars, respectively, but also galactomannan

Phenolic content of bark extracts
The yield of ethanol-water extraction and the phenolic composition and anti-oxidant activity of the bark extracts are given in Table 2. The extraction yield of 22.8% and 25.4% for the barks of A. peregrina and A. colubrina respectively was very similar to the content of the polar extractives determined by sequential solvent extraction (Table 1).
There was a substantial difference in the extract composition between the two species. The total phenolic content was high, especially for A. colubrina (682 mg GAE/g extract vs. 583 mg GAE/g extract) that contained also substantially more flavonoids (445 mg CE/g extract vs. 148 mg CE/g extract). On the contrary, the tannin content was much higher in the A. peregrina bark extract (587 mg CE/g extract vs. 98 mg CE/g extract). The antioxidant activity of both bark extracts was similar corresponding on average to 253 mg Trolox g -1 extract or 59 mg Trolox g -1 of bark.

Anatomical features
A. peregrina and A. colubrina could be differentiated by the adhesion of the bark to the cambium and xylem since the bark of A. peregrina could be pulled out easily. This difference, which should be related with cambial features and has been referred for other species e.g. for Eucalyptus spp. [28].
The bark anatomy of both Anadenanthera species was very similar. [15] also observed little anatomical variation in the bark of different genus of Mimosaceae subfamily. The bark of A. peregrina and A. colubrina showed overall common features: a conspicuous scaly rhytidome with several periderms with little phellem development interspersed by cortical tissues. This type of rhytidome with successive periderms including bands of cortical parenchyma was described by [16] for A. peregrina and other species of the same family i.e. Bowdichia virgilioides. These cortical parenchyma cells were particularly obvious in the outgrowths of A.colubrina (Fig 2A) and their form and radial alignment could mistake them with phellem cells of the periderm. [15] also referred in cork of Mimosaceae and above the scale, the presence of small cells which preserve their embryonic shape and meristematic state and may be able to divide periclinally resembling phellogen cells; the little suberization of the cork was also noticed by the same author. The cortex below the innermost periderm featuring a more or less complete ring of irregularly shaped sclereids was also noticed by [14], [15] and [16]. The phloem included a narrow conducting region with anatomical features that were similar in both species and that in general agree with the observations of [16] for the Mimosoideae. The nonconducting phloem showed a conspicuous dilatation tissue formed by parenchyma cell division and expansion, resulting from the phloem adjustment to tree radial growth [36] and are in agreement with observations for the Leguminoseae by (15) and [16].
Thick walled sclereids were present in large proportion, especially in A. colubrina often including large prismatic crystals. The presence of crystals accompanies sclerefication and contributes to the mechanical strength of the bark [42,15].
Numerous crystals were observed in the phloem of both species namely in the axial parenchyma close to the fibres (Fig 3A and 3C). Crystals in the axial parenchyma seem to be a characteristic of the Mimosaceae subfamily [15] and were described for A. peregrina [43,16]. Although some authors refer that crystals may have a diagnostic value to differentiate species within a genus [44,45,46], [47] considers that variability is also large between individuals in the same species. Starch grains were observed in parenchyma cells of A. colubrina ( Fig 3B) and their presence in the axial parenchyma in the majority of the studied species examined by [16].
A conspicuous feature of these barks was the presence of extensive phenolic deposits in e.g. tanniferous cells especially in the rhytidome and in the non-conducting phloem (Fig 3D and  3E) that seemed overall more abundant in A. peregrina bark. This is clearly in agreement with the traditional use of these barks as a tannin source, and with the chemical features as discussed further on.
Overall the anatomical structure of the barks of A. peregrina and A. colubrina was very similar and the small differences that were observed do not allow a bark based taxonomic differentiation of both species.

Chemical composition
There are no previous studies and references on the chemical composition of the barks from A. peregrina and A. colubrina. It is however known that they contain a large amount of tannins, especially the bark of A. peregrina, and that they are used in the leather and dyeing industries [17,18,19].
The determination of extractives (Table 1) confirms the high content of polar extractives that can be extracted by an ethanolic aqueous solution ( Table 2). The microscopic observations also showed the high amount of deposits and inclusions that are a conspicuous feature of the bark structure of these species (Fig 3D and 3E). It was also confirmed that the bark extract of A. peregrina contains a much higher tannin content that represents about 59% of the extract (in catechin equivalents) while tannins in the bark of A. colubrina represent only about 10% of the extract ( Table 2).
The valorization of A. peregrina bark is therefore certainly possible for production of tannins that will represent approximately 17% of the bark (173.3 mg CE/g bark). The bark extract of A. colubrina is very rich in total phenolics (ca. 68% in GAE of the extract) that include mainly flavonoids (45% in CE of the extract) ( Table 2). Flavonoids are also valued as bioactive supplements and therefore their extraction from A. colubrina bark represents also a potential valorization route. However the antioxidant properties of the bark extracts of both species are only of moderate intensity with an average IC 50 value of 13 mg ml -1 compared with 2 mg ml -1 for Trolox (Table 2).
Since the barks of A. peregrina and A. colubrina have a small proportion of phellem in their periderms, their content in suberin is low (Table 1). Suberin is a typical structural component of bark periderms where its presence is specific to the wall of phellem cells for which it is a chemical fingerprint [31,54,55]. Therefore when the proportion of phellem in a bark is small, then the content in suberin is also small. This has been shown for several species e.g. Pinus sylvestris and Picea abies [56], E. globulus [57] or Tectona grandis [33].
The main structural chemical components of the barks of A, peregrina and A. colubrina are lignin, which represented 31.0% and 31.1% respectively of the extractive and ash-free barks, and by cellulose and hemicelluloses. Xylan and galactoglucomannans should be the major hemicelluloses, while glucose was the predominant monosaccharide obtained after acid hydrolysis (Table 2). It should be noted that only neutral monosaccharides were determined and it is to expect the presence in the hemicelluloses of acetyl groups and uronic acids.
The high mineral content of both barks should result from the frequent presence of crystals, presumably of calcium oxalate, that are clearly observed namely in the cells of the axial parenchyma (Fig 3A and 3C).

Conclusions
The barks of the two species of the genus Anadenanthera (A. peregrina and A. colubrina) are very similar in relation to the structural arrangement, the type and morphology of cells, and tissue distribution in phloem, rhytidome and periderms. Both species were distinguished by the sclereids in the phelloderm forming a more obvious ring in A peregrina and by the type of cortical parenchyma cells in the rhytidome in A. colubrina that resemble phellem cells. However the bark anatomy cannot per se be used as a taxonomic indicator.
The chemical composition of A. peregrina and A. colubrina barks was studied for the first time. The most striking characteristic is the very high content of polar extractives rich in phenolic compounds and with good antioxidant activities. Significant differences were found in the content of polar extractives with A. peregrina bark showing a higher content of tannins and A. colubrina a higher content of total phenolics and flavonoids.
Their extraction will represent an important valorization of these barks and therefore contribute to improve the overall economic potential and sustainability of A. peregrina and A. colubrina.