Taxonomic Value of Leaf Epidermal Markers in Discriminating Some Medicinal Tree Species of Apocynaceae Juss †

: Apocynaceae is a useful family comprising trees notable for different medicinal remedies. Consequent to their importance vis-à-vis scarcity in the forest, they are being sold in various Nigerian markets by herb sellers mostly in sterile and fragmentary forms. Hence, the medicinal plants are subjected to adulteration and substitution. Frequently, identification of the plants by users is basically with the aid of floristic markers, which are not readily available for such purpose. It, therefore, becomes pertinent to carry out the taxonomic revision of these trees to provide additional markers that will contribute to their effective identification for correct use. Various docu-mentations have been made on members of apocynaceae and are properly placed on their respective taxa using epidermal traits. However, such information is scarce for Alstonia boonei , Holarrhena floribunda, Rauvolfia vomitoria , Thevetia nerifolia , and Vocanga africana. This study therefore aimed at providing epidermal taxonomic markers that could be employed in delimiting the species as an alternative when the fruit or floral parts are wanting. Leaf epidermises of five (5) species of apoc-ynaceae representing 5 genera were studied under a Biological microscope with a camera attachment. Data obtained were statistically analyzed. The epidermal cell was penta or hexagonal in A. boonei and V. africana . The stomatal length varied from 20.88 µm ( R. vomitoria ) to 25.92 µm ( T. nerifolia ) and 18.96 µm ( R. vomitoria ) to 29.28 µm ( V. africana ) on the abaxial and adaxial layers respectively. All the epidermal characters on the adaxial layer were significantly different ( p < 0.05) among the species. Anticlinal walls were sinuated in H. floribunda and T. nerifolia while in R. vomi-toria , it was straight to wavy. V. africana and A. boonei anticlinal walls were straight. This study represents the first account of epidermal characterization of the members of apocynaceae in Nigeria and is of taxonomic importance in setting boundaries among the species.


Introduction
Apocynaceae Juss. is one of the important families in angiosperm founded in 1789.About 1900 species representing 215 genera have been identified worldwide [1].According to [2], the family was delimited into five subfamilies, comprising of Secamonoideae, Apocynoideae, Rauvolfioideae, Periplocoideae, and Asclepiadoideae.Plants of this family are greatly diversified in lifeform to trees, shrubs, climbers and rarely herbs [3][4][5].Notable diagnostic morphological feature of the family is the production of pod-like fruits.The leaves are usually sessile or petiolate having variable shapes of lanceolate, ovate, linear, obovate, elliptic, or oblong [1,6,7].
Various works have been published on the epidermal characterization of many taxa of the family apocynaceae.Wriohtia tinctoria, Ervatamia divaricata, and Catharanthus swere delimited based on their amphistomatic leaves from six other apocynaceous species after studying the foliar epidermis of ten Indian species comprising of nine genera by [8].
Seven species representing seven genera were investigated by [1] and recommended the merging of Apocynaceae and Asclepiadaceae to one big family.Nerium oleander was distinguished from other species by having stomatal crypt [9].Despite these wonderful documentations, the taxonomic relationship among the members of apocynaceae remains unsettled and incomplete.This can be deduced from the fact that some of the African species in the family such as Astonia boonei, Holarrhena floribunda, Rauvolfia vomitoria, Thevetia neriifolia, and Vocanga africana are still being left out.These species are among the trees commonly used for traditional medicine in Nigeria.The bark of Alstonia boonei in a tincture with the bark of Enantia chlorantha for the treatment of malaria and yellow fever [10].According to [11], the decoction of root and bark of Rauvolfia vomitoria in tandem with some species of meliaceae is efficacious in the treatment of coated tongue disease.
Often time, identification of these medicinal trees is achieved using the floral and fruiting components.However, the existence of flowers is seasonal and is therefore useless for taxonomic discrimination purpose during off-flowering and fruiting seasons.Due to their wide application in ailment treatment, they are usually sold in the market either fragmentary or sterile conditions [12].Hence, they are highly prone to substitution and adulteration, which is very inimical to effective application.Given the various medicinal uses of these species, there is a need for thorough taxonomic revision to provide additional markers for proper discrimination of the taxa.This study therefore aimed at providing important epidermal taxonomic markers that could be employed in delimiting the species as an alternative when the fruit or floral parts are wanting.

Sources of Plant Samples and Epidermal Peels Preparation
Fresh leaf samples of the species were collected from Onigambari Forest reserve and the University of Ibadan, Nigeria.Identification and authentication of the samples were carried out in the Forest Herbarium, Ibadan.According to [13], leaf samples were first preserved in 50% ethanol before subjection to epidermal characterization.Mature leaves were randomly selected, cut into sizeable sections, and soaked in concentrated nitic acid ranging from 8 to 24 h depending on the leaf texture [14].
Swollen of the leaf surfaces with the appearance of air bubbles are an indication of the readiness of the epidermal layers for separation.Samples with swollen surfaces and air bubbles were then transferred into clean glass Petri-dishes containing water while the adaxial and abaxial layers were separated using dissecting needle and forceps.The peels were cleaned using a camel-air-brush in water and preserved in storage bottles containing 50% ethanol [13][14].

Preparation of Slides, Assessment of Epidermal Characters, and Data Analysis
Epidermal peels were first washed in water before staining them with appreciable drops of safranine [13].For clear visibility, peels were counterstained using toluidine blue and the excess stains were removed by washing with water twice.The samples were subjected to a series of ethanol concentrations of 50%, 70%, 80%, 90%, and 100% for approximately 3 min to dehydrate To completely remove all traces of stains, water and ethanol, the peels were treated using absolute xylene.Each epidermal peel was then mounted on a slide using 25% glycerol for the feasibility of the internal structure.The slides were studied under a CIWA XSP-35TV biological microscope.Photomicrographs were taken using with 200 W Electronic Eyepiece.Quantitative variables such as length and breadth of the epidermal cell and stomata were measured using an ocular micrometre.
Guard cell area (GCA) and Stomal index (SI) were estimated according to [15].Data were subjected to analysis of variance (ANOVA).

Results
Qualitative epidermal markers of the Nigerian species of apocynaceae are shown in Table 1.Stomata were present on both the abaxial and adaxial surfaces of all the species except for Alstonia boonei, which lacked stomata on the adaxial layer (hypostomatic).Paracytic stomata were identified in all the species except in A. boonei, where stomatal crypts were found (Figure 1).Generally, stomata were more distributed in the abaxial surface compared to the adaxial layer of all the species.Stomata were abundant in an abaxial layer of R. vomitoria, H. floribunda, and T. nerifolia but were scant in V. africana and A. boonei.
All the epidermal markers were significantly different (p < 0.05) among the species except for the length of the guard cell and length of stomata on the abaxial leaf surface (Table 2).041 * LE = length of epidermal cell, BE = breadth of epidermal cell, LS = Length of stomata, BS = breadth of stomata, LGC = length of guard cell, WGC = width of guard cell, GCA = guard cell area, SD = Stomatal density, SI = Stomatal index; * = significant at 5% probability level; ns = not significant at 5% probability level.

Discussion
There has been some documentation in which epidermal markers were used to discriminate plant taxa [14,[16][17][18][19]. Results obtained from this study clearly demonstrated that epidermal markers could provide a reasonable value for discriminating the selected medicinal tree species of apocynaceae.For instance, only Alstonia boonei of all the medicinal species considered was hypostomatic.This marker could be used as a veritable means of discriminating it from other taxa.The result agrees with [20], in which hypostomatic distribution of stomata was used as a marker to set the boundary between the species of East African apocynaceae.
Another important and unique epidermal marker peculiar to A. boonei is the stomatal crypt.Stomatal scripts are also known as sunken stomata that are located in depressed portions of the epidermis, which forms a narrow-mouthed or deep longitudinal groove [21].The stomatal crypt can contain one or more stomata and at times with trichomes or wax accumulations.The presence of sunken stomata in A. boonei can be linked to its wide range of ecological distribution, which occurs both in the savannah and wet regions [22].The effectiveness of stomatal crypt for apocynaceae taxa delimitation has been reported.Nerium indicum was said to be taxonomically distinct from Catharanthus roseus and Tabernaemontana divaricata by having stomatal crypts [23].N. indicum was the first species of apocynaceae to be reported with stomatal crypts characteristics while A. boonei observed in the present study happened to be the second species with such important taxonomic trait.
Other useful qualitative markers with taxonomic value based on the result include stomatal abundance, anticlinal wall and crystal types, which varied among the species.Such markers could be employed at both specific and generic level of the medicinal species as previously reported in some plant's families [13,17].The overlap in the epidermal cell shape of these species makes it unsuitable as a marker for taxonomic purpose in the taxa.
Holarhena floribunda was separated from other species by having the largest value for some of the stomata and epidermal quantitative markers, especially on the abaxial leaf surface.Additionally, Thevetia nerifolia was singled out with the highest value of the stomata index on both the abaxial and adaxial layers.Therefore, the epidermal and stomatal markers which are similar among the species could be translated as the affinity that exists within the family while those that were variable could be termed as diagnostic features among the species.This corresponds to the literature [12,[24][25][26] in which stomatal and epidermal markers were used to solve taxonomic problems among the medicinal plants.According to a report by [12], the stomatal index is very reliable and useful in delimiting some medicinal tree species.

Conclusions
Aside from the fact that this study is the first account of epidermal characterization of the members of apocynaceae in Nigeria, various epidermal markers have been identified to be useful for the discrimination of the medicinal tree species in the taxa.Of worthy note is the stomatal crypt, which is being reported for the first time in Nigerian Alstonia boonei.This, therefore, calls for further study at the molecular level to complement the existing findings generated from the epidermal markers and consideration for separating the species from the current family.

Table 1 .
Qualitative epidermal markers of the Nigerian species of apocynaceae.

Table 2 .
Quantitative epidermal markers (abaxial layer) of the Nigerian species of apocynaceae.

Table 3 .
Quantitative epidermal markers (adaxial layer) of the Nigerian species of apocynaceae.BE = breadth of epidermal cell, LS= Length of stomata, BS = breadth of stomata, LGC = length of guard cell, WGC = width of guard cell, GCA = guard cell area, SD= Stomatal density, SI= Stomatal index; * = significant at 5% probability level.