TAXONOMIC REVISION OF NIGERIAN SPECIES OF Capsicum L. BASED ON SOME MORPHOLOGICAL CHARACTERS

ABSTRACT


INTRODUCTION
The genus Capsicum in Nigeria has not been thoroughly revised, classified and identified especially using morphological characters. There is a dearth of information on the exact number of Capsicum species and varieties found in the country. At the moment, no satisfactory revision of the morphology of the Nigerian genera of Capsicum is available. Apart from this challenge, it has been observed that some authors have misrepresented some Nigerian species of Capsicum due to lack of proper identification. E.g. in Edeoga et al. (2010), 'tatashe' (local name for C. annuum) was regarded as a variety of C. frutescens. The boundaries between some of the species are still ill-defined, with many of the taxa proving to possess not more than slight morphological variations from those already described. Even where the revisions of the genera are in existence, the situation is further complicated by the researchers who always either treat different members of the genera as varieties of particular species or considered them as different species on the basis of morphological differences (Schilling and Andersen, 1990;Edmonds and Chweya, 1997;Grubben and El Tahir, 2004). Mainly, the disagreements among taxonomists on Capsicum taxonomy include species boundaries and importance of some morphological characters over others. It is agreeable that when classification is confused, so is nomenclature and literally any information about such taxa is unspecific and definitely, less useful.
Capsicum crops are perennial crops with densely branched stems and an average height of 0.5 -1.0 meters, which are usually grown annually (Young and Tarawou, 2014). They are important crops not only because of their economic importance but also for the nutritional value of their fruits, being a major source of natural colours and antioxidant compounds (Ogunlade et al., 2012). In fact, Iwegbue et al. (2011) stated that support for increased production and consumption of fresh vegetables such as Capsicum annuum is an important goal. Pepper is a largely widespread spice with annual world production in the year 2004 evaluated to 23 million tons from a total of 1.54 million ha (Djieto-Lordon et al., 2014). Despite the importance of Capsicum spp (Adepoju et al., 2020), there is currently no consensus classification of Capsicum itself. The infrageneric taxa proposed by Kuntze (1891) and Bitter (1921) have later been recognised asthe segregate genera: Witheringia, Brachistus, Saracha Ruiz and Pav. TuboCapsicum (Wettst.) Makino, Aureliana (Hunziker, 2001). More recently, different classical and molecular cytogenetic analyses, crossing experiments, enzymatic studies, and chloroplast and nuclear DNA sequence studies (Scaldaferro et al., 2006), have allowed considerable progress in the characterization of infrageneric groups in Capsicum.
At present, there is no worldwide accepted formal infrageneric classification of Capsicum. Two attempts at grouping the species were made based on cytogenetic studies (Moscone et al., 2007), and a combination of data from enzyme, crossing and molecular studies (Walsh and Hoot, 2001). In both studies, the informal classification was still considered provisional despite more than 50% of the species having been analysed. Barboza (2011) designated lectotypes for 14 species names of the genus, and these were synonymised under their accepted names in Capsicum. In addition, a new name in Capsicum was proposed. In each case, the locality information given for the lectotype corresponded with the information found on the specimen itself.
Anatomy of Capsicum in Nigeria has been studied by Mbagwu et al. (2007), Nwachukwu et al. (2007), Adedeji et al. (2007) and Edeoga et al. (2010). Edeoga et al. (2010) studied the role of leaf exomorphology in the taxonomy of Capsicum annum and C. frutescens and reported the anatomical markers for only these two. C. chinense which is also a common species in Nigeria was not included in the study while the authors did not elucidate the details of the trichomes of the two taxa that they examined. Adedeji et al. (2007) embarked on the study of the organographic distribution and taxonomic importance of trichomes in the family Solanaceae involving only six Nigerian species of t he family namely; Capsicum frutescens L., Solanum pimpinellifolium (Jusl.) Mill., S. macrocarpon Linn., Solanum torvum Sw., Solanum nigrum Linn. an d Nicot-iana tabacum Linn. based on the findings, these authors suggested that lycopersicum which had earlier been regarded as a species of Solanum, should be placed in a separate genus.
This study sought to undertake a vegetative and reproductive morphological revision of Capsicum species in Nigeria with a view to examining their taxonomic status as well as provide markers for their identification.

Taxonomic Treatments of the Sample
A total of 23 characters were drawn out from the leaves, fruits and seeds of the five cultivars of Capsicum. The qualitative characters obtained were first quantified by scoring presence as "1" and absence as"0". Quantitative readings were taken in 30 replicates; one from each individual of a cultivar, which equals a total of 150 samples. Means and levels of significance (Duncan's) were determined by the use of SPSS statistical software, the 19.0 version. Thereafter, the scores of both qualitative and quantitative characters were used as characters to perform a cluster analysis on the five cultivars, each of which was taken as an operational taxonomic unit (OTU). A dendrogram was constructed using PAST statistical software (Hammer et al., 2001) by adopting a hierarchical cluster analysis using Ward's method applying squared Euclidean Distance. Using both the qualitative and quantitative morphological characters obtained, a dichotomous key was constructed for the purpose of diagnosing the five varieties.  Qualitative leaf morphological features in the five cultivars of Capsicum studied were fairly constant, all the leaves being simple, lanceolate in shape with sparse to fair pubescence, entire margin, pinnate venation, acuminate tip, and oblique-cuneate to cuneate base. On the other hand, the features of the fruits and the seeds (Figure 1 and 2; Table 2) were fairly diagnostic. While the seed shape was generally discoid and flattened, the fruit shape ranged from campanulate in C. chinense to blocky in C. annuum and elongate in the other three cultivars of C. fructescens. The seed colour observed for most of the cultivars was straw, with those of C. annuum and C fructescens var. sombo being brown and cream-brown respectively. Surface texture was generally smooth except for the seeds of C. annuum which had rough texture (Table 2).  The quantitative results of leaf morphology are presented in Table 3 while those of the fruits and seeds are in Table 4. Mean lamina length which ranged between 6.61 cm (in C. fructescens var. ijosi) and 8.92 cm (in C. fructescens var. bawa), showed a significant difference across the five cultivars studied, but this was not observed for the other four leaf morphological features (Table 4). The mean fruit length and width were also observed to be diagnostic among the cultivars of Capsicum studied. C. fructescens var. ijosi had the shortest fruit length (i.e. 1.21 cm), a mean value which was significantly shorter than the fruits in the others, while C. fructescens var. bawa with mean fruit length of 10.70 cm was significantly longer than the fruits of the others. The highest fruit width with the mean value of 3.20 cm was observed in C. annuum, which along with that of C. chinense were significantly wider than the other fruits observed. Lastly, C. fructescens var. ijosi which recorded the shortest mean fruit value (0.60 cm) was also next to C. fructescens var. sombo (0.58 cm), the fruits of both, being significantly narrower than the other three (Table 4).  Figure 1 shows the dendrogram obtained when a cluster analysis was performed on the morphological characters from leaves, fruits and seeds of the five cultivars of Capsicum studied. From this dendrogram, three groupings are evident at 66.67% distance as follows: a direct (same level) cluster of C. fructescens var. bawa and C annuum; a middle branch containing C. chinense alone; and a first branch of C. fructescens var. sombo and C. fructescens var. ijosi. The dendrogram is suggestive of some closeness in distance between C. chinense, C. fructescens var. bawa and C. annuum as compared to C. chinense and the other two cultivars.

Biosystematic implications of the morphological features observed in Capsicum
Relatively little work had been carried out on the morphology of the Nigerian species of Capsicum. Evaluation of the morphological and leaf epidermal features of C. annum and C. frutescens carried out by Nwachukwu et al. (2007) showed certain characteristics that were of some taxonomic value. According to these authors, the vegetative features of habit and height of C. annum separated it from C. frutescens. Okwulehi and Okoli (1999) and Edeoga and Eboka (2000) had earlier used comparative morphology of different species of Capsicum in establishing relationship among various taxa while Okeke and Nwachukwu (2001) reported morphological markers in the family Euphorbiaceae. The results of the present study are partly at variance to those of Nwachukwu et al. (2007) because the authors described the leaf tips and leaf bases of C. annuum and C. fructescens as mucronate and round respectively. This probably may be as a result of misidentification or use of improper description chart. Some taxonomists are of the opinion that much of the proliferation of synonyms in Capsicum had been due to differences in fruit characters. The findings of the present study with regard to morphological features appear to be in consonance with this position. There seems to be no agreement yet with regard to the number of species of Capsicum present in West Africa. Wilson (1959Wilson ( , 1961 agreed to the presence of only two species, C. annum and C. frutescens while some other taxonomists proposed that all other purportedly recognized species were forms of either C. annum or C. frutescens. It can be deduced from Figure. 1 that morphological variations between the cultivars studied may have some relation to their quality of Capsaicin contents. Based on their fruit taste (hotness), the five cultivars can be listed in increasing order of their hotness (and hence, capsaicin content) as C. annuum, C. fructescens var. bawa, C. chinense, C. fructescens var. sombo and C. fructescens var. ijosi (Nwokem et al., 2010) Interestingly, the cluster obtained followed this arrangement (Figure 1) with ijosi and sombo clustering as hot-taste cultivars, bawa and C. annuum clustering as mildtaste cultivars while C. chinense alone clustered as a cultivar with intermediate taste. This finding supports that of Adepoju et al. (2019) in which two Nigerian pepper varieties of C. fructescens (ijosi and sombo) formed a taxonomic cluster, different from the cluster formed by bawa, annuum and chinense, based on their seed protein profiles.
The infrageneric classification of Capsicum proposed by McLeod et al. (1982) based on isozyme data and flower colour suggested two groups; C. annuum as one; and other species (fructescens and chinense) as the other. This study partly agrees with the findings of McLeod et al. (1982) in that C. chinense clustered separately as a species and with just one of the varieties of C. fructescens, while C. annuum was distinct. However, contrary to the findings of McLeod et al. (1982), this study reveals that C. annuum may be closer to C. chinense than to C. fructescens, two varieties which clustered far away from C. annuum.
Based on the morphological results obtained, from this study, qualitative and quantitative features of the fruits and seeds in particular can be said to be of some value for both classificatory and diagnostic purposes in the genus Capsicum. Table 5 is a morphology-based key, usable for the identification of the five cultivars studied.

CONCLUSION
Evaluation of vegetative and reproductive morphological features of Nigerian Capsicum has perfectly confirmed extant clusters of taxa based on their fruit capsaicin content (which is responsible for fruit hotness). It has thus assisted to reaffirm earlier taxonomic groupings of C. chinense as a separate species from C. annuum but not C. fructescens from the two. This study has established some concordance between fruit capsaicin content in Capsicum and infrageneric taxonomic groupings based on conventional morphological characters. Variations in vegetative and reproductive morphological features in Nigerian species of Capsicum have hereby been documented in form of unambiguous artificial key for proper identification of the taxa.