Solenostemon rotundifolius [(Poir.) J. K. Morton] (Lamiaceae), is commonly called Chinese potato, Innala, Hausa potato, Zulu round potato, Sudan potato, Saluga, Fabirama or Frafra potato. It is believed to have originated from Central or East Africa but spread throughout tropical Africa and  into  South-east Asia.  It  is  cultivated as tuber crop in many African countries including Burkina Faso, Ghana, Nigeria, Togo, Mali (in west Africa), Cameroon and Chad (in central Africa) and some parts of South and East Africa (Schippers, 2000; Gouado et al., 2003; Sugri et al., 2013). In Asia, S. rotundifolius is reported to be cultivated in Sri Lanka, South India and Java (Jayakody et al., 2005).

S. rotundifolius is an annual herbaceous plant, 15 to 30 cm high, with ascending or prostrate stems and thick leaves having aromatic smell (Sugri et al., 2013). It is specially adapted to the Sahelian region of West Africa. Local varieties of S. rotundifolius produce many (up to 70/plant) small sized tubers; 3.78 cm long and 1.53 cm width (Nanéma et al., 2009). The potential yield reported in West Africa ranged from 7 to 20 T/ha (Enyiukwu et al., 2014). The tubers contain significant rate of reducing sugar (26 mg/100g), protein (13.6 to 14.6 mg/100g), crude fat (1.2%), crude fiber (1.6%), phosphorus (36 mg/100g), calcium (29 mg/100g), vitamins A and C, respectively 13.6 mg/100g and 10.3 mg/100g, and antioxidants (Anbuselvi and Balamurugan, 2013; Anbuselvi and Priya, 2013). They are commonly consumed as a curry, baked or fried, or cooked (Agyeno et al., 2014).  

Besides its nutritional attributes, S. rotundifolius holds strong economic potential and could be financially rewarding to the farm economy (Enyiukwu et al., 2014). During the period of availability, the quantity of tubers sold varied from 16 to 32 Kg/day/person in the main markets of Ouagadougou in Burkina Faso. According to the period and the tubers availability, the prices varied from 1.2 to 3 USD/Kg. This important variation of prices (250%) showed the economic potential of frafra potato (Nanéma et al., 2017).

S. rotundifolius is a very important food crop which can contribute to improving food security. Besides its agricultural importance, it has ornamental, medicinal, culinary and many other uses (Kwarteng et al., 2017). Therefore, as mentioned by Chivenge et al. (2015) and Mamadalieva et al. (2017), there are many neglected crops that have the potential to contribute to food security but investigation should be done to clearly demonstrate their potentialities and the priorities in term of research on these crops.  It is now a consensus that understanding variability within plant genetic resources is one of the key steps for a sustainable use and conservation of its potentialities. Some research activities were already carried out on S. rotundifolius germplasm and contributed to identify a set of useful traits that could be used as descriptors for this crop (Opoku-Agyeman et al., 2007;  Nanéma et al., 2009). Some other works focused on the identification of the main morphotypes within S. rotundifolius genetic resources based on tuber skin color (Prematilake, 2005; Sugri et al., 2013).

A description based on one trait could lead to some confusions between the morphotypes. The objective of this study is to contribute to a better description of the main morphotypes of S. rotundifolius cultivated in Burkina Faso based on a set of qualitative morphological traits. This will give some useful tools for the identification of the morphotypes for research activities on agronomic, economic or nutritional potentialities of S. rotundifolius.

Plant material

Three accessions (E02, E35 and E20) representing three morphotypes of S. rotundifolius (respectively A, B and C) were used for this study (Table 1). These accessions were identified based on morphological variation observed during our previous research on S. rotundifolius genetic resources. The accession E02 was collected in the province of Passoré in the North region of Burkina Faso. The accessions E35 and E20 were collected in the province of Nahouri in the South region of Burkina Faso. For each accession, 30 tubers were randomly selected for the experiment (Table 1).

Morphological traits observed at vegetative stage

Height morphological traits were observed during the vegetative stage. The color of stems (COSt) and the color of leaves (COL1) of young plant were scored at juvenile  stage  (Table  2).  During the plant growth, the foliage color (FCO), the thickness of leaves (TLE), and the color of apical leaves (CAL) were observed. The variation of leaves color when injured by insects (LCI), the main stem (SMS) and the secondary stems (SSS) stature were observed before flowering period.

Morphological traits observed at flowering stage and after harvesting

A total of seventeen morphological traits were observed on the inflorescence, the stems and the tubers after flowering. Nine of them were observed on the inflorescence. These traits were the number of flower buds per cluster (NFB), the color of flower buds (CFB), the type of inflorescence (TIN), the level of branching of inflorescence (LBI), the arrangement of clusters of flowers on the rachis (AFR) and the flowers persistence on the rachis (FPE). The color of sepals (COSe), and the color of petals (CPE) were observed on three randomly selected flowers per plant. The color of rachis (CRA) was also observed. Besides the morphological traits observed on the inflorescence, two were observed on the stems and the leaves. These were the shape of the section of main stems (SSS) and the color of leaves of adult plant (COL2). Six traits were observed on the tubers. After harvesting, the position of tuberous roots (PTR), the tuber shape (TSH), the tuber skin color (TSC), the tuber flesh color (TFC), the presence of lateral tubers (PLT) and the texture of tuber skin (TTS) (Table 2).

Data analysis

The individuals with missing data  were eliminated. A consensus value for each morphological trait was identified based on the individual score of each plant. The morphotypes were then described based on the consensus value for all the morphological traits.

Variability of the morphotypes at juvenile stage

A variability was observed between young plant of the morphotypes for stems and leaves color (Table 3). The stems and leaves of young plants of morphotypes “A” and “C” were green (Figures 1 and 2) while the young plants of morphotype “B” were red (Figures 3 and 4).

Variability of the morphotypes at vegetative growth stage

Variability was observed between the morphotypes at vegetative stage (Table 4). The color of foliage was green for morphotype “A”, dark green for morphotypes “B” and light green for morphotype “C”. The color of apical leaves did not vary from the color of whole foliage. No particular color was observed on the leaves when injured by insects.  The  morphotypes  also  varied  for  the  level  of leaves thickness. The leaves of the morphotype “A” were very thick while those of the morphotypes “B” and “C” were relatively less thick (Figure 5).

Variability of the morphotypes at flowering stage

The morphological traits observed at flowering stage revealed important variability between the morphotypes. The inflorescence was an apical spike. Some inflorescences with primary branching were observed for the morphotypes “A” and “B” (Table 5). The flower buds color was green with reddish pigmentation for the morphotype “B” (Figure 6) but it was green for the morphotypes “A” (Figure 7) and “C” (Figure 8). The number of flower buds per cluster was four for the morphotypes “A” and “C” but it was three for the morphotype “B”. The clusters of flowers were opposite on the rachis for all the morphotypes. The flowers buds and the flowers were no persistent on the rachis. After flowering, the color of rachis was reddish for the morphotype “A” but was green for the morphotypes “B” and “C”. The morphotype “A” developed elongated flowers with green-reddish sepals and white-purple petals (Figure 9). The sepals of the flowers of the morphotype “B” was reddish and the petals were very purple (Figure 10). For the morphotype “C”, the sepals were green and the petals were reddish (Figure 11). The morphological traits observed on the stems and the leaves after flowering did not vary. The section of stems was quadrangular for all the morphotypes and the leaves were dark green.

Variability of the morphotypes for the morphology of the tubers

The tuber skin and flesh color, and the tuber shape were the discriminant traits observed on tubers (Table 6). The tubers of the morphotype “A” were black skin (Figure 12) and the flesh color was white (Figure 13). The tubers of the morphotype B, skin color was red (Figure 14) and the tuber flesh color was white-yellow (Figure 15). The tubers of the morphotype “C” skin color was white-yellow (Figure 16) and the flesh color was yellow (Figure 17). Tubers were oblong for morphotypes “A” and ovoid for the morphotypes “B” and “C”.  For all the morphotypes, the tuberous roots were observed on the upper parts of the roots. The tubers were rough and some tubers presented lateral tubers.

In Asia, some varieties of S. rotundifolius produce hairy tubers (Jayakody et al., 2005). This difference could be due to genetic factor or to  growth  conditions. For  all  the morphotypes, many branched tubers were identified. Solenostemon rotundifolius produced small size tubers and the presence of branched tubers make peeling very difficult. According to Enyiukwu et al. (2014), branched tubers make the crop unattractive and reduce the marketability of the tubers.

The described morphotypes were identified in S. rotundifolius germplasm in Burkina Faso. Based on some morphological traits used in previous works, the three morphotypes were reported to be cultivated in West Africa (Sugri et al., 2013; Agyeno et al., 2014). The morphotype “A” and “B” are cultivated in Sri Lanka respectively as “Bola” and “Dik” (Prematilake, 2005, Jayakody et al., 2005). Some research activities highlighted the influence of genetic variability on tuber nutritional, medicinal potentialities and their behavior in conservation (Jayakody et al., 2005; Priya and Anbuselvi, 2013; Parmar et al., 2017; Mamadalieva et al., 2017; Azad et al., 2017). A clear classification of accessions used in future research on S. rotundifolius could contribute to a better analysis of the results. This include nutritional, agronomical and economical potentialities as well as genetic diversity.

Future research activities could also be focused on the existence of variability within the morphotypes. Other aspects could be the description of flower morphology. Similar studies already led to the revision of Borassus classification (Bayton et al., 2006).

This study described three morphotypes of S. rotundifolius cultivated in Burkina Faso based on a set of qualitative morphological traits. These morphological traits can be used as descriptors for S. rotundifolius. It is an important step towards full description of the morphotypes of this important tuber crop. The future studies on morphological variability could focused on flower structure and microscopic description of plant tissues. A complete description of the morphotypes will provide useful information for a sustainable management of genetic resources of S. rotundifolius.

The authors have not declared any conflict of interests.

This work was supported by the University Ouaga I Pr Joseph KI-ZERBO.