Characteristics of three morphotypes of Solenostemon rotundifolius [ ( Poir . ) J . K . Morton ] based on qualitative morphological traits

Solenostemon rotundifolius [(Poir.) J. K. Morton] (Lamiaceae) is commonly called Fabirama, Frafra potato or Innala. It is a tuber crop cultivated in many countries in Africa and Asia. Its tubers contain significant rate of carbohydrates, proteins, fibers, vitamins and antioxidants. Besides its nutritional attributes, S. rotundifolius has also strong agronomic and economic potentialities and could be financially rewarding to the farm economy. However, S. rotundifolius is currently a minor crop in African agro ecosystem. It is a priority to contribute to a better preservation and a sustainable use of its genetic resources. Understanding morphological variability is a key step for S. rotundifolius genetic resources management. Previous studies identified three morphotypes based on tuber skin color but a complete description of these morphotypes is not yet done. This study is a contribution to a better description of the main morphotypes of S. rotundifolius cultivated in Burkina Faso. Three accessions representing three morphotypes were described based on twenty five qualitative morphological traits assessed at different stages of plant growth. Significant variability between the morphotypes was observed for young plant color, leaves morphology, color and form of inflorescence as well as tuber skin and flesh color. This work provided useful tools for the characterization of S. rotundifolius genetic resources.


INTRODUCTION
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).
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).

Study area and experimental design
The research was carried out in the research farm of the Faculty of Earth and Plant Sciences of the University Ouaga I Joseph KI-ZERBO in Ouagadougou (12°21′56″ N; 1°32′01″ W).A total rainfall of 665.1 mm was registered during the period of the experiment (July 2016 to January 2017).The experiment was laid out in Fisher blocks with three replications.The replication consisted in three lines of height plastic buckets perforated at the bottom to improve drainage.Each plastic buckets (9 L) contained a mix of sun (1/3) and potting soil (2/3).The spacing was 50 cm between the lines, 40 cm between the plants and 50 cm between the blocks.One tuber was planted per bucket on 25 July 2016.An additional irrigation was bring after the rainy season from October 2016 to January 2017.

Morphological traits
A total of 25 qualitative morphological traits were assessed at different stages of plant growth.These traits were identified during our previous research activities.The different variants were identified and scored per plant.At all, the morphological traits were observed on 24 plants per morphotype.
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Stage Morphological traits
Young  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.

DISCUSSION
Three morphotypes of S. rotundifolius were described  The main stem was ascending or prostate for the morphotypes "A" and "B" and generally ascending for the morphotype "C".The secondary stems of all the morphotypes were prostrate.based on morphological traits observed on leaves, stems, inflorescences and tubers.At juvenile stage, the presence red coloration on young plant was identified to be the main difference between the morphotypes.The variation  morphotypes "A" (Figure 7) and "C" (Figure 8).(1938), the reddish color observed on Lamiaceae species are due to the presence of anthocyanins (mainly cyanidine saccharides).
During the vegetative growth stage, the main variation was observed for leaves thickness and the stature of main stem.All the morphotypes produced thick leaves.As suggested by Edison et al. (2006), leaves morphology is an important parameter for water regulation in plant tissues.The strong thickness of leaves of the morphotype "A" could be considered as a particular adaptation potential of this morphotype to water scarcity conditions.Besides the leaves morphology, the stature of stems was identified as a discriminant parameter.Previous research activities revealed the presence of pigmentation on S. rotundifolius leaves (Agyeno et al., 2014).
The most significant discriminant traits were observed on the inflorescence.These parameters can be sufficient to identify the morphotypes.Previous works on S. rotundifolius variability mentioned the color of the petals of the morphotypes "A" and "B" (Nanéma et al., 2009).This work is the first report on the flowering of the morphotype "C".The other morphological traits observed after flowering did not reveal differences between the morphotypes.It was the stem section, that was quadrangular and the color of leaves that was green.The quadrangular stem section of S. rotundifolius was also mentioned by Agyeno et al. (2014).However, variability of stem section can be observed within the same morphotype (Nanéma et al., 2009).Pigmentation on leaves was also mentioned at flowering stage.These traits seem to be influenced by S. rotundifolius growth conditions.
In addition to the tuber skin color, variability was observed for tuber flesh color.Current literature mentioned three morphotypes according to the tuber skin color: -the morphotype with white skin tuber; the morphotype with red skin tuber; and the morphotype with black or brown skin tuber (Prematilake, 2005;Nanéma, 2010;Sugri et al., 2013).Chevalier and Perrot (1905) mentioned these morphotypes respectively as varieties "alba", "rubra" and "nigra".These authors suggested the presence of a fourth variety with black skin tuber called "javanicaminum".Some research activities were carried out on S. rotundifolius based on the tuber skin color as the main criteria of identification of the varieties (Jayakody et al., 2005;Priya andAnbuselvi, 2013, Taye et al., 2012).After harvest, the tuber skin color is the most accessible criteria for identification of the morphotypes but any error of color appreciation could lead to ambiguous results.Additional morphological traits could be the tuber flesh color and tuber shape.
The tubers of all the morphotypes were rough and glabrous.Other research activities in West Africa also mentioned the lack of hairiness on tubers of S. rotundifolius (Opoku-Agyeman et al., 2007;Agyeno et al., 2014).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).

Conclusion
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.
leaves (CAL) Variation of leaves color when injured by insects (LCI) Stature of main stem (SMS) Stature of secondary stems (SSS) Flowering stage Number of flower buds per cluster (NFB) Color of flower buds (CFB) Type of inflorescence (TIN) Level of branching of inflorescence (LBI) Arrangement of clusters of flowers on the rachis (AFR) Flowers persistence on the rachis (FPE) Color of sepals (COSe) Color of petals (CPE) Color of rachis (CRA) Shape of the section of main stems (SSS) Color of leaves of adult plant (COL2) After harvesting Position of tuberous roots (PTR) Tuber shape (TSH) Tuber skin color (TSC) Tuber flesh color (TFC) Presence of lateral tubers (PLT) Texture of tuber skin (TTS) 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

Figure 5 .
Figure 5. Thickness of leaves of S. rotundifolius; a: leaf of morphotype A; b: leaf of morphotype B; c: leaf of morphotype C.The main stem was ascending or prostate for the morphotypes "A" and "B" and generally ascending for the morphotype "C".The secondary stems of all the morphotypes were prostrate.

Figure 7 .
Figure 7. Spike of morphotype A of S. rotundifolius presenting flower buds; a: flower buds.

Figure 9 .
Figure 9. Spike of the morphotype A of S. rotundifolius sowing flowers; a: flower.

Figure 10 .
Figure 10.Spike of the morphotype B of S. rotundifolius sowing flowers; a: flower.

Figure 11 .
Figure 11.Spike of the morphotype C of S. rotundifolius sowing flowers; a: flower.

Figure 12 .
Figure 12.Black skin color of tubers of the morphotype A.

Figure13.
Figure13.White flesh color of tubers of the morphotype A.

Figure 14 .
Figure 14.Red skin color of tubers of the morphotype B.

Figure15.
Figure15.White-yellow flesh color of tubers of the morphotype B.

Figure 16 .
Figure 16.White-yellow skin color of tubers of the morphotype C.

Figure 17 .
Figure 17.Yellow flesh color of tubers of the morphotype C.

Table 1 .
List of the accessions used for the morphological characterization of the morphotypes of S. rotundifolius.: A, B and C are names of the morphotypes of S. rotundifolius; E02, E35 and E06 are accessions' numbers in the genes bank of the University Ouaga I Pr Joseph KI-ZERBO (Burkina Faso); GPS: Global Positioning System. Legend

Table 2 .
Morphological traits observed on the morphotypes of S. rotundifolius.

Table 3 .
Variability between morphotypes at juvenile stage.

Table 4 .
Variability between morphotypes at vegetative growth stage.

Table 5 .
Variability between morphotypes at flowering stage.

Table 6 .
Variability of the morphotypes for the morphology of the tubers.