INTRODUCTION
Cassava (Manihot esculenta
Crantz) from the Euphorbiaceae
family is an important tuber crop cultivated in many countries around the world
and more particularly in West Africa (N’Zué et
al., 2014; Agré et al., 2015). It draws
its importance firstly from its tuberous roots which are rich in starches and
its leaves which are rich in protein. It remains an undeniable asset for a
region that is less and less self-sufficient in terms of food (Gnonloufin et al.,
2011). It also draws its importance from its great ease of cultivation and its
various energy products. Ethanol obtained by starch fermentation could
completely replace gasoline (N’Zué et al., 2014; Agré
et al., 2015; Adjebang-Danguah
et al., 2016).
In Côte d’Ivoire, cassava represents the second food crop after yam with
an annual production of around 6.3 million tonnes (Faostat,
2022). This crop is well integrated into all cropping systems. The Ivorian
people have made it a staple food and there is a great varietal diversity favoring different types of local dishes (attiéké, bêdêkouman, placali, etc.). In production areas, cassava accessions are
designated by vernacular names linked to the phenotype or the name of the place
of origin or of a person who introduced it into a locality. This method of
designation often gives rise to confusion, since the same accession can have
different names depending on the production area. Also, cultural practices
linked to the use of cuttings infected by viruses from year to year promote the
progressive loss of varietal diversity of cassava (Amoakon
et al., 2023).
As well, the abandonment of certain traditional or local accessions by
farmers due to factors such as their low yield, their long production cycle and
their susceptibility to diseases (viruses, anthracnose, root rot) and pests
(mites, mealybugs, nematodes), contribute largely to the loss of cassava
genetic resources. In addition, the loss of soil fertility, the action of bush
fires, the difficulty of supplying healthy planting material among others cause
the decline in national yield. This constitutes a major threat to food
security, particularly for small farmers practicing subsistence agriculture (Segnou, 2002). These farmers, contribute to the spread of
viral-infected material through the exchange of infected cuttings, leading to a
constant reduction in yields (Singa et al., 2008). The result is a gradual
abandonment of local accessions in favor of improved
ones, leading to the erosion of local cassava genetic resources. However, the
preservation of these local accessions constitutes an important issue for
sustainable agricultural development in Sub-Saharan Africa. In the current
context of climate change, one of the strategies likely to combat this erosion involves
the collection, analysis and organization of the existing diversity in
production areas. This allows not only knowledge of the existing cultivars, but
also to guide the methods of conservation and management of these genetic
resources in varietal improvement programs.
The study of the diversity of accessions from the South of Côte d'Ivoire
will allow us to better understand the genetic diversity of these local
accessions and the agronomic potential in order to better exploit them in a
varietal improvement program to increase national production. Many studies of
characterization have been carried out on the genetic resources of cassava from
Côte d’Ivoire (N'Zué 2007; N'Zué
2014; Djaha et
al., 2017; Doubi et al., 2021; Kouakou et al., 2022; Kouakou et al.,
2023), however, cassava accessions newly collected in farmers' fields in the
regions of South of Côte d'Ivoire had not yet been characterized in terms of
morphological and agronomic traits.
Thus, the general objective of the present study was to study the agromorphological diversity of cassava accessions
cultivated in the South of Côte d'Ivoire for their efficient exploitation.
Specifically, the aim was to (i) characterize the
accessions on an agromorphological level and (ii)
establish the structure of the agromorphological
variability for rational conservation and management purposes.
MATERIALS AND METHODS
Site of study
The study was conducted in Toumodi region of
Côte d'Ivoire on the experimental site of Swiss Center
for Scientific Research (SCSR, Côte d’Ivoire). The experimental site is located
at 6°25’0’’ N et 5°4’60’’ W and the area of the study is
a transition zone, located between forest and savanna. The climate is
equatorial, with a big rainy season starting from March to July and a short one
starting from August to October. The average annual rainfall is 1200 mm of rain
spread over 5 to 6 months. The soil is ferralitic and
rich in humus.
Plant Material
The plant material consists of 47 local accessions of cassava collected
in the south of Côte d'Ivoire from farmers in the areas of Abidjan, Bonoua, Grang-Lahou, Aboisso and Dabou. These
accessions were referenced by their vernacular names given by the farmers. The number of accessions collected in the regions are summarised in the
table 1.
DISCUSSION
Plant breeding based on classical methods uses
morphological traits of plants growing in the field as basis for identification
(Fukuda et al., 2010; Robooni et al.,
2014). It has been effectively used as a powerful tool in the classification of
cultivars and the study of their taxonomic status (Elisabeth, 2011). Also,
collection characterization based on agromorphological
traits is the preliminary phase for molecular studies to be much more precise. Agromorphological characterization are still used successfully in numerous characterization
and agronomic evaluation studies, thus allowing easier and faster
differentiation of phenotypes especially in cassava (N’Zué
et al., 2014, Setiawan and Sebayang, 2022; Diaguna
et al., 2022; Kouakou
et al., 2023). This study permitted
to evaluate the level of cassava variability newly collected in farmers' fields
in the regions of South of Côte d'Ivoire by using morphological and agronomic
traits.
The analysis carried out showed a significant variation within the studied
accessions, thus allowing a preliminary assessment of the structuring of the
diversity of cassava collected in the South of Côte d'Ivoire.
Multivariate analysis are statistical methods used in diversity analyses.
In this study, multivariate analysis was used to elucidate the nature and
degree of divergence of the cassava accessions. The PCA, applied to the 47
accessions on the basis of characters, results in the existence of a total
variability of 75.04% within the cassava collection. This variability is
greater than that observed by Ephrem et
al. (2014) in Central African Republic and Kouakou
et al. (2023) in Côte d’Ivoire based
on agromorphological characterization. Indeed, these
authors respectively obtained a variability of 55% for the first six cumulative
axes and 57.45% for the first four cumulative axes having eigenvalues greater
than 1. This high variability within the cassava accessions in the area of
Southern Côte d'Ivoire could be explained by regular introductions of cassava
varieties from neighboring countries of Côte d'Ivoire
such as Ghana, Togo and Nigeria, by cultivation practice based on use of
several assessions in the same field and the continuous exchanges of plant
material between farmers from different localities (Missihoun
et al., 2012; Ferguson et al., 2019; Kouame et al., 2023).
Ascending Hierarchical Clustering (AHC) performed in this study revealed three groups or three morphological
clusters. Similar results on cassava were obtained by N’Zué et al. (2014), Djaha et al. (2017), and Kouakou
et al. (2022, 2023) who obtained
three clusters by assessing respectively the morphological diversity of 159, 44
and 200 cassava accessions from Côte d’Ivoire. These results therefore indicated that cultivated
cassava in Côte d'Ivoire could be grouped into three
distinct genetic clusters. Three distinct genetic clusters were obtained
again by Yusuf et al. (2016) by
assessing the morphological diversity of 22 cassava accessions from several
regions in Riau province, Indonesia. Swathy et al. (2023) obtained similar results
in India.
Within these three groups, we found accessions collected in different areas
under the same names but phenotypically different. Indeed, within villages,
exchanges of plant material are frequent between producers and most often
different accessions have the same vernacular name or that several names could
be given to a single cultivar on the farm as reported by Elias et al. (2001). This situation is the
basis of several duplicates among the cassava accessions as detected by N’Zué et al. (2014)
in collection from three zones in Côte d'Ivoire.
The variables that discriminated the three
clusters were the height of the plant, the
number of roots per plant and the length of the petiole. Our results are similar to those of Agre et al.
(2015) who through the same quantitative parameters revealed a large
variability within 116 cassava genotypes from Benin. Djaha
et al. (2017) also showed that in
their study on 44 cassava accessions, a total variability based on the height
of the first branch, plant height, number of lobes and petiole length. These
results show the relevance of the choice of the different parameters in our
study.
CONCLUSION
The study of the diversity of 47 cassava accessions
based on 8 descriptors showed significant variability. This diversity was
structured into 3 groups characterized by the height of the plant, the length
of the petiole and the number of roots per plant. Thus, the collection of
accessions from the South of Côte d’Ivoire has different groups of cassava. A
group of plants with height (305.57 cm), with an average number of roots per
plant (6.31) and a long petiole (24.84 cm). A groups of cassava accessions with
a low number of roots per plant (4.17) and small size (260 cm) with small
petiole (17.07 cm) and a group of intermediate sized (294 cm) with a high
number of roots per plant (7.68) with medium-sized petioles (18.45 cm). This
description highlights the heterogeneity that exists within the cultivars held
by farmers. The significant diversity highlighted offers great potential for
the varietal improvement of cassava. This diversity could be used for food as
well as for the development of varieties with multiple uses. The results of
this study also showed that some accessions with the same vernacular names are
genetically different. The study of the genetic diversity of cassava accessions
from South of Côte d'Ivoire, through molecular markers, would deepen our
knowledge of this genetic pool. However, it would be important to extend the
prospecting area throughout the country in order to discover other accessions
of cassava with characters of interest which could be valued in a varietal
selection program. Also, complementary studies like molecular characterization
are necessary.
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