Advances in Bioscience and Biotechnology, 2024, 15, *-*
https://www.scirp.org/journal/abb
ISSN Online: 2156-8502
ISSN Print: 2156-8456
Genetic Diversity in Cowpea (Vigna unguiculata
(L.) Walp) under Two Growing Conditions*
Omotola Oluwakemi Dairo1,2
Department of Plant Breeding and Seed Technology, Federal University of Agriculture, Abeokuta, Nigeria
College of Education and Liberal Art, Wilmington University, Delaware, USA
Email: Omotee.dairo20@yahoo.com
1
2
Abstract
How to cite this paper: Dairo, O.O. (2024)
Paper Title. Advances in Bioscience and
Biotechnology, 15, *-*.
https://doi.org/10.4236/***.2024.*****
This study explores the use of genetic variability for advancing the genetic
improvement of Cowpea (Vigna unguiculata (L.) Walp), particularly in response to insect infestation stress. Over a period spanning 2015 to 2017, forty
accessions of cowpeas were evaluated to determine their variability under
both insecticide spray and no insecticide spray conditions at the Teachings
and Research Farms, Federal University of Agriculture, Abeokuta. The experimental design was a randomized complete block design in three replicates.
The accessions were evaluated for plant height, leaf length, leaf width, number of days of 50% flowering, number of pods per plant, pod length, number
of seeds per plant, 100-seed weight, and seed yield. Data collected were subjected to principal component and single linkage cluster analyses. Principal
axis I (PCA1) accounted for 39% and 35% under insecticide spray and no insecticide spray respectively to the total variation in the accessions. Plant
height with a factor score of 0.38, leaf length (0.41), number of leaves (0.37),
and 100-seed, weight (0.30) was related to PCAI under insecticide spray while
leaf width (0.32). Pod length (0.37) and number of seeds/plant (0.38) were
significant to PCA1 under no insecticide spray. Notably, accessions such as
SAMPEA6, SAMPEA10, IFE-Brown, and IFE-BPE exhibited consistent performance across both conditions, while others displayed condition-specific
attributes. For instance, NGB1063, NGB1152, and NGB1093 demonstrated
distinct traits under insecticide spray, while NGB1146 and NGB1124 exhibited notable characteristics under no insecticide spray conditions. Therefore,
identifying these forty accessions with desirable traits hold promise for future
genetic improvement efforts of cowpea cultivation in Nigeria and beyond.
Received: **** **, ***
Accepted: **** **, ***
Published: **** **, ***
Copyright © 2024 by author(s) and
Scientific Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
Keywords
Cowpea, Vigna Unguiculata, Genetic Diversity, Cowpea Accessions
*
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Breeding program to improve cowpea genetics.
**** **, 2024
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1. Introduction
Cowpea (Vigna unguiculata) is an important crop in many countries of tropical
Africa, Asia, and South America. The grains and leaves are rich and cheap sources
of protein. They are used alongside cereal and root vegetables in tropical Africa
[1] [2]. On average cowpea grains contain 23% - 25% protein and 50% - 67%
starch on dry matter bases [3]. Cowpea has several products such as immature
pods, and immature and mature seeds from a single planting. Careful and positive attention to cowpeas would support 850 million people in sub-Saharan Africa
with high incidences of undernourishment [4] [5]. Also, cowpea has many beneficial and desirable horticultural characteristics usually non-food associated. It is
also an efficient nitrogen-fixing, heat and drought-tolerant plant.
In most African countries, cowpea is either grown alone or intercropped with
various other crops such as maize, millet, sorghum, beans, pigeon peas, and cotton [6] [7]. In intercropping production systems, when grown alongside other
crops, cowpeas spread out and help stop weeds from growing, protecting the soil
from washing away. In addition, some types of cowpeas stop a harmful plant
called Striga hermonthica from growing on cereal crops [8].
Genetic diversity is particularly important for agricultural productivity and
development [9]. Despite its importance, genetic diversity within cowpea remains relatively narrow, highlighting the need for comprehensive studies to elucidate the genetic variation and potential for improvement [7]. This paper aims
to investigate the genetic diversity and characterize cowpea accessions to facilitate breeding programs for enhanced crop performance and adaptation.
Genetic diversity refers to the diversity within species and is the foundation of
the genetic improvement of crops. Genetic diversity serves as a way for crops to
adapt to changing environments. Lately, there have been attempts to save and
study this variety to improve breeding programs, but more research is still
needed [10]. The study on genetic diversity in cowpeas is essential for accessing
and exploiting genetic variation within the species, thereby justifying introgression and ideotype-breeding programs to enhance crop performance [11]. The
first step of any breeding program is to identify crop plants that exhibit exploitable variation for the traits of interest [12]. A quantitative assessment of the genetic divergence among germplasm and the contribution of different traits to genetic divergence provides an essential and effective contribution of different traits
to genetic divergence provide essential and effective information to a breeder in
his hybridization program and thereby genetic improvement of yield.
The primary objectives of this study are to determine the genetic diversity
among forty accessions of cowpea under two growing conditions (Insecticide
and Non-Insecticide) and to identify the characters contributing to genetic variation among these accessions.
2. Literature Review
Cowpea (Vigna unguiculata. Walp) is an important food and fodder legume in
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the world cultivated in the tropics and subtropics and it is a major food crop in
Africa, Latin America, and India because it is high in protein, which is good for
improving plants [13]. The average yield of cowpeas in West Africa was estimated as 483 kg/ha [14]. World production was estimated at 11.8 million ha
with 5.4 million tons of dried grains; Africa alone provides 91% of the global
production thus 10.7 million ha from West Africa [15].
Germ-plasm assessment is very important for gene bank managers since it allows more efficient sampling of available resources for crosses and also removes
obvious duplicate materials. Diversity in the available gene pool is the foundation of all plant improvement programs. Cowpea is a particularly valuable component of low-value input farming system for resource-poor-farmers because of
its productivity and high yield stability in the face of abiotic stress (drought,
heat, low soil fertility), and the ability of the crop to enhance soil fertility for
succeeding cereal of tuber crops grown in rotation with its greater tolerance to
heat, drought, and low soil fertility [16]. Nevertheless, the potential production
yield is still below 50%, despite its widespread cultivation. There remains untapped potential for yield improvement and adaptation to environmental stresses.
Genetic diversity assessment is crucial for efficient sampling of available resources and forms the foundation of all plant improvement programs.
2.1. Origin, Domestication and Distribution
Cowpea, among the oldest crops, has sparked debates about its origin. Some
people believe that cowpeas originated from West Africa because both wild and
cultivated species abound in the region. Others believe that it originated in Southern Africa. Its production has spread to East and Central Africa, India, Asia, and
South and Central America [17]. Cowpeas are grown in more than 100 countries
around the world, and Nigeria is the top producer, making 6.5 million metric
tons each year [13].
Nowadays, it is widely cultivated globally and consumed by a lot of people
[18]. A lack of archaeological evidence has resulted in contradicting views supporting Africa, Asia, and South America as origin. Some literature indicates that
cowpea was introduced from Africa to the Indian subcontinent approximately
2000 to 3500 years ago, at the same time as the introduction of sorghum and
millet [19]. While others suggest it reached Europe and North Africa from Asia
before 300 BC. Speculations are that the Northern part of the Republic of South
Africa (former Transvaal region) was the center of speciation of V. unguiculata,
determined by the presence of the earliest primitive wild varieties. They further
hypothesized that the species moved northwards from the Transvaal to Mozambique and Tanzania, where the subspecies pubescence evolved. Cowpea now
thrives in various climates, nourishing about 110 million people worldwide. The
species V. unguiculata includes domesticated forms, i.e., V. unguiculata ssp. un-
guiculatavar. unguiculata, wild annual forms, i.e., spp. unguiculata var. spontanea (Schweinf.), and 10 wild perennial subspecies [20] [21].
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Domesticated cowpea includes five cultivar groups Morphological analysis
contrasted evolved cultivars and more primitive cultivars according to seed size,
on the one hand, and early and late flowering (under inductive conditions) on
the other hand. The latter character was markedly correlated with the ovule
number. Photosensitive and early-flowering photo-independent cultivars had 11
- 17 ovules per pod and late-flowering photo period-independent cultivars had
16 - 25 ovules per pod [3]. However, this organization of the domesticated gene
pool was poorly correlated with isozyme data, which only showed more diversity
in primitive cultivars [22].
The African origin of cowpea was suggested by Richard as early as 1847 [23],
and since Piper discovered the wild prototype of cowpea in 1913 [24], no one
has contested it since wild cowpea plants are found only in tropical Africa and
Madagascar, but not in Asia [25]. However, where the crop was first domesticated is still uncertain, and different centers of diversity and origin of the cowpea
have been proposed, i.e., Ethiopia [22] [26] [27], West Africa [28]-[33], and
Eastern and Southern Africa [34]. “Diffuse” domestication in the savanna after
the dispersal of cereals was also hypothesized [2] [10] [35]. Cowpea’s tough nature, able to thrive even in poor soils and partial shade, makes it a favorite for
farmers who mix it with cereals and rotate crops [36].
2.2. Classification of Cowpea
Verdcourt [37] and Marechal et al. [30] classified cowpea as follows;
Kingdom: Plantae
Order: Fabales
Family: Fabaceae
Subfamily: Faboideae
Genus: Vigna
Species: V. unguiculata
Cowpea belongs to the genus Vigna and is classified under the Fabaceae family. The Vigna genus includes multiple species, yet the precise count varies according to different authors. All cultivated cowpeas are grouped under V. unguiculata, which is further divided into four semigroups: Unguiculata, Biflora,
Sesquipedalis, and Textiles [30] [38] [39].
2.3. Uses
Cowpea seeds serve as a nutritious dietary staple for both humans and livestock.
Bressani reported nutrient content of cowpeas includes protein (24.8%), fat
(1.9%), fiber (6.3%), carbohydrate (63.6%), thiamine (0.000074%), riboflavin
(0.000042%) and niacin (0.002 81%) [40]. Cowpeas can also gain advantages by
using their wild relatives that can breed with them, similar to what was seen with
common beans [18]
Vigna unguiculata ssp. sesquipedalis offers a special type of vegetable pod that’s
vine-like, keeps growing, and stays fresh for a long time [41]. The young green
leaves serve as a vital food source in Africa and are cooked and eaten like spiDOI: 10.4236/***.2024.*****
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nach. The undeveloped pods are utilized much like snap beans, often mixed into
various foods. The green cowpea seeds are commonly boiled and enjoyed as a
fresh vegetable, or they can be preserved through canning or freezing processes.
Furthermore, the dry, mature seeds of cowpeas are suitable for cooking and canning purposes alike. In many areas of the world, cowpea is the only available
high-quality legume hay for livestock feed. Cowpeas may be used in green or as
dry fodder. It is also used as a green manure crop, a nitrogen-fixing crop, or for
erosion control [42]. It is very good for quick growth and establishment, and for
increasing organic matter and improving soil structure. It has excellent heat tolerance and good drought tolerance. It can also be used for intercropping with
other main crops like pearl millet (Pennisetum glaucum) or sorghum (Sorghum
bicolor).
2.4. Cowpea Characterization
The trade of both seeds and processed cowpea foods presents lucrative opportunities for individuals in both urban and rural settings to generate steady income. Beyond its importance for food and feed, the spreading indeterminate or
semi-erect bushy cowpea varieties provide ground cover, thus suppressing weeds
and providing some protection against soil erosion.
The role of the crop in soil fertility restoration as well as its compatibility with
many crop mixtures, has made it a common component of most cropping systems of the savanna zone of tropical Africa. The roots and root nodules decay to
enrich the soil for the benefit of the subsequent crop. As a result of this, cowpea
forms an important component of most cereal-legume cropping systems.
Another important feature of cowpeas is that they fix atmospheric nitrogen
through symbiosis with nodule bacteria (Brady rhizobium spp.). In so doing, it
provides 80% - 90% of its nitrogen requirements and allows adequate yields in
nitrogen-deficient soils where non-nodulated crops such as cereals fail. Besides,
cowpea is a drought-tolerant crop, capable of producing reasonable yield where
other legumes and cereals fail.
Cowpeas yields are low because the environments where they are produced
are characterized by various abiotic and biotic stresses. However, even under optimal conditions, the yields are variable and unpredictable, partly due to variability in the growth and development of individual plants. Understanding the
extent, distribution, and nature of this variation would be useful in the development of cowpea genotypes within the increased yield potential and improved
adaptation to environmental stresses. Phenotypes and genetic diversity can be
evaluated using morphological characters, and biochemical or molecular markers (DNA markers).
3. Materials and Methods (Experimental Materials)
Forty (40) accessions of cowpeas collected from the Germ-plasm Unit of the National Centre for Genetic Resources and Biotechnology (NACGRAB), Ibadan, were
used for the study in Table 1. The genotype included four cultivated materials.
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Table 1. Species of the cowpea accessions used in the study.
Species and Ascessions
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Ascessions
Species
NGB964
V. spontanea
NGB1070
V. spontanea
NGB1132A
V. spontanea
NGB1152
V. spontanea
NGB963
V. spontanea
NGB1044
V. spontanea
NGB1058
V. spontanea
NGB1105
V. spontanea
NGB1115
V. spontanea
NGB1126
V. spontanea
NGB1090
V. spontanea
NGB1109
V. spontanea
NGB1027
V. spontanea
NGB1028
V. spontanea
NGB1094
V. spontanea
NBG1127
V. spontanea
NGB1163
V. spontanea
NGB1014
V. spontanea
NGB1113
V. spontanea
NGB1118
V. spontanea
NGB1140
V. spontanea
NGB1069
V. spontanea
NGB1088
V. spontanea
NGB1093
V. spontanea
NGB1160
V. spontanea
NGB1162
V. spontanea
NGB1063
V. spontanea
NGB952
V. spontanea
NGB1087
V. spontanea
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Continued
NGB1124
V. spontanea
NGB1153
V. spontanea
NGB1116A
V. spontanea
NGB1146
V. spontanea
NGB1164
V. spontanea
NGB1168
V. spontanea
NGB1173
V. spontanea
Ife BPE
V. spontanea
Ife Brown
V. spontanea
SAMPEA10
V. spontanea
SAMPEA6
V. spontanea
a. The information presented in Table 1 was collected and compiled as part of the cowpea
experiment.
3.1. Experimental Site and Field Design
The experiment was carried out at the Research Farm of the Federal University
of Agriculture, Abeokuta (FUNAAB). Abeokuta is located in a forest transition
zone and the experimental site falls on latitude 3˚23'E and longitude 7˚20'N. The
experimental field, 30 m × 40 m (1200 m), was cleared manually with the use of
cutlasses and hoes and plowed manually with the use of a hoe. It was laid out in
a randomized complete block design with three replicates. Each block, 7 m × 40
m was divided into two groups of 40 single-row plots. The plots were 2.5 m long,
separated at 0.75 m, and were assigned randomly and independently within the
groups. One group was controlled for insect pests while the other was not. The
groups were 2 m apart within the blocks.
Field experiment was carried out and seeds of the accessions were planted per
hill along the rows at 0.50 m apart and the emerging seedlings were thinned to
one plant per hill at 2 weeks after sowing. A total of 5 plants were maintained
per plot. Metaforce at 60 ml /20L of water was sprayed at 2 weeks after planting
and subsequently. No fertilizer application was done and weeding was done as
necessary.
3.2. Character Evaluation
At 50% flowering till maturity, the three inner plants along the row were measured
for yield and yield-related characters. Eleven characters were evaluated and they included plant height, leaf length, leaf width, number of leaves, days to 50% flowering,
number of pods per plant, pod length, number of seeds per pod, seeds per plant,
100-seed weight, and overall seed yield in Table 2. Measurement was done accordingly using meter rule vernier calliper and sensitive electronic weighing balance.
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Table 2. Eleven quantitative characters were evaluated in the cowpea accessions.
Element Quantitative measure
Elements
Measurement
Plant height (cm)
Measured from ground level to the shoot apex of the plant.
Leaf length (cm)
Measured from the tip of the leaf to the point of attachment to the petiole.
Leaf width (cm)
Measured as the widest part of the leaf.
Number of leaves
Was counted.
Days to 50% of flowering
Days from sowing till the appearance of flowers on 50% of the plants per plot.
Number of pods per plant
Was counted at maturity.
Pod length (cm)
Measured from the tip of the pod to the point of attachment to the pedicel.
Number of seeds per pod
Was counted on matured pods.
Number of seeds per plant
Was estimated as the product of the number of seeds per pod and the number of pods per plant.
100-seed weight (g)
Weight of 100 seeds.
seed yield (g)
Weight of total seeds harvested per plant.
b. The data presented in Table 2 was collected and analyzed as part of the cowpea experiment.
3.3. Data Analysis
The values observed were averaged over a single plant basis per plot. Data collected were considered independently as a factorial design under plots with insecticide and without insecticide. Analysis of variance was used to determine
significant differences among the cowpea accessions for the characters evaluated.
Principal component analysis was used to determine characters that contributed significantly to variation among the cowpea accessions and bi-plots between the first and second principal component axes were used to describe the
genetic diversity among the accessions. Also, single linkage cluster analysis was
used to generate a dendrogram that summarizes the diversity among the accessions into a single cluster.
4. Results
Factor scores of the characters evaluated in the cowpea accessions under two
growing conditions are presented in Table 3. Principal axis I (PCA1) accounted
for 39% of the total variation of the cowpea accessions under insecticide spray
and 35% under no insecticide spray. Considering the first three principal axes,
70% and 72% of the observed variation in the accessions were revealed under
insecticide spray and no spray respectively. Characters contributing significantly
to the variation under the two growing conditions include several pods per plant
and several seeds per pod. In addition, high factor scores (>3.00) were observed
in plant height (0.38), leaf length (0.41), number of leaves (0.37), and 100-seed
weight (0.30) under insecticide spray and leaf width (0.32), pod length (0.37) and
number of seeds per plant (0.38) under no insecticide spray.
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Table 3. Factor scores from the first three principal axes of characters evaluated in forty cowpea accessions under insecticide and
no insecticide spray.
Insecticide/No Insecticide
Character
PCA1
PCA2
PCA3
Plant Height
0.38
−0.11
−0.31
0.26
−0.12
0.49
Leaf length
0.41
−0.08
−0.29
0.37
−0.03
0.25
Leaf width
0.27
−0.32
−0.36
0.21
0.20
0.25
Number of leaves
0.37
−0.28
−0.35
0.33
−0.01
0.25
Number of pods/plant
0.32
0.42
0.15
0.17
0.34
−0.12
Pod length
0.01
0.37
0.27
0.18
0.33
0.27
Number of seeds/pod
0.33
0.43
0.27
0.15
0.02
−0.32
Number of seeds/plant
0.20
0.38
0.39
0.30
0.11
−0.15
Days to 50% flowering
0.26
0.29
0.24
0.26
0.51
0.25
100-seed weight
0.30
0.19
0.32
−0.45
−0.4
0.45
Seed yield
0.27
0.20
0.30
−0.44
−0.44
0.44
Eigen value
4.33
3.84
2.11
2.37
1.31
1.71
Variation
0.39
0.35
0.19
0.22
0.12
0.16
Cumulative
0.39
0.35
0.59
0.56
0.70
0.72
c. Principal component axis (PCA) analysis was employed in the generation of these factor scores derived from the evaluation of
characters in forty cowpea accessions under insecticide and no insecticide spray conditions.
Leaf width (0.32) number of seeds per plant (0.39) and seed yield (0.30) contributed significantly to 19% of the variation observed in the accessions under
insecticide spray in PCA 2. Whereas leaf length (0.37), number of leaves (0.33),
100-seed weight (0.45), and seed yield (0.44) contributed more to the 22% variation observed in the accessions under no insecticide spray in PCA 2. Days to
50% flowering were not related to any of the first three principal axes.
4.1. Discussion
The existence of genetic variation in available germ-plasm is a prerequisite for
the improvement of desirable characters through selection and breeding [41].
The genetic variability of cowpeas in response to environmental stress was considered in the study. This is necessary to identify superior genotypes for breeding programs.
Genetic diversity exists among the cowpea accessions under the two growing
conditions. Ousters observed among the genotypes indicated that the cowpea genotypes responded differently within the growing conditions. However, it was observed that some of the genotype’s responses to the two growing conditions were
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similar. These genotypes were Extinct and consistently grouped as similar genotypes under the two growing conditions. The genotype included the improved and
cultivated materials, SAMPEA6, SAMPEA 10, IFE-Brown, and IFE-BPE.
Also distinct among the genotypes were NGB1063, NGB1152, and NGB1093
under insecticide spray, and NGB1146 and NGB1124 Under no insecticide
spray. As revealed foam, the principal component axes NGB1152 can be selected
for a higher number of leaves and leaf width while NGB1063 and NGB1152 are
desirable for several seeds/plant, 100-seed weight, and seed yield under insecticide spray. NGB1124 is desirable for leaf width, 100-seed weight, and seed yield
while NGB1146 can be selected for higher leaf length, number of leaves, and
number of seeds/plant under no insecticide spray. The cultivated genotypes are
superior for leaf length, number of leaves, number of pods/plants, number of
seeds/pod, 100-seed weight under insecticide spray, and 100-seed weight and
seed yield under no insecticide spray.
4.2. Conclusion
Exploiting genetic diversity in different environmental stresses will help to identify
and select promising crop plants for breeding programs. The selected genotypes
can be used to develop improved and adaptable genotypes across the environment.
Additionally, exploring how cowpea seeds look and their colors, along with studying their genes, gives a deeper understanding of cowpea diversity. This is crucial
for pinpointing desirable traits and supporting breeding efforts [42] [34].
In the study, NGB1063, NGB1093, NGB1152, NGB1146, and NGB1124 were
identified as potential cowpea genotypes that can be used for the genetic improvement of cowpea. Ultimately, this study represents a significant step towards
boosting food security and advancing sustainable agriculture practices not only
within Nigeria but also globally. Additionally, Leveraging local, underused cowpea varieties can expand the genetic pool, leading to stronger and more diverse
cowpea varieties.
Acknowledgements
I acknowledge the Germ-plasm Unit of the National Centre for Genetic Resources and Biotechnology (NACGRAB), Ibadan, for providing the cowpea accessions used in this study. Additionally, I express my appreciation to the Federal University of Agriculture, Abeokuta, for granting access and support to their
research farm, where the experimental work was conducted. Special thanks are
also due to Dr. O.A. Oduwaye for his invaluable guidance and support throughout
the course of this research endeavor. His expertise and encouragement greatly
contributed to the successful completion of this study.
Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.
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Appendix
Note: Bi-plot between PCA1 and PCA2 revealing genetic diversity within the forty accessions of cowpea under insecticide spray
was presented in Figure 1, six groups were revealed among the accessions, SAMPEA6, IFE-Brown, IFB-BPE, and SAMPEA 10
were clustered. Also, NGB1063, NGB1093, and NGB1152 were at a distinct distance from the forty cowpea genotypes.
Figure 1. Bi-plot between principal axes 1 and 2 of forty accessions of cowpea under insecticide spray.
Note: Figure 2 revealed genetic clusters among the forty cowpea accessions under no insecticide spray. Similarly, SAMPEA6,
IFE-Brown, IFE-BPE, and SAMPEA1O were clustered together and distinctly from the accessions. However, NGB1124 and
NGB1146 were revealed as other distinct cowpea accessions under no insecticide spray.
Figure 2. Bi-plot between principal axes 1 and 2 of forty accessions of cowpea under no insecticide spray.
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(a)
(b)
Note: The dendrogram generated among the cowpea accessions under two growing conditions from single linkage cluster analysis is presented in Figure 3. Under the two growing conditions, no cluster was formed at 100% similarity coefficient. The first cluster was
formed at an 88% similarity coefficient between NGB1146 and NGB1153 under insecticide spray and between NGB1115 and NGB1126 under no insecticide spray. The dendrogram revealed the diversity of NGB1093, NGB1152, IFE-BPE, SAMPEA6, SAMPEA10
and IFE-Brown under insecticide spray and NGB1124, IFE-BPE, SAMPEA10, SAMPEA6,
IFE-Brown and NGB1146 under no insecticide spray.
Figure 3. Dendrogram of 40 accessions cowpea evaluated under insecticide spray (a) and
no insecticide spray (b).
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