Heliyon 8 (2022) e12059Contents lists available at ScienceDirect
Heliyon
journal homepage: www.cell.com/heliyonResearch articleImprovement in cowpea variety Videza for traits of extra earliness and
higher seed yield
Innocent Kwaku Dorvlo b, Godwin Amenorpe a,b,*, Harry Mensah Amoatey b, Samuel Amiteye b,
Jacob Teye Kutufam a, Emmanuel Afutu c, Elvis Asare-Bediako c, Alfred Anthony Darkwa c
a Biotechnology and Nuclear Agricultural Research Institute (BNARI), Ghana Atomic Energy Commission (GAEC), Accra, Ghana
b Department of Nuclear Agriculture and Radiation Processing. School of Nuclear and Allied Sciences, College of Basic and Applied Sciences, University of Ghana, Accra,
Ghana
c Department of Crop Science, School of Agriculture, College of Agriculture and Natural Sciences, University of Cape Coast, Cape Coast, GhanaA R T I C L E I N F O
Keywords:
Mutagenesis
Development
Extra early
Radiosensitivity
Cowpea
Agriculture* Corresponding author.
E-mail address: gamenorpe@yahoo.com (G. Ame
https://doi.org/10.1016/j.heliyon.2022.e12059
Received 2 August 2021; Received in revised form
2405-8440/© 2022 The Author(s). Published by Els
nc-nd/4.0/).A B S T R A C T
The cowpea variety Videza, which was used as the control, matures early (70 days after planting), although it
produces low yields. Gamma irradiation mutagenesis was used to induce Videza into extra-early maturing and
higher yielding mutant genotypes. A single seed descend population was developed for radio-sensitivity test, and
a Lethal Dose 50 (LD50) of 240.5 Gy was determined, and applied from a cobalt-60 (60Co) source, to acutely mass
irradiate 1800 Videza seeds, at the Ghana Atomic Energy Commission. The irradiated seeds (M1) were planted to
produce M2 seeds bearing plants and subsequently advanced to M3 plants for selection of nine induced plants
based on extra earliness and significantly higher seed yields than the parental control. It took 48 days after
planting (DAP) for the genotype coded P1N02#1 to reach 50 % maturity followed by 52 DAP for genotypes with
codes P4N03#3; P3N01#5; P5N05#6, P4N14#7, P5N07#8, P5N05#10 and 54 DAP for genotype P4N14#11.
P1N06#9 had the highest yield (97.38 g/plant), followed by P5N05#10 (95.97 g/plant), P1N08#13 (81.24 g/
plant), P2N09#12 (73.94 g/plant), P6N10#19 (70.83 g/plant), P1N06#20 (65.36 kg/plant), P5N07#14 (61.23
g/plant), P4N14# (58.05 g/plant) and P1N08#17 (56.23 g/plant). M3 seeds were advanced to M4 plants for a
Preliminary Yield Trial which revealed that induced plants P5N05#10 (1235 kg/ha), P2N09#12 (1206 kg/ha),
P5N07#14 (1185 kg/ha), P1N06#20 (1171 kg/ha), P1N06#9 (1051 kg/ha), P1N08#13 (1041 kg/ha), and
P6N10#19 (999 kg/ha) outperformed the control (517 kg/ha) and two other commercial varieties. Overall, the
two highest performing candidates for further evaluation for varietal release were P5N05#10 and P2N09#12.1. Introduction
Cowpea [Vigna unguiculata (L) Walp] is a native of West Africa, where
numerous wild species are found (Ng, N., 1995; Fatokun et al., 2018).
Because of its nutritional importance, cowpea cultivation is common in
developing countries, particularly in the semi-arid tropics and certain
temperate regions (Timko et al., 2008). Cowpea seeds are excellent
source of carbohydrate (50–60 %) and a crucial source of protein (18–35
%) (Stancheva et al., 2017; Addo-Quaye et al., 2011; Fatokun et al., 2018;
Sharma and Lavanya, 2002). The seeds’ crude protein content range from
23 % to 32 % (Diouf, 2011). Additionally, a significant number of min-
erals like vitamin A, iron, zinc, and calcium are present in cowpea (Quayenorpe).
20 June 2022; Accepted 24 Nov
evier Ltd. This is an open access aet al., 2009; Prinyawiwatkul et al., 1996). The production of the crop and
the value chain provide smallholder farmers, particularly women, with a
means of subsistence income (Odendo et al., 2011). Cowpea production
is, however, severely constrained in Sub-Saharan Africa by persistent
droughts that particularly harm the flowering and pod-filling stages
(Owade al et., 2020). As a result, cowpea yields are extremely low in
West Africa. Notably, extra early maturing genotypes that can escape
drought periods prevent the effects of moisture stress limitation on pro-
tein or free amino acid formation and concentration in seeds. Agbicodo
et al. (2009) indicated that drought conditions during the early stages of
flowering and pod filling can cause yield losses of up to 80 %. Farmers
strategically prefer to plant early maturing varieties at specific times soember 2022
rticle under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
I.K. Dorvlo et al. Heliyon 8 (2022) e12059that the flowering and pod-filling stages can avoid drought and pressure
periods for pest invasion (Selvan et al., 2021; Ehlers and Hall, 1997).
Farmers therefore, choose early maturing and high yielding crops in
order to reduce the consequences of severe drought (Fatokun et al.,
2012).
Unfortunately, because this species self-pollinates, the genetic di-
versity of cowpea is typically low. Gomes et al. (2020), Wamalwa et al.
(2016), Kuruma et al. (2008) and Reis and Frederico (2000) who
observed low levels of genetic variety based on morphological markers
among cowpea accessions in Kenya further emphasized the assertion of
the crop's narrow genetic variability. Until a spontaneous mutation takes
place, the genetic makeup of self-pollinated plants like cowpea remains
fixed throughout generations. Unssless a natural or spontaneous muta-
tion occurs, the level of stability for homozygote lines can endure for
generations. It is said that the frequency of spontaneous mutation is fairly
low (Singh et al., 2013). The genetic diversity of cowpeas has barely
increased due to the slow rate of spontaneous mutation. Therefore, it is
crucial to use mutagenesis to increase genetic diversity in self-pollinated
crops (Ronald, 2011; Singh et al., 2013). Faster external influences are
typically used in artificial mutagenesis to hasten mutagenesis and the
development of genetic variation. The selection of a mutagen for muta-
genesis is largely influenced by previous achievements recorded for a
crop species, as well as the cost and accessibility of mutagens (Bado et al.,
2015). As high as 3, 218 mutant varieties of crops, including cowpea,
have been developed and logged in the FAO and IAEA Mutant Varieties
Database (MVD, 2012). For instance, Girija et al. (2013) observed
cowpea seeds that had been exposed to gamma rays (20, 25, 30 kR) as
well as EMS (20, 25, 30 mM) treatment. These researchers identified
unique mutants with larger seeds and new seed coverings. Additionally,
Gunasekaran et al. (1998) noted variance in various agronomic variables
in M2 populations that had been subjected to gamma radiation or
ethidium bromide mutagenization. According to the findings, gamma
radiation causes mutations more effectively than ethidium bromide
(Gaafar et al., 2016).
Furthermore, Horn et al. (2017) found four mutants with widespread
adaption in Namibia at M7 and radiated four elite cowpea cultivars there.
On two types of cowpea, Singh et al. (2006) conducted a thorough
mutagenesis utilizing three chemical mutagens: ethyl methane sulfonate
(EMS), methylmethane sulfonate (MMS), and sodium azide (SA). A wide
range of macro mutations were seen in the offspring of both kinds in the
M2 generation. The enhancement of cowpeas has been accomplished
through the use of physical and chemical mutagenesis techniques, and
the mutants that have been produced have met local farmers and con-
sumers’ expectations. Gamma radiation can be used to produce
extra-early and early-maturing cowpea mutants because of its high
penetration strength and energy levels (Amenorpe, 2010; Mba et al.,
2010; Tshilenge-Lukanda et al., 2012; Nakatsuka and Koishi 2018; Ndi-
nelao et al., 2018; Singh, 2007).
Induced mutation is a useful complementary approach to traditional
plant breeding, especially when it is desired to enhance a few readily
identifiable traits in a variety that is well-adapted (Navabi et al., 2016). In
mutation breeding, novel traits are introduced through a modest modifi-
cation to the basic genotype of the parental variety (Nakatsuka and Koishi
2018; Ndinelao et al., 2018). Increased genetic variability due to induced
mutation has improved conventional breeding in many crops by widening
the genetic base (Singh et al., 2006). The mutagenized population of the
M1 generation may have subtle induced mutations as a result (Kolar et al.,
2015). Therefore, in order to increase trait expression, M1 plants must be
advanced at least to theM2 generation. According to Clement et al. (2015),
Singh and Sharma (2016), and Reddy et al. (2013), it is preferable to begin
screening traits at the M2 generation while the plant population is still
small rather than in subsequent generations to forestall obscurity of
judgement posed by a larger population. Early detection of helpful mutants
demonstrates the efficacy of doses used and suggests a chance of finding2
additional useful mutants in subsequent generations (Amenorpe, 2010).
The most popular, affordable, and effective method for phenotypically
identifying mutations is visual screening (Shu et al., 2012). Therefore, the
visual screening for quantitative traits, such as number of days to flower-
ing, the number of pods per plant, seed yield per plant is preferably carried
out in M2 generation. Omoigui et al. (2006) and Horn et al. (2016) studied
the induced variability in cowpea and observed significant differences in
the number of days to flowering, days to maturity, number of branches per
plant, number of pods per plant, 100 seed weight, seed yield per plant in
the M2 generation.
In general, cowpea varieties are categorized as extra early if they
reach maturity 60 days or less after planting (DAP), early if they do so in
61–80 days, and late if they reach maturity in more than 80 DAP (Singh
et al., 2007; Dugje et al., 2009; Hall et al., 2003; Egbe et al., 2010; Owusu,
2015). Twelve of the thirteen cowpea varieties that were released in
Ghana are categorized as early maturing based on this earliness rating.
The cultivars grown in Ghana have the following maturation times and
yields: i. Nhyira (65–68 days; yield 2,460 kg/ha) (CSIR-CRI, 2006); ii.
Asuntem (CSIR, 2005); iii. Asentenap (2,500 kg/ha) (CSIR-CRI, 1991); iv.
Zaayura (64–67 days; 600 kg/ha) (CSIR-CRI, 2008); v. Marfo-Tuya
(66–70; 600 kg/ha) (Monyo and Boukar, 2013); vi. Apagbaala (60 days;
600 kg/ha) (Monyo and Boukar, 2013); vii. Padi-tuya (64–67 days; 400
kg/ha) (Monyo and Boukar, 2013; CSIR-CRI, 2015); viii. Songotra (62–65
days; 600 kg/ha) (Monyo and Boukar, 2013); ix. Bawutawuta (69–75
days; 400 kg/ha) (Monyo and Boukar, 2013); x. Hewale (64–72 days); xi.
Videza (68–77 days or 70 days based on average) (Adu-Dapaah and Addy,
2015) and xii. Asomdwee (65–72 days) (Owusu et al., 2018)). Only Benpla
(55–60; 1,255 kg/ha) is an extra early variety (Quaye et al., 2011). The
yield of the early varieties range from 400 kg/ha (Padi-tuya and Bawu-
tawuta) to 2,500 kg/ha (Nhyira and Asentenap) and that for the extra early
variety Benpla is estimated as 1,255 kg/ha. It is noteworthy that the extra
early variety Benpla yields higher than most of the early varieties. This
observation provides some indication that extra early varieties could be
developed without significantly compromising on yield levels.
In order to circumvent the limitations imposed by periods of pro-
longed drought, the aim of this study was to develop cowpea genotypes
that combine the traits of extra earliness and high seed yield by muta-
genizing the parental variety Videza using gamma irradiation. Using
Videza as parental control, selected M1 generations that showed extra
earliness and high seed yield were advanced to M2 and then to M3.
Cowpea varieties with extra early maturation and greater yields have
enormous economic potential for revitalizing the cowpea agrobusiness
since such improved varieties would allow the crop to avoid dry spells
and irregular rainfall patterns brought on by climate change (Egbadzor
et al., 2013).
2. Materials and methods
2.1. Research site, experimental design and agronomic practices
A field experiment was conducted on the research field of the
Biotechnology and Nuclear Agriculture Research Institute (BNARI) of the
Ghana Atomic Energy Commission (GAEC). The research area is situated
in latitude 05 400 and longitude 0 130 West, 76 m above sea level. It is
located in the coastal savannah zone and receives between 750 and 1000
mm of precipitation each year (Essel et al., 2016). According to Afram
(2020), the major soil type is a well-drained sandy loam called Savannah
Ochrosol (Ferric Acrisol, locally known as Haatso series), which is
derived from quartzite schist. Irradiated seeds were planted the following
day in the field at the BNARI Research Farm at aspacing of 80 cm  40
cm. Each plot comprised five rows of six plants/row at a seeding rate of
one seed per hill. The experiment was set up in a Randomized Complete
Block design with three replications. Seeds and seedlings were watered
twice per week to ensure adequate soil moisture for germination and
I.K. Dorvlo et al. Heliyon 8 (2022) e12059
Figure 1. A. Number of days to flower. B. Number of days to maturity. Colours: Blue: induced plants; Yellow: Mean value of all induced plants; Green: un-
induced plants.growth. The biological effects of the gamma irradiation on the plants
were studied following the protocols established at the Plant Breeding
and Genetics Laboratory of the Joint FAO/IAEA (Spencer-Lopes et al.,
2018; Essel et al., 2016; Afram, 2020).2.2. Plant material and mass irradiation
A farmer-preferred variety of cowpea, Videza was used for the study.
This was obtained from the best cowpea farmer in the Volta Region of
Ghana, precisely in Avenorpedo (Akatsi-North District). This cowpea
variety was first planted on the Research Farm of the Biotechnology and
Nuclear Agriculture Research Institute (BNARI) between March 2018 to
April 2018 and pods were harvested from the best performing single
plant to ensure homogeneity of seeds. The harvested seed was multiplied
from the best performing single selected plant, planted in the BNARI
Research Farm, between July 2018 to September 2018, to obtain
approximately 8,000 seeds. After radio-sensitivity test, mass irradiation
was carried out as follows: Thousand eight hundred (1800) cowpea seeds
were acutely irradiated inMarch 2019 at LD50 of 240.51Gy, at a dose rate
of 300 Gy/h from Cobalt-60 (60Co) source at the Radiation Technology
Centre (RTC), Ghana Atomic Energy Commission (GAEC), Accra, Ghana.
Zero (0Gy) dose served as control. Seeds with a moisture level of 12.8 %,
100 % germination, and in a dry, healthy, quiescent state were used.2.3. Field establishment and selection at M1, M2 and M3 generations
On the field, the control seeds, irradiated seeds (M1), inducedM2 and
M3 seeds, were sown at a depth of 2 cm with spacings of 40  75 cm.
Manual weed control was used. 30 induced plants were tagged at the M1,
M2 generations, seeds harvested separately, and dried to yield seeds for
the M3 generation. These plants flowered and reached maturity earlier
than the parental control plant.3
2.4. Preliminary evaluation of twelve genotypes for yield at M4 generation
The experiment was conducted during the minor season within the
coastal savanna agro ecological zones. Nine induced plants (P5N05#10,
P2N09#12, P5N07#14, P1N06#20, P1N06#9, P1N08#13, P6N10#19,
P1N08#17, P4N14#7) selected on the bases of early maturity were
evaluated with three commercial varieties (Videza, the control), Hawale
and Zamzam under Randomized Complete Block Design (RCBD) with
three replicates. The field had three blocks which were separated by 1 m
alleys. Each replicate plots were also separated by 1 m alleys on each
block. Each plots dimension was 6.11m 6.11m. Each plot had ten rows
by ten columns of cowpea plants. One seed was sown per hill at a spacing
of 60 cm  60 cm apart.2.5. Statistical analyses
All induced plants from M1 to M3 populations were compared with
their parental control for differences using excel and means determined
for M3 populations. The preliminary yield trial was analysed for signif-
icant differences with GenStat Release 12.1 and significantmeans were
separated by least significant difference values.
3. Results
3.1. Number of days to flowering and maturity at M3 generation
Figures 1A andBdisplay the parental control (green bar) andM3plants’
maturation and days to 50% flowering. The control plant required 45 days
toflower and 70 days tomature, but the obtainedmutant plants took 30–45
days and 48–60 days, respectively, toflower andmature. It took an average
of35and58days forplants toflower ormature, respectively.On thebasis of
increased earliness and maturity compared to their overall mean (yellow
I.K. Dorvlo et al. Heliyon 8 (2022) e12059
Figure 2. A. 100-seed weight. B. Total seeds per plant. Colours: Blue: induced plants; Yellow: Mean value of all induced plants; Green: un-induced plants.
Figure 3. A. Number of branches per plant. B. Number of pods per plant. Colours: Blue: induced plants; Yellow: Mean value of all induced plants; Green: un-
induced plants.
4
I.K. Dorvlo et al. Heliyon 8 (2022) e12059
Figure 4. A. Pod length. B. Number of seed per pod. Colours: Blue: induced plants; Yellow: Mean value of all induced plants; Green: un-induced plants.bar) and control (green bar), eight induced mutant plants were chosen
(P1N06#20, P6N10#19, P6N10#18, P1N08#17, P2N09#16, P3N01#15,
P5N07#14, P1N08#13, and P2N09#12).
3.2. A100-seed weight and total seeds per plant at M3 generation
The number of seeds per plant and the weight of 100 seeds are pre-
sented in Figures 2A and B. The control plant's 100-seed weight (green
bar) was 15.25 g, and each plant had a total of 48.65 g of seeds. The
overall weight of the seeds per plant varied from 19.12 to 97.38 g, while
the weight per 100 seeds varied from 12.69 to 21.61 g. The average
weight of 100 seeds per plant was 16.78 and 51.75 g. Ten induced plants
significantly outweighed the mean value (yellow bar) in terms of 100-
seed weight, and majority of these plants had significantly more total
seeds per plant. On the basis of total seed production above the mean,
nine plants (P1N06#9, P5N05#10, P1N08#13, P2N09#12, P6N10#19,
P1N06#20, P5N07#14, P4N14#7, and P1N08#17) were chosen. The
majority also hadmuch higher 100-seed yields than themean and control
plants.
3.3. Number of branches and pods per plant at M3 generation
Figures 3A and B show the number of branches and pods per plant.
The control plant (green bar) had 5 branches and 30 number of pods per
plant. Number of branches and pods per plant ranged from 3 to 7 and 26
to 47, respectively. The mean (yellow bar) number of branches was 5 and
38.15 for pods per plant. Nine induced plants selected (P3N01#15,
P1N02#1, P4N03#3, P4N14#7, P5N05#6, P1N06#20, P5N07#14,5
P1N08#13 and P2N09#12) on the basis of higher number of branches
per plant than mean and control. Most of these plants also had higher
number of pods per plant.3.4. Pod length and number of seed per pod at M3 generation
Figures 4A and B illustrate the variation in pod length and the number
of seeds per pod. The control plant (green bar) contained 13.0 seeds per
pod and a pod length of 16.50 cm. The number of seeds per pod varied
from 12.25 to 22.75, and the length of the pods ranged from 12.75 to
26.8 cm. The average pod length (yellow bar) was 18.11, and there were
17 seeds per pod. On the basis of having longer pods than the mean and
control, eight induced plants (P1N06#20, P5N07#8, P5N05#10,
P5N05#6, P2N09#12, P1N08#17, P4N14#7, and P6N10#18) were
chosen. The majority of these plants also produced more seeds per pod.3.5. Preliminary evaluation of twelve genotypes for yield at M4 generation
The yield per hectare was highly significant (F pr.< 0.001) (Figure 5).
The CV %, mean and range values for yield per hectare were 38.23;
872.47 and 268.6–1235 respectively. Moreover there are significant
difference between control and induced plants, as induced plants
P5N05#10 (1235 kg/ha), P2N09#12 (1206 kg/ha), P5N07#14 (1185
kg/ha), P1N06#20 (1171 kg/ha), P1N06#9 (1051 kg/ha), P1N08#13
(1041 kg/ha) and P6N10#19 (999 kg/ha) had exceptional higher values
than the control (517 kg/ha) and the rest.
I.K. Dorvlo et al. Heliyon 8 (2022) e12059
Figure 5. Mean field performance of genotypes.4. Discussion
Negative environmental effects worsen climate-related yield in-
stabilities in broad-leaved and grain legume crops more than they do in
cereals (Dhankher and Foyer, 2018; Reckling et al., 2018). The likelihood
of famine is predicted to increase due to rising temperatures, intensifying
droughts, strong precipitation events, and insect and pathogen in-
festations that frequently accompany these factors (Long et al., 2015).
Ghanaian cowpea producers consequently support vigorous research
efforts to cut the number of days needed for flowering and maturation
while increasing crop yields. The shorter gestation period varieties would
enhance the sustainability of economic yields even with little rain before
biotic and abiotic stress conditions set in to reduce yields as pertains in
late maturing crops.
Although Cober and Curtis (2003) asserted that the activity of floral
promoters and inhibitors mediate flowering time antagonistically and
can even be sensed in unexpanded leaves or buds, photoperiod is the
most significant environmental variable regulating time to flowering in
cowpea. At least seven main genes, each of which is associated with an
average flowering delay of up to six days, have been identified as con-
trolling the time to flowering in cowpea (Boukar et al., 2019; Ishiyaku
et al., 2005). However, Bernard (1971) discovered that the two main
genes in soybeans that regulate flowering and maturity are independent,
but the gene for lateness is connected to the colour of the pubescence.
The late allele at each locus was only partially dominant in the majority
of combinations, but gamma irradiation mutagenesis might change the
order of the late allele at each locus, as well as the promoters and in-
hibitors that regulate flowering time, to cause extra early flowering in
cowpea (Tripathi et al., 2020).
Extra early cowpea varieties mature in less than 60 days after
planting, early varieties mature between 61 and 80 days after planting,
and late varieties reach maturity in more than 80 days after planting
(Singh et al., 2007; Dugje et al., 2009; Hall et al., 2003; Egbe et al., 2010;
Owusu, 2015). The study showed that some induced plants have the
ability to mature earlier than their mutagenized parents. For instance,
genotype P4N14#7 matures extra-early at 54 days after planting (DAP)
and flowers very early at 30 days after planting (DAP), but the control
flowers 15 days later at 45 DAP and matures 16 days later at 70 DAP. The
shorter flowering period and shorter days to maturity showed genetic
diversity brought on by gamma radiation in the desired direction,
allowing selection of mutants with extra-early maturation. Similarly,
Horn et al. (2013) discovered significant genetic variation among
induced cowpea genotypes at M5 based on flowering and maturity times.
It was found that "Bira" mutant plants matured 62 days earlier than the
control plant (74 days). Gamma rays-induced artificial genetic disruption6
of the parental genomemay result in heritable insertions and or deletions
of specific nucleotides or DNA sequences.
It was also observed that eight induced plants flowered extra earlier
than the mean value of 33 DAP for induced plants. Most of these extra
early mutant genotypes are earlier maturing than their parental control.
For example P1N02#1 took 48 days to mature was followed by P4N03#3
(52 days), P3N01#5 (52 days), P5N05#6 (54 days), P4N14#7 (54 days),
P5N07#8 (54 days), P5N05#10(54 days) and P4N14#11 (54 days), with
mean value of 54.55 days which is extra earlier than the control of 70
days. The selection for extra flowering and early maturing plants was
consistent with Laskar and Khan (2017) and Wu et al. (2005) who stated
that most quantitative traits observed at the M2 generation are likely to
recur in subsequent generations. Similar result was reported by Horn
(2016) in cowpea, Tulmann and Alve (1997) in soyabeans and Karim
et al. (2008) in chickpea. Therefore, the variability generated through
gamma-radiation-induction was screened for useful extra early maturing
cowpea genotypes such as P1N02#1 (which matures 48 days), followed
by P4N03#3 (52 days), P3N01#5 (52 days), P5N05#6 (54 days),
P4N14#7 (54 days), P5N07#8 (54 days), P5N05#10 (54 days) and
P4N14#11 (54 days), which took 10–15 days to mature earlier than their
parents.
By completing their life cycle within the brief rainy period before
severe weather sets in, these extra early genotypes offer the opportunity
to avoid seasonal drought and harmful heat stress in evaluated and
released to farmers as varieties. Compared to late maturing varieties,
extra early maturing cowpea mutant varieties may be better able to fend
off insect and disease attacks. According to Singh et al. (2007) and
Bozokalfa et al. (2017), many small-scale farmers deliberately favour
shortened planting dates over longer durations in order to prevent
complete crop failure. Bozokalfa et al. (2017) reported that farmers grow
early maturing crop varieties because these varieties provide early har-
vest than the late maturing varieties. Farmers adopt early maturing va-
rieties because such varieties are more reliable in the food security
strategy of Sub-Saharan Africa. Increasing harsh unfavourable climate
changes are rendering most late maturing varieties less useful due to
significant decline in their efficient development and productivity. More
extra early maturing varieties are, therefore, required to support efforts
at mitigating the negative effects of climate change on achieving optimal
level of food security (Egbe et al., 2010; Mligo, 1989). Extra early
maturing cowpea varieties are ideal for intercropping with other crops
because they offer less competition for growth resources than late
maturing varieties (Ntare, 1990; Kamara et al., 2018). Besides,
extra-early varieties open the possibility of successful single cropping in
areas with short rainy seasons, double or triple cropping in areas with
relatively longer rainfall. Extra early maturing cowpea varieties are
I.K. Dorvlo et al. Heliyon 8 (2022) e12059amenable to relay cropping after millet, sorghum or maize, as well as
intercropping with cereals and root and tubers (Egbe et al., 2010; Mligo,
1989).
For many farmers in the Sub-Saharan region, cowpea is a crucial cash
crop. Therefore, an increase in agricultural output and seed size can
significantly increase farmers' and their household's income. In this
study, seed size was measured in terms of weight per 100 seeds, a statistic
whose value constantly rises as seed size does (Herniter et al., 2019;
Langyintuo et al., 2004). It was observed that total seed per plant had
strong positive correlation (0.67) with a 100-seed weight. This implies
that higher yielding cowpea genotypes could invariably produce higher
100 seed weight. The 100-seed weight in mutant genotypes ranged from
12.69 to 21.61 g. With a mean of 16.78 g, the mutant genotypes
compared favourably with varieties cultivated in Ghana. The 100-seed
weight of cowpea varieties reported in Ghana range from 7.3 g to 40.1 g,
with a mean of 17.4 g for larger seeds.Moreover, these mutants being extra
early in maturity, have advantage of being cultivated multiple times in
the year than most existing varieties. However, compared to more
advanced regions, yield performance of cowpea in Ghana is generally
low. Herniter et al., 2019 indicated that cowpea varieties grown in for
example the United States produce higher 100-seed weight of 20–25 g
(Herniter et al., 2019).
This study also revealed that total seeds per plant ranged from 19.12 –
97.38 g. The control plant produced 15.28 g 100-seed weight and 16.78 g
total seeds per plant at the M3 generation. It should be noted, however,
that the observed seed size range for the induced mutant plants is higher
than most of the previously reported weight of 12.2 g per 100-seeds in
northern Ghana (Langyintuo et al., 2004). Some seed sizes were also
larger than 14.4 g per 100-seeds in southern Ghana (Mishili et al., 2009).
The observed increase in seed size could be due to mutagenic effect of the
gamma rays applied. It was also observed from this study that some
induced plants had higher 100-seed weight and total seed per plant than
their mean and control at the M3 stage. The mutant genotype P6N10#19
had higher 100-seed weight (21.60 g) and total seed per plant (97.38 g)
than the mean and control. The increase in 100-seed weight and total
seed per plant above the parent plant showed that variability has been
induced in the desired direction and would offer the possibility for
selecting high seed yielding mutants. The effectiveness of gamma radi-
ation in causing increase in yield is extensively reported (Raina et al.,
2020; Laskar and Khan, 2017; Badr et al., 2014; Wu et al., 2005). Badr
et al. (2014) reported the potential of increasing growth and yield related
parameters by gamma radiation. Chemical mutagens such as sodium
azide have also been proven to increase genetic variability in cowpea
genotypes with respect to control (Raina et al., 2020).
It was observed that total seed weight per plant has strong correlation
(0.67) with 100-seed weight. Selection for higher total seed weight per
plant therefore, included most genotypes (P5N21 (22.80), P10N23
(21.71), P10N28 (21.31), P10N29 (20.96), P6N10 (20.77), P5N04
(20.34), P6N09 (20.11), P5N11 (20.00), P6N22 (19.92), P1N03 (17.60)
with higher 100-seed weight. Moreover, the possibility of recurrent se-
lection of higher total seed weight per plant genotypes across generations
confirm the notion that quantitative traits observed in the M2 generation
are repeated in subsequent generations (Laskar and Khan, 2017; Wu
et al., 2005). Similar result was reported by Horn (2016) in cowpea,
Tulmann and Alve (1997) in soyabeans and Karim et al. (2008) in
chickpea. Therefore in M3 generation, the recurrent selection of nine
induced genotypes (P1N06#9 (97.38 g), P5N05#10 (95.97 g),
P1N08#13 (81.24 g), P2N09#12 (73.94 g), P6N10#19 (70.83 g),
P1N06#20 (65.36 g), P5N07#14 (61.42 g), P4N14#7 (58.05 g) and
P1N08#17 (56.23 g) on the basis of total seed weight above their control
and mean (51.75 g) are highly likely to be repeated in subsequent gen-
erations. The total seed weight per plant had strong correlation (0.67)
with 100-seed weight. Therefore, selection for one trait might strongly
lead to the selection for other. In this regard, on the basis of 100-seed
weight six genotypes were selected among the top eight genotypes
(P6N10#19 (21.61 g), P1N06#20 (20.25 g), P5N07#14 (20.22 g),7
P1N06#9 (20.20 g), P5N05#10 (19.56 g), P5N07#8 (17.96 g),
P1N08#13 (17.58 g), P1N02#1 (17.31 g).
Plants with more branches were observed to have a greater number of
pods per plant at a high positive correlation (0.66). This result is
consistent with the significant positive correlation (0.988) between the
number of branches and pods/plant obtained by Akram et al., 2016 and
Horn et al., 2016). The M3 genotypes (P4N03#2, P1N02#4, P3N01#5,
P4N03#3, P5N07#8, P4N14#11, P3N01#15, P6N10#19, P1N02#1 and
P1N06#20) which had more branches also had more number of pods per
plant than the mean and control plants.
An increase in the pod length was observed in some of the selected M3
plants compared to the control. Pod length and the number of seed per pod
showed positive relationship (0.79). M3 plants with the longest pod length
also produced highest number of seeds per pod. A similar result was reported by
Raina and Khan (2020) and Pathak (1991). Gamma irradiation had been
found to cause either an increase or decrease in pod length of chickpea (Khan
et al., 2005); soybean (Justin et al., 2012) and cowpea (Wani et al.,
2018), The nine mutant genotypes (P5N07#8, P4N14#7, P4N14#11,
P4N03#2, P5N05#6, P1N06#9, P5N07#14, P6N10#19, P5N05#10)
produced higher number of seeds per pod than mean and control. These
mutant genotypes are, therefore, good candidates for multi locational
trials under different agro-ecological zones, for release as new varieties.
The major challenge for cowpea production in sub-Sahara Africa is
poor seed yield. The continuous evaluation of mutant genotypes for high
and stable yield varieties is the guaranteed approach to solving regional
food security problems (Aliyu and Makinde, 2016). The yield per hectare
obtained among the developed mutant genotypes was highly significant
(F pr. < 0.001). The yield ranged from 268.6 to 129.7 kg/ha in all the
genotypes. The significant difference between control and the mutants
P5N05#10, P2N09#12, P5N07#14, P1N06#20, P1N06#9, P1N08#13
and P6N10#19 affirm the successful creation of mutants is from a single
seed descend populations effected by gamma irradiation. This is because
a single seed descend population of a self pollinated plant cannot produce
significant differences in any trait under the same environmental con-
dition unless induced by a mutagen. It is known that gamma irradiation
influences growth hormones and cause increase in EBT (eriochrome
black-T) production leading to an increase in plant vigour, flag leaf area
and number of grain yield (Singh and Datta, 2010). The best performing
genotype P5N05#10, exhibits extra early maturing and high yielding
with an average of 38 pods per plant and 25.80 cm pod length compared
to the parental control which produces 30 pods per plant and 15.5 cm
pod length. P5N05#10 should therefore, be advanced via multi-locatonal
trials and released as both extra early maturing and higher yielding
variety.
5. Conclusion
From the early maturing, commercial control variety Videza, this
work successfully developed extra early maturing, high seed production
cowpea mutants. The reference variety Videza matures early, at 70 days
after planting, however some induced mutant genotypes could mature
even earlier, at 48 days after planting (DAP), as was seen for P1N02#1,
52 DAP for P4N03#3 and P3N01#5, and 54 DAP for P5N05#6,
P4N14#7, P5N07#8, P5N05#10, and P4N14#11. The control variety
yields as low as 48.65 g/plant but some induced mutant genotypes yield
as high as 97.38 g/plant in P1N06#9, followed by P5N05#10 (95.97 g),
P1N08#13 (81.24 g), P2N09#12 (73.94 g), P6N10#19 (70.83 g),
P1N06#20 (65.36 g), P5N07#14 (62.42 g), P4N14#7 (58.23 g) and
P1N08#17 (56.23 g). Genotype P5N05#10 takes only 54 DAP to produce
95.97g of seeds per plant whilst the control takes 70 days to produce
48.24 g of seeds per plant. P5N05#10 is, therefore, both early maturity
and high yielding. P5N05#10 also produced an average of 38 pods per
plant and 25.80 cm pod length compared to the parental control with 30
pods per plant and 15.5 cm pod length. After preliminary yield trial, it
was observed that the mutant plants P5N05#10 (1235 kg/ha),
P2N09#12 (1206 kg/ha), P5N07#14 (1185 kg/ha), P1N06#20 (1171
I.K. Dorvlo et al. Heliyon 8 (2022) e12059kg/ha), P1N06#9 (1051 kg/ha), P1N08#13 (1041 kg/ha) and
P6N10#19 (999 kg/ha) had exceptional higher yields than the control
(517 kg/ha) and two other commercial varieties, an indication that
indeed a mutant has been created from a single seed descend populations
by the gamma irradiation. It is recommended that the two best genotypes
P5N05#10 and P2N09#12 could be advanced through multi-locational
trials under different agro-ecological zones and released as both extra
early maturing and high yielding cowpea variety.
Declarations
Author contribution statement
Innocent Kwaku Dorvlo, M. Phil degree: Conceived and designed the
experiments; Performed the experiments; Analyzed and interpreted the
data; Contributed reagents, materials, analysis tools or data; Wrote the
paper.
Godwin Amenorpe, PhD; Harry Mensah Amoatey, Professor; Samuel
Amiteye, PhD: Conceived and designed the experiments; Analyzed and
interpreted the data; Wrote the paper.
Jacob Teye Kutufam, First Degree: Performed the experiments.
Emmanuel Afutu, PhD; Elvis Asare-Bediako, Professor: Analyzed and
interpreted the data; Contributed reagents, materials, analysis tools or
data.
Alfred Anthony Darkwa, PhD: Analyzed and interpreted the data;
Wrote the paper.
Funding statement
This research did not receive any specific grant from funding agencies
in the public, commercial, or not-for-profit sectors.
Data availability statement
Data will be made available on request.
Declaration of interest's statement
The authors declare no conflict of interest.
Additional information
No additional information is available for this paper.
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