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VI. Limitations to Development of Root Growth Models

VI. Limitations to Development of Root Growth Models

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mental and endogenous factors, and genetics are only beginning to be

studied. Nevertheless, models serve the useful purpose of focusing clear

attention on deficiencies in our knowledge and, along with field and laboratory investigations, are an important part of the progress being made in

understanding root growth and function.


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COWPEA (Vigna unguiculata [L.]




A. 0. Latunde-Dada

Department of Crop Production

College of Agricultural Sciences

Ogun State University


Ogun State, Nigeria

I . Introduction

Insect Pests

Ill. Fungal Pathogens

IV. Tissue Culture Technology

A. Somaclonal Variation and Cellular Selection

B. Embryo Culture

C. Somatic Hybridization

D. Genetic Transformation

V. Conclusions and Epilogue




The cowpea, Vigna unguiculata (L.) Walp., is perhaps the most important papilionaceous grain legume in the Third World, particularly Africa.

Although conjectural claims to the contrary abound (Vavilov, 1951 ;

Steele, 1976; Zhukovskii, 1962, Piper, 1913; Sauer, 1952), the dominant

worldview is that the crop originated in West Africa, probably in the

subhumid savanna grasslands of Nigeria, the area of its greatest diversity

(Faris, 1965; Rawal, 1975; Lush and Evans, 1981; Ng and Marechal, 1985).

It is, however, agreed that the cowpea was domesticated about 4000 years

ago from the wild progenitors V . unguiculata ssp. dekindtiana vars. dekindriana (in sub-Sahelian West Africa) and mensensis (in the humid and


Copyright B 1990 by Academic Press. Inc.

All rights of reproduction in any form reserved.



subhumid zones) and distributed thereafter throughout sub-Saharan

Africa, reaching the Middle East by 2000 B.C. and India by 1500 b . c . (Lush

and Evans, 1981; Steele, 1976). The Indian cultigroups of V. unguiculata

ssp. unguiculata, namely, Biflora (the catjang bean) and Sesquipedalis (the

yardlong or asparagus bean), arose probably by selection from the early

cowpea domesticates (Faris, 1965). The crop is believed to have been

introduced into Mediterranean Europe before 300 B.C. and reached the

Americas only recently, during the Great Slave Trade of the sixteenth and

seventeenth centuries A.D. (Steele, 1976). The cowpea is today cultivated

throughout sub-Saharan Africa, Southeast Asia, Latin America, and

the United States (Fig. 1). Nigeria is the world's largest cowpea producer, with production levels ranging between 40% (Rachie, 1985) and

75%(Cobley and Steele, 1977) of the world's output.

The cowpea plant is a herbaceous annual with habits ranging from

determinate to completely indeterminate types. The stem is nonpubescent

and may be erect, semierect, prostrate, or twining. The inflorescence is a

simple raceme borne on an elongated axillary peduncle on which the

flower opening is acropetal. The pod, a legume, contains seeds that are

arranged in a single row within the pod wall. The taproot system develops

both secondary and tertiary roots along its length, all of which are capable

of nodulation, hence symbiotic nitrogen fixation. The mutualistic interaction between the cowpea root and soil bacteria of the genus Rhizobium

results annually in an estimated production level of 198 kg/ha of nitrogen,

a level higher than that of Arachis hypogaea (124 kg/ha) or Glycine max

+ dispersal rouln

FIG. 1. Major cowpea-growing areas of the world, with West Africa as the center of




(103 kg/ha) (Nutman, 1975). The cowpea plant by this feat not only provides for its own nitrogen requirements but also contributes to the replenishment of nitrogen in tropical soils, and assists in cutting the everincreasing capital outlay on nitrate fertilization when intercropped with

tropical cereals. The cowpea plant is drought tolerant.

Vigna unguiculata is diploid with a chromosome number of 2n = 22. It

belongs to the tribe Phaseoleae of the subfamily Papilionoideae. It is

classified further into the subtribe Phaseolinae and group Phaseolastrae.

The genus Vigna is pantropic, comprising 7 subgenera and about 170

species (Cobley and Steele, 1977). The taxonomy of the cowpea at levels of

taxon below the generic appears complex.

Marechal’s approach to the classification (Marechal e t al., 1978;

Marechal, 1982) is given in Table I . Sexual incompatibility barriers exist in

the Phaseolinae and so far interspecific crosses are yet to be reported with

Vigna unguiculata (Baudoin and Marechal, 1985). Nevertheless, these

authors regard V . neruosa (section Catiang) and V . frutescens (section

Liebrechtsia) to be close enough to introgress with the cowpea: no hybrid

has been reported between these close relatives and the cultivated cowpea. All members of the cultigroups of V . unguiculata ssp. unguiculata are

interfertile and also sexually compatible with the wild varieties of the

subspecies dekindtiana (Lush and Evans, 1981; Ng and Marechal, 1985).

In Nigeria, the cowpea is cultivated solely for its seed, which is harvested dry and eaten either boiled or processed into different dish forms.

The seed contains about 25% (w/w) protein (Bressani, 1985) and constitutes an important and cheap source of protein for most people in West

Africa, especially the poor. Moreover, the cowpea seed, compared with

some other pulses, i s relatively low in toxic substances and antimetabolites. These qualities have for long promoted its acceptability among the

peoples of Nigeria. The cowpea crop is well adapted to the savanna

grasslands of Nigeria. About 80% of the country’s output comes from this

region (Emechebe, 1981). In this region, it is cultivated either solely or in

mixtures with cassava and such cereal crops as maize, sorghum, and

millets. Sole crops of cowpea are more commonly cultivated in the southern humid Rain Forest Belt, where, in spite of an annual rainfall pattern

and distribution that can theoretically support three crops of cowpea, only

20% of the Nigerian output is produced. Figure 2 shows the attempts made

at the Ogun State University, Ago-Iwoye, Nigeria (about 50 km south of

Ibadan, and in the Rain Forest Belt of Nigeria), to produce three crops of

cowpea over two consecutive seasons (A. 0. Latunde-Dada, unpublished


Cowpea production, in the Rain Forest Belt of Nigeria and elsewhere in

Africa is limited by a number of constraints. The grain output from most



Table 1

Taxonomy of the Cowpea, Vignu unguiculutu Walp."





















V . unguiculata


V. unguiculata




Related taxonomic forms

(a) Caesalpinioideae

(b) Mimosoideae

(a) Cajaninae

(b) Glycinae



(a) Macrorhyncha

( b ) Plectotropis

(c) Sigmoidotropis

(d) Ceratotropis

(e) Haydonia

(0 Lasiocarpa

(a) Vigna

(b) Comosae

(c) Reticulatae

(d) Liebrechtsia

(e) Macrodontae

(a) V . neruosa

(b) V . frutescens (in section Liebrechtsia)

(a) V . unguiculata ssp. unguiculata spp. srenophylla

(b) V . unguiculata spp. tenuis

(c) V . unguiculata spp. dekindtianab


(catjang bean)

(b) SESQUIPEDALIS (asparagus or yardlong bean)


Sources: MarCchal et a / . (1978); Markchal, (1982).

Vigna unguiculata ssp. dekindtiana comprises the following varieties: (a) dekindtiana,

(b) pubescens, (c) mensensis, and (d) protracta.

local varieties is between 100 and 300 kg/ha (IITA Research Briefs, 1984).

Apart from the obvious genetic inadequacies (such as extreme viny growth

habit, compulsive photoperiodism, low flowering and pod-setting abilities,

and low yields) of these local varieties, other problems abound. These

include susceptibility to insect pests, lack of resistance to viral, bacterial,

and fungal diseases, lack of tolerance of excessive moisture levels, weed

infestation, and inadequate soil nutrient supply. These problems have

been reviewed elsewhere (Summerfield et al., 1974; Thottappilly and

Rossel, 1985; Muleba and Ezumah, 1985; Poku and Akobundu, 1985).













FIG.2. Annual rainfall distribution pattern for Ago-lwoye. Horizontal arrows represent

periods during which cowpea (vars. TV x 3236, Ife Brown, IT82E-60, and IT81D-994)

was cultivated. During period 1, plants were largely free from foliar pathogens and insect

pests; pods required additional artificial drying, however. During period 11, the incidence

of Ootheca mutabilis and Aphis cracciuoru was high and damage due to web blight, Pythium

stem rot, and anthracnose was severe in the susceptible varieties. Pods were picked compulsorily when light yellow and were dried artificially. Web blight was the greatest challenge,

especially in Ife Brown and IT83E-60, during period 111. Additional drying was unnecessary,

and yields were highest during this period.

Advances have been made in the various ramifications of cowpea research

during the past two decades to ameliorate the gloomy situation just presented. The achievements of two agricultural research institutes situated

in the Rain Forest Belt of Nigeria are worth considering.

The Institute of Agricultural Research and Training (IART) of the

Obafemi Awolowo University of Ibadan was responsible for earlier

research during the 1960s that led to the development and release of

the varieties Westbred and Prima, and later Ife Brown and Ife Bimpe

(Franckowiak et al., 1973; Ojomo, 1975). The varieties Ife Brown and Ife

Bimpe are short, determinate, and early, flowering within 35 days of

sowing. When given adequate care, Ife Brown yields about 1700 kg/ha

(Singh and Ntare, 1985) but lacks demonstrable resistance to insect pest

and disease problems.

Since its inception in 1967, the International Institute of Tropical Agriculture (IITA), also in Ibadan, has virtually taken the lead in Nigeria and

indeed international cowpea research. The VITA series of cowpea varieties released originally by IITA in the 1970s have since been supplanted in



Nigeria by such popular IITA varieties as TV X 3236, IT82E-60, IT82D716, IT84E-124, and lately IT84S-2246-4. IITA continues to draw on the

rich genetic diversity of the cowpeas and has succeeded in a seemingly

endless release of lines and varieties that combine high-yielding characteristics with multiple pest and disease resistance (Table 11), and without any

apparent impairment of the seed protein levels. The modern IITA varieties

of cowpea grown in the Rain Forest Belt are day-neutral, early, determinate, short, and erect and do not require staking. Moreover, they have

peduncles that elongate rapidly during the early reproductive phase to

display the maturing pods high up and above the leaf canopy. These pods

mature uniformly, thereby facilitating synchronous harvesting. In general,

the potential yield of these IITA varieties is 2 t/ha; nevertheless, there are

plans and projections afoot to develop varieties with yield potentials in the

Table I1

Some IITA Cowpea Varieties Bred for Multiple

Resistance to Disease and Insect Pests”

Insect pest resistance‘











1I .










TV x 3236






























a Sources: “Varietal Improvement of Cowpea.”

IITA Crop Production Training Series. Singh, S. R.

1986. Trop. Grain Legume Bull. 32, 10-24.

Varieties I , 2,3,4,5,7,8,9. 10, and 12 possess

multiple resistance to diseases. IT81D-I 137, for instance, is resistant to a total of 1 1 fungal, bacterial,

and viral diseases. TV x 3236 and IT84E-124 are

resistant to at least 7 such diseases.

A, B, C, and D represent, respectively, leaf

hoppers, aphids, thrips, and bruchids; + denotes

resistance; a blank space denotes lack of resistance

to the insect pest.



region of 6 tlha (IITA Research Briefs, 1984). However, it must be stated

that this projected target may be rather overambitious for a field crop such

as cowpea growing for 60-odd days.

A yield potential of between 3 and 4 t dry seed/ha may be more feasible.

The attainment of even this lower yield requires, on the one hand, a

reevaluation of the nagging problems of insect pests and fungal diseases

peculiar to the various ecological zones. Second, it also requires the

adoption of a technology that in consonance with conventional plant

breeding methods used so far at IITA seeks to both increase the genetic

diversity of the cowpea and improve the selection of desirable characteristics. These two considerations are explored in this chapter.


Eight out of the 31 insect pest species identified in the cowpea by Singh

and Jackai (1985) constitute the major preharvest entomological pest problem in the Rain Forest Belt of Nigeria. These insects are:

1. The cowpea leaf beetle, Ootheca mutabilis (Shalberg), which, though

a sporadic pest, may cause total crop loss through foliage destruction. It is

also an important virus vector.

2. The cowpea bud thrip, Megalurothrips sjostedti (Trybom) = Taeniothrips sjostedti (Trybom), which may cause total crop loss through

interference with flowering.

3. The cowpea aphid, Aphis craccivora (Koch), which causes up to 35%

damage when pods are infested and also transmits a number of virus


4. The cowpea pod borer, Maruca testulalis (Geyer), the caterpillars of

which may cause 60% or more damage when coincident with those of

Lasperyresia ptychoru (Meyrick), a minor lepidopterous pest; and the

following cowpea coreids or pod suckers:

5. Acanthomia tomentosicollis (Stalberg) = Clavigralla tomentosicollis


6. Acanthomia horrida (Germar) = Clavigralla shadabi (Dolling)

7. Anocplocnemis curvipes (Fabricius)

8. Riptortus dentipes (Fabricius)

These coreid bugs feed massively on young developing pods and foliage of

cowpea and may cause up to 60% yield loss. Figure 3 illustrates the

activities of these pests in relation to the phenology of the crop.

Sources of resistance have been identified within the massive collection



FIG.3. Spans of activity of the major preharvest insect pests of cowpea, V. unguicduta,

in relation to the crop’s phenology. Shaded areas of bars denote span of peak activity.

Source: Singh, S. R. 1980. In “Biology of Breeding for Resistance to Arthropods and

Pathogens in Agriculture Plants” (M. K. Harris, ed.), Texas A & M Univ. Bulletin, MP-1451,

p. 398-42 1 .

of cowpea germplasm (about 12,000 accessions) at IITA for thrips

( M . sjostedti), aphids ( A . cracciuora),and the major harvest insect pest,

Callosobruchus maculatus (Fabricius), a curculionid beetle. These

resistance genes have been bred successfully in different combinations

into such cowpea lines as TVx3236, IT81D-1020, IT82D-716, IT81D-994,

and IT84E-124. The list of IITA varieties with insect pest resistance is

given in Table 11.

Sources of resistance to Ootheca mutabilis, Maruca testulalis, and the

coreid pod suckers are as yet unidentified. For these pests, chemical

control by insecticide applications remains the only pragmatic means of

protection. Given the medium-sized farms replete in an underdeveloped

country such as Nigeria, and the current depressingly low economic fortunes of the country, the rising costs of farm inputs continue to cut rather

deeply into the farmer’s profit margin. Moreover, the prices of such insecticides as Cymbush Sherpa, Decis, and Thiodan, which are prominent

items on the IITA’s production package for cowpeas, are high enough to

edge these all-important chemicals out of the reach of the small-time

farmer and thus depress his yield. In a virtually insect-pest-susceptible

variety like Ife-Brown, this could mean total crop loss, and even in IITA

advanced breeding lines a fourfold reduction in yield (IITA Research

Briefs, 1984). For this class of farmer, efforts that are aimed at introducing



into existing cowpea varieties and lines effective and durable resistance to

these destrutive insect pests remain the only panacea to the problem. Such

research must aim at introducing, for example, genes for antibiosis, nonpreference mechanisms as well as leaf pubescence for the control of

Ootheca mutabilis, genes for antibiosis for the control of Maruca testulalis, and genes for scabrous and pubescent pods for the control of the

coreid pod suckers. These may be achieved, theoretically, by attempting

crosses between the cowpea and other closely related Vigna species (such

as V . uexillata) as well as wide crosses with the more distantly related

genera of the Phaseoleae. The existence of incompatibility barriers has

been highlighted earlier.


Fungal diseases account for losses in the cowpea surpassed only by the

damage due to insect pests (Emechebe and Shoyinka, 1985). Out of these,

anthracnose and brown blotch (caused by Colletotrichum lindemuthianum

[Sacc. & Magn.] Bri. & Cav., and C . capsici [Syd.], respectively), leaf

spots (incited by Cercospora canescens Ell. & Mart., and Pseudocercospora cruenta [Deighton] Sacc.), web blight (caused by Rhizoctonia

s o l d [hill. & Delacr.] Bourd & Galz), and wilts (caused by Fusarium

oxysporum f.sp. tracheiphilum Schlect, ex Fr. and Sclerotium rolfsii

Sacch.) have been identified as the most damaging on cowpea in the Rain

Forest Belt of Nigeria (Emechebe and Shoyinka, 1985). Sources of resistance to some of these diseases have been identified in the IITA germplasm

accessions and varieties and lines such as TVx3236, IT82E-716, IT82D699, IT83D-326-Z, IT8 1 D- 1 137, IT835-1 1, and IT8 1D- 1020 have been bred

and selected from multiple resistance to various fungal diseases. More

significantly, these lines combine this characteristic with degrees of insect

pest resistance and high grain yield potentials also.

Resistance to web blight, a destructive foliar disease, remains largely

unresolved. This disease is caused by the imperfect fungus Rhizoctonia

solani (with Thanatephorus cucumeris [Frank] Donk. as its basidiomycete

teleomorph) and is a major problem in cowpea production in the humid

Rain Forest Belt. No cowpea variety is wholly resistant to this disease, to

this author’s knowledge. The development and deployment of an effective

and durable means of protection through breeding methods is therefore


Although descriptive studies abound on the biology and control of a

number of the fungal diseases mentioned (Williams 1975; Emechebe, 1981;

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VI. Limitations to Development of Root Growth Models

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