Cowpea: Origin, Taxonomy and Varieties | India

After reading this article you will learn about: 1. Origin of Cowpea 2. Taxonomy of Cowpea 3. Botany 4. Qualitative Genetics 5. Descriptors List for Characterisation and Evaluation 6. Breeding Objectives 7. Breeding Procedures 8. Breeding For Resistance to Diseases 9. Breeding For Resistance to Insect Pests 10. Varieties 11. DNA Markers and Transgenic Plants.

Origin of Cowpea:

All the evidences indicate that cowpea originated in Africa. The exact place of domestication is uncertain. Ethiopia, Central Africa, Central and South Africa and West Africa, all have been considered as probable centers of domestication as reviewed by Ng and Marechal (1985).

In India, cowpea is known since Vedic times. According to Simmonds (1976) West Africa and India both are modern centers of diversity for this crop. However, it is generally agreed that the cowpea is of African origin as conspecific wild forms are found in Africa but are absent in Asia.

Cowpea (Vigna unguiculata, 2n = 2x = 22) also called as southern-pea and black-eyed pea, is well adapted to the tropics. The major cowpea growing countries are Nigeria, Niger, Burkina Faso, Ghana, Kenya, Uganda, Malawi, Tanzania (all in Africa) and India, Sri Lanka, Burma, Bangladesh, Philippines, Indonesia, Thailand, etc. In India, the estimated area is about 50,000 hectares.

Cowpea is primarily used in the form of dry seeds, fodder, green pod, green manure, and cover crops.

Taxonomy of Cowpea:

Most nomenclature problems in the cultivated cowpeas and related wild species have been resolved to great extent. Experts agree that cowpeas belong to the botanical species Vigna unguiculata (L.) Walp. There are more than 20 synonyms for V. unguiculata. There are three cultivated and two wild sub-species of cowpea.

These are as follows:

Some authors do not consider the three cultivated subspecies as distinct and group them under one subspecies V. unguiculata subsp unguiculata and differentiate them by the intraspecific category/cultigroup. The subspecies unguiculata, cylindrica and sesquipedalis are renamed by Marechal and his colleagues as cultigroups Unguiculata, Biflora and Sesquipedalis, respectively.

Cultigroup Unguiculata is the most diverse of the cultivated subspecies unguiculata and has the widest distribution. It is commonly called cowpea and is grown in Africa, India and Brazil. These are prostrate, semi-erect, erect or climbing. Pods are coiled, round, crescent or linear. Pods are 20-30 cm long and small seeded.

Biflora is commonly called catjang bean and is used as dry seeds and fodder. It is frequently prostrate and sometimes climbing. Pods are usually smaller (7-13 cm) and are held upright on the raceme axis. Pods are more or less erect. Seeds are small and kidney shaped (5-6 mm long).

Sesquipedalis is known as yard-long or asparagus bean. Its pods and sometimes leaves also are used as vegetable. It is mostly climbing. Flowers are larger. Plants are trailing or climbing. Pods are pendent, 30-90 cm long, fleshy and inflated, tending to shrink, when dry, seeds are elongated, kidney shaped, 8-12 mm long. Such cultivars are found in Indonesia, Philippines, Sri Lanka and are also grown in India.

The relationship between cultigroups Biflora, Sesquipedalis and Unguiculata is shown in Fig. 17.1 as illustrated by Som and Hazra (1993).

Cultigroup Textiles is an old primitive cultivar and is rarely found in farmers’ fields.

Botany of Cowpea:

Growth habit ranges from erect, determinate, non- branching type to prostrate or climbing, indeterminate, with profuse branching. It has strong tap root system with several lateral roots.

Stems are cylindrical and slightly ribbed, twisting, sometimes hollow and glabrous. Stems may be green or pigmented (purple). Leaves are alternate, trifoliate, with one symmetrical terminal leaflet and two asymmetrical leaflets. Petioles are 3-25 cm long with a swollen pulvinus at the base.

Inflorescence is an un-branched axillary raceme bearing several flowers at the terminal end of peduncles. The peduncles vary from 5 to 60 cm in length and are slightly twisted and ribbed. Calyx is longitudinally ribbed, tubular with 2-15 mm long sub-equal lobes.

The corolla is papilionaceous with an erect standard petal spreading at anthesis. The pigmentation pattern of corolla varies from white to solid mauve with yellow spots near the base of the standard petal. The wings are adherent to the boat-shaped keel, enclosing the androecium and gynoecium.

The stamens are diadelphous (9+1). Anthers are bright yellow. Ovary is monocarpellary, unilocular with many ovules. Pods are pendent or vertically attached to the raceme axis. They are mostly linear, although curved and coiled shapes are also found. The length of pods may vary from less than 11 to more than 100 cm.

The diploid chromosome number is 2n = 2x = 22. Mukherjee (1968) studied the pachytene chromosomes and reported that the 11 bivalent complement, consisted of 1 short (19 µm), 7 medium (26-36µm), and 3 long (41-45 µm) chromosomes.

Chromosomes are small and difficult to manipulate. Advanced cytogenetic techniques, such as fluorescent staining of chromosomes, silver staining of nucleolar organising regions, and in situ hybridization are beginning to be employed and promise to be useful to plant breeding programs in the future.

A linkage map for cultivated cowpea has been constructed that spans 916 cM over 12 linkage groups and includes 133 RAPDs, 19 RFLPs, 25 AFLPs, and 3 morphological markers.

Cowpea flowers are large and showy. Flowers open only once between 7 and 9 am. On cloudy days the flowers may open even in the afternoon. Though the flowers open late in the morning, the dehiscence of the anthers is much earlier. It may vary from 10 pm to 0.45 am.

The dehiscence is influenced by environmental factors like presence of moonlight, a clear sky and a dry warm atmosphere. During dark nights the dehiscence tends to be delayed. Due to dehiscence taking place before the opening of flowers, the cowpea is strictly self-pollinated in nature.

Since the dehiscence of anthers is much in advance of the blooming, the emasculation needs to be carried out in mature flower buds in the preceding evening. The flower buds likely to bloom the next day (recognised by large size, the yellowish colour of the back of the standard petal) is selected for emasculation. The bud is held between the thumb and the fore-finger with the keel side uppermost.

A needle is run along the ridge where the two edges of the standard unite. One side of the standard is brought down and secured in position with thumb. Same thing is done with one of the wings. After this the exposed keel is slit on the exposed side, about 1/16 inch from the stigma.

A section of keel is also brought down and secured in position under the end of thumb. Now 10 stamens are seen. They are removed with pointed forceps. Afterwards, the disturbed parts of standard, wing and keel are brought in original position as far as possible. To prevent drying out of the emasculated bud, a leaflet may be folded and pinned around the bud. A tissue paper can be used to cover and protect the bud.

Pollination is done next morning from a freshly opened flower. The standard and wings of male flower are removed. By slight depression of the keel, stigma covered with pollen grains protrudes out. This itself can be used as a brush for pollination. Cowpea flowers are highly sensitive and drop off easily with slight mechanical disturbance or injury. Therefore, much labour and time is devoted to get enough crossed seed.

Under improved technique of Rachie (1975), the time taken for both emasculation and pollination has been reduced substantially and the pod set has increased from 18.6 to 26.1 per cent. A simple technique of emusculation and pollination has been standardized at IIVR, Varanasi by research fellows working for thesis/project work.

In this process of emasculation, top portion of corolla of an unopened, immature bud is removed and 10 anthers are pulled out from the top of the bud. A cotton pad is put on the emasculated bud to protect it from cross-pollination and also for easy identification next morning for pollination. Emasculation is done in late afternoon.

For pollination next morning, freshly opened flower from male parent is taken. The portion of keel containing anthers and style with stigma is taken out from the freshly opened flower. Anthers and style from this keel cap are removed. The keel cap is put on the stigma of emasculated flower bud. The pollen mass collected inside the keel cap pollinates the emasculated flower bud (P.K. Singh-Personal Communication).

Qualitative Genetics of Cowpea:

Several scientists (R.L. Fery, S.C. Harland, J.D. Franckowiak, L.N. Barker, A.R. Saunders, W.J. Spillman, O.A. Ojomo, J.D. Norton, N.K. Sen, P.N. Patel, B.B. Singh etc.) have contributed to the understanding of qualitative genes of cowpea. Quite often different gene symbols were assigned to the same gene and some-times, no gene symbols were allotted.

Fery (1985 a) has compiled an exhaustive list of 159 genes and proposed standard gene symbols based on standard gene nomenclature rules being followed by the International Committee on Gene Symbols and Tomato Genetics Cooperative.

A few important genes from that list are given in Table 17.1:

Descriptors List for Characterisation and Evaluation of Cowpea:

Gupta (1995) have given the following descriptors for cowpea and other legume vegetables:

1. Characterisation Traits:

1.1. Botanical type (specify)

1.2. Growth habit:

i. erect (bushy)

ii. Semi-erect

iii. Semi-spreading

iv. spreading

v. trailing (postrate)

1.3 Growth Pattern:

i. determinate

ii. semi-determinate

iii. indeterminate

1.4  Basic plant vigour:

i. highly vigorous

ii. vigorous

iii. average vigour

iv. poor

v. very poor

1.5 Raceme Position:

i. mostly above canopy

ii. upto upper 1/3^rd of canopy

iii. throughout the canopy

1.6 Pod attachment of peduncle:

i. pendent

ii. 30^o to 60^o from erect

iii. erect

1.7 Utility type:

i. vegetable type

ii. dual-1 (vegetable + seed) type

iii. seed type (pulse type)

iv. dual-2 (seed + fodder) type

v. fodder type

vi. dual-3 (fodder + vegetable) type

2. Flowering Characteristics:

2.1. Days to first flowering

2.2. Days to 50% flowering

2.3. duration of flowering:

i. short

ii. medium

iii. long

2.4. Number of flushings

3. Maturity Traits:

3.1. Days to first pod picking (green stage)

3.2. Days to first pod maturity

3.3. Days to 50% maturity

3.4. Days to complete maturity

3.5. Senescence:

i. uniform maturity

ii. overlapping maturity (over 2 consecutive flushings)

iii. highly variable maturity range

4. Plant Canopy Characteristics:

4.1. Plant height (cm)

4.2. Number of primary branches

4.3. Leafiness

4.3.1. Leaf size:

i. large

ii. medium

iii. small

4.3.2. Leaf density:

i. dense

ii. medium

iii. sparse

4.3.3. Leaf shape:

i. normal

ii. moderately dissected

iii. highly dissected

4.4. Number of clusters per plant

5. Pod Characteristics:

5.1. Number of pods per cluster

5.2. Pod length (cm)

5.3. Pod shape:

i. straight

ii. slightly curved

iii. highly curved

5.4. Pod fibrousness (at green picking stage):

i. very soft

ii. average

iii. highly fibrous

6. Seed Characteristics:

6.1. Number of seeds per pod

6.2. Seed size:

i. big

ii. medium

iii. small

6.3. Seed shape:

i. round

ii. oval

iii. rhomboid

iv. crow-edge

v. others (specify)

6.4. Seed colour description (specify)

6.5. 100-seed weight (g)

7. Shattering:

i. non-shattering

ii. slightly shattering

iii. medium shattering

iv. shattering

v. highly shattering

8. Yield Traits:

8.1. Yielding ability aspect (yield potential) (visual score): based on number of seeds per pod, pods per cluster, clusters per plant etc.:

i. good

ii. above average

iii. average

iv. below average

v. poor

8.2. Seed yield per plant (g)

8.3. Seed yield per plot (kg/ha)

8.4. Seed production potential (visual score): Based on factors narrated in 8.1 plus no. of pods/plant, plant canopy structure etc.

8.5. Vegetable pods production (visual score)

8.5.1. Potential:

i. high

ii. medium

iii. low

8.5.2. Duration of picking:

i. extended over long period

ii. extended over short period

iii. uniform single picking

9. Disease/Pest Status:

0-7 Scale (specify disease/pest)

0. immune

3. low susceptibility

5. medium susceptibility

7. highly susceptible

10. Quality Traits:

10.1. Protein content

10.2. Cooking ability (time to cooking, minutes)

Breeding Objectives of Cowpea:

1. High green pod yield (vegetable type varieties)

2. High seed yield (dry-seed type varieties)

3. High fodder yield (fodder type varieties)

4. Dual purpose (seed and vegetable type and seed and fodder)

5. Earliness

6. Appropriate plant type (erect, determinate for vegetable and seed type cultivars and spreading type for fodder type cultivars)

7. Wider adaptability

8. Photo-insensitive

9. Short tender pods for whole pod processing

10. Long, tender and string-less pods for fresh consumption

11. Varieties suitable for inter-cropping

12. Resistance to diseases:

(i) Anthracnose (Colletotrichum lindemuthianum)

(ii) Cercospora leaf spot (Cercospora cruenta)

(iii) Powdery mildew (Erysiphe polygoni)

(iv) Fusarium wilt (Fusarium oxysporum)

(v) Ascochyta blight (Ascochyta phaseolorum)

(vi) Bacterial blight (Xanthomonas campestris)

(vii) Bacterial pustules (Xanthomonas phaseoli)

(viii) Cowpea yellow mosaic virus

13. Resistance to insects:

(i) Hairy caterpillar

(ii) Leaf hoppers

(iii) Aphids

(iv) Thrips

(v) Bruchids

(vi) Pod borer

(vii) Pod sucking insects

14. Better seed quality (acceptable to consumers):

Medium to large seed size, uniformly white/creamy/light red without black/brown scar around hilum

Breeding Procedures of Cowpea:

The pedigree system of breeding is the most common method used by cowpea breeders. This method has been successful in developing cowpea cultivars with new combinations of characteristics and resistance to diseases. Single plant selections are carried out within large F₂/F₃ populations. Individual plant progenies are planted in one or more rows, 4-6 m in length and 1.5 m apart.

The backcross breeding procedure has been found efficient for transferring single-gene resistance to specific diseases into cowpea cultivars. For example, this procedure has been used to transfer the Cls gene, which provides resistance to Cercospora leaf spot into the susceptible cultivar Colossus in USA.

A combination backcross-pedigree breeding procedure has also been used successfully to transfer a desired trait from a relatively un-adapted genetic background into a well-adapted commercial cultivar which is lacking in a particular trait.

In this approach only one or two and rarely three backcrosses are made and after that the material is handled as per conventional breeding procedure. This method has been found successful by the author himself at Pantnagar in transferring resistance to yellow mosaic virus from a wild soybean, Glycine soja to cultivated soybeans.

Mutation breeding in cowpea has been utilized on a limited scale through irradiation (9-40 kR) by gamma rays to isolate mutants with increased yield and earliness. Mutants with resistance to cowpea aphids (ICV 11 and ICV 12) have been obtained from ICV 1 seeds irradiated with 20k rad of gamma rays.

Surprisingly the mutants differed from ICV 1 in several traits, including having semi-erect rather than spreading growth habit, and longer pods with 19 seeds compared with only 13 seeds/pod for ICV 1 and aphid resistance of ICV 11 was shown to be conferred by a dominant gene.

Breeding For Resistance to Diseases in Cowpea:

The success of a resistance breeding programme depends on identification of stable sources of resistance, their use in large number of crosses and handling of segregating generations in a disease conducive environment, preferably a disease sick plot/hot spot location.

Anthracnose, Cercospora leaf spot, Ascochyta blight and mosaics are serious diseases of cowpea. The major work on disease resistance breeding on cowpea is being carried out at International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Sources of resistance to different diseases in cowpea are given in Table 17.2.

Several improved breeding lines have been developed at IITA which have resistance to bacterial blight and bacterial pustules, e.g., TV_X 1850-01 E, IT 90 K-284-2, IT 90 K- 277-2, IT 86 D-719 and IT 8ID-1228-11. TV_X 3236 is highly resistant to anthracnose and brown blotch.

Breeding line H 8-8-27 developed by University of California has resistance to race 4 of fusarium wilt. Cowpea cultivars IT 82 D-889, IT 835-818, IT 83 D-442 and IT 85 F- 867-5 are reported to be resistant to CPMV, CABMV, CGMV, CMV and SBMV.

Breeding For Resistance to Insect Pests in Cowpea:

Aphids are a serious problem in dry regions, reducing yield not only directly but also indirectly by transmitting viral diseases. Resistant varieties (based on antibiosis) have been developed. Inheritance studies have shown the involvement of a single dominant gene for resistance.

Thrips can cause yield losses up to 100 per cent. The sources are available. Resistance is controlled by two recessive gene pairs. Similarly, donors for bruchid have been identified and inheritance studies have revealed that bruchid resistance is controlled by two recessive gene pairs.

Resistance to pod borer (Maruca testulalis) is dominant and the trait is probably controlled by several genes. The sources of resistance to different insect pests in cowpea are given in Table 17.3. These have been identified by systematic screening of 6000 lines at IITA.

DNA Markers and Transgenic Plants of Cowpea:

Indirect marker-assisted selection can be useful for applying selection pressure in early generations for traits that are difficult to select directly or for shuttle breeding, where it is not possible to directly screen for resistance to a pest due to quarantine restrictions. Isozymes have not been useful for indirect selection in cowpea due to extremely low levels of polymorphisms in cultivated cowpea.

DNA markers should be more effective for developing a linkage map for cowpea. A cowpea linkage map has been developed from a cross between an improved cultivar and a wild subspecies (Vigna unguiculata ssp. dekindtiana). This map consists of 87 random genomic and 5 cDNA RFLPs, 5 random amplified arranged in 10 linkage groups.

Also, a genetic linkage map has been constructed within the cultivated gene pool of cowpea. A cross was made between inbred breeding lines from IITA (IT 84S-2049) and UCR (524B a stable line from ‘CB5’X ‘CB3’), and the map is based on an F₈ recombinant inbred population (94 individuals).

The map consists of 180 loci, comprising 133 RAPDs, 19 RFLPs, 25 AFLPs, and 3 morphological markers. The markers identify 12 linkage groups spanning 932 cM with an average distance of 6.2 cM between markers. Linkage groups ranged from 4 to 268 cM in length and from 2 to 41 markers, respectively. This DNA-linkage map could be useful for indirect marker-assisted selection.

Genetic engineering has considerable potential for making possible unique types of progress in cowpea breeding. For cowpea, it will not replace traditional breeding methods but could provide genes from other species that confer useful resistance to insects and some other traits to cowpea.

Genes coding for resistance to insect pests of cowpea need to be identified, cloned, and transmitted into cowpea, such that the toxic proteins are expressed in the right place, at the right time, and at the right concentration to confer effective resistance. In addition, it is necessary to develop bioassays to test the effectiveness of specific proteins, such as through the rearing of target insect pests on artificial diets.

Some proteins have the potential to confer insect resistance, including the Bacillus thuringiensis d-endotoxin, and various protease inhibitors and lectins. In collaboration with L. E. N. Jackai of IITA, his research group has developed and used bioassay systems to test the effects of these proteins on several insect pests.

They have demonstrated that two B. thuringiensis gene products [cry IA(b) and cry II(a)] are effective against Maruca testulalis, that the cysteine proteinase inhibitor E-64 and a lectin from wheat are effective against Clavigralla tomentosicollis, and that an α-amylase inhibitor can control Callosobruchus maculatus. Unfortunately, an effective system for the genetic transformation of cowpea has not yet been developed to permit the exploitation of these genes.

Progress is being made in the development of genetic transformation systems for cowpea using an in vivo system involving DNA electroporation into axillary buds and systems involving micro projectile bombardment or co-cultivation with Agrobacterium tumefaciens as reviewed by Hall et al. (1997).

Varieties of Cowpea:

Pusa Phalguni:

It is a selection from a promising introduction Dolique Du Tonkin (ex Canada). The plants are dwarf with bushy habit. It gives two flushes of dark green erect pods of 10-12 cm length with small cylindrical white seeds. Pods get ready for harvesting in about 60 days. Yield potential of green pods is 75 q/ha. It is best suited for February-March sowing.

Pusa Barsati:

It is a selection from exotic materials from Philippines. It is an early cultivar (45 days to first green pod picking) suitable for growing during the rainy season. It gives 2-3 flushes of light green pendent pods. The pods are 25-28 cm long and contain large green seeds. Green pod yield potential is about 75 q/ha.

Pusa Do Fasali:

It is bushy type. It has been developed from cross of Pusa Phalguni and a long podded introduction from Philippines. It flowers in 35-40 days after sowing. Pods are light green, erect, about 18 cm long. Plants are bushy. It is photo-insensitive and is suitable for sowing in spring- summer and rainy seasons. Green pod yields are lower in summer plantings than those in the rainy season. The green pod yield potential is about 80 q/ha.

Yard Long Bean (V. sesquipedalis):

It is commonly grown in home gardens in Uttar-Pradesh. It is not suitable for transportation as pods break easily. Pods are about 50 cm long. The plants are viny type and mature late (100 days). Green pod yield potential is 100 q/ha.

Pusa Komal (Sel. 1552):

It has been bred at IARI, New Delhi. It has been recommended for release by the all India coordinated vegetable improvement workshop, 1983. It is a dwarf and bushy cultivar suitable for planting in spring-summer and rainy seasons. Pods are light green, 25-30 cm long. It flowers in 45 days and gives 2-3 flushes. It is resistant to bacterial blight. The green pod yield potential is 100 q/ha.

Pusa Rituraj:

This is a selection in germplasm by National Bureau of Plant Genetic Resources, New Delhi. This is photo-insensitive and can be grown in summer and rainy seasons. First picking is possible in 40-50 days. The plant is bushy. Pods virtually cover the foliage. Pods are 20-25 cm long. It is a dual purpose variety as pods and seeds (brown) both can be consumed. Green pod yield potential is 80 q/ha.

Bhagya Lakshmi:

It has been developed at KAU, Vellanikara. Pods are light green, 30 cm long, borne in cluster of 2-4, resistant to anthracnose, 65 q/ha.

Arka Samrudhi (Arka Garima x Pusa Komal):

Developed at IIHR, Bangalore, photo-insensitive, erect, bushy, 15-18 cm long, green pods.

Arka Garima (V. Unguiculta TUV 762 x V. Unguiculata ssp. sesquipedalis):

Developed at IIHR, Bangalore, tall, vigorous, trailing, light green 20-25 cm long pods.

Narendra Lobia 1:

This variety was evolved at NDUAT, Narendranagar, Faizabad from a cross of L 1552 (now released as Pusa Komal) X Varanasi Local following pedigree method of breeding. It has been released by the U.P. State Variety Release Committee meeting on 7.4.1995. It has determinate plant habit. Plant height is 40-45 cm.

Foliage is green with large leaves. Green pods are 28-32 cm long with purple terminal end. Each pod contains 10-12 seeds. Seeds are bold (18 g/100 seeds) with black hilum. Edible pod maturity is 45-48 days and seed maturity 75-80 days. It is photo- insensitive and can be grown in summer and rainy both the seasons. Green pod yield potential is 90 q/ha.

International Programme:

Cowpea improvement programme forms a major part of research programmes of International Institute of Tropical Agriculture (IITA) located at Ibadan, Nigeria. IITA came into existence in the year 1967. Cowpea improvement programme at IITA in the initial stages concentrated primarily on germplasm collection, evaluation, maintenance, and breeding for disease resistance.

Later on, the emphasis shifted on breeding for insect resistance, early maturity, improved plant types and desired seed quality. There are about 12,000 accessions of cowpea and about 200 accessions of wild Vigna at IITA. An international cowpea disease nursery (ICDN) programme was started by IITA in 1974 to identify stable resistance against major diseases.

The promising lines coming out of this programme were VITA 1, 3, 4 and 5. Breeding efforts in 1980s aimed at developing extra-early cowpea varieties and bush type varieties, combined with resistance to diseases and insects in focus. This gave rise to development of IT 82E-16, -18, IT 82D-807, -828, -889, and -950 having resistance to six major viral diseases (CYMV, CAbMV, CuMV, CMeV, SBMV, CGM).

Extra early varieties (60 days maturity) developed in this programme are IT 82E-32, -9, -56, -5, -60 and bush type varieties are IT 81D-1228-13, -14 and -15. From 1988, the main breeding objectives have been improved drought and heat tolerance and multiple pest and disease resistance. At IITA, every year four generations of breeding lines are advanced and within two years F₆/F₇ lines combining major attributes are developed.

Future Breeding Opportunities:

Cowpea cultivars with complete resistance to the major pests and diseases are required. Achieving adequate levels of resistance to several pests and diseases will require a team effort between classical breeding programs and molecular genetics programs working on cowpea in developing and more developed countries and at IITA.

An effective genetic transformation system is needed. A range of insect-resistance genes and screening methods are available. Consequently, progress in increasing insect resistance of cowpea may be rapid, but inadequate resistance to flower thrip may continue to be a major constraint in Africa.

There is a critical need to develop and use physiological information in breeding and agronomic programs involved in increasing the performance and productivity of cowpea production systems because conventional empirical approaches are making slow progress with this crop.

The main opportunities for improvement arise from the choice of sowing dates and plant spacing’s and development of cultivars that result in:

(1) Adequate plant establishment,

(2) Attainment of full canopy ground cover by first flowering, and

(3) An adequately long and effective reproductive period during the part of the season when temperatures, solar radiation, and soil moisture will support photosynthesis and grain production.

Consideration should be given to breeding cultivars of cowpea and other crop species for the climatic conditions of the next century, especially the projected increases in atmospheric CO₂ and temperature. Elevated CO₂ increases productivity of cowpea and most other C₃ species, but it does not overcome the detrimental effects of high night temperature on reproductive development.

It has been hypothesized that cultivars of C₃ species with heat tolerance during reproductive development, high HI, high photosynthetic capacity per unit leaf area, small leaves, and low leaf area per unit ground area under present levels of CO₂ will be most responsive to elevated atmospheric CO₂ under both hot and intermediate temperatures.