|Keywords:||Leguminosae; International Institute of Tropical Agriculture (IITA); Cell-/Tissue culture; Genetic engineering; Abiotic stress; Disease/pest resistance.|
|Correct citation:||Ogbuinya, P.O. (1997), "Advances in Cowpea Research." Biotechnology and Development Monitor, No. 33, p. 1012.|
Cowpea, a high protein food legume produced in several African countries is adapted to a wide array of soils and moisture conditions. However, average yields cannot be contemplated unless cowpea is accompanied by either heavy doses of insecticides or specific management measures such as intercropping or mixed cropping. The International Institute of Tropical Agriculture (IITA) and collaborators are currently working on varietal improvement based on host plant resistance. However, cowpea is produced mostly by smallholder farmers who have not adopted these newly developed varieties.
Cowpea (Vigna unguiculata) is a leguminous grown throughout the
African continent as well as in some parts of SouthEast Asia and Latin
America. Though native to West Africa, this legume has become a part of
the diet of about 110 million people. Having a protein content of about
25 per cent, this legume is a cheap source of protein for the poor. In
the West African region, where more than 70 per cent of the total world
production is grown, cowpea has become an integral part of the farming
The ratio for this widespread integration into farming systems lies in the multiple secondary use qualities of cowpea. The creeping varieties, for instance, are applied in weed control; the stalks are important fodder material, harvested in the dry season when fresh grass and other fodder materials are not available; and cowpea is an atmospheric nitrogen fixing legume which aids soil fertility when intercropped with cereals.
Although yields of 2500 kg/ha are achievable, several constraints have kept farmers' yields constantly low at levels between 350 and 700 kg/ha. Yields are lowered due to pests, diseases and the nonavailability of improved cowpeas. Insect pests have remained the most important setback to cowpea production, because each phase attracts an array of insect pests. For instance, aphids such as Oetheca mutabilis and Aphids craccivora dwell on the crop in the period between germination and flower initiation. Afterwards, until maturation of the plant, it is endangered by flower thrips pod borers and pod suckers. Finally, from harvesting until storage and marketing, the seeds are under attack from bruchid species such as Callosobruchus maculatus. In Nigeria, IITA estimates that bruchids damage causes annual losses of US$ 30 million. Therefore, for a good yield, regular spraying with pesticides is advocated. But attempts by merchants to preserve the seeds through chemical means have often had undesirable ecological and health consequences. In 1996, Nigeria witnessed rampant cases of food poisoning from cowpea stored with highly toxic pesticides. Since cowpea is cultivated mostly by resource poor farmers in small holdings, solutions to production and storage problems should be on a lowinput and economically sound base. New techniques should also increase yield and improve tolerance to drought prone ecosystems.
Research at IITA using traditional plant breeding to introduce insect resistance has yielded limited success. After screening over 8,000 cultivated cowpea genotypes, the researchers were unable to identify resistance to the most problematic pests in the cultivars. This is because the cultivated cowpea varieties simply do not have genes for resistance to certain insect pests that could be used for crossbreeding. However, excellent sources of resistance were found in wild species, namely Vigna vexillata and Vigna oblongitelia. But attempts to transfer the wild genes into the cultivated species by traditional crossing have proved to be very difficult. After the crosses the resulting embryos abort spontaneously within four to five days.
A breakthrough was achieved in 1995 when the problem of aborting embryos was solved using an in vitro rescue technique. For this, an immature embryo arising from the fertilization in the cross between V. vexillata and V. oblongitelia was transferred into a culture medium. Researchers achieved a 70 per cent plant recovery rate from culturing the rescued embryos. Applying this technique made it possible to transfer genes from wild species into cultivated cowpea varieties, including resistance to the storage weevil (Callosobruchus maculatus), leafhoppers and aphids. Partial resistance to other pests and diseases has also been incorporated into elite varieties conferring multiple resistance. In vitro rescue of embryos is generally used in obtaining interspecific or intergenetic hybrids. With similar techniques it has been possible to grow hybrid in vitro embryos from barley and rye.
IITA also uses genetic engineering to transfer genes from edible legumes into cowpea. Desirable traits such as hardness or hairiness which inhibit insect feeding were found in African yam bean (Sphenostylis steriocarpa), Bambara groundnut (Vigna subterranea) and Hyacinth bean (Lablab purpureus). To insert the genes responsible for the respective traits into cowpea, two different techniques are applied. In the first approach the foreign DNA sequence is forcibly introduced into the cowpea genome by accelerated microparticle bombardment. The second method is based on the use of Agrobacterium tumefaciens as a vector to create transgenic cowpea. IITA scientists obtained cowpea embryos through somatic embryogenesis. To conduct a gene transfer, the embryos were then cultivated with Agrobacterium tumefaciens. However, developing a regeneration medium for transformed tissue was the most limiting phase of the process. For this, problems arising from somaclonal variation, which is the spontaneous genetic mutation of plant cells in tissue culture, had to be solved. Based on a new culture medium a method was developed that could prevent somaclonal variation. It has been repeated and confirmed at two laboratories cooperating with IITA, the John Innes Centre, UK and the Portici University, Italy. However, so far it has not been possible to develop a regeneration method that works for all varieties.
To obtain a better understanding of the genetic makeup of cowpea and its wild relatives, IITA and collaborators are constructing genetic maps using restriction fragment length polymorphism (RFLP). The genetic maps will help scientists to identify the approximate location of specific genes for desired traits within the plant genome. This would limit the amount of plant DNA that must be searched to isolate a given gene, and also help to accelerate traditional breeding.
Photoinsensitive and droughtresistant varieties
Cowpea varieties used in traditional cropping systems are photoperiodsensitive. This important feature regulates the synchronization of crop growth with the occurence of the raining season. However, the IITA breeding programme also strives to develop erect and semierect cowpeas that mature early, resist drought, are insensitive to variations in day length and produce higher yield. One of the most successful of IITA's erect and photo insensitive varieties matures in 90 days. It is currently used in North Nigeria, the Philippines and other parts of South East Asia in rice based mixed cropping systems. Farmers sow this cowpea in their fields immediately after rice is harvested and when the field normally would lie fallow. Using only the residual moisture in the soil the legumes mature yielding a valuable extra harvest. These varieties are drought resistant and have been distributed in wartorn, drought prone Mozambique. Other varieties can be intercropped in cereal (sorghum and pearl millet) farming systems of the Sudan Savanna and southern Sahel.
Another high yielding variety, IITA IT89KD, is the result of a fiveyear research effort which started in 1991 when IITA scientists from Kano, Nigeria, initiated an evaluation of cowpea varieties for adaptation to dry season planting (January to May). This variety, when planted around the end of January, matures by the end of April. It could therefore be harvested well before the onset of the rains in Kano and other similar ecosystems in northern Nigeria. Cowpea yields range between 1,500 and 3,000 kg/ha and fodder yields reach 4,000 to 6,000 kg/ha.
Limitations and constraints
However, despite the breakthrough in research at IITA most of the cowpea farmers are not familiar with these newly developed varieties. There is a big gap between research results and their transmission to rural farmers. Most farmers are still cultivating the traditional varieties, even though the newly bred cultivars seem to better meet their needs.
In 1994, a survey conducted by IITA and the University of Ibadan tried to investigate farmers' and consumers' preferences with regard to IITA's newly developed cowpea varieties. Based on the assessment of different characteristics such as absence of weevils, cooking and peeling time and taste, the IITA developed cultivars were preferred.
At present, the poor extension system prevalent in most SubSaharan African countries is seen as the most important constraint to distributing the newly developed varieties. In Nigeria, the World Bank assisted agricultural development programmes have implemented an extension system to deliver research packages to farmers. However, this effort has not met the expectations of the farmers. As a result, many farmers who could be willing to adopt the new technologies are still handicapped by lack of knowledge or by the shortage of the improved seed material.
Patrick Osondu Ogbuinya
Ebony State Agricultural Development Project, Onueke, P.O. Box 165,
Ezza South L.G.A.,
Ebonyi State, Nigeria. Email email@example.com
IITA (1995), Annual report and research highlights. Ibadan, Nigeria: IITA.
Personal communication with Michael Okocha (University of Agriculture Umudike, Nigeria).
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