The genetic, morphological and physiological evaluation of African cowpea genotypes

English: Cowpea (Vigna unguiculata) is a staple food crop of significant economic importance worldwide. Cowpea is valued for the high vitamin and mineral content present in young leaves, pods and seed. The crop is unique in that it provides food, cash and fodder. However, cowpea production is limited by numerous biotic (insects, microbial and fungal diseases, and other pests) and abiotic (high temperature, drought) factors. Because of its widespread use, numerous initiatives have been undertaken to improve various agronomic and nutritional traits of cowpea. Analysis of genetic relationships among and within crop species is a prerequisite for any genetic improvement and central to successful breeding programmes. This can be achieved through characterisation of germ plasm either using morphological, biochemical or DNA markers. Fifteen qualitative morphological traits and 12 quantitative morphological traits were used in this study to group 20 cowpea accessions from three different African regions into clusters according to their genetic distance, in order to quantify the extent of phenotypic and genetic diversity among these accessions and identify desirable groups that could be utilised in breeding programmes. The results showed a relatively high level of dissimilarity among the accessions for most of the morphological traits analyzed, especially for accessions from different countries. This indicates better possibilities for genetic improvement of the crop through selection and cross breeding. However, a very high level of similarity was revealed between many accessions from the same region for most of the characters studied. Makueni, Gacaga, M.66, Kamurugu 1, Bafoussam 1, MTA22, AS-94 and Okhalweni were the group of accessions different from the other accessions for most of the traits studied and can therefore be used for cowpea improvement programmes through cross breeding. Drought is a serious environmental stress affecting cowpea production throughout the world. Twenty cowpea accessions were characterised for drought tolerance using three different screening methods (pot screening method, cell membrane stability and stomatal behaviour). For the pot screening method, seeds were planted in pots in the glasshouse at the University of the Free State. After germination plants were kept well watered, until they had completed the early development stage, after which 500 ml of water was given per day per pot up to the flowering stage when the watering was terminated. After 21 days, each plant was scored using the International Board for Plant Genetic Resources (IBPGR) descriptors for cowpea. The results showed a wide range of drought tolerance among the studied accessions and this suggests the possibility of breeding cowpea drought tolerant cultivars. Bafoussam 1 and Bafoussam 2 were found to be highly drought tolerant and could be valuable materials for drought tolerance improvement in cowpea. Drought tolerance potential of the accessions was also characterised based on their ability to close their stomata, and the number of stomata per unit leaf area under water deficit conditions. The results showed that cowpea (Vigna unguiculata) in Kenya, South Africa and Cameraan were rich in drought tolerant accessions. The general trend showed that accessions from the humid areas of Kenya and Cameraan had a relative high stomata density and also had the lowest ability to regulate the degree of their stomatal opening under drought stress. Meanwhile accessions from the arid and semi-arid areas of South Africa, Cameroon, and Kenya did show a relatively good ability to regulate the degree of their stomatal opening under the imposed drought stress and had a relatively low stomatal density. Materials with good drought tolerance included Bafoussam 4, MT22, AS-94, Balen, Hluhluwa, Mpenbeni, Bafoussam 5 and K.80. However, Bafoussam 4 was the only accession found to have both a low stomatal density and a good ability to regulate the degree of its stomatal opening under the imposed drought stress. Therefore it was not possible to confirm any correlation between the ability to regulate the degree of stomatal opening and stomatal density among the accessions studied. The results of the cell membrane stability test also showed that there was significant variation among the accessions for electrolyte leakage. Kamurugu 2, Ken-Kunde, Kasuku, and K.80 had the highest percentage level of injury and were found to be the most susceptible to drought stress, when the accessions were exposed to simulated osmotic stress. Bafoussam 2, Okhalweni, and Bafoussam 4 had the lowest percentage level of injury and were found to be the least susceptible to drought stress. The results of the three techniques seemed to be promising in discriminating the accessions tested for their drought tolerance ability as presented in Appendix. However, extensive field screening is required to confirm the results obtained in this study