GENETIC ANALYSIS OF DROUGHT TOLERANCE IN COWPEA [VIGNA UNGUICULATA (L.) WALP] - ABSTRACT
Agriculture in SSA is under serious threat due to water shortage, population pressure and climate change. Cowpea, a protein-rich legume crop which is an important component in the diets of rural and urban people of the tropics and sub-tropical regions of the world thus plays a significant contribution to sustainable food and nutrition security in SSA. Cowpea is predominantly grown after the rainy seasons under reducing soil moisture which predisposes the crop to drought which may occur early, mid and/or late in the cropping season causing reduction in yield. Increasing the level of drought tolerance in existing cowpea varieties grown by farmers would enable them to obtain more and stable yield from their cowpea fields. The objectives of this study were to: (i) identify the impact of drought on cowpea production and farmers‘ preferred traits in cowpea varieties (ii) assess the diversity of cowpea germplasm for drought tolerance (iii) assess the combining ability of cowpea lines under drought and well-watered conditions and (iv) and determine the gene action controlling drought tolerance in cowpea. Result of a Participatory Rural Appraisal (PRA) conducted in fifteen cowpea growing communities of Kano State, Nigeria established that drought, pests and diseases were major constraints to cowpea production. Farmers indicated that drought causes up to 73% and 81% reduction in grain and fodder yields respectively. Consumer-based traits such as large seed, short cooking time and dual-purpose varieties which increase farmers‘ income were identified as important preferred traits as well as traits for biotic and abiotic tolerances in new cowpea varieties. Ninety-one cowpea varieties were screened for tolerance to drought using the wooden box screening technique. The aim of this study was to identify parents to be used for genetic studies. Twenty lines were selected based on their responses to the screening and were mated in a North Carolina Design II design to generate 100 single F1 crosses.
The F1 progenies and their parents were evaluated under drought and well-watered conditions at two locations. Grain yield of the F1 progenies ranged between 2533 kg ha-1 for TVu6707 x TVu9797 and 18 kg ha-1 for TVu11986 x TVu2736 under drought stress, 3786 kg ha-1 for TVu6707 x TVu9797 and 45 kg ha-1 for TVu633 x TVu2736 under well-watered conditions. General Combining Ability (GCA) and Specific Combining Ability (SCA) mean squares were significant for grain yield and other traits across all research environments indicating that both additive and non-additive gene effects were important in the control of grain yield and other drought adaptive traits across all research environments. The contribution of GCA (71%) to the total sum of squares was higher than that of SCA (21%) for grain yield under drought stress indicating that additive gene action was more important in the inheritance of grain yield under drought stress. Similarly, the superior positive GCA (GCA-female and GCA-male) effects for 100-seed weight, number of seeds per pod, Normalized Difference Vegetation Index (NDVI) measured at the different growth stages, number of pods and seeds per plant under drought stress indicated that additive gene action was more important in the inheritance of these traits under drought stress. The lines TVu79, TVu6707, TVu9693 and TVu9707 were identified as general good combiners with outstanding positive GCA effects for grain yield under drought stress. They can be used as parents to generate improved cowpea varieties for drought tolerance. Considering both mean yield and stability performance, TVu8670 x Sanzi, IT89D-288 x TVu8670, TVu6707 x TVu79 and TVu8670 x TVu79 can further be advanced for development of novel drought tolerant varieties.