Breeding for Grain Quality Traits in Cowpea [Vigna Unguiculata (L) Walp]

Abstract

This study was conducted in the Northern guinea and Sudan savannas of Zaria and Kano, Nigeria to: (i) identify cowpea production constraints and assess farmer and consumer perceptions and preferences on grain quality traits in cowpea, (ii) assess the variability of grain nutritional values (protein, iron and zinc contents) of cowpea accessions from Africa and other parts of the world, (iii) determine the mode of inheritance of iron and zinc contents in cowpea grains and (iv) determine the relationship between zinc concentration and yield components. Farmers pointed out inadequate improved cowpea seed at planting time (rainy season) and lack of resistant varieties to pod sucking and pod boring insects as the major constraints to cowpea production in the areas. Farmers preferred a cowpea variety that combines high yield with rough texture, white or brown colour. Consumers‘ grain quality preferences were consistent with those of farmers, except for women who prioritized cooking time and oil consumption. Significant genetic variations were observed in: zinc content (1.01 to 329.15 mg/kg); iron content (10.01 to 386.3 mg/kg); protein content (1.72 to 29.93%) and other physicochemical properties of grain. Many of these variations can be generated by conventional breeding methods to address the nutritional needs in developing countries. In addition, 3 accessions: TVu-13088, TVu-13495 and TVu-9725 that combined the largest number of desirable nutritional attributes were identified which can be nominated for anti-nutritional factor testing prior to recommendation for infant diets formulation and other use. For nutrient enhancement, five accessions each were identified for protein, zinc and iron contents. Genetic diversity of 169 cowpea accessions using 119 SNP markers clustered the accessions into two main groups on genetic distances (0.00 to 0.212) with small genetic differentiation (0.26 to 0.45) between African and USA cowpea accessions. This indicates that the entire genetic diversity in the African germplasm might already have spread over cowpea-growing regions in the world as a whole, though not completely within any single region. Polymorphism Information Content (PIC) values ranged from 0.2366 (7344_500 SNP) to 0.427 in two SNP markers (4749_1972 and 14929_258). Weak negative correlation existed between iron content and fat (r= -0.18, P < 0.007), iron content and carbohydrate (r= -0.18, P < 0.007) but iron content was positively correlated with protein (r= 0.26, P < 0.001) content. Fat content was negatively correlated with ash content (r= -0.13, P < 0.05) and protein content (r= -0.85, P < 0.001). Ash content correlated positively with protein (r= 0.14, P < 0.05) and negatively with carbohydrate (r=-0.23, P < 0.001). Protein was negatively correlated with carbohydrate (r= -0.29, P < 0.01). Zinc concentration showed weak negative significant (r= 0.03, P < 0.05) correlation with number of pods per plant. No significant correlation was observed between zinc concentration and grain weight. Similarly negative correlation was observed between zinc content and number of pod per plant implying an increase in zinc content may lead to decrease in pod yield. Generation mean analysis of the six basic generations was significant (P < 0.001) among the generations. The significant variation of generation mean performance with transgressive segregants among the progenies and two fold increases in mean zinc concentration of F2 population over the high zinc content parent (82.54 mg/kg > 37.70 mg/kg), implies that cowpea can be enhanced with essential micronutrients using conventional approach. Additive [a] and additive by dominance [ad] model explained the inheritance of iron and zinc content in cowpea grain. An indication of maternal effect was observed in iron content inheritance indicating non suitability to make selection in early generation of selfing. Both seed weight and plant height are predominantly under complementary gene actions in this study, suggesting the possibility of considerable amount of heterosis for seed weight and fodder yield which are the determinants for choosing cowpea varieties,. The study revealed that: Variability of grain nutritional traits (Zn,Fe and Protein) among cowpea was observed. Genetic pattern of iron and zinc content elucidated and Possibility of enhancement in cowpea using conventional approach is realized.