Yield stability, correlation and homogeneity of advanced cowpea lines in Uganda
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Genetic stability coefficients, selection indices and homogeneity are important parameters that guide breeders in deciding what variety to release. The objectives of this study were to determine the yield stability, correlation coefficients and homogeneity of advanced cowpea lines. A total of 30 genotypes were tested against six checks in three locations (MUARIK, Kabanyolo) in central, NaSARRI in Serere in eastern and Abi-ZARDI in Arua in west Nile for three seasons (2017A, 2017B and 2018A). The experiments were laid in a 6x6 alpha lattice design with three replications. Grain yield per hectare was used to determine the stability and performance of the genotypes across locations and seasons, while yield and its components were used for correlation and path analysis. Anthocyanin coloration, leaf, pod, seed characteristics and key agronomic traits were used for homogeneity and variability tests. Single-site and multi-location ANOVA were used to determine variance components and F-test to detect significance of the main effects of environment (E), genotype (G) and genotype by environment interactions (GxE). Further analysis of stability was visualized using the genotype and genotype by environment interaction (GGE) biplot and the Additive Main Effect and Multiplicative Interaction (AMMI) models. The homogeneity of the genotypes was analyzed using the off-type procedure, while the coefficient of genotypic variation (CGV) was calculated as per standard procedures suggested by Burton (1952) and Johnson et al. (1955) to determine the variability among the genotypes. Phenotypic and genotypic correlation coefficients were computed according to the formula given by Webber and Moorthy (1952) and tested for significance at P < 0.05 according to Yate and Fisher (1954), while path coefficient analysis was employed to disentangle the direct and indirect effects of yield components on grain yield according to the formula given by Dewey and Lu (1959). ANOVA depicted highly significant differences among the genotypes, locations, seasons and GEI for grain yield. Based on AMMI analysis, environmental effect accounted for the most variation (84.7%) in the phenotype followed by GE (9.45%) and genotypes (4.45%), alluding to the complex inheritance of grain yield in cowpea. The polygon view and the average environment coordination view of the GGE biplot revealed Ayiyi as the wining genotype in the major mega environment and the most stable and high yielding across environments respectively. The genotypes Ayiyi, WC64 and ALEGIxACC2 yielded higher than the checks and were very stable. The other genotypes, G36 (WC 36), G3 (ACC12xSECOW3B), G32 (WC16), and G14 (MU9) did not outperform the checks but displayed high yield stability and the mean yields were above the overall average. These genotypes were considered desirable for advancement to National Performance Trial for potential release as new improved cowpea cultivars. Strong, positive and highly significant genotypic correlations were observed between grain yield per plant and number of branches (r = 0.97), number of pods per plant (0.94), 100 seed weight (0.91), number of peduncles per plant (0.9) and peduncle length (0.87). The 100 seed weight had the highest direct and positive effect on grain yield per plant (0.701) followed by number of pods per plant (0.214), number of peduncles (0.206) and maturity date (0.201) at genotypic level, signifying their importance as selection indices for the improvement of cowpea lines. Negative direct effects were observed on grain yield with days to flowering (-0.332), leaf length (-0.368), leaf width (-0.227), number of branches (-0.011), pod width (-0.021) and peduncle length (-0.251). Basing on these results, the number of pods per plant, 100 seed weight, number of peduncles per plant, and peduncle length could be used for the selection of superior cowpea genotypes. All the genotypes were homogeneous for flower colour, growth habit, leaf/peduncle anthocyanin and pod colour. However, leaf colour and leaf size showed significant variations among the plants examined. Based on CGV, there was moderate level of variability among the genotypes since the highest CGV observed was between 10% - 20% for number of pods per plant, number of branches, number of peduncles, peduncle length, 100 seed weight, grain yield per plant, number of seeds per pod, days to maturity and days to 50% flowering. Based on the overall research output, the genotypes, Ayiyi, Alegi*ACC2, NE23, NE37, WC36, WC64, and the checks NAROCOWPEA1, NAROCOWPEA3 and NAROCOWPEA4 were the most stable and high yielding, had the best yield attributes and were generally homogeneous but unique from the other genotypes. These genotypes could be subjected to participatory variety selection and national performance trials (NPT), thereby promoting the process of their release as varieties or their use as parents for introgression of useful genetic variants into other elite cultivars.