PhD

Legume pod borer is one of the most important post-flowering pests of cowpea in the tropics, which acts as a major limiting factor in cowpea cultivation in all seasons. The present investigation was conducted in the Department of Plant Breeding and Genetics, College of Agriculture, Vellayani during 2002 to 2004, with the objective of studying the nature of inheritance and magnitude gene effects of yield and legume pod borer resistance in cowpea.
Fifty varieties of cowpea were evaluated for resistance to legume pod borer and yield. ANOVA revealed significant variability for all the damage measurements of legume pod borer, the related biochemical and morphological traits and yield characters. Major yield contributing characters like number of pods per plant, grain yield and 100 seed weight showed high coefficients of variation. All the damage related biochemical and morphological traits except content of chlorophyll ‘a’ in the leaf tissues exhibited high heritability and high genetic gain. Important yield contributing characters also showed high heritability coupled with high genetic gain. This results indicate that the underlying additive gene action provides immense scope for improvement through selection.
Plant resistance indices served as the selection criteria for identifying the testers for L XT crossing programme. The plant resistance indices were minimum for T45, T47, and T49 which were selected as testers.
Significant positive correlation was noticed between the damage parameters, but significant negative correlation was noticed with non-glandular trichome density, peduncle length and ratio of chlorophyll ‘a’ to ‘b’. Plant resistance index was positively correlated with all damage parameters and negatively correlated with peduncle length, density of non-glandular trichomes on pod wall and ratio of chlorophyll ‘a’ to ‘b’. Grain yield per plant exhibited highly significant positive correlation with number of pods per plant, number of inflorescences per plant, number of seeds per pod and 100 seed weight. Path coefficient analysis revealed that number of pods per plant followed by 100 seed weight and number of seeds per pod exerted the maximum positive direct effect on grain yield. Pod length contributed to yield through positive indirect effects through all other characters.
Selection indices were worked out for the fifty genotypes for selection of lines for L XT analysis. The genotypes T2, T4, T6, T7 and T8 were selected on the basis of index scores. Mahalanobis D2 statistic was used to group the fifty genotypes into ten clusters. Wide range of genetic divergence was noticed among the 50 genotypes.
The mean performance of parents, estimates of heterosis, general combining ability of parents and specific combining ability of the crosses were evaluated through line X tester analysis. The significance of line X tester interaction suggested the involvement of different gene effects for most characters.
L2 and L3 showed high mean values for major yield characters among the lines. Among the testers, T2 exhibited the least estimates for damage measurements and highest mean values for number of pods per plant and grain yield. L4 X T3 showed high mean values for yield characters and low mean values for damage parameters among the crosses. L1 X T2 followed L4 X T3 with high mean values for important characters like grain yield. L3 X T2 also exhibited high mean values for yield traits combined with low scores for damage measurements.
Desirable negative heterosis was noticed for days to flowering in all the crosses. Positive and significant estimates of all three types of heterosis for number of pods per plant was noticed in seven crosses. Three crosses had positive and significant estimates of heterosis for number of inflorescences per plant, number of pods per inflorescence and grain yield. Negative and significant relative heterosis for percentage flower bud infestation and percentage pod infestation was noticed in three crosses, while for number of damaged seeds per 25 pods, two crosses showed negative and significant relative heterosis. None of the crosses expressed significant negative heterobeltiosis for any of the damage parameter.
Grain yield per plant, number of pods per plant, number of inflorescences per plant, pod length and number of seeds per pod exhibited significant gca effects. Among the lines, L4 and L1 showed good gca effects for important yield characters. Among the testers, T2 displayed desirable gca effects for damage parameters and yield traits.
High sca effect were observed for grain yield per plant, percentage flower bud infestation, percentage pod infestation and number of pods per plant. L5 X T2 and L2 X T3 exhibited good sca effects for damage parameters, yield traits and morphological characters. L2 X T2 and L3 X T3 displayed desirable sca effects for several yield characters.
Crosses contributed maximum towards the total variability for all characters except number of branches per plant and leaf chlorophyll content. For number of branches and leaf chlorophyll content, the proportional contribution of lines was the highest followed by crosses. For all the damage measurements, proportional contribution of crosses were followed by that of lines.
Predominance of one or multiple epistatic interactions was generally observed for all characters. Additive gene effects were significant for days to 50 per cent flowering, number of pods per inflorescence, number of seeds per pod, 100 seed weight, plant height, and crude fibre content of pods. For all characters except peduncle length, the direction of dominance effects and domimance X dominance interactions suggest the presence of non-allelic duplicate gene action in their expression. For peduncle length, complimentary gene action plays a major role.
The significance of additive gene effects and desirable direction of dominance X dominance epistatic interactions underlines the suitability of exploiting heterosis and selection in an efficient manner. The magnitude and direction of the gene effects underlying the pest damage parameters offers a favourable background for the breeder to develop legume pod borer resistant cowpea types, through recombination breeding and / or selection.