PhD

Yard long bean, V. unguiculata ssp. sesquipedalis (L.) Verdc., is a distinct form of cowpea grown as a vegetable crop in southern Asia and Far East for its immature pods. Incidence of pests and diseases is considered to be a major limiting factor affecting the production of yard long bean. Many viruses are known to infect yard long bean and cause serious losses. This research programme was carried out at the Department of Plant Breeding and Genetics, College of Agriculture, Vellayani during 2001-2005 with the objective to study the genetic basis and inheritance pattern of important quantitative and qualitative characters, including yield and black-eye cowpea mosaic resistance of yard long bean.
Fifty genotypes of yard long bean collected from various agroclimatic regions were evaluated adopting randomized block design with three replications. Analysis of variance revealed significant differences for almost all the characters. High GCV was observed for pods per cluster, yield per plant, pod weight, pods per plant and clusters per plant, which indicate that there exists high genetic variability and better scope for improvement of these characters through selection. The characters clusters per plant, pods per cluster, pods per plant, primary branches per plant, pod yield per plant, pod weight, pod length, seeds per pod and main stem length had high heritability coupled with high genetic advance. In the present study high heritability and low genetic advance was noted for days to 50 per cent flowering and pod breadth.
Yield per plant showed strong positive genotypic correlation with pods per cluster, pods per plant, pod weight, pod length, pod breadth and seeds per pod. A negative correlation was noted for days to 50 per cent flowering, days to first harvest and primary branches per plant. The characters pods per cluster, pods per plant, pod weight, pod length, pod breadth, seeds per pod and main stem length had positive direct effects while length of harvest period had negative direct effect.
Mahalanobis D2 analysis clustered the 50 genotypes into 4 groups with genotypes from different eco-geographic locations being grouped in the same clusters. Maximum divergence was shown between the clusters II and IV. The grouping of genotypes by selection indices followed almost the same pattern as there clustering pattern in the D2 analysis.
The same fifty genotypes were screened for resistance under artificial incidence of black-eye cowpea mosaic. Vulnerability indices for the various genotypes were estimated which indicated sufficient variability of resistance. The lowest index was observed for the accession Thiruvananthapuram local-3, which was grouped as moderately resistant.
Ten parents were selected and raised in a partial diallel cross so that each parent was involved in five crosses. The F1 along with the parents and a standard check variety were raised in RBD with three replications and observations on various characters recorded. Perusal of the data recorded significant positive heterosis for all yield related traits and significant negative heterosis for days to 50 per cent flowering indicating earliness.
The crosses VS86 x VS132, Kozhicode local-2 x Thiruvananthapuram local-10 and Thiruvananthapuram local-9 x Thiruvananthapuram local-12 had significant heterosis for maximum number of characters. These three crosses also had significant standard heterosis for all the characters studied.
The parents VS86, Thiruvananthapuram local-9, Thiruvananthapuram local-10 and Thiruvananthapuram local-12 had significant gca effects for days to 50 per cent flowering. Lola had positive significant gca effect for pod length. The parents Kozhicode local-2 and VS132 had significant positive gca effects for pod weight. VS86 was the only parent with significant and positive gca effect for pods per plant. The parents VS132 and Thiruvananthapuram local-12 had significant positive gca effects while Thiruvananthapuram local-3 and Thiruvananthapuram local-4 had significant negative gca effects for yield per plant.
Significance of scale A and B for most of the characters suggested that the simple additive-dominance model was inadequate for defining the inheritance of these characters. Presence of non-allelic interactions was noticed for days to 50 per cent flowering. Hybridisation and selection can be resorted to for improving the character of earliness in this crop.
The positive significance of dominance x dominance interactions for pod weight points out that a breeding strategy for improving pod weight should be based on direct selection or hybridization and selection for high pod weight.
Presence of all three types of digenic interaction was observed for pods per plant. Pod yield per plant also displayed all three types of digenic interactions. The same direction of dominance effect and dominance x dominance interactions for days to flowering, pod length, pods per plant, pod yield per plant suggests non-allelic complimentary gene action in the expression of the character. The negative significance of dominance x dominance interaction for pod yield per plant suggests a limited scope of improvement through heterosis breeding for this trait.
The direction of dominance effects and dominance x dominance interactions suggests the presence of non-allelic duplicate gene action for pod weight, pod breadth, pods per cluster, seeds per pod, root weight per plant and days to first harvest in their expression.
The activity of defense related enzymes namely, peroxidase, polyphenol oxidase and phenylalanine ammonialyase exhibited a wide variation in the genotypes studied. A comparison of the vulnerability indices of thirteen genotypes with their respective PAL activity revealed its declining trend with the increasing value of vulnerability index. This suggests the presence of a correlation between the disease resistance and the PAL activity of the plants. This particular trait can hence be utilized to screen for resistance.
The results suggest ample scope of improvement of yield through selection based on the characters pod weight and pod length. Hybridization was found to increase the activity of biochemical components relating to resistance to black-eye cowpea mosaic virus.