Assessment of genetic variability and inter-character association in the germplasm of cowpea (Vigna Unguiculata L. Walp) in hot arid climate

Cowpea [Vigna unguiculata (L.) Walp.] is an annual food legume crop and it is an integral part of traditional cropping systems in the arid and semi-arid regions of the tropics. In the hot arid climate soil is dry, the air is dry and yearly precipitation is very low. Cowpea due to its quick growth and rapid ground cover is an essential component of sustainable subsistence agriculture in marginal lands and drier regions of the tropics, where rainfall is scanty and soils are sandy with little organic matter. In Rajasthan cowpea is of great importance because of its short duration, high yield potential and quick growing habits along with high protein content.
       
The study of variability in accessions of cultivated crops could provide vital information for the establishment of breeding programme, especially when intraspecific hybridization are necesssary for the incorporation of new features like water stress tolerance, resistance to biotic stress include insects, diseases, parasitic weeds and nematodes. On account of diverse uses of cowpea the varietal requirements are also diverse from region to region. The measurement and evaluation of variability are essential in drawing meaningful conclusion from a given set of phenotypic observations (Mehdi and Khan., 1994; Marwede et al., 2004). Hence, to have a thorough comprehensive idea it is necessary to have an analytical assessment of yield components. Since heritability is also influenced by environment, the information on heritability alone may not help in pin pointing characters enforcing selection. Nevertheless the heritability estimates in conjunction with the predicted genetic advance will be more reliable (Johnson et al., 1955).
       
The study of character association in plant breeding is the assessment of regression of one trait on another and subsequently on productivity. Therefore, the present research work was attempted to estimate the magnitude of present variability and inter-character association among the yield and yield contributing traits.

Thirty-eight cowpea germplasm accessions were collected from different agro climatic zones conserved at Regional Seed Gene bank, ICAR- NBPGR, Regional Station- Jodhpur were evaluated in randomized block design (RBD) with three replications for two consecutive years (environments) viz., Kharif 2013 (E₁) and Kharif 2014 (E₂) at Research farm of NBPGR, Regional Station, Jodhpur, India, which is situated at about 28° 35' N, longitude of 70°18' E and an altitude of 226 m above mean sea level. The recommended agronomic packages of practices were followed to raise good crop. The data were recorded for  seed yield per plant (g), 100-seed weight (g), number of seeds per pod, pod length (cm), number of pods per plant, peduncle length (cm), number of clusters per plant and plant height (cm) on five randomly selected plants of each accession as per the standard descriptors described for cowpea. The mean of all the traits of plants at each plot was subjected to analysis of variance as per the method suggested by Panse and Sukhathme (1967). The estimate of genotypic variance and phenotypic variance were worked out according to the method suggested by Johnson et al., (1955) using mean square values from the ANOVA table. Phenotypic and genotypic coefficients of variance were calculated based on the method advocated by Burton (1952). Heritability percentage in broad sense was estimated as per the method described by Lush (1940) and traits were classified as having high (>60), moderate (31-60) and low heritability (0-30) as per the method of Robinson et al., (1949). Genetic advance was estimated according to the method suggested by Johnson et al., (1955). Traits were classified as having high (>20), moderate (10-20) and low (0-10) while correlation coefficients and path coefficient analysis were calculated using the formulae suggested by Falconer (1964) and Dewey and Lu (1959) respectively.

Variability, heritability and genetic advance
 
In the present investigation, the genotypes exhibited considerable amount of variability for all the nine traits studied in both the environments. The estimates of genotypic coefficient of variation were lesser than the estimates of phenotypic coefficient of variation for all the traits in both the environments indicating the environmental influence over the characters studied. In the present study high GCV and PCV estimates were observed for seed yield per plant, 100 seed weight, number of pods per plant, number of clusters per plant, number of branches per plant and plant height in both the environments (Table 1). This indicated that there was greater diversity for these characters in cowpea. Hence direct selection based on these traits would be effective. The high PCV and GCV were earlier reported in cowpea by Tamgadge et al., (2008) and Vir and Singh A.K. (2014) plant height, 100-seed weight and seed yield per plant traits, similarly by Manggoel et al., (2012) for number of pods per plant and seed yield per plant.  These results were in conformity with the report of Vavilapalli et al., (2013), Vir and Singh. (2014) and Khan et al., (2015) in cowpea.
 

               
High heritability coupled with high genetic advance was observed for seed yield per plant, 100 seed weight, pod length, number of pods per plant, peduncle length, number of clusters per plant, number of branches per plant and plant height in both the environments (Table 1) except for the number of seeds per pod which showed medium heritability with medium genetic advance in both the environments. High heritability coupled with high genetic advance values were reported in cowpea by Khan et al., (2015) for test weight, plant height, number of branches per plant, number of pods per plant, pod length and seed yield per plant. Sarath et al., (2017) reported high heritability coupled with high genetic advance for plant height, pod length, number of seeds per pod and seed yield per plant. These findings were also supported by Vir and Singh (2014) and Kumar et al., (2017) in cowpea.
 
Correlation and Path analysis
 
Estimation of phenotypic correlation coefficient between different pair of traits under study revealed that not all traits were correlated to each other or with single plant yield. Considering the correlation between seed yield per plant and other characters, it was found that seed yield per plant was positively correlated with pod length, number of pods per plant, number of clusters per plant, number of branches per plant and plant height in E₁ environment (Table 2). In E₂ environment seed yield per plant was positively correlated with 100 seed weight, pod length, number of pods per plant, peduncle length, number of clusters per plant, number of branches per plant and plant height. Hence, these characters have to be given importance during the selection programme to improve the yield potential of the crop. Significant and positive phenotypic correlation was observed between pod length and 100 seed weight, number of branches per plant and pod length, number of cluster per plant and number of pods per plant, number of branches per plant and number of pods per plant and number of branches per plant and number of cluster per plant. Sapara et al., (2014) observed positive phenotypic correlation of seed yield per plant with number of pods per plant. Positive phenotypic correlation of seed yield per plant with test weight, pod length, number of clusters per plant, plant height and number of seeds per pod was also reported by Sharma et al., (2016).
 

       
From genotypic correlation coefficient analysis it was observed that seed yield per plant was positively correlated with pod length, number of pods per plant, number of clusters per plant, number of branches per plant and plant height in both the environments. Positive genotypic correlation was reported for 100 seed weight with number of seeds per pod and pod length, number of seeds per pod with peduncle length, pod length with number of clusters per plant and number of branches per plant, number of pods per plant with peduncle length, number of clusters per plant and number of branches per plant, number of clusters per plant with number of branches per plant in both the environments. Manggoel et al., (2012) observed positive genotypic correlation of seed yield per plant with number of peduncles per plant, 100 seed weight and number of pods per plant. Similar kind of association was revealed by Meena et al., (2015) for plant height, pods per plant, pod length, seeds per pod and 100 seed weight. 
       
In present study, path coefficient was computed for seed yield per plant taking remaining 8 independent characters. Partitioning of the total correlation into direct and indirect effects would provide actual information on the contribution of traits and thus form the basis for selection to improve seed yield. In the present investigation (Table 3), the positive direct effect showed by number of branches per plant, number of cluster per plant and plant height in E1 (Kharif 2012) environment. The number of branches per plant followed by plant height, number of pods per plant, test weight and number of cluster per plant showed positive direct effect on seed yield per plant in E2 (Kharif 2013) environment. These results are in accordance with Sharma et al., (2016) and Patel et al., (2016) for plant height and number of pods per plant. The positive direct effect of green pod yield per plant with number of pods per plant and pod length was observed by Sapara et al., (2014). The results were also supported by Manggoel et al., (2012), Hitiksha et al., (2014), Vir and Singh (2014) and Meena et al., (2015).
 

Within in the range of materials used in this study, there existed substantial genetic variability in the character studied to warrant selection in the cowpea accessions for seed yield improvement. The high genetic variance components and heritability estimates couple with positive correlation coefficients and high direct effects of 100 seed weight, number of pods per plant, number of clusters per plant, plant height and number of branches per plant on seed yield, these traits were identified in this study and could be listed in selection criteria for good parental lines in a cowpea breeding program.