Genetic characteristics and path coefficient analysis in ten groundnut varieties ( Arachis hypogaea L . ) evaluated in the Guinea Savannah agro-ecological zone

Nine improved varieties of groundnut developed at the Institute of Agricultural Research, Samaru and one locally cultivated variety were evaluated for their breeding potentials in the Guinea Savannah agroecological zone. Considerable variability was observed for basal stem diameter, biological yield, days to first and 50% flowering, plant height, number of leaves/plant, branches/plant and100-seed weight, showing a wide scope for improvement through selection. Coefficient of variation at phenotypic and genotypic levels were close in magnitude for number of branches/plant, plant height, days to first flowering and grain yield suggesting the presence of additive gene effects. High heritability estimates coupled with genetic advance were noted for number of branches/plant, plant height, days to first flowering and grain yield. Correlation studies revealed that grain yield correlated positively with all except the phenological traits. The path analysis implicated biological yield, failed pegs/plant, number of leaves/plant, and basal stem diameter as having substantial influence on grain yield in groundnut. Thus, selection of breeding lines based on the biological yield, failed pegs, number of leaves/plant and basal stem diameter could give a better scope for maximum grain yield in groundnut. Considering the grain yield, four of the varieties, ICIAR, SAM NUT 22, SAM NUT 23, SAM NUT 21 and SAM NUT 11 were identified as promising in Makurdi environment.


INTRODUCTION
Groundnut (Arachis hypogaea L.) is one of the most important legume crops cultivated in the sub-Saharan Africa for cheap source of vegetable oil, good quality feedstuff, improvement of soil health through nitrogen fixation as well as a source of fuel for the rural population.
In developing countries such as Nigeria where the cost of animal protein is prohibitive, groundnut serves as a good alternative source of animal protein for improving the nutritional status of Nigerians (Asibuo et al., 2008).Groundnut (also called peanut) is an ancient oilseed crop which belongs to the family, Leguminosae and subfamily Papilionoidae.Weiss (2000) reported that South America is credited as the primary centre of diversity of groundnut but presently, its production has spread to other parts of the world.Globally, the leading groundnut producers includes, India (33.4% of global production) China (27.80%),USA (9.3%), Senegal (4.2%), Indonesia (4.2%), Nigeria (3.3%), Myanmar (3.0%), Sudan (2.7%) and Argentina (2.0%) (Isleib et al., 1994).Groundnut is a bio-energy feedstock that supplies most of the important food components.Compared to other sources of food like milk (cow), egg (fowl), beef, mutton, redgram, processed groundnut gives protein (25.33%), carbohydrate (10.2%), fats (40.5%), and caloric value (5000 to 6000) (Wrenshall, 1949).Groundnuts are rich sources of vitamins such as Vitamin E and the members of the Bcomplex, especially thiamine, riboflavin, and nicotinic acids, but the amounts of Vitamins A, C, and D are negligible (Wrenshall, 1949;Talawar, 2003).Economic analyses of this crop revealed its prospects as a major source of income to farmers (Awoke, 2003;Taru et al., 2010).Okolo and Utoh (1999) reported that the average yield of groundnut in Nigeria per ha is within the range of 500 to 3000 kg compared with China where the average ton per ha has reached 1.4 metric (FAO, 2004).One possible way to wean groundnut from subsistence farming and integrate it into a commercially important crop as in the days of groundnut pyramids (Echekwu, 2003), there is need to develop high yielding varieties to replace the existing traditional ones.In this regard, efforts have been made at the Institute for Agricultural Research, Samaru, Nigeria to develop some new varieties of groundnut but their yielding potentials have not been tested under the Guinea Savannah agro-ecological zone.
Knowledge of the genetic variability in a population and partitioning the variance into the components provides useful information for improvement of the desirable traits.Coefficient of variation at genotypic and phenotypic levels, heritability and genetic advance have been calculated for different yield parameters in groundnut by several research workers (khan et al., 2000;Meta and Monpara, 2010;Zaman et al., 2011) and have shown the importance of these parameters in enhancing success in a planned breeding program.High heritability estimate coupled with high genetic advance for a character could serve as a better criterion for selection (Johnson et al., 1955).Correlation is a biometrical approach which brings out the intensity of the association between two pairs of characters and provides information on those components that could serve as criteria for selection of candidates in a breeding program.Traits that are positively correlated with yield are considered effective because selection for such traits would result in the simultaneous improvement in yield (Mahalakshmi et al., 2005;Zaman et al., 2011;Khan et al., 2000).
Path analysis provides an efficient way of partitioning the correlation coefficient into direct and indirect effect to allow the selection of traits that have direct implications for yield (Dewey and Lu, 1959).Khan et al. (2000) reported that the 100-kernel weight had the highest direct positive effect on pod yield, followed by pods per plant and sound mature kernel, whereas, the study by Zaman et al. (2011) showed that seed yield was influenced by number of mature pods per plant, followed by nut size, shelling percentage, days to 50% flowering and days to maturity.
The objective of the present study was to evaluate 10 groundnut varieties for genetic variability, genetic parameters, heritability and genetic advance to provide information that might be useful for groundnut breeding program.

MATERIALS AND METHODS
The experimental materials comprised nine improved varieties of groundnut viz, SAM NUT 10, SAM NUT 11, SAM NUT 21, SAM NUT 22, SAM NUT 23, ICIAR 19 BT, NC-7, GH 119-20, and ICGV 93030 were collected from Institute for Agricultural Research, Samaru, Zaria, and one local variety (OLAMU).These varieties were selected based on their promising agronomic attributes.Evaluation of the varieties was done during the 2010 cropping season at the Teaching and Research Farm of the University of Agriculture, Makurdi (Lat.7°41' N, Long.8°35' E), Nigeria.Makurdi falls within the Southern Guinea Savannah agro-ecological zone.The experiment was laid in a randomized block design with four replications.The plot size was 5 × 4 m, with four ridges in a plot, and the distance between one plant and another within a row and between rows was maintained at 0.30 and 0.50 m, respectively.Observations were taken randomly on four plants in the two middle rows.All standard agronomic practices (viz, weeding, fertilization, etc) and insect control were applied to ensure the full expression of the varieties potentials.Harvesting was done 116 days after planting, sun-dried for 5 days and the biological yield (grain weight plus Haulm weight) determined before the nuts were decorticated.Other observations taken included plant height (cm), number of leaves/plant, number of branches/plant, basal stem diameter (cm), pods/plant, 100-seed weight and grain yield/plant (g).Data collected on the following variables were subjected to analysis of variance (ANOVA) using Genstat Discovery Edition 3 software, and significant means separated using least significant difference as described by Obi (1996).Heritability was estimated using the procedure of Singh and Chaudhury (1979), genetic advance was calculated as a percentage of the mean at 5% selection intensity, while path coefficient analysis was calculated according to Dewey and Lu (1956)

RESULTS AND DISCUSSION
The result obtained from the ANOVA is presented in Table 1.The result showed significant differences among the varieties for all the characters studied except biological yield and failed pegs indicating the presence of large magnitude of genetic variability.Conversely, biological yield and failed pegs however showed no significant differences evidencing low variation in these traits.Our results are comparable with the reports of Saleh and Masiron (1994) who also found highly significant differences in pod yield, kernel yield, 100-seed weight, number of pods/plant and days to flowering and  no significant difference in shelling percentage among the entries evaluated.Similarly, findings were also reported by Meta and Monpara (2010) and Zaman et al. (2011).
Among the entries evaluated in the present study, ICIAR 19 BT gave the highest grain yield (39.0 g/plant).This was closely followed by SAM NUT 22 (38.75 g/plant), and SAM NUT 11 (34.0 g/plant), while GH119-20 (21.25 g/plant) had the least grain yield.Similarly, OLAMU (95.00 g/plant) and ICIAR 19 BT (390.0 g/plant) produced the least and highest biological yields, respectively.The variety, SAM NUT 11 had the heaviest kernel with average 100-seed weight of 48.70 g.This was followed by SAM NUT 21 and SAM NUT 22 with average 100-seed weight of 44.60 g each.Variety GH119-20 (30.0 g) recorded the lowest kernel weight when average 100-seeds weight was considered.A wide range of variation was observed for number of leaves/plant (180.0 to 1470.0), biological yield (71 to 1140.0) failed pegs/plant (24.0 to 464.0) and plant height (28.0 to 90.0) as shown in Table 1.
Coefficient of variation at both genotypic and phenotypic levels was relatively high for number of branches/plant, failed pegs/plant, biological yield/plant, plant height, number of leaves/plant, 100-seed weight, days to first flowering and grain yields/plant (Table 2), evidencing high variation in the material.
Apparently, grain yield/plant, number of branches/plant, days to first flowering as well as days to 50% flowering recorded genotypic coefficient of variation (GCV) and phenotypic coefficient of variation (PCV) values that were comparable in magnitude suggesting the minimum influence of environment on the expression of these traits.This result indicates that the phenotypic variation observed in the material was largely due to genotypic effects.Whereas, the magnitude of PCV was considerably higher than GCV for biological yield and failed pegs/plant suggests the preponderances of environmental effects.Consequently, the relatively high environmental coefficient of variation (ECV) observed in biological yield, failed pegs, number of leaves/plant and 100-seed weight shows higher environmental influence than genetic factor controlling these characters.
Good estimate of heritability is a useful tool that helps the breeder to determine traits that can serve as criteria for selection of parental lines in a breeding program.According to Allard (1960), heritability is the proportion of the genetic variation that can be attributed to genetic causes.When considered alongside with genetic advance, heritability estimates give a better assessment of the reliability of the traits and allows for easy prediction of the genetic progress to be achieved in a breeding program (Johnson et al., 1955).In the present study (Table 2), number of branches/plant (97023) had the highest heritability estimate, followed by days to first flowering (96.30), plant height (90.43 cm), days to 50% flowering (81.69), grain yield (56.52 tons/ha), and basal stem diameter (56.0), in that order.Meta and Monpara (2010) reported high heritability in plant height and days to maturity.While 100-seed weight and number of leaves/plant gave moderate heritability values and low heritability was obtained for failed pegs and biological yield.Expectedly, number of branches/plant (101.11),plant height (54.51 cm), days to first flowering (28.60) and grain yield (27.13 g/plant) had high genetic advance suggesting the presence of additive gene effects.Effective selection is only possible when the additive gene effect is substantial and environment relatively low.Thus, these results showed that selection of breeding lines based on number of branches/plant, plant height, days to first flowering and grain yield would be more  rewarding than using biological yield, days to 50% flowering which had low genetic advance.The tendency of some of the yield components that gave high heritability values as found in the present study to be utilized as the criteria for selection is espoused by the reports of other legume crops workers (Manggoel et al., 2012;Shaahu et al., 2012;Vange and Egbe, 2009).The correlation coefficients of 10 varieties of groundnut are presented in Table 3. Grain yield was positively associated with all the traits except the phenological traits (days to first flowering and days to 50% flowering each.This result appears to suggest that grain yield is linearly related to all the traits that have positive correlation with it, and  illustrates the fact that increase in these traits would lead to increase in grain yield (Manggoel et al., 2012, Sadeghi andNiyaki, 2012;Zaman et al., 2011;Khan et al., 2000).This result on the interrelationship of traits appears to suggest that although the phenological traits gave high heritability estimates, however, its negative correlation with grain yield makes it a poor criterion for selection.This inference is further corroborated by the low genetic advance values obtained for these traits.Significant correlation was observed between grain yield and biological (r = 0.65*), indicating that selection based on this trait would improve grain yield.Kobraee et al. (2010) had reported significant correlation between grain yield and biological yield in Chickpea.Meta and Monpara (2010) also observed a negative association between grain yield and days to 50% flowering.Highly significant correlation was recorded for number of branches/plant and number of leaves/plant (r = 0.98**), whereas the association between branches/plant and plant height was negative (r = -0.67*).Other correlations between pairs of traits that are of some interest to the breeder are shown in Table 3.The result of the path analysis showing the direct and indirect effects of characters on grain yield is shown in Table 1.Path analysis revealed that biological yield/plant (0.81), failed pegs/plant (0.78), number of leaves/plant (0.46), and basal stem diameter (0.44) exerted positive and highest direct influence to grain yield (Figure 1), indicating that selection based on these traits will be helpful in improving the grain yield of groundnut.While the contribution of 100 seed weight to grain yield was positive but negligible.This result is in contrast to the reports of Khan et al. (2000) and Manggoel et al. (2012) who reported that 100-seed weight has substantial influence on grain yield.However, number of branches/plant and plant height contributed negatively to grain yield, implying their poor association with grain yield.

Figure 1 .
Figure 1.Path analysis showing the cause and effect relationship of yield and yield components of 10 groundnut varieties evaluated during the 2010 planting season.
to partition the total correlation into direct and indirect effects using Analysis for Moment of Structures for Windows Version 16 (AMOS Development Corp., House, USA) software program.

Table 1 .
Characteristics of 10 groundnut varieties evaluated during 2010 planting season in Makurdi.

Table 2 .
Basic statistics and estimates of genetic components for 10 groundnut varieties evaluated during 2010 planting season at Makurdi.

Table 3 .
Correlation coefficients of 10 groundnut varieties evaluated at Makurdi during the 2010 planting season.

Table 4 .
Direct and indirect effects of yield traits on grain yield of 10 groundnut varieties evaluated during 2010 planting season.