Trait association and direct and indirect effects of yield-attributing traits on pod yield in advanced breeding lines of Groundnut (Arachis hypogaea L.)

Introduction

Groundnut (Arachis hypogaea L.) is one of the most important oilseed and food legume crop cultivated worldwide. It is a self-pollinated allotetraploid species (2n = 4x = 40) belonging to the family Fabaceae and is believed to have originated in South America, particularly Brazil and adjoining regions (Bertioli et al., 2019) [2] . The kernels are rich in oil (40-56%), protein (20-30%), carbohydrates (10-20%) and essential micronutrients such as vitamin E, niacin, folate, magnesium, phosphorus, calcium, zinc and iron (Arya et al., 2016) [1] . In India, groundnut is one of the principal oilseed crops cultivated under both rainfed and irrigated ecosystems. Gujarat, Rajasthan, Andhra Pradesh, Telangana, Karnataka and Tamil Nadu are major groundnut growing states contributing significantly to national production. In Telangana, groundnut is predominantly cultivated during the rabi season under irrigated conditions and the crop occupies a substantial area in districts such as Nagarkurnool, Wanaparthy, Gadwal and Nalgonda. Despite improvements in cultivation practices and varietal development, the productivity of groundnut remains lower than its realizable potential due to the occurrence of several biotic and abiotic stresses. Among these, foliar fungal disease like late leaf spot (LLS), caused by Nothopassalora personata (formerly Phaeoisariopsis personata), is considered one of the most destructive foliar fungal diseases worldwide (Sudini et al. 2015) [19] , insect pest like groundnut leaf bud borer (Anarsia ephippias) was economically important pest causing severe damage to foliage, terminal buds and reproductive structures (Wightman and Rao, 1993) [20] and among physiological constraints fresh seed dormancy plays a crucial role in preventing pre harvest sprouting, which otherwise adversely affects seed viability, pod quality, oil composition and market value. Previous studies have shown that fresh seed dormancy is largely governed by major genes with significant influence of environmental factors (Khedikar et al. 2018) [11] . A dormancy period of about 15-20 days is considered optimum for balancing protection against in situ germination and ensuring rapid seed germination during subsequent sowing. Understanding the interrelationship among traits is essential for effective crop improvement. Correlation analysis provides information regarding the magnitude and direction of association among characters, while path coefficient analysis partitions correlation into direct and indirect effects, thereby identifying traits contributing substantially towards pod yield. Since pod yield is a complex quantitative trait governed by several interacting component characters, indirect selection through associated traits often improves breeding efficiency. Keeping these aspects in view, the present investigation was undertaken to evaluate 90 advanced breeding lines along with six commercial checks viz., Kadiri-6, K1812, Vishista, Girnar-4, TAG-24 and GJG-32 for yield and yield attributing traits, foliar fungal disease resistance, fresh seed dormancy and leaf bud borer tolerance under field conditions.

Materials and Methods

Ninety advanced breeding lines along with six commercial checks like Kadiri-6, K-1812, TAG-24, GJG-32, Girnar-4 and Vishista, were evaluated in an alpha lattice design with two replications. Each line was sown in five rows of 5 m length, 30 cm between rows and 10 cm between plants at the AICRP Groundnut Centre, Regional Agricultural Research Station (RARS), Palem, Nagarkurnool, Telangana, during the rabi, 2025-26. The experimental site has a semi-arid climate and is located at 16°51' N latitude, 78º25' E longitude and 642 m above mean sea level in the Southern Telangana Zone (STZ). Observations were recorded for all yield related traits like germination percentage, days to 50 per cent flowering, number of primary branches per plant, number of pods per plant, days to maturity, pod yield per plant (g), seed yield per plant (g), shelling percentage, late leaf spot score at 90 DAS, leaf bud borer incidence at 60 DAS (%), Intensity of fresh seed dormancy (%) at 7 DAS and duration of fresh seed dormancy (days) for each genotype.

Observations recorded 1. Germination percentage:

It was recorded by counting the number of seeds that germinated on the observation date and was expressed as a percentage of the total number of seeds sown using the following formula:

Days to 50 per cent flowering:

The number of days from the date of sowing to the day on which 50 per cent of the plants in a plot produced at least one open flower.

Number of primary branches per plant:

Branches arising directly from the main stem were counted on five randomly selected competitive plants in each genotype at maturity and the average value was recorded.

Number of pods per plant:

The total number of mature pods harvested from each of the five randomly selected competitive plants was counted and the average number was recorded.

Days to maturity:

It was recorded as the number of days from sowing until approximately 75-80 per cent of the pods attained physiological maturity, as indicated by darkening of the inner shell surface.

After harvesting and drying, pods from each of the five selected competitive plants were weighed individually using an electronic balance. The average pod weight was expressed in grams per plant.

Seeds obtained after shelling the pods harvested from each of the five selected plants were weighed and the average seed weight per plant was recorded in grams.

Shelling percentage:

It was calculated as the proportion of seed weight to pod weight and expressed as a percentage using the following formula:

Shelling percentage (%) = Seed yield Pod yield × 100

9. Late leaf spot score at 90 DAS: Late leaf spot disease was assessed at 90 days after sowing (DAS) using the standard 9-point disease rating scale developed by Subrahmanyam et al. (1995) [15] , where lower scores indicate higher levels of resistance.

10. Leaf bud borer incidence at 60 DAS: Leaf bud borer incidence was recorded at 60 DAS by estimating the percentage of leaf damage caused by Anarsia ephippias.

Observations were recorded from 35 randomly selected plants of each genotype and the mean percentage of leaf damage was calculated.

11. Intensity of fresh seed dormancy (%) at 7 DAS: Fresh seed dormancy intensity was assessed seven days after sowing freshly harvested seeds. It was calculated as the percentage of seeds that failed to germinate using the following formula:

Intensity of fresh seed dormancy (%) = 100 -Germination (%)

12. Duration of fresh seed dormancy (days): It was recorded as the number of days required for freshly harvested seeds of each genotype to attain approximately 75 per cent germination under laboratory conditions. The experimental data were subjected to analysis of variance (ANOVA) for an alpha lattice design as described by Patterson and Williams (1976) [16] using R statistical software with the agricolae package (de Mendiburu, 2023) [6] to test the significance of genotypic differences among the groundnut genotypes. Genotypic and phenotypic correlation coefficients among different pairs of traits were estimated following the procedure described by Falconer and Mackay (1996) [7] . The estimated correlation coefficients were further partitioned into direct and indirect effects through path coefficient analysis as proposed by Wright (1921) [21] and elaborated by Dewey and Lu (1959) [5] . Correlation and path coefficient analyses were carried out using INDOSTAT software.

Results and Discussion

Analysis of variance

Analysis of variance revealed significant genetic variability among the 96 groundnut genotypes for most of the traits studied. At 1% level, highly significant differences were observed for the majority of traits, while days to 50% flowering and number of primary branches per plant were significant at the 5% level and days to maturity was nonsignificant. Replication effects were non-significant for all traits, indicating uniform experimental conditions. Significant treatment effects and block effects for a few traits confirmed substantial genetic variability and the effectiveness of the alpha lattice design in accounting for environmental variation.

Genotypic correlation analysis

The genotypic correlation coefficients among yield and yield-attributing traits are presented in Table 2. Pod yield per plant exhibited significant positive associations with the number of primary branches per plant (r = 0.294**), seed yield per plant (r = 0.994***), number of pods per plant (r = 0.990***), germination percentage (r = 0.560***), shelling percentage (r = 0.421***) and duration of fresh seed dormancy (r = 0.330). In contrast, pod yield exhibited a significant negative association with late leaf spot score at 90 DAS (r = -0.282**), whereas its associations with days to 50 per cent flowering (r = -0.041), plant height (r = -0.114), days to maturity (r = -0.105) and leaf bud borer incidence at 60 DAS (r = -0.048) were non-significant. Among the component traits, number of pods per plant showed a very strong positive association with seed yield per plant (r = 0.988***), shelling percentage (r = 0.394***) and duration of fresh seed dormancy (r = 0.343***), but a significant negative association with late leaf spot score (r = -0.279**). Similarly, seed yield per plant was positively associated with shelling percentage (r = 0.481***) and duration of fresh seed dormancy (r = 0.342***). Overall, the results indicated that seed yield per plant, number of pods per plant, germination percentage, shelling percentage and duration of fresh seed dormancy were the major traits positively associated with pod yield and can be effectively utilized as selection criteria in groundnut breeding programmes. Similar results were reported by Madhuri et al.

(2022) [12] , Reddy et al. (2023) [14] and Sravanti et al. (2024) [18] .

Path coefficient analysis

Genotypic and phenotypic path coefficient analyses revealed that seed yield per plant exerted the highest positive direct effect on pod yield at both genotypic (6.7890) and phenotypic (1.0410) levels, indicating its major contribution to yield improvement. At the genotypic level, bud borer incidence at 60 DAS (0.2670), duration of fresh seed dormancy (0.1690) and plant height (0.0750) also showed positive direct effects. In contrast, number of pods per plant (-5.3990), shelling percentage (-0.5510) and days to maturity (-0.2950) exhibited negative direct effects. However, the strong positive correlation of number of pods per plant with pod yield (r = 1.0143) was mainly due to substantial positive indirect effects through plant height and number of primary branches per plant. At the phenotypic level, most traits exhibited negative direct effects except seed yield per plant and days to maturity. The low residual effects at the genotypic (0.1999) and phenotypic (0.1691) levels indicated that the studied traits explained about 96.0% and 97.1% of the variation in pod yield, respectively. Overall, path coefficient analysis identified seed yield per plant as the most important direct selection criterion for improving pod yield in groundnut. Similar results were reported by Madhuri et al. (2022) [12] , Reddy et al. (2023) [14] and Sravanti et al. (2024) [18] .

Conclusion

The present investigation revealed significant genetic variability among the 96 groundnut genotypes, indicating ample scope for genetic improvement. Pod yield per plant showed strong positive associations with number of primary branches per plant (r = 0.294**), number of pods per plant (r = 0.990**), seed yield per plant (r = 0.994**), germination percentage (r = 0.560**), shelling percentage (r = 0.421**) and duration of fresh seed dormancy (r = 0.330**). Significant negative association was observed with late leaf spot score at 90 DAS (r = -0.282**). Path coefficient analysis identified seed yield per plant as the major direct contributor to pod yield with the highest positive direct effect at both genotypic (6.789) and phenotypic (1.041) levels. The low residual effects (0.1999 and 0.1691) indicated that the studied traits explained 96.0% and 97.1% of the variation in pod yield, respectively. Therefore, seed yield per plant, number of pods per plant, germination percentage, shelling percentage and duration of fresh seed dormancy can be used as effective selection criteria for improving pod yield in groundnut breeding programmes.

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