Morphological traits governing drought tolerance in groundnut (Arachis hypogaea L.): A multi-generational evaluation

Introduction

Groundnut (Arachis hypogaea L.) is an important food and oilseed crop valued for its high oil (47-53%) and protein (25-36%) content. As a self-pollinated tetraploid (2n = 4x = 40) with a geocarpic fruiting habit, it thrives across diverse agro-ecological regions and serves as a major source of edible oil, industrial raw material and nutrient-rich fodder. Its wide adaptability and short duration make it suitable for multiple cropping seasons, thereby supporting crop improvement programmes. Globally, groundnut is the fifth most cultivated oilseed crop, occupying 27.9 million hectares with an annual production of 47 million tonnes with an average yield of 1676 kg/ha. India ranks second in production, with major contributions from Gujarat, Rajasthan, Madhya Pradesh, Karnataka, Tamil Nadu, Andhra Pradesh and Maharashtra.

Gujarat is leading state in area (169.53 lakh ha) followed by Rajasthan (89.06), during the year 2023-24. (Ministry of Agriculture and Farmers Welfare, Govt. of India). Global groundnut agriculture covers 4.96 million hectares and produces 10.03 million tons with a productivity of 1616 kg/ha (FAOSTAT, 2023) [11] . Despite its significance, productivity remains low because nearly 80% of the crop is grown under rainfed conditions, making it highly vulnerable to erratic and insufficient rainfall. With the demand for vegetable oils projected to double by 2040, enhancing groundnut productivity under water-limited environments has become essential. Drought is the most critical abiotic stress limiting groundnut yield, compromising flowering, pod formation and kernel filling. Conventional selection for yield under drought is often unreliable due to high genotype × environment interactions. Therefore, understanding the genetic basis of morphological traits associated with drought tolerance is necessary to design efficient breeding strategies. Generation mean analysis enables partitioning of additive, dominance and epistatic gene effects, providing insights into trait inheritance. This study evaluates multiple generations of groundnut to identify key morphological determinants of drought tolerance and the gene actions governing their expression. Despite the importance of morphological traits in drought tolerance, limited information exists on how these traits are inherited across generations in groundnut. Thus, the present investigation focused solely on morphological observations to elucidate trait expression under irrigated and drought condition and to estimate the nature of gene action governing these traits.

Materials and Methods

The present investigation was conducted at the field of AICRP on summer groundnut, Mahatma Phule Krishi Vidyapeeth, Rahuri, Ahmednagar (MS) during Kharif-2023, Summer-2024 and Kharif-2024. Six generations (P 1 , P 2 , F 1 , F 2 , BC 1 and BC 2 ) were developed from cross TAG-24 × ICG 4670 for drought tolerance and pod yield. The experimental material consisted drought tolerant parent as donor and recipient parent is high yielding and wider adaptability of groundnut genotypes. The cross TAG-24 × ICG 4670 was effected in Kharif-2023 to produce the F 1 seeds and breeding material like F 1 , F 2 , BC 1 and BC 2 and advancement was attempted in Summer-2024. During Kharif-2024, final evaluation trial conducted for study of inheritance of drought tolerance related traits and generation mean analysis, for yield and yield contributing characters under both irrigated and drought (rainout shelter) condition The characters viz., days to 50 percent flowering, days to maturity, number of branches per plant, number of mature pods per plant, haulm yield per plant, dry pod yield per plant, hundred kernel weight, shelling percentage and harvest index on dry weight basis were analysed for this study. Each character was analysed separately and the ANOVA was constructed as per Panse and Sukhatme (1995) [26] . The data was analyzed for mean performance of the generations. Gene action has been worked by generation mean analysis as per six parameter model given by (Hayman, 1958) [14] . Adequacy of additive dominance effect was detected by individual scaling test, three tests of scale were carried out to detect presence or absence of gene interaction by using formulae given by Mather (1949) [20] and Hayman and Mather (1957) [15] . Cavalli's (1952) Joint scaling test was applied to test the adequacy of additive-dominance model. Whenever, the model was found inadequate, Hayman (1958) [14] six parameter model was used to estimate the different gene effects. Statistical analysis of data was done by using software package INDOSTAT.

Results and Discussion

Analysis of Variances

The analysis of variance for nine characters is presented in Table 1 for two crosses under both irrigated and drought (rainout shelter) condition. Under irrigated condition (Table 1), the analysis of variance revealed significant differences among treatments in cross TAG-24 × ICG 4670 i.e., parents and their derived generations (F 1 , F 2 , BC 1 and BC 2 ) for all traits studied. This indicates the presence of substantial genetic variability among the parents and their progenies. Under drought condition (Table 1), the analysis of variance showed significant differences among treatments in cross TAG-24 × ICG 4670 i.e., parents and their respective generations (F 1 , F 2 , BC 1 and BC 2 ) for the majority of traits, indicating considerable genetic variability under moisture stress. However, trait such as haulm yield exhibited nonsignificant differences, suggesting limited genetic variation for this character.

Mean performance of parents and generations

The mean performance of parents and their crosses for pod yield per plant and its contributing traits in groundnut are presented in Table 2.1 and 2.2 for irrigated and drought condition respectively. Higher values are desirable for all traits under study except for days to flowering, days to maturity for which lower values are preferred. The trait wise results are discussed as below:

Cross: TAG-24 × ICG 4670

Days to 50% flowering Under irrigated condition, the mean values for days to 50% flowering across parents and generations ranged between 35.86 and 45.33 days. Among the parental genotypes, ICG 4670 exhibited the earliest flowering (35.86 days), whereas TAG-24 showed the latest (45.33 days). Among the derived generations, BC 2 recorded the earliest flowering (37.26 days), followed by F 1 (40.53 days), BC 1 (41.56 days) and F 2 (42.80 days). This trend suggests that backcross generations, particularly BC 2 , may contribute to the development of early flowering lines under irrigated conditions, which could be beneficial for fitting into shorter cropping durations. Under drought stress, the mean values for days to 50% flowering among the parental and hybrid generations ranged from 33.80 to 41.33 days. Among the parents, ICG 4670 exhibited the earliest flowering (33.80 days), whereas TAG-24 recorded the latest (41.33 days). Among the derived generations, the BC 2 population showed the earliest flowering (36.16 days), followed by F 1 (36.46 days) and BC 1 (38.53 days). The F 2 generation exhibited the most delayed flowering (39.38 days). These findings suggest that certain backcross generations, particularly BC 2 , may possess drought escape mechanisms through earlier flowering, which is a critical adaptive trait under water-limited environments.

Days to Maturity

Under irrigated condition, the mean values for days to maturity among the parents and derived generations varied from 102.67 to 112.30 days. The BC 1 generation recorded the earliest maturity (104.26 days), followed by F 1 (107.60 days), BC 1 (107.73 days) and F 2 (112.30 days). Among the parental lines, ICG 4670 matured earlier (102.67 days), while TAG-24 required the longest duration to reach maturity (111.66 days). These results suggest that the F 1 and backcross generations have inherited early maturity traits from the early-maturing parent, ICG 4670, which can be advantageous for escaping terminal stress in water-limited or late-sown conditions. Under drought stress, the days to maturity among the parental and hybrid generations ranged from 97.66 to 106.00 days. The BC 2 generation matured the earliest (100.18 days), followed by F 2 (102.18 days), F 1 (102.20 days) and BC 1 (104.20 days). Among the parents, ICG 4670 matured earlier (97.66 days), while TAG-24 again required a longer duration (106.00 days). Compared to irrigated conditions, a slight advancement in maturity was observed across most generations, indicating a stress-induced acceleration of phenological development. This early maturity under stress, particularly in F 1 and BC 2 , may serve as an important drought-escape strategy.

Number of branches/plant

Under irrigated condition, the number of branches per plant varied from 4.86 to 7.40 across the parents and their derived generations. The highest number of branches was recorded in the BC 1 generation (6.31), followed by F 2 (6.24), F 1 (5.40) and BC 2 (4.68). Among the parents, TAG-24 exhibited a higher branching habit (7.40), while ICG 4670 recorded the lowest (4.86). The increased number of branches in BC 1 and F 2 generations under optimum water availability indicates the presence of heterosis and favorable genetic recombination contributing to enhanced vegetative growth. Under drought stress, the number of branches per plant declined across all generations, ranging from 4.96 to 6.04. The F 2 generation recorded the highest number of branches (6.04), followed by F 1 (5.13), BC 2 (5.10) and BC 1 (4.96). Among the parents, ICG 4670 maintained a higher branching ability (5.86), while TAG-24 exhibited the lowest value (5.53). The reduction in branch number under drought conditions indicates the negative impact of water deficit on vegetative growth. However, the relatively higher number of branches maintained by the F 2 and F 1 generations suggests a better adaptability and potential to sustain vegetative vigor under stress environments.

Number of mature pods/plant

Under irrigated condition, the number of mature pods per plant varied widely among generations, ranging from 14.33 to 20.40. The BC 1 generation recorded the highest number of mature pods (18.10), followed closely by F 2 (17.95), F 1 (17.06) and BC 2 (14.58). Among the parents, TAG-24 exhibited a better pod set (20.40), while ICG 4670 had a comparatively lower pod number (14.33). The higher pod numbers observed in BC 1 and F 2 suggest heterotic advantage and transgressive segregation, indicating that the hybrids inherited and combined favorable alleles for reproductive efficiency from both parents.

Under drought stress, the number of mature pods per plant was considerably reduced across parent first related generations. While, P 2 related generations show considerable improvement in pod set. It was ranging from 16.20 to 19.73. Despite the stress conditions, the BC 2 generation recorded the highest pod number (18.50), followed by F 2 (18.35), F 1 (17.33) and BC 1 (17.18). Among the parents, ICG 4670 produced more pods (19.73) than TAG-24 (16.20). The relatively higher pod production in BC 2 and F 2 indicates better reproductive resilience and potential drought adaptability. This trait is critical for yield stability under stress conditions.

Under irrigated condition, haulms yield per plant ranged from (18.59 g) to (29.54 g) across parents and generations. The highest haulm yield was observed in the F 2 generation (26.30 g), followed by F 1 (25.60 g), BC 1 (23.74 g) and BC 2 (20.39 g). Among the parental lines, TAG-24 recorded a higher haulm yield (29.54 g) as compared to ICG 4670 (18.59 g). The significantly higher biomass accumulation in F 2 and F 1 suggests the influence of heterosis. This increased vegetative biomass is indicative of better growth and nutrient assimilation, which can indirectly support improved reproductive performance as well. Under drought condition, haulms yield per plant exhibited a noticeable reduction in first parent i.e. TAG-24 (21.56 g) and simultaneously increase in second parent i.e. ICG 4670 (23.45 g). The mean was ranging from (21.56 g) to (23.45 g) across parents and generations. The BC 2 generation maintained the highest haulm yield (22.74 g) followed by BC 1 (22.63 g), F 2 (22.20 g) and F 1 (21.86 g). Among the parents, ICG 4670 recorded a higher haulm biomass (23.45 g) compared to TAG-24 (21.56 g). The overall decline in biomass under water-limited conditions highlights the adverse effect of drought on vegetative growth. However, the ability of P 2 and BC 2 to retain relatively higher haulm yields under stress signifies better physiological resilience and suggests their utility in drought tolerant breeding programs.

Under irrigated condition, the dry pod yield per plant exhibited substantial variation among different generations, ranging from 15.52 g to 19.87 g. The F 2 generation recorded the maximum yield (19.87 g), followed by F 1 (18.08 g), BC 1 (17.64 g) and BC 2 (15.52 g). Among the parental lines, TAG-24 produced a higher pod yield (19.27 g), whereas ICG 4670 recorded a lower yield (15.58 g). These results suggest that the F 2 and F 1 generations possess enhanced yield potential, likely due to hybrid vigor and favorable genetic combinations. High pod yield under optimal water availability reflects improved partitioning of assimilates towards reproductive structures, making these generations promising candidates for further evaluation. Under drought stress, a noticeable decline in dry pod yield per plant was observed across all generations except P 2 and BC 2 , with values ranging from 16.51 g to 19.67 g. Among the generations, BC 2 recorded the highest pod yield (18.26 g), followed by F 1 (17.85 g), F 2 (17.54 g) and BC 1 (16.80 g). In comparison, the parental line TAG-24 yielded (16.51 g), whereas ICG 4670 showed the highest yield (19.67 g). The reduction in pod yield except P 2 and BC 2 under water-deficit conditions underscores the negative impact of drought on reproductive output. Nonetheless, the BC 2 and F 1 generations maintained relatively higher yield which indicating their superior adaptability and potential for use in drought resilient breeding programs.

Under irrigated condition, the 100-kernel weight among parents and generations ranged from (24.86 g) to (32.84 g). The highest kernel weight was observed in the BC 1 generation (32.65 g), followed by F 1 (30.02 g), F 2 (28.38 g) and BC 2 (26.05 g). Among the parents, TAG-24 recorded a higher kernel weight (32.84 g) as compared to ICG 4670 (24.86 g). The relatively greater seed mass in BC 1 and F 1 may be attributed to hybrid vigour and enhanced translocation of assimilates towards seed development under optimal moisture. Under drought stress, the 100 kernel weight exhibited a significant reduction compared to irrigated conditions except P 2 and BC 2 , with values ranging from (23.80 g) to (27.43 g). The BC 2 generation maintained the highest kernel weight (26.80 g), followed by F 1 (24.95 g), F 2 (24.69 g) and BC 1 (23.80 g). Among the parents, TAG-24 recorded (24.54 g), while ICG 4670 showed the highest value (27.43 g).

Under irrigated condition, the shelling percentage varied from (64.23%) to (71.98%) across the parental lines and their derived generations. The BC 1 generation exhibited the highest shelling percentage (69.50%), followed by F 1 (68.11%), F 2 (66.75%) and BC 2 (64.55%). Among the parents, TAG-24 recorded a shelling percentage of (71.98%), while ICG 4670 showed a lower value of (64.23%). The superior shelling performance of the BC 1 and F 1 generations under optimal moisture suggests efficient kernel filling and improved seed to pod ratio. High shelling percentage under irrigated conditions is a desirable trait for enhancing marketable yield and improving economic returns. Under drought conditions, a slight reduction in shelling percentage was observed across all generations except P 2 and BC 2 , with values ranging from (63.50%) to (66.37%). The BC 2 generation maintained the highest shelling percentage (64.86%), followed by F 2 (64.68%), BC 1 (63.75%) and F 1 (63.61%). Among the parents, TAG-24 recorded 63.50%, whereas ICG 4670 registered the highest (66.37%). Although drought stress led to marginal declines in shelling efficiency, the BC 2 and F 2 generations retained relatively higher values, suggesting their resilience in maintaining seed recovery.

Under irrigated condition, the harvest index ranged from (37.11%) to (41.85%) among the parental lines and their generations. The highest value was recorded in the BC 1 generation (38.59%) followed by F 1 (38.50%), F 2 (37.48%) and BC 2 (36.71%). Among the parents, TAG-24 had a harvest index of (41.85%), while ICG 4670 recorded the lowest (37.11%). The superior performance of BC 1 and F 1 indicates efficient translocation of assimilates towards the reproductive sink under optimal water availability. A higher harvest index under irrigation reflects better resource use efficiency and is an important trait for improving overall productivity.

Under drought stress, the harvest index exhibited a noticeable decline across all generations except P 2 and BC 2 , ranging from (34.33%) to (38.81%). The BC 2 generation showed the highest value (37.51%), followed by F 2 (37.41%), F 1 (36.02%) and BC 1 (34.86%). Among the parents, TAG-24 and ICG 4670 recorded harvest index of 34.33% and 38.81% respectively. Although overall values were lower than under irrigated conditions, BC 2 and F 2 generations maintained relatively higher values reflecting their efficiency in biomass partitioning under moisture limited environments. A stable harvest index under drought is indicative of genotypes with better stress resilience and yield potential.

Estimates of scaling tests for detecting non-allelic interactions

The results on scaling test (A, B, C and D) in respect of yield, yield contributing and drought tolerance related traits have been tabulated in (Table 3) under irrigated and drought stress (rainout shelter) condition. The result for cross: TAG-24 × ICG 4670 is mentioned under two conditions as follow:

Under irrigated condition (Table 3) Days to 50% flowering For the days to 50% flowering, the scaling test 'D' (6.76) was positively significant in cross TAG-24 × ICG 4670. Joint scaling test was found highly significant, indicating the presence of epistasis.

Days to maturity

In case of days to maturity, scaling tests 'D' (12.61) was positively significant in cross TAG-24 × ICG 4670. The joint scaling test was found highly significant for days to maturity in cross TAG-24 × ICG 4670, indicating inadequacy of additive dominance model to explain all the genetic variation.

Number of mature pods/plant

Scaling test 'B' (-2.23) was negatively significant. While, scaling test 'D' (3.23) for cross TAG-24 × ICG 4670 was positively significant. Estimates of genetic effects for this trait by joint scaling test indicated inadequacy of additive dominance model to explain all the genetic variations as the chi-square values of all the crosses were significant.

Haulm yield/plant

For haulm yield per plant, scaling tests 'C' (9.89) and 'D' (6.47) were significant, showing additive dominance model was inadequate to explain all the genetic variations.

Dry pod yield/plant

In case of dry pod yield per plant, scaling tests 'C' (10.46) and 'D' (6.58) were positively significant. Estimates of genetic effects for this trait by joint scaling test indicated inadequacy of additive dominance model to explain all the genetic variations as the chi-square values of both the crosses were significant.

kernel weight

For 100 kernel weight, no scaling tests were found significant in cross TAG-24 × ICG 4670. All joint scaling tests were found non-significant, indicating adequacy of additive dominance model to explain all the genetic variation.

Shelling percentage

In case of shelling percentage, all joint scaling tests were found non-significant showing additive dominance model was inadequate to explain all the genetic variations.

Harvest index All scaling test 'A', 'B', 'C' and 'D' were non-significant,

showing adequacy of additive dominance model to explain all the genetic variation as the chi-square values of both the crosses were non-significant.

Under drought condition (Table 3) Days to 50% flowering For Days to 50% flowering, scaling tests 'C' (9.46) and 'D' (4.06) were positively significant, showing additive dominance model was inadequate to explain all the genetic variations.

Days to maturity

All scaling test 'A', 'B', 'C' and 'D' were non-significant which shows adequacy of additive dominance model to explain all the genetic variation as the chi-square values of both the crosses were non-significant.

Number of mature pods/plant

Scaling tests tests 'C' (2.50) and 'D' (2.01) were positively significant, showing additive dominance model was inadequate to explain all the genetic variations as the chisquare values of both the crosses were significant.

Haulm yield/plant

All scaling tests 'A', 'B', 'C' and 'D' were non-significant which shows adequacy of additive dominance model to explain all the genetic variation as the chi-square values of both the crosses were non-significant.

Dry pod yield/plant

In case of dry pod yield per plant, scaling tests 'A' (0.43) and 'B' (0.21) were positively significant. Estimates of genetic effects for this trait by joint scaling test indicated inadequacy of additive dominance model to explain all the genetic variations as the chi-square values of both the crosses were significant.

kernel weight

All scaling tests 'A', 'B', 'C' and 'D' were non-significant which shows adequacy of additive dominance model to explain all the genetic variation as the chi-square values of both the crosses were non-significant.

Shelling percentage

All scaling tests 'A', 'B', 'C' and 'D' were non-significant which shows adequacy of additive dominance model to explain all the genetic variation as the chi-square values of both the crosses were non-significant.

Harvest index

For harvest index, no scaling tests were found significant, indicating adequacy of additive dominance model to explain all the genetic variation.

Estimates of gene effects for yield and yield contributing characters

The estimates of m (mean), major genetic effects additive [d] and dominance [h] and non-allelic gene interactions (i, j and l) based on six parameter model (Hayman, 1958) [14] for pod yield, yield contributing traits for cross: TAG-24 × ICG 4670 (Table 4 andTable 5) under both irrigated and drought stress condition respectively. The parameter [m] was significant for all the characters under study.

Cross: TAG-24 × ICG 4670

The gene effects estimated by using perfect fit model in respect of traits associated with pod yield in groundnut has been presented in Table 4 and Table 5 for irrigated and drought condition respectively.

Days to 50% flowering

Under irrigated condition, the estimates of genetic parameters, it was observed that 'd' (4.30) was positively significant while 'h' (-13.60) was negatively significant. The interaction components 'l' (18.13) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. Under drought stress, from estimates of genetic parameters, it was observed that 'd' (2.36) was positively significant while 'h' (-9.23) was negatively significant. The interaction component 'i' (-8.13) was observed negatively significant, while 'l' (6.80) was estimated positively significant.

Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. The presence of additive, dominance and epistatic interactions for this trait were earlier reported by Kalaimani and Thangavelu (1996) [17] , Jayalakshmi and Reddy (2002) [16] , Suneetha et al., (2006) [36] and Boraiah et al., (2015) [6] . While, Parameshwarappa and Kumar (2007) [27] reported dominance x dominance genetic effects for days to flowering.

Days to maturity

Under irrigated condition, from the estimates of genetic parameters, it was observed that 'h' (-24.80) was negatively significant. The interaction components, 'i' (-25.23) was observed negatively significant, while 'l' (30.76) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', revealing epistasis was predominantly of duplicate type. Under drought stress, from estimates of genetic parameters, it was observed that significant mean (m) effects were observed, but additive (d) and dominance (h) effects were non-significant, indicating an absence of non-allelic interactions for this trait. Role of additive and non-additive gene action for days to maturity were earlier reported by, Talwar et al., (1983) [37] in soybean crop while in groundnut by Basu et al., (1988) [5] , Suneetha et al., (2006) [36] , Gaurav et al., (2010) [12] Pavithradevi (2013) [29] , Prabhu et al., (2016) [30] and Shinde et al., (2016) [34] .

Number of branches/plant

Under irrigated condition, from estimates of genetic parameters, it was observed that 'd' (1.63) was positively significant while 'h' (-3.70) was negatively significant. The interaction component 'i' (-2.96) was observed negatively significant, however 'l' (4.03) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. Under drought stress, from estimates of genetics parameters, it was observed that 'h' (-4.60) was negatively significant. The interaction component 'i' (-4.03) was observed negatively significant, while 'l' (5.56) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', revealing epistasis was predominantly of duplicate type. The duplicate type of epistasis was earlier noticed by Rahangdale and Raut (2002) [31] , Amrita et al., (2014) [2] and Nagarajan et al., (2022) [24] .

Number of mature pods/plant

Under irrigated condition, from estimates of genetic parameters, it was observed that 'd' (3.51) was positively significant while 'h' (-6.76) was negatively significant. The interaction component 'i' (-6.46) was observed negatively significant, however 'l' (9.96) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. Under drought condition, significant mean (m) effects with high magnitude were observed. Additive (d) (-1.76) effect was negatively significant, but non-allelic interactions were absent, suggesting the preponderance of dominant gene effects for number of mature pods per plant. It revealed their potential in controlling this character in this cross which supports the earlier findings of Kaw and Menon (1983) [18] , Cecon et al., (1985) [8] and Nagabushanam et al., (1992) in groundnut, also, similar results were reported by Rahangdale and Raut (2002) [31] and Nagarajan et al., (2022) [24] .

Under irrigated condition, the estimates of genetic parameters, it was observed that 'd' (5.34) was positively significant while 'h' (-13.42) was negatively significant. The interaction components 'l' (16.01) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. Under drought stress, from estimates of genetic parameters, it was observed that significant mean (m) effects were observed, but additive (d) and dominance (h) effects were non-significant, indicating an absence of non-allelic interactions for this trait. This was in conformity with previous findings such as Nigam S. N. (2014) observed both additive and non-additive gene effects; while, Patil et al., (2017) observed non-additive gene effects.

Under irrigated condition, from estimates of genetic parameters, it was observed that 'd' (2.11) was positively significant while 'h' (-13.50) was negatively significant.

The interaction component 'i' (-13.16) was observed negatively significant, however 'l' (15.86) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', with presence of duplicate epistasis. Under drought stress, from estimates of genetic parameters, it was observed that 'd' (-1.46) and 'h' (-2.27) were negatively significant. The interaction component 'l' (0.18) was estimated positively significant. Opposite sign observed for genetic component dominance 'h' and dominance x dominance 'l', revealing epistasis was predominantly of duplicate type. Gaurav et al., (2010) [12] , Shoba et al., (2010) [35] and Nagarajan et al., (2022) [24] reported additive gene action to be involved in the inheritance of this trait. However, non-additive gene action holds good for this trait were reported by Mothilal and Ezhil (2010) [22] , Savithramma et al., (2010) [32] , Pavithradevi (2013) [29] and Azad et al., (2014) [4] . Although, importance of both additive and non-additive gene action in the inheritance of this trait was observed by Makne and Bhale (1989) [19] in groundnut.

Under irrigated condition, significant mean (m) (26.14) effect with high magnitude were observed. Additive (d) (5.15) effect was positively significant, but non-allelic interactions were absent, suggesting the preponderance of additive gene effects for 100 kernel weight. Under drought condition, significant mean (m) (23.54) effects with high magnitude were observed. Additive (d) (-1.44) effect was negatively significant, but non-allelic interactions were absent, suggesting the preponderance of dominant gene effects for 100 kernel weight. Role of additive and non-additive gene action for 100-kernel weight were earlier reported by Senthil and Varman (1998) [33] , Gaurav et al., (2010) [12] , Pavithradevi (2013) [29] and Prabhu et al., (2016) [30] respectively. Interestingly, Hariprasanna et al., (2008) [13] reported predominance of additive gene action in the expression of 100-seed weight.

Under irrigated condition, significant mean (m) (66.99) effect with high magnitude were observed. Additive (d) (3.87) effect was positively significant, but non-allelic interactions were absent, suggesting the preponderance of additive gene effects for shelling percentage. Under drought stress, from estimates of genetic parameters, it was observed that significant mean (m) (66.41) effect was observed, but additive (d) and dominance (h) effects were non-significant, indicating an absence of non-allelic interactions for this trait. This was confirmed with earlier findings, Dobaria et al., (2003) [10] observed both additive and dominant genetic effects in case of shelling out-tum. However, Adamu et al., (2008) [1] reported major role of non-additive gene action as well as dominance x dominance genetic interaction for shelling percent.

Under irrigated condition, significant mean (m) (38.80) effect with high magnitude were observed. Additive (d) (2.36) effect was positively significant, but non-allelic interactions were absent, suggesting the preponderance of additive gene effects for harvest index. Under drought condition, significant mean (m) (41.48) effects with high magnitude were observed. Additive (d) (-2.23) effect was negatively significant, but non-allelic interactions were absent, suggesting the preponderance of dominant gene effects for harvest index. Previously, the additive gene effect for harvest index trait is supported by Chavadhari et al., (2017) [9] . Nonetheless, Apparao (2000) [3] reported predominant non-additive genetic effects governing this trait.

Conclusion

In nutshell, in the present investigation, it can be inferred that morphological traits showed substantial variability under irrigated and drought stress condition and exhibited distinct inheritance patterns. Dry pod yield and number of mature pods per plant were primarily controlled by additive and additive × additive gene effects, indicating the effectiveness of early-generation selection. In contrast, days to 50% flowering, days to maturity, number of branches per plant, number of mature pods, haulm yield and dry pod yield were governed largely by dominance and dominance × dominance interactions, suggesting that selection and breeding procedures should be modified to exploit this type of non-additive genetic variance by delaying the selection to later generations. Overall, integrating morphological traits with physiological and yield-based indicators can greatly strengthen drought screening and accelerate breeding progress towards developing drought-resilient groundnut cultivars.

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