Root system architecture under salt stress
Root scanning revealed clear genotypic differences in root morphology across salinity levels (Fig 1). Under low salinity (50 mM), KDMB-5004, KDMB-5005, KDMB-5011, KDMB-5023 and KDMB-5025 showed well-developed roots with uniform thickness and visible lateral emergence, indicating mild stress impact. Under high salinity (150 mM), KDMB-5010, KDMB-5011, KDMB-5014, KDMB-5023 and KDMB-5025 exhibited pronounced alterations root twisting, surface cracking and cortical fragmentation though KDMB-5023 and KDMB-5025 retained relatively better structural organization, indicating superior resilience. This aligns with known glycophyte responses to NaCl-induced osmotic and ionic stress, consistent with
Chaurasia et al., (2024), who reported progressive declines in root volume, surface area and tip number with increasing NaCl, alongside tolerance indices ranging from 0.38 to 0.91 across accessions. The epidermal and cortical damage reflects reduced cell turgor and impaired expansion, restricting absorptive capacity
(Farooq et al., 2024). Iqbal et al., (2024) linked mungbean salt tolerance to restricted root Na
+ transport and sustained K
+ retention and the resilience of KDMB-5023 and KDMB-5025 likely reflects similar mechanisms, supporting vascular continuity and water/assimilate transport under salinity (
Munns and Tester, 2008).
Cluster analysis (Mahalanobis D2)
Grouping of genotypes into clusters
Mahalanobis D² cluster analysis grouped the 20 mungbean genotypes into four distinct clusters, reflecting considerable genetic diversity among the evaluated material (Table 1; Fig 2). Cluster II was the largest, comprising 11 genotypes (KDMB-5009, KDMB-5010, KDMB-5012, KDMB-5013, KDMB-5014, KDMB-5017, KDMB-5023, KDMB-5025, KDMB-5030, KDMB-5003 and KDMB-5006), indicating a high degree of similarity among these genotypes for the studied traits. Cluster I contained six genotypes (KDMB-5004, KDMB-5007, KDMB-5015, KDMB-5020, KDMB-5028 and KDMB-5005), while Cluster IV comprised two genotypes (KDMB-5011 and KDMB-5018). Cluster III was a monogenotypic cluster containing only KDMB-5001, indicating its high genetic divergence from all other genotypes.
Cluster mean values
Cluster mean analysis revealed substantial variation across clusters (Table 2). Cluster IV showed the highest means for root and shoot growth traits root length (6.21 cm), shoot length (18.13 cm), plant height (24.34 cm), root fresh weight (0.72 g), shoot dry weight (0.64 g), total dry weight (0.80 g) and vigour index (16.33) indicating superior vigour under salinity. Cluster III led in leaf area, primary branches, fresh weight and root dry weight, reflecting stronger biomass accumulation. Cluster II showed intermediate performance, while Cluster I recorded the lowest values overall, including vigour index (8.79), indicating poor salinity performance.
Inter- and intra-cluster distances
Intra-cluster distances ranged from 1.61 (Cluster III) to 2.89 (Cluster II), reflecting greater within-cluster variability in larger clusters. Inter-cluster distances ranged from 3.33 (Clusters I-II) to 8.49 (Clusters I-IV), indicating maximum divergence between Clusters I and IV, followed by Clusters I and III (8.34) (Table 3). The markedly larger inter- versus intra-cluster distances confirm substantial genetic diversity among the evaluated genotypes.
Clustering of 20 mungbean genotypes into four groups confirmed substantial genetic diversity, consistent with
Meena et al., (2025), whose D
2-based clustering and PCA (first three components >62% variation) showed similar results. KDMB-5001’s monogenotypic cluster reflects high divergence, aligning with
Rahangdale et al., (2023). The maximum inter-cluster distance (Clusters I-IV, D
2 = 8.49) suggests these crosses would maximize recombination and yield superior segregants, supporting
Bužinskienė et al. (2023), who linked distant-cluster crosses to high heterosis. Superior growth/vigour traits in Cluster IV and biomass traits in Cluster III mark them as promising parents, consistent with
Iqbal et al., (2024).
Principal component analysis
PCA retained three components (Eigenvalue >1.0) explaining 92.40% of total variation (Table 4; Fig 3A). PC1 (62.58%) showed strong positive loadings for shoot length, plant height, root dry weight, total dry weight and vigour index, representing overall seedling growth under stress. PC2 (15.85%) was linked to primary branches and fresh weight traits, with root length contributing negatively. PC3 (13.95%) reflected dry matter accumulation (shoot/total dry weight, root fresh weight), with negative loadings for leaf area and primary branches. The biplot (Fig 3B, C) identified KDMB-5010 (G20) as a distinct outlier with exceptional biomass, while KDMB-5018 and KDMB-5011 showed superior shoot growth and vigour. KDMB-5005, KDMB-5020, KDMB-5028, KDMB-5007 and KDMB-5009 clustered in the negative quadrant, indicating poor performance. This high variance capture confirms strong trait interdependency with PC1’s dominance aligning with
Azam et al., (2023) and PC2/PC3 patterns mirroring osmotic-ionic stress partitioning
(Iqbal et al., 2024). The biplot separation corroborates cluster findings, consistent with
Sivakumar et al., (2020). Plant height contributed most to genetic divergence (32.50%), followed by shoot length (18.87%), vigour index (15.90%) and leaf area (15.21%) (Fig 3D), with total fresh weight, shoot fresh weight and root length contributing moderately (3.10-7.42%); remaining traits were negligible (≤0.08%). Together, plant height, shoot length, vigour index and leaf area accounted for 82.47% of divergence, aligning with
Chaurasia et al., (2024). The negligible dry-weight contribution suggests shoot morphology drives inter-genotypic discrimination, while dry matter differences manifest within salinity levels, consistent with
Talakayala et al., (2025). These findings support plant height, shoot length, vigour index and leaf area as efficient, non-destructive selection criteria for early-generation salinity tolerance breeding in mungbean.
Stress tolerance indices
Considerable genotypic variation was observed across all nine stress indices. KDMB-5018, KDMB-5025, KDMB-5030 and KDMB-5013 consistently showed high STI, GMP, YSI, YI and HM values, indicating stable performance under salinity, while KDMB-5006 recorded the lowest SSI (-0.83) and SSPI (-9.43), confirming its tolerance. Conversely, KDMB-5017, KDMB-5005 and KDMB-5028 exhibited high SSI, TOL and SSPI, indicating greater susceptibility. Based on mean ranks across all indices, KDMB-5020 emerged as the most tolerant genotype (mean rank: 6.33), followed by KDMB-5030 (7.00), KDMB-5014 (7.11), KDMB-5025 (7.22) and KDMB-5001 (7.89) (Fig 4). The most susceptible were KDMB-5010 (17.56) and KDMB-5007 (17.44). This multi-index approach confirmed that no single index fully captures salinity tolerance, reinforcing the value of integrating complementary indices consistent with
Sarkar et al., (2025), who showed that combining multiple indices with PCA and cluster analysis improves screening reliability in lentil under abiotic stress. The superior STI, GMP and HM values of KDMB-5018 and KDMB-5025 indicate genotypes sustaining productivity under both stress and non-stress conditions, a key criterion for variety development (
Fernandez, 1992). This aligns with
Pour et al., (2019), who established that high STI, MP, GMP and HM identify genuinely stress-tolerant, high-yielding genotypes, while SSI > 1 denotes above-average susceptibility correctly classifying KDMB-5017, KDMB-5005 and KDMB-5028 as susceptible here. The negative SSI and SSPI of KDMB-5006 suggest possible stress-induced compensatory responses, warranting further investigation.
Multi-trait genotype ideotype distance index (MGIDI)
MGIDI analysis revealed substantial variation in multi-trait performance under salinity (Fig 5A). KDMB-5001 recorded the lowest MGIDI value (3.210), ranking first as closest to the ideotype, followed by KDMB-5018 (3.459) and KDMB-5011 (3.500). Other relatively low-scoring genotypes (4.640-4.780) included KDMB-5017, KDMB-5010, KDMB-5003, KDMB-5023, KDMB-5013 and KDMB-5025, while KDMB-5006, KDMB-5015, KDMB-5007, KDMB-5005 and KDMB-5020 (5.307-5.482) showed the greatest deviation from the ideotype. Selection differential analysis showed strong positive gains for biomass traits, particularly under medium/high salinity: TDW-High (188.64%), SDW-High (173.57%), SDW-Medium (163.96%), TDW-Medium (147.01%), SFW-Low (303.91%) and TFW-Low (278.16%) (Fig 5B). Factor contributions showed FA1 capturing medium/high-salinity biomass, FA2 low-salinity dry matter, FA3 fresh biomass and root dry weight, FA4 shoot/plant height under stress and FA5 seedling vigour at low salinity. A near-100% goal percentage across traits indicated minimal genetic trade-offs, confirming MGIDI’s reliability for simultaneous multi-trait selection. By integrating twelve traits across three environments, MGIDI identified KDMB-5001, KDMB-5018 and KDMB-5011 as most ideotype-proximal, consistent with
Debnath et al., (2024). KDMB-5001’s ranking is further validated by its monogenotypic cluster status and strong stress index performance. Similar utility was reported by
Zahedi et al., (2026) in wheat and
Chaity et al., (2026) in rice (genetic gains of 28.51% and 25.86%). The high biomass gains and near-universal goal achievement confirm simultaneous trait improvement with minimal trade-offs, an advantage over conventional mean-rank methods (
Olivoto and Nardino, 2021), corroborated by
Palaniyappan et al., (2025) in fodder maize.