Variance analysis of seed yield
The analysis of variance of seed yield (g plant
-1) of soybean genotypes evaluated (Table 3) revealed that a highly significant difference in soybean seed yield due to the single factor (drought stress levels (S), soybean genotypes (G) and the interaction of drought stress levels and soybean genotype (S x G).
Tukey’s HSD (Table 4) showed that mean seed yield under stress (S
1, 11.09±0.57 g plant
-1; S
2, 9.41±0.51 g plant
-1; S
3, 9.65±0.54 g plant
-1) was consistently lower than under non-stress (S
0, 17.75±0.69 g plant
-1). Genotype G
12 (Grobogan, 21.40±1.69 g plant
-1) and G
15 (Dega
-1, 18.26± 1.62 g plant
-1) exhibited the highest yields, while G
1 (Detam
-2, 4.70±0.39 g plant
-1) and G5 (Detam
-1, 6.39±0.46 g plant
-1) produced the lowest. Genotypes with consistent high yield (G
12, G
15, G
16, G
18, G
20) or low (G
1, G
5, G
10, G
11) under non-stress conditions showed similar patterns under stress.
Yield reductions varied among genotyps across stress levels (Fig 1), consistent with
Fekadu et al., (2022) in barley. The smallest reductions occurred in G
4 (S
1: -1.77%), G
1 (S
2: -12.58%) and G
5/G
1 (S
3: -28.15%/-28.39%), while G
8, G
11 and G
4 experienced the greatest reductions (-70.95%, -66.94% and -60.17%, respectively). G
1 (Detam
-2) exhibited the least and G8 (Anjasmoro) the most, yield loss across all stress levels.
However, yield reduction alone is not a definitive indicator of drought tolerance.
Akbar et al., (2018) dan
Purbowahyuani et al., (2019) emphasized that high yields under optimal conditions reflect genetic potential, essential in evaluating stress tolerance.
Pertiwi et al., (2022) further noted that tolerance indices prioritize the ability to express genetic potential under non-stress conditions. Thus, yield decline under stress may result from environmental influences rather than poor adaptability. Supporting this,
Shahrokhi et al., (2020) indicated that genotypes maintaining high seed yield under both stress and non-stress conditions, reflected in indeces like STI, tends to exhibit superior drought resilience.
Drought tolerance and adaptation indices
Drought tolerance refers to a plant’s ability to sustain productivity under water-limited conditions
(Shoaib et al., 2022), while adaptation involves physiological mechanisms like stomatal closure and osmoprotectant accumulation
(Haghpanah et al., 2024; Seleiman et al., 2021). The Stress Adaptation Index (SAI) is a key tool for identifying drought-resilient soybean genotypes, emphasizing high yield potential, as initially propsed by
Slamet and Suyamto (2001). Common indices used to assess drought response include STi, SAI, SSI, DRI, YSI, YI, MP, GMP, HM and TOL
(Kisman et al., 2021; Riduan et al., 2022; Suhartina et al., 2021; Wasae, 2021).
Table 5 shows considerable variation in drought tolerance indices across genotypes and stress levels (PEG6000 10%, 20% and 65% FC). STI values ranged from 0.1-1.8 with 10-20% of genotypes scoring >1.0 depending on stress level. SAI ranged from 0.1-3.2 with 40% of values >1.0 under all stress conditions. GMP, MP and HM each identified 10-20% exceeding 1.0, while YSI showed higher proportions (40-60%) of genotypes with values >1.0. YI values mostly fell below 1.0 with only 5% exceeding that threshold under S1. TOL and SSI values varied widely but did not indicate consistent tolerance trends. These results are consistent with previous findings in wheat by
Eid and Sabry (2019),
Khosravizad (2023) and
Sedghiyeh et al., (2025).
Correlation analysis among all drought indices
Pearson correlation analysis is essential for identifying effective drought indices to determine genotypes with high yield potential and drought tolerance
(Kumar et al., 2018). According to
Anwar et al., (2011), indices that strongly correlate with seed yield under both stress and non-stress conditions are considered the most reliable indicators.
As shown in Table 6, significant positive correlations were observed among several drought indices, STI, SAI, GMP, MP, HM, DRI and YSI, with seed yields under normal (Yp) and drought stress conditions (Ys
1, Ys
2, Ys
3). These correlations varied with drought severity, as indicated by yield differences between Yp and Ys. These results suggest that these indices are effective in identifying drought-tolerant genotypes capable of maintaining higher yields under stress. These findings align with previous research in wheat (
Eid and Sabry, 2019;
Khosravizad, 2023;
Sedghiyeh et al., 2025).
Tolerant/adapted/stable large-seeded soybean genotypes under drought stress
Biplot analysis enables effective identification of superior genotypes by simultaneously evaluating yield performance and drought-related indices under varying stress levels. In this current study, biplots constructed using two principal components (PC
1 and PC
2) illustrated genotype performance under stress levels of 10% PEG6000 (S
1), 20% PEG6000 (S
2) and 65% field capacity (S
3) (Fig 2). PC1 represented overall yield stability and drought tolerance (Yp, Ys, STI, SAI, MP, GMP, HM, DRI, YSI), while PC2 captured drought susceptibility, distinguishing genotypes with high yield only in non-stress conditions. Genotypes with high PC
1 and low PC
2 were considered optimal (
Golabadi and Arzani, 2006).
Using
Fernandez (1992) classification, genotypes were grouped based on yield performance: Group A (tolerant), Group B ( non-stress specific), Group C (stress specific) and Group D (susceptible). Under S
1, G
12 (Grobogan), G
15 (Dega-1), G18 (KH-1) and G
16 (Biosoy-2) were tolerant. Under S
2, G
12 (Grobogan), G
15 (Dega-1), G
13 (Burangrang) and G
18 (KH-1) remained in Group A, while under S
3, only G
12 (Grobogan) and G
15 (Dega-1) maintained their tolerat classification. These genotypes, especially KH-1, showed enhanced drought adaptation (
Kisman et al., 2022).
Group B include G
7 (Argomulyo) under S
1 and also G
16 (Biosoy-2), G
8 (Anjasmoro) under S
2 and S
3. Group D, comprising drought-susceptible genotypes with low yield under all conditions, included G
8, G11 (Kemuning-1), G
2 (Denasa-2) and G
17 (Detap) at S
1; G
11, G
4, G
8, G
3 (Denasa-1) at S
2; and G
4, G
3, G
9 (Local A) at S
3. Genotypes G
1, G
5 (Detam-1), G10 (edamame Ryoko 75) and G
19 (Kaba) were consistently identified in Group C across all stress levels. Prior research also recognized Grobogan and Dega-1 for their drought resilience
(Saputra et al., 2015; Sukmasari, 2018;
Wahono et al., 2018), with Grobogan noted performance in both drought and waterlogged conditions (
Sukmasari, 2018).