We studied fifteen drought tolerance-related indicators of seed cotton output under drought stress conditions for five consecutive years (from 2016 to 2020) in order to analyze the impacts of drought stress on eleven Egyptian cotton materials.
1. Combined ANOVA and genetic parameters:
The data of combined ANOVA and genetic parameters for each trial individually for seed cotton yield (Kentar/Feddan) is presented in Table 4. The combined ANOVA table showed that seed cotton yield was significantly affected (p ≤ 0.05 or 0.01) by genotype (G), years (Y), and GY interaction in both irrigation conditions. The effects of E, G, and GY interaction collectively explained 83.94% and 74.45% of the total cotton yield variation under normal irrigation and water-deficit stress conditions, respectively. The G (39.01%) explained most of the total SS, followed by the GY interaction (35.33%) under normal irrigation conditions, while the opposite was true for water-deficit stress conditions (21.40% and 45.83%, respectively). In normal irrigation and water-deficit stress conditions, seed cotton yield displayed low and moderate coefficient of variation (CV%) values of 8.45% and 14.78%, respectively. According to ANOVA analysis, which assumes a random-effects model, all genetic parameters calculated for seed cotton yield were higher in normal irrigation conditions compared with water-deficit stress conditions, except for error variance. The variance due to GY interaction was greater than the other variances in both irrigation conditions. The values of H2 were high (H2 > 0.60) and moderate (0.30 < H2 < 0.60) for seed cotton yield under normal irrigation and water-deficit stress conditions, respectively (Table 4).
Table 4
Combined ANOVA and genetic parameters across five years for seed cotton yield of 24 genotypes under normal irrigation and water-deficit stress conditions.
Source of Variation
|
df
|
Normal irrigation conditions
|
Water-deficit stress conditions
|
Sums of Squares (SS)
|
Mean of
Squares
|
SS%
|
Sums of Squares (SS)
|
Mean of
Squares
|
SS%
|
Years (Y)
|
4
|
87.25
|
21.81**
|
9.60
|
43.93
|
10.98**
|
7.23
|
Replication/Y
|
10
|
34.38
|
3.44**
|
3.78
|
13.91
|
1.39ns
|
2.29
|
Genotype (G)
|
10
|
354.43
|
35.44**
|
39.01
|
130.07
|
13.01*
|
21.40
|
G x Y
|
40
|
321.00
|
8.03**
|
35.33
|
278.60
|
6.97**
|
45.83
|
Error
|
100
|
111.51
|
1.12
|
12.27
|
141.38
|
1.41
|
23.26
|
CV%
|
8.45
|
14.78
|
Genetic Parameters
|
VG
|
1.83
|
0.40
|
VGY
|
2.30
|
1.85
|
VE
|
1.12
|
1.41
|
VPh Mean
|
2.36
|
0.87
|
H2 Mean
|
77.36
|
46.45
|
VG: genotypic variance; VGY: genotype x year interaction variance; VE: error variance; VPh mean: phenotypic variance on entry-mean basis; H2 mean: broad-sense heritability on entry-mean basis (%). Statistically significant differences at *p ≤ 0.05 and **p ≤ 0.01; ns: indicate the non-significant difference.
2. Mean Performance and GxY heatmap analysis:
Mean seed cotton yield comparisons in both irrigation conditions showed significant differences among evaluated genotypes in each growing year. Over the five years studied, normal irrigation conditions resulted in a significant increase in seed cotton yield when compared to water-deficit stress conditions (Fig. 1). The average environmental seed cotton yield of genotypes ranged from 7.94 (G7 in 2019) to 16.33 (G4 in 2018) and from 4.72 (G7 in 2018) to 12.61 (G1 in 2020) under normal irrigation and water-deficit stress conditions, respectively. Based on the mean of all investigated genotypes, the growing years 2018 (13.35) and 2019 (8.81) had the highest seed cotton yield compared with other years under normal irrigation and water-deficit stress conditions, respectively.
GY heatmap analysis of seed cotton yield was used to create a visual comparison of the effects of the growing years on the genotypes in both irrigation conditions, as well as to determine the range of water-deficit stress responses detectable in these genotypes (Fig. 1). The GY heatmap analysis of both irrigation conditions revealed two dendrograms: the five years on top, and that influenced the distribution of eleven cotton genotypes on the left. In both irrigation conditions, the top dendrogram classified the growing years into two distinct clusters. The first cluster included the 2019 and 2020 years, as well as the 2020 year under normal irrigation and water-deficit stress conditions. The second cluster included the remaining years in both irrigation conditions. As for the left dendrogram, eleven genotypes could be classified for five and seven clusters in normal irrigation and water-deficit stress conditions, respectively. Genotypes within the cluster have the least variance and genetic distance, whereas genotypes between clusters differ and have the greatest genetic distance.
The G4 genotype in the second cluster gave the best seed cotton yield in most growing years, followed by the genotypes in the fifth cluster (G2, G9, and G10) under normal irrigation conditions. The genotype G7 in the third cluster had the best cotton yield in 2016, 2017, and 2018 years under normal irrigation conditions. Based on the heat map under water-deficit stress conditions, the G3 and G4 genotypes in the sixth and fifth clusters, respectively, were among the best performers of cotton yield across most growing years, followed by the genotypes in the second cluster (G9 and G10). The G1and G2 genotypes in the fourth and seventh clusters recorded the highest seed cotton yield in the 2020 and 2017 years, respectively, and moderate to low cotton yield in the other years. In contrast, the other genotypes in the other clusters were intermediate or low in GY interactions in both irrigation conditions. Generally, the G8 and G6 genotypes recorded the lowest seed cotton yield in normal irrigation and water-deficit stress conditions, respectively.
Drought Tolerance Indices:
Fifteen drought tolerance indices based on seed cotton yield potential and response were calculated, to assess the drought tolerance of eleven cotton genotypes under normal irrigation (Yp) and water-deficit stress (Ys) conditions (Table 5). The low values of the SSI, TOL, YR, ATI, and SSPI indices indicate that the genotypes are low sensitive to water stress. In comparison, the high values of the MP, GMP, STI, YI, YSI, DI, SNPI, RDI, HM, and GOL indices indicate that the genotypes are drought-tolerant. The investigated genotypes showed significant differences in seed cotton yield under normal irrigation and water-deficit stress conditions. Over five growing years, the seed cotton yield of eleven genotypes decreased under water-deficit stress compared to normal irrigation conditions. Seed cotton yield ranged from 9.79 Kentar/Feddan (G8) to 15.45 Kentar/Feddan (G4) under Yp conditions, and from 6.52 Kentar/Feddan (G6) to 9.47 Kentar/Feddan (G3) under Ys conditions.
Lower SSI, TOL, YR, ATI, and SSPI values, as well as higher YSI, RDI, and GOL values were recorded by the genotypes G1, G3, and G8. As a result, these genotypes were identified as the most drought-resistant and desirable under Ys based on these indices. The YI, DI, and SNPI indices were high in G1, and G3 during the Ys, and G4 during Yp. However, the genotypes G6, and G7 by the indices of SSI, YR (high), Yi, YSI, DI, SNPI, RDI, and GOL (low), and the genotypes G4 and G10 by TOL, ATI, and SSPI indices (high) were identified as drought-susceptible.
The genotypes G4, G9, and G10 exhibited the highest values by MP, GMP, STI, and HM indices with high productivity under Yp, and moderate-to-high productivity under Ys. Therefore, these genotypes were classified as drought tolerant in both irrigation conditions. Opposite, the genotypes G6, G7, and G8 showed low MP, GMP, STI, and HM values with low productivity in both Yp, and Ys, but the G7 genotype had moderate productivity in Yp. As a result, these findings suggest that these genotypes are more sensitive to drought. With the exception of the previously identified sensitive and tolerant genotypes, all drought tolerance indices in this study classified the remaining genotypes as semi-tolerant or semi-sensitive to drought stress.
Table 5
Comparison of drought indices for eleven cotton genotypes based on seed cotton yield (Kentar/Feddan) under normal irrigation (Yp) and water-deficit stress (Ys) conditions (averaged over five years).
Genotypes
|
Drought Tolerance Indices
|
Yp
|
Ys
|
SSI
|
TOL
|
YR
|
ATI
|
SSPI
|
MP
|
GMP
|
STI
|
YI
|
YSI
|
DI
|
SNPI
|
RDI
|
HM
|
GOL
|
G1
|
11.88
|
8.59
|
0.77
|
3.29
|
0.28
|
7.41
|
13.14
|
10.24
|
10.10
|
0.65
|
1.07
|
0.72
|
0.77
|
17.60
|
1.13
|
9.97
|
6.22
|
G2
|
12.88
|
8.4
|
0.97
|
4.48
|
0.35
|
10.40
|
17.89
|
10.64
|
10.40
|
0.69
|
1.05
|
0.65
|
0.68
|
16.28
|
1.02
|
10.17
|
4.75
|
G3
|
11.84
|
9.47
|
0.56
|
2.37
|
0.20
|
5.60
|
9.47
|
10.66
|
10.59
|
0.72
|
1.18
|
0.80
|
0.94
|
21.21
|
1.25
|
10.52
|
8.99
|
G4
|
15.45
|
8.97
|
1.17
|
6.48
|
0.42
|
17.02
|
25.88
|
12.21
|
11.77
|
0.88
|
1.12
|
0.58
|
0.65
|
16.73
|
0.90
|
11.35
|
3.77
|
G5
|
12.6
|
7.53
|
1.12
|
5.07
|
0.40
|
11.02
|
20.25
|
10.07
|
9.74
|
0.61
|
0.94
|
0.60
|
0.56
|
14.16
|
0.93
|
9.43
|
3.97
|
G6
|
11.35
|
6.52
|
1.19
|
4.83
|
0.43
|
9.27
|
19.29
|
8.94
|
8.60
|
0.47
|
0.81
|
0.57
|
0.47
|
12.13
|
0.89
|
8.28
|
3.70
|
G7
|
12.22
|
6.87
|
1.22
|
5.35
|
0.44
|
10.94
|
21.37
|
9.55
|
9.16
|
0.54
|
0.85
|
0.56
|
0.48
|
12.72
|
0.88
|
8.80
|
3.57
|
G8
|
9.79
|
7.03
|
0.79
|
2.76
|
0.28
|
5.11
|
11.02
|
8.41
|
8.30
|
0.44
|
0.87
|
0.72
|
0.63
|
14.34
|
1.12
|
8.18
|
6.09
|
G9
|
13.87
|
8.51
|
1.08
|
5.36
|
0.39
|
12.99
|
21.41
|
11.19
|
10.86
|
0.75
|
1.06
|
0.61
|
0.65
|
16.13
|
0.96
|
10.55
|
4.18
|
G10
|
14.13
|
8.4
|
1.13
|
5.73
|
0.41
|
13.93
|
22.89
|
11.27
|
10.89
|
0.76
|
1.05
|
0.59
|
0.62
|
15.77
|
0.93
|
10.54
|
3.93
|
G11
|
11.69
|
8.1
|
0.86
|
3.59
|
0.31
|
7.79
|
14.34
|
9.90
|
9.73
|
0.60
|
1.01
|
0.69
|
0.70
|
16.17
|
1.08
|
9.57
|
5.51
|
Minimum
|
9.79
|
6.52
|
0.56
|
2.37
|
0.20
|
5.11
|
9.47
|
8.41
|
8.30
|
0.44
|
0.81
|
0.56
|
0.47
|
12.13
|
0.88
|
8.18
|
3.57
|
Maximum
|
15.45
|
9.47
|
1.22
|
6.48
|
0.44
|
17.02
|
25.88
|
12.21
|
11.77
|
0.88
|
1.18
|
0.80
|
0.94
|
21.21
|
1.25
|
11.35
|
8.99
|
Mean
|
12.52
|
8.04
|
0.99
|
4.48
|
0.36
|
10.13
|
17.90
|
10.28
|
10.01
|
0.65
|
1.00
|
0.64
|
0.65
|
15.75
|
1.01
|
9.76
|
4.97
|
The genotypes and drought tolerance indices key names can be found in Tables 1 and 3, respectively.
Principal component analysis:
Principal component analysis (PCA) was used to identify drought-tolerant and sensitive genotypes, as well as to gain a clear understanding of the relationships between drought tolerance indices in both irrigation conditions. Out of all PCs, the two first main PCs (PC1 and PC2) were kept for the final analysis because they both have eigenvalues greater than one and explain 99.50% of the total variance of all analyzed variables. The PC1 explains 54.67% of the total variance of variables and is highly positively correlated with indices of SSI, YR, TOL, SSPI, and ATI, and positively correlated with indices of MP, GMP, and STI under Yp (Fig. 2A). While, the PC2 accounted for 44.83% of the total variation of analyzed variables and strongly positively correlated with indices of STI, MP, GMP, HM, and YI, and positively correlated with other indices under Yp, and Ys, except for SSI, and YR (Fig. 2B). Generally, PC1 and PC2 are positively correlated with STI, MP, GMP, HM, and YI indices in both irrigation conditions.
A perfect positive correlation had observed between YS and YI, between SSI and YR, between TOL and SSPI, between GMP and STI, as well as between YSI and RDI, because the angles between them are zero (Fig. 3). Our findings revealed that most drought indices had below 90-degree angles (sharp angled), indicating a positive correlation between these variables. A high and positive correlation (smallest sharp angles) was recorded among Yp with TOL, ATI, SSPI, MP, GMP, STI, YI, and HM indices, as well as among Ys with MP, GMP, STI, YI, DI, SNPI, and HM indices. A strong positive association was observed among SSI, TOL, YR, SSPI, and ATI indices, among MP, GMP, STI, YI, SNPI, and HM indices, among YSI, DI, SNPI, RDI, and GOL indices, and among YI, DI, SNPI, and HM indices, suggesting that these indices are closely associated in the ranking of the genotypes. ATI had highly positively correlated with MP, GMP, STI, and HM indices. The other relationships between the drought tolerance indices were positive (low) or negative, depending on whether the angles between them were acute (large) or obtuse, respectively (Fig. 3).
As shown in Fig. 3, the PCA analysis for seed cotton yield and drought tolerance indicators also allowed cotton genotypes to be divided into four groups based on their phenotypic similarities under normal irrigation and water-deficit stress conditions. The first quarter (the first group) was occupied by the genotypes G4, G9, and G10 using STI, MP, GMP, HM, ATI, SSPI, and TOL, which showed the highest PC1 and PC2 as well as the highest and moderate seed cotton yield in Yp and Ys, respectively. The second group comprised genotypes G5, G6 and G7 using SSI and YR, which were located in the fourth quarter (the highest PC1 and the lowest PC2) and showed medium cotton yield in Yp. The genotypes G8, and G11, which were discovered in the third quarter, formed the third group (the lowest PC1, and PC2), and had low to moderate grain yield performance in both conditions, and associated with YSI, and RDI. The genotypes G1, G2, and G3 by YI, SNPI, DI, GOL, RDI, and YSI had the lowest PC1 and the greatest PC2 in the fourth group (the second quarter), which exhibited a high and moderate yield response in Ys and Yp, respectively.