3.1 Variability of traits among entries
Under low-N stress condition, significant (p ≤ 0.05) environment, genotype, and genotype × environment interaction effects were obtained for GY and some traits (Table 2). Significant environment effect was obtained for GY and other traits except EA, while genotypic effect was significant for GY and other traits, except ASI, EHT, SG and EPP. Genotype × environment interaction effect was significant for GY and other traits, except EPP. Significant GCA and SCA effects were obtained for GY and all traits, although GCA effect was twice as large as SCA effect. Significant GCA × environment interaction effects were obtained for GY and other traits, except AD, SD, ASI and PA. Also, SCA × environment interaction effect was significant for GY and some traits, but not ASI, EA and EPP. The repeatability estimates ranged from 9.0% for ASI to 63.7% for PA (Table 2).
Under high-N condition, significant environment effect was obtained for GY and other traits, except EHT, and PA, while genotype effect was significant for GY and other traits except ASI and EA. Significant genotype × environment interaction effect was obtained for GY and other traits, except ASI, PHT, EHT, EA and SG (Table 2).General combining ability effect was significant for GY and all measured traits, while significant SCA effect was obtained for GY and other traits, except ASI, EA and SG. Significant GCA effect for GY and other traits was twice as large as SCA effect. Significant GCA × environment interaction effect was obtained for GY and other traits, except ASI, PHT, EHT, EA and EPP, while SCA × environment interaction effect was obtained for GY and other traits, except ASI, PHT, EHT, PA, EA and SG. The repeatability estimates ranged from 61.9% for ASI to 95.7% for AD (Table 2).
Under DS condition, significant environment effect was obtained for GY and other traits under DS condition, while genotypic effect was significant for GY and other traits, except AD, SD, ASI and PHT. Genotype × environment interaction effect was significant for GY and other traits, except PA and EPP. Significant GCA and SCA effects were obtained for GY and other traits except EPP. Also, significant GCA effect for GY and other traits was twice as large as SCA effect. Significant GCA × environment interaction effect was obtained for GY and other traits, except ASI, PA, and EPP. SCA × environment interaction effect was significant for GY and other traits, except PA, EA, SG and EPP. Repeatability estimates ranged from 16.2% for SD to 87.3% for PA (Table 3).
Under WW condition, significant environment effect was obtained for GY and other traits, except ASI, PHT, EHT, and PA, while genotypic effect was significant for GY and other traits except EA. Significant genotype × environment interaction effect was obtained for GY and other traits, except PHT and EHT. Partitioning the genotypic effect into its components revealed significant GCA and SCA effects for GY and other traits, except SCA effect for EA. The GCA effect was twice as large as SCA effect, except AD and PA. Significant GCA and SCA × environment interaction effect was obtained for GY and other traits, except PHT, and EHT. Repeatability estimates ranged from 17.2% for ASI to 93.8% PHT (Table 3).
Table 2
Mean squares for grain yield and other selected yield traits of quality protein maize singe-cross hybrids evaluated under low-N and high-N conditions.
Sources of variation
|
Df
|
GY
|
AD
|
SD
|
ASI
|
PHT
|
EHT
|
PA
|
EA
|
SG
|
EPP
|
|
|
(kg/ ha)
|
|
(days)
|
(cm)
|
(1–9 rating)
|
|
Low-N condition
|
Environment (Env.)
|
1
|
53374970.9***
|
7.19**
|
127.21***
|
73.92***
|
108991.8***
|
47000.2***
|
74.72***
|
0.48
|
35.56***
|
1.715***
|
Replication (Env.)
|
4
|
1528234.6***
|
3.29***
|
3.58***
|
0.77***
|
1188.6***
|
806.4***
|
2.16***
|
13.82***
|
4.32***
|
0.030***
|
Genotypes (G)
|
77
|
2043058.3***
|
6.07***
|
8.74***
|
0.89
|
500.8*
|
136.4
|
2.50***
|
2.05*
|
1.28
|
0.023
|
G × Env.
|
77
|
953526.1***
|
15.91**
|
2.92***
|
0.77***
|
305.6***
|
99.1***
|
1.22***
|
1.23***
|
1.12***
|
0.019
|
GCA
|
12
|
3542622.4***
|
4.25***
|
24.16***
|
2.07***
|
1406.0***
|
285.5***
|
2.80***
|
5.49***
|
3.78***
|
0.027*
|
SCA
|
65
|
1766215.7***
|
1.27***
|
5.89***
|
0.67***
|
333.7***
|
108.8***
|
2.45***
|
1.42***
|
0.82**
|
0.023**
|
GCA × Env.
|
12
|
1388534.4***
|
0.75
|
1.36
|
0.32
|
239.9*
|
160.6***
|
0.67
|
3.62***
|
2.24***
|
0.030*
|
SCA × Env.
|
65
|
873216.9***
|
1.37***
|
3.21***
|
0.86
|
317.7***
|
87.8***
|
1.32***
|
0.79
|
0.92***
|
0.017
|
Residual
|
260
|
224099.7
|
0.76
|
1.05
|
0.34
|
116.9
|
42.0
|
0.63
|
0.70
|
0.47
|
0.015
|
Repeatability
|
|
0.572
|
0.689
|
0.605
|
0.090
|
0.34
|
0.40
|
0.637
|
0.347
|
0.231
|
0.129
|
High-N condition
|
Environment (Env.)
|
1
|
64381860.5***
|
20.94***
|
35.01***
|
1.80**
|
586.70*
|
68.46
|
1.80
|
44.31***
|
-
|
0.494***
|
Replication (Env.)
|
4
|
70049903.5***
|
3.49***
|
5.41***
|
0.85***
|
2917.90***
|
1557.48***
|
4.84***
|
0.76***
|
-
|
0.018*
|
Genotypes (G)
|
77
|
7571910.5***
|
2.04***
|
2.19***
|
0.34
|
684.12***
|
136.75***
|
1.17*
|
0.41
|
-
|
0.013**
|
G × Env.
|
77
|
2721611.9***
|
0.87***
|
0.88***
|
0.30
|
48.89
|
9.72
|
0.71*
|
0.39
|
-
|
0.008*
|
GCA
|
12
|
16048738.4***
|
6.97***
|
7.32***
|
0.44*
|
1778.97***
|
217.35***
|
1.93***
|
0.75*
|
-
|
0.022***
|
SCA
|
65
|
6006957.7***
|
1.13***
|
1.24***
|
0.32
|
481.99***
|
121.87***
|
1.03***
|
0.35
|
-
|
0.012***
|
GCA × Env.
|
12
|
3348073.9***
|
1.29***
|
1.30***
|
0.25
|
39.97
|
8.45
|
1.27**
|
0.51
|
-
|
0.007
|
SCA × Env.
|
65
|
2605957.4***
|
0.80***
|
0.81***
|
0.31
|
50.53
|
9.96
|
0.61
|
0.36
|
-
|
0.008*
|
Residual
|
260
|
667675.4
|
0.29
|
0.32
|
0.24
|
89.39
|
35.78
|
0.52
|
0.34
|
-
|
0.005
|
Repeatability
|
|
0.882
|
0.957
|
0.940
|
0.619
|
0.878
|
0.942
|
0.901
|
0.853
|
|
0.657
|
Grain yield (GY), Silking date (SD), Anthesis date (AD), Anthesis-silking interval (ASI), Ear height (EHT), Plant height (PHT), Ear aspect (EA), plant aspect (PA), number of ears per plant (EPP), Stay-green characteristics (SG) |
*, **, and *** = significant at 5, 1 and 0.1% probability levels, respectively. |
Table 3
Mean squares for grain yield and other selected yield traits of quality protein maize singe-cross hybrids evaluated under drought stress and well-watered conditions.
Sources of variation
|
Df
|
GY
|
AD
|
SD
|
ASI
|
PHT
|
EHT
|
PA
|
EA
|
SG
|
EPP
|
|
|
(kg/ ha)
|
(days)
|
(cm)
|
(1–9 rating)
|
|
Drought stress condition
|
|
|
|
|
|
|
|
|
|
|
Environment (Env.)
|
1
|
8617993.7***
|
313.44***
|
450.17***
|
12.34***
|
168500.3***
|
27724.6***
|
4.33**
|
1120.03**
|
148.92***
|
0.409**
|
Replication (Env.)
|
4
|
106585.0***
|
5.96***
|
5.83***
|
0.90***
|
1136.7***
|
708.4***
|
0.57*
|
11.80**
|
0.32***
|
0.260***
|
Genotypes (G)
|
77
|
2014378.4***
|
3.70
|
4.21
|
0.69
|
451.9
|
170.1**
|
3.78***
|
3.14**
|
1.98***
|
0.052*
|
G × Env.
|
77
|
933106.1***
|
8.82***
|
4.48***
|
0.62***
|
395.9***
|
95.3***
|
0.47
|
1.24*
|
0.30*
|
0.038
|
GCA
|
12
|
2276957.3***
|
2.86***
|
7.81***
|
1.68***
|
1404.8***
|
418.9***
|
2.80***
|
9.28**
|
3.68***
|
0.046
|
SCA
|
65
|
1965902.3***
|
3.24***
|
3.88***
|
0.54**
|
369.7***
|
124.3***
|
3.96***
|
2.01**
|
1.67***
|
0.059
|
GCA × Env.
|
12
|
825007.1***
|
7.10***
|
7.74***
|
0.48
|
657.6***
|
137.1**
|
0.60
|
2.44**
|
0.56**
|
0.052
|
SCA × Env.
|
65
|
953062.9***
|
2.53***
|
3.88***
|
0.65***
|
347.6***
|
87.5**
|
0.45
|
1.02
|
0.26
|
0.036
|
Residual
|
260
|
285425
|
0.96
|
1.08
|
0.34
|
170.29
|
50.06
|
0.45
|
0.87
|
0.35
|
0.05
|
Repeatability
|
|
0.576
|
0.295
|
0.162
|
0.186
|
0.432
|
0.477
|
0.873
|
0.576
|
0.769
|
0.188
|
Well-watered conditions
|
|
|
|
|
|
|
|
|
|
Environment (Env.)
|
1
|
46600226.0**
|
77.95**
|
82.93**
|
0.08
|
69.2
|
1.7
|
0.42
|
147.80**
|
-
|
0.647**
|
Replication (Env.)
|
4
|
6951604.4**
|
9.11**
|
11.10**
|
0.39**
|
2831.9**
|
1813.1**
|
7.01**
|
5.47**
|
-
|
0.005**
|
Genotypes (G)
|
77
|
7177054.7**
|
5.10*
|
5.78**
|
0.56**
|
896.8**
|
228.0**
|
2.30**
|
0.65
|
-
|
0.014*
|
G × Env.
|
77
|
2626067.5**
|
15.36**
|
4.09**
|
0.70**
|
13.4
|
7.8
|
1.27**
|
0.64*
|
-
|
0.009**
|
GCA
|
12
|
15339084.5**
|
3.21**
|
23.03**
|
1.12**
|
2111.6**
|
287.7**
|
3.78**
|
1.55**
|
-
|
0.029**
|
SCA
|
65
|
5670218.4**
|
3.15**
|
4.13**
|
0.51**
|
672.5**
|
216.9**
|
2.03**
|
0.48
|
-
|
0.012**
|
GCA × Env.
|
12
|
2653646.5**
|
3.73**
|
5.40**
|
0.61*
|
12.8
|
3.5
|
2.18**
|
0.84*
|
-
|
0.011*
|
SCA × Env.
|
65
|
2620976.0**
|
3.04**
|
3.85**
|
0.71**
|
13.5
|
8.6
|
1.10**
|
0.6
|
-
|
0.009**
|
Residual
|
260
|
495870
|
0.40
|
0.51
|
0.29
|
73.1
|
35.5
|
0.61
|
0.48
|
-
|
0.005
|
Repeatability
|
|
0.660
|
0.414
|
0.447
|
0.172
|
0.938
|
0.892
|
0.466
|
0.501
|
-
|
0.235
|
Grain yield (GY), Silking date (SD), Anthesis date (AD), Anthesis-silking interval (ASI), Ear height (EHT), Plant height (PHT), Ear aspect (EA), plant aspect (PA), number of ears per plant (EPP), Stay-green characteristics (SG) |
*, **, and *** = significant at 5, 1 and 0.1% probability levels, respectively. |
3.2 Relative contributions of additive and non-additive gene effects
The GCA and SCA effects obtained for GY and other traits indicated that both additive and non-additive gene actions were involved in the inheritance of those traits under low-N and DS conditions. Estimates of relative contributions of both gene actions were determined as proportion of additive variances to the total genetic variance (Baker, 1978). In this study, the proportion of additive gene action obtained for GY was greater under optimal (73%) than under low-N (67%), DS (54%) and across low-N and DS conditions (63%) (Table 4). In general, additive gene effect was largely responsible for the inheritance of GY and other traits under low-N and DS, except AD under DS condition (Fig. 1). Under low-N, additive gene action was more important in the inheritance of GY and other traits, ranging from 54.0% for EPP to82.0% for SG (Table 4, Fig. 1). Under DS condition, the proportion of additive gene effect was greater than non-additive gene effect for GY and other traits, except AD, and PA (Fig. 1). Averaged across low-N and DS conditions, proportion of additive gene effect for GY and other traits was greater than non-additive gene effect, except PA (52.0%) (Fig. 1). The proportion of additive gene effect ranged from 53.0% for PAto88.0% for SG across low-N and DS conditions (Table 4) The additive gene effect contributed 63.0% of the total genetic variance in GY across low-N and DS conditions. Under optimal conditions, additive gene effect or GY and other yield traits was greater than non-additive gene action, ranging from 58.0% (EHT) to 88.0% (SG) (Table 4). Also, additive gene effect contributed 73.0% of the total genetic variance in GY (Table 4).
Table 4
Proportion of additive and non-additive gene actions for grain yield and selected yield traits of QPM inbred lines evaluated under low-N, drought stress, and optimal conditions.
Traits
|
LN
|
DS
|
ACR
|
OPT
|
|
GCA
|
SCA
|
GCA
|
SCA
|
GCA
|
SCA
|
GCA
|
SCA
|
GY
|
0.67
|
0.33
|
0.54
|
0.46
|
0.63
|
0.37
|
0.73
|
0.27
|
AD
|
0.77
|
0.23
|
0.47
|
0.53
|
0.76
|
0.24
|
0.86
|
0.14
|
SD
|
0.80
|
0.20
|
0.67
|
0.33
|
0.76
|
0.24
|
0.86
|
0.14
|
ASI
|
0.76
|
0.24
|
0.76
|
0.24
|
0.78
|
0.22
|
0.58
|
0.42
|
PHT
|
0.81
|
0.19
|
0.79
|
0.21
|
0.83
|
0.17
|
0.79
|
0.21
|
EHT
|
0.72
|
0.28
|
0.77
|
0.23
|
0.77
|
0.23
|
0.64
|
0.36
|
PA
|
0.53
|
0.47
|
0.41
|
0.59
|
0.48
|
0.52
|
0.65
|
0.35
|
EA
|
0.79
|
0.21
|
0.82
|
0.18
|
0.82
|
0.18
|
0.68
|
0.32
|
SG
|
0.82
|
0.18
|
0.69
|
0.31
|
0.88
|
0.12
|
0.88
|
0.12
|
EPP
|
0.54
|
0.46
|
0.44
|
0.56
|
0.55
|
0.45
|
0.65
|
0.35
|
Grain yield (GY), Silking date (SD), Anthesis date (AD), Anthesis-silking interval (ASI), Ear height (EHT), Plant height (PHT), Ear aspect (EA), plant aspect (PA), number of ears per plant (EPP), Stay-green characteristics (SG), Across high-N and well-watered (OPT), Drought stress (DS), Low soil nitrogen (Low-N), Across low-N and DS conditions (ACR). |
3.3 General combining ability effect of inbred lines
Significant GCA effect for GY and other yield traits was obtained for inbred lines under low-N, and DS conditions. Under low-N condition, significant positive GCA effect of GY was obtained for inbred lines CRIZEQ–49, CRIZEQ-77, CRIZEQ-42 and CRIZEQ-44, while significant negative GCA effect was obtained for inbred lines CRIZEQ-5, CRIZEQ-24, CRIZEQ-40, CRIZEQ-45 and CRIZEQ-54. Under DS conditions significant positive GCA effect of GY was obtained for inbred lines CRIZEQ-44, CRIZEQ-14, CRIZEQ-77 and CRIZEQ-49, while significant negative effect was obtained for CRIZEQ-5, CRIZEQ-25, CRIZEQ-42, CRIZEQ-45 and CRIZEQ-54 (Table 5). Averaged across low-N and DS conditions, inbred lines CRIZEQ-49, CRIZEQ-44 and CRIZEQ-77 manifested significant positive GCA effects for GY, while significant negative GCA effect was obtained for CRIZEQ-25, CRIZEQ-24, CRIZEQ-5, CRIZEQ-45, CRIZEQ-40 and CRIZEQ-54.Averaged across high-N and WW conditions, inbred lines CRIZEQ-42, CRIZEQ-25, CRIZEQ-77, CRIZEQ-49 and CRIZEQ-44 manifested significant positive GCA effect for GY, while significant negative GCA effect for GY was obtained forCRIZEQ-55, CRIZEQ-24, CRIZEQ-54, CRIZEQ-40 and CRIZEQ-5.Inbred linesCRIZEQ-77, CRIZEQ-44 and CRIZEQ-49manifested significant positive GCA effects for GY across low-N and DS conditions.
Averaged across low-N and DS conditions, significant negative GCA effect for AD was obtained for inbred lines CRIZEQ-5, CRIZEQ-40, CRIZEQ-44, CRIZEQ-54 and CRIZEQ-55. Significant positive GCA effect for SD was obtained for inbred lines CRIZEQ-14, CRIZEQ-40, CRIZEQ-44 and CRIZEQ-54, while significant negative GCA effect for ASI was manifested by inbred lines CRIZEQ-14, CRIZEQ-44 and CRIZEQ-46.Also, significant negative GCA effect for PHT and EHT was obtained for inbred lines CRIZEQ-5, CRIZEQ-14, CRIZEQ-40, CRIZEQ-42 and CRIZEQ-44.Inbred lines CRIZEQ-44, CRIZEQ-49 and CRIZEQ-77, manifested significant negative GCA effect for PA while inbred linesCRIZEQ-5, CRIZEQ-40, CRIZEQ-54 and CRIZEQ-55 had significant positive GCA effect. Significant GCA effect for EA was obtained for inbred lines CRIZEQ-5, CRIZEQ-24, CRIZEQ-40, CRIZEQ-42, and CRIZEQ-55, while inbred lines CRIZEQ-14, CRIZEQ-44, CRIZEQ-45, CRIZEQ-46, CRIZEQ-49 and CRIZEQ-77 manifested significant negative GCA effect for EA. Inbred lines CRIZEQ-24, CRIZEQ-45, CRIZEQ-55 and CRIZEQ-77 had significant negative GCA effect for SG, while significant positive GCA effect was obtained for inbred lines CRIZEQ-5, CRIZEQ-40, CRIZEQ-44, CRIZEQ-46, and CRIZEQ-49. Significant positive GCA effect for EPP was obtained for inbred lines CRIZEQ-49 and CRIZEQ-55 (Table 5).
Table 5
General combining ability effect for grain yield and selected yield traits for QPM inbred lines evaluated under low-N and drought stress conditions
Inbred
|
GY
|
AD
|
SD
|
ASI
|
PHT
|
EHT
|
PA
|
EA
|
SG
|
EPP
|
|
(kg/ ha)
|
|
|
|
|
|
|
|
|
|
|
LN
|
DS
|
ACR
|
OPT
|
|
|
|
|
|
|
|
|
|
CRIZEQ-5
|
-384.54**
|
-216.70**
|
-300.62**
|
-1052.27***
|
-0.18**
|
-0.02
|
0.16**
|
-7.73***
|
-5.08***
|
0.23***
|
0.57**
|
0.28**
|
-0.029*
|
CRIZEQ-14
|
-53.45
|
159.31**
|
52.93*
|
145.21
|
-0.11
|
-0.20**
|
-0.10**
|
-0.58
|
-1.21**
|
0.04
|
-0.12*
|
0.00
|
-0.041***
|
CRIZEQ-24
|
-202.00**
|
-56.96
|
-129.48**
|
-431.74***
|
0.15**
|
0.17**
|
0.02
|
4.49***
|
3.45***
|
-0.07
|
0.22**
|
-0.37**
|
0.008
|
CRIZEQ-25
|
-14.19
|
-143.20**
|
-78.70*
|
334.79***
|
0.38***
|
0.50***
|
0.12**
|
1.64*
|
0.15
|
-0.03
|
0.02
|
0.05
|
-0.007
|
CRIZEQ-40
|
-115.01*
|
-145.35**
|
-130.18**
|
-195.80*
|
-0.65***
|
-0.84***
|
-0.2
|
-4.40***
|
-1.71***
|
0.24***
|
0.26**
|
0.31**
|
-0.010
|
CRIZEQ-42
|
91.71*
|
-150.68**
|
-29.48
|
251.76**
|
0.32***
|
0.25***
|
-0.07
|
1.52*
|
-1.11**
|
-0.03
|
0.28**
|
-0.08*
|
0.021
|
CRIZEQ-44
|
303.06**
|
196.77**
|
249.92**
|
449.82***
|
-0.49***
|
-0.75***
|
-0.26**
|
-4.39***
|
-1.96***
|
-0.18***
|
-0.40**
|
0.15**
|
0.015
|
CRIZEQ-45
|
-112.39*
|
-112.19*
|
-112.29**
|
-29.09
|
0.82***
|
0.81***
|
-0.01
|
-0.79
|
1.02*
|
0.05
|
-0.28**
|
-0.12**
|
-0.005
|
CRIZEQ-46
|
-67.63
|
84.16
|
8.26
|
7.62
|
0.28***
|
0.05
|
-0.23**
|
5.18***
|
2.25***
|
-0.04
|
-0.30**
|
0.09*
|
-0.016
|
CRIZEQ-49
|
229.18**
|
146.04**
|
187.61**
|
238.93**
|
-0.02
|
0.14*
|
0.17**
|
5.22***
|
1.62***
|
-0.14**
|
-0.41**
|
0.24**
|
0.026*
|
CRIZEQ-54
|
-99.59*
|
-164.12**
|
-131.85**
|
-156.86*
|
-0.34***
|
-0.30***
|
0.04
|
-1.07
|
1.67***
|
0.29***
|
-0.09
|
0.02
|
0.003
|
CRIZEQ-55
|
-78.26
|
-6.19
|
-42.22
|
-491.36***
|
-0.18**
|
-0.08
|
0.10**
|
-4.62***
|
0.54
|
0.11*
|
0.46**
|
-0.25**
|
0.028*
|
CRIZEQ-77
|
503.11**
|
409.12**
|
456.11**
|
928.99***
|
0.01
|
0.26***
|
0.25**
|
5.53***
|
0.37
|
-0.46***
|
-0.22**
|
-0.31**
|
0.005
|
Grain yield (GY), Silking date (SD), Anthesis date (AD), Anthesis-silking interval (ASI), Ear height (EHT), Plant height (PHT), Ear aspect (EA), plant aspect (PA), number of ears per plant (EPP), Stay-green characteristics (SG), Low-N (LN), Drought stress (DS), Averaged across high-N and well-watered (OPT), Averaged across low-N and drought stress (ACR). |
*, **, and *** = significant at 5, 1 and 0.1% probability levels, respectively. |
3.4 Heterotic grouping of inbred lines
Classification of the inbreds into heterotic groups was based on HGCAMT and illustrated with a dendrogram. Under low-N condition, the inbred lines were classified into three heterotic groups at 40.0% level of dissimilarity (r2 = 0.4). Inbred lines CRIZEQ-5, CRIZEQ-40, and CRIZEQ-44 were assigned to heterotic group I. Heterotic group II comprised eight inbred lines, CRIZEQ-14, CRIZEQ-46, CRIZEQ-54, CRIZEQ-24, CRIZEQ-25, CRIZEQ-42, CRIZEQ-45, and CRIZEQ-55, while two inbred lines CRIZEQ-49 and CRIZEQ-77 were classified into heterotic group III (Fig. 2). According to Pswarayi and Vivek (2008), identification of an inbred tester should be based on significant positive GCA effect for grain yield, inbred lines must be assigned to a heterotic group and must have high grain yield per se. Based on these interpretations, inbred lines CRIZEQ-49 and CRIZEQ-77 belonging to heterotic group III were identified as inbred testers (Fig. 2).
Under DS condition, the inbred lines were assigned to three heterotic groups at 40.0% dissimilarity level. Heterotic group I comprised of four inbred lines, CRIZEQ-5, CRIZEQ-40; CRIZEQ-54 and CRIZEQ-55. Five inbred lines CRIZEQ-14, CRIZEQ-46, CRIZEQ-49, CRIZEQ-44, and CRIZEQ-77 were assigned to heterotic group II, while CRIZEQ-24, CRIZE-25, CRIZEQ-42 and CRIZEQ-45 were also assigned to heterotic group III. Based on criteria proposed by Pswarayi and Vivek (2008), CRIZEQ-14, CRIZEQ-49, CRIZEQ-44 and CRIZEQ-77 were identified as testers belonging to heterotic group II (Fig. 3).
Averaged across low-N and DS conditions, the inbred lines were assigned to two heterotic groups at 30.0% (r2 = 0.3) dissimilarity level. Heterotic group I comprised of six inbred lines, CRIZEQ-5, CRIZEQ-14, CRIZEQ-54, CRIZEQ-55 and CRIZEQ-44, while inbred lines CRIZEQ-24, CRIZEQ-25, CRIZEQ-42, CRIZEQ-45, CRIZEQ-46, CRIZEQ-49 and CRIZEQ-77 were also assigned to heterotic group II. Inbred lines CRIZEQ-14 and CRIZEQ-44 belonging to group I, and CRIZEQ-49 and CRIZEQ-77 belonging to group II were testers based on method proposed by Pswarayi and Vivek (2008) (Fig. 4).
3.5 Yield performance of hybrids under contrasting conditions
Under low-N, high-N, DS and WW conditions, significant differences were obtained among the hybrids for GY and other yield traits. Under low-N condition, GY performance of the hybrids ranged from 1092 kg/ ha (CRIZEQ-5 × CRIZEQ-24) to 4373 kg/ ha (CRIZEQ-42 × CRIZEQ-77), while that of the checks ranged from 1226 kg/ ha (Standard-3) to 1725 kg/ ha (Standard-1), with a mean of 2797 kg/ ha (Table 6).Compared to high-N conditions, GY reduction due to low-N stress ranged from 39.3% (CRIZEQ-24 × CRIZEQ-77) to 61.3% (CRIZEQ-40 × CRIZEQ-42), while that of the checks ranged from 37.3% (Standard-3) to 64.0% (Standard-1), with a mean reduction of 48.3%. The top performing hybrid (CRIZEQ-42 × CRIZEQ-77) out-yielded the best check (Standard-1) by 60.5% (Table 6). The ASI ranged from 2.68 (CRIZEQ-44 × CRIZEQ-45) to 4.21 (CRIZEQ-5 × CRIZEQ-25), while for the checks, ASI ranged from 3.12 (Standard-1) to 4.17 (Standard-2), with an average of 3.31. The range in PA was from 2.67 (CRIZEQ-24 × CRIZEQ-77) to 5.65 (CRIZEQ-5 × CRIZEQ-24), while values of the checks ranged from 3.07 (Standard-2) to 5.83 (Standard-1), with a mean of 3.91. Estimates of EA ranged from 1.90 (CRIZEQ-44 × CRIZEQ-45) to 4.05 (CRIZEQ-45 × CRIZEQ-46), while the EA of the checks ranged from 2.07 (Standard-3) to 4.09 (Standard-1), with a mean of 2.86. The range in EPP of the test hybrids was from 0.72 (CRIZEQ-5 × CRIZEQ-24) to 0.93 (CRIZEQ-44 × CRIZEQ-54), while that of the checks ranged from 0.71 (Standard-1) to 0.82 (Standard-3), with a mean of 0.83. The SG rating of the test hybrids ranged from 1.90 (CRIZEQ-5 × CRIZEQ-25) to 4.20 (CRIZEQ-45 × CRIZEQ-46), while SG for the checks ranged from 2.72 (Standard-3) to 3.92 (Standard-1), with a mean of 3.41. The BI of the test hybrids ranged from − 13.56 (CRIZEQ-5 × CRIZEQ-24) to 9.02 (CRIZEQ-44 × CRIZEQ-77), while BI of the checks ranged from − 4.71 (Standard-3) to -9.55 (Standard-1).
Under high-N condition, GY of the test hybrids ranged from 2543 kg/ ha (CRIZEQ-5 × CRIZEQ-24) to 7711 kg/ ha (CRIZEQ-44 × CRIZEQ-77), while values of GY for the checks ranged from 1956 kg/ ha (Standard-3) to 4786 kg/ ha (Standard-1), with a mean of 5352 kg/ ha (Table 6). The top performing hybrid had yield advantage of 73.9% over the best check. Under this condition, ASI ranged from 2.33 (CRIZEQ-14 × CRIZEQ-49) to 3.19 (CRIZEQ-14 × CRIZEQ-44), while ASI for the checks ranged from 2.60 (Standard-1) to 2.85 (Standard-3), with a mean of 2.73. The PA rating of the test hybrids ranged from 2.56 (CRIZEQ-24 × CRIZEQ-42) to 4.41 (CRIZEQ-5 × 25), while that of the checks ranged from 3.17 (Standard-2) to 3.62 (Standard-3) with a mean of 3.34. Also, EA ranged from 1.83 (CRIZEQ-40 × CRIZEQ-42) to 2.94 (CRIZEQ-24 × CRIZEQ-54), while EA of the checks ranged from 2.01 (Standard-2) to 2.44 (Standard-1), with a mean of 2.30. The EPP ranged from 0.70 (CRIZEQ-5 × CRIZEQ-24) to 1.01 (CRIZEQ-46 × CRIZEQ-49) for the test hybrids, whiles EPP for the checks ranged from 0.90 (Standard-1) to 0.95 (Standard-3), with a mean of 0.93.
Under DS condition, GY ranged from 1072 kg/ ha (CRIZEQ-5 × CRIZEQ-40) to 4020 kg/ ha (CRIZEQ-24 × CRIZEQ-77), and that of the checks ranged from1501 kg ha− 1(Standard-3) to 2094 kg/ ha (Standard-1), with a mean of 2583 kg/ ha (Table 7). The top performing hybrid (CRIZEQ-24 × CRIZEQ-77) had yield advantage of 47.9% over the best check (Standard-1). Reduction in GY due to DS of the test hybrids ranged from 28.8% (CRIZEQ-24 × CRIZEQ-77) to 78.1% (CRIZEQ-42 × CRIZEQ-44), while reduction in GY of the checks ranged from 45.8% (Standard-1) to 67.6% (Standard-2) with a mean of 49.5%. The ranged in ASI of the test hybrids was from 3.11 (CRIZEQ-46 × CRIZEQ-77) to 3.71 (CRIZEQ-14 × CRIZEQ-49), while ASI of the checks ranged from 3.43 (Standard-1) to 4.15 (Standard-3) with a mean of 3.53. The PA of the test hybrids ranged from 2.56 (CRIZEQ-24 × CRIZEQ-77) to 5.78 (CRIZEQ-5 × CRIZEQ-40), while that of the checks ranged from 5.01 (Standard-3) to 5.68 (Standard-1) with a mean of 4.43. The EA ranged from 3.07 (CRIZEQ-46 × CRIZEQ-77) to 5.99 (CRIZEQ-5 × CRIZEQ-40), while EA for the checks ranged from 4.26 (Standard-1) to 5.31 (Standard-3) with a mean of 4.39. The range in EPP was from 0.52 (CRIZEQ-5 × CRIZEQ-40) to 0.96 (CRIZEQ-14 × CRIZEQ-49), while EPP for the checks also ranged from 0.69 (Standard-1) to 1.24 (Standard 2). The SG rating of the test hybrids ranged from 2.81 (CRIZEQ-24 × CRIZEQ-77) to 4.57 (CRIZEQ-40 × CRIZEQ-42), while that of the checks ranged from 3.26 (Standard-1) to 4.45 (Standard-3). The BI of the test hybrids ranged from − 11.55 (CRIZEQ-5 × CRIZEQ-40) to 10.26 (CRIZEQ-24 × CRIZEQ-77), while BI of the checks ranged from − 2.26 (Standard-1) to -8.98 (Standard-3).
Under WW condition, GY ranged from 2313 kg/ ha (CRIZEQ-5 × CRIZEQ-24) to 74041 kg/ ha (CRIZEQ-44 × CRIZEQ-77), while GY of the checks ranged from 3864 kg/ ha (Standard-1) to 4663 kg/ ha (Standard-2), with a mean of 5276 kg/ ha (Table 7). Under this condition, the top test hybrid out-yielded the best check by 68.8%. The ASI of the test hybrids ranged from 2.44(CRIZEQ-5 × CRIZEQ-40) to 3.43 (CRIZEQ-24 × CRIZEQ-77), while ASI for the checks ranged from 2.82 (Standard-3) to 4.08 (Standard-1), with a mean of 2.93.The PA rating of the hybrids ranged from 2.56 (CRIZEQ-24 × CRIZEQ-42) to 4.47 (CRIZEQ-5 × 24), while that of the checks ranged from 3.17 (Standard-2) to 4.54 (Standard-1) with a mean of 3.46. The EA of the test hybrids ranged from 1.93 (CRIZEQ-14 × CRIZEQ-44) to 2.93 (CRIZEQ-40 × CRIZEQ-42), while EA of the checks ranged from 2.05 (Standard-3) to 2.56 (Standard-1), with a mean of 2.33. The range in EPP of the test hybrids was from 0.72 (CRIZEQ-5 × CRIZEQ-24) to 1.03 (CRIZEQ-44 × CRIZEQ-49), while EPP of the checks ranged from 0.88 (Standard-1) to 1.01 (Standard-3), with a mean of 0.94 (Table 7).
Table 6
Performance of grain yield and selected yield traits of 15 quality protein maize hybrids (top10 and bottom 5) and three checks evaluated under low-N and high-N conditions.
Hybrids
|
GY
(kg/ ha)
|
YR (%)
|
ASI
(days)
|
PASP
(1–9 rating)
|
EASP
(1–9 rating)
|
EPP
|
SG
|
BI
|
|
LN
|
HN
|
|
LN
|
HN
|
LN
|
HN
|
LN
|
HN
|
LN
|
HN
|
LN
|
LN
|
P-42 × P-77
|
4372.7
|
7231.1
|
39.5
|
3.46
|
2.47
|
2.86
|
2.70
|
2.98
|
2.16
|
0.89
|
0.98
|
2.97
|
7.89
|
P-44 × P-77
|
4266.9
|
7711.1
|
44.7
|
3.22
|
2.50
|
2.77
|
2.78
|
2.54
|
2.37
|
0.90
|
0.96
|
3.48
|
9.02
|
P-46× P-49
|
3911.6
|
7447.4
|
47.5
|
3.27
|
2.56
|
3.45
|
3.19
|
2.78
|
2.38
|
0.89
|
1.01
|
3.99
|
3.71
|
P-24 × P-77
|
3769.1
|
6210.7
|
39.3
|
3.57
|
2.64
|
2.67
|
2.69
|
2.54
|
2.57
|
0.86
|
0.95
|
2.90
|
7.27
|
P-44× P-54
|
3632.6
|
6908.2
|
47.4
|
2.84
|
2.42
|
3.42
|
2.92
|
2.12
|
2.51
|
0.93
|
0.99
|
3.78
|
7.89
|
P-14 × P-49
|
3488.2
|
5851.9
|
40.4
|
3.01
|
2.33
|
3.95
|
3.50
|
2.64
|
2.36
|
0.88
|
0.90
|
3.84
|
2.91
|
P-14 × P-44
|
3441.3
|
6581.2
|
47.7
|
2.96
|
3.19
|
3.13
|
3.03
|
1.85
|
1.97
|
0.84
|
0.93
|
2.97
|
7.29
|
P-44 × P-45
|
3425.7
|
6012.4
|
43.0
|
2.68
|
2.97
|
3.62
|
3.19
|
1.90
|
2.12
|
0.83
|
0.93
|
3.52
|
5.17
|
P-42 × P-54
|
3377.9
|
6307.7
|
46.4
|
3.25
|
2.97
|
4.21
|
2.94
|
2.83
|
1.98
|
0.86
|
0.92
|
3.50
|
2.94
|
P-24 × P-42
|
3362.2
|
6845.0
|
50.9
|
2.95
|
2.69
|
3.14
|
2.56
|
2.75
|
2.36
|
0.90
|
1.00
|
3.03
|
6.04
|
P-40 × P-42
|
2091.0
|
5405.5
|
61.3
|
3.38
|
3.05
|
4.41
|
3.75
|
3.12
|
1.83
|
0.76
|
0.96
|
2.91
|
-5.56
|
P-45 × P-46
|
1961.4
|
4458.3
|
56.0
|
2.99
|
2.47
|
4.81
|
3.67
|
4.05
|
2.42
|
0.80
|
0.92
|
4.20
|
-7.69
|
P-5 × P-25
|
1930.7
|
4112.0
|
53.0
|
4.21
|
3.63
|
4.23
|
4.41
|
2.34
|
2.49
|
0.87
|
0.90
|
1.90
|
-1.03
|
P-24 × P-54
|
1657.3
|
3412.2
|
51.4
|
3.51
|
2.80
|
5.17
|
4.24
|
3.23
|
2.94
|
0.78
|
0.86
|
4.18
|
-9.00
|
P-5 × P-24
|
1091.5
|
2543.1
|
57.1
|
4.21
|
2.39
|
5.65
|
3.89
|
3.95
|
2.33
|
0.72
|
0.70
|
3.24
|
-13.56
|
Standard-1
|
1725.1
|
4786.3
|
64.0
|
3.12
|
2.60
|
5.83
|
3.86
|
4.09
|
2.44
|
0.71
|
0.90
|
3.92
|
-9.55
|
Standard-2
|
1449.1
|
2551.1
|
43.2
|
4.17
|
2.65
|
3.07
|
3.17
|
3.21
|
2.01
|
0.79
|
0.92
|
3.35
|
-6.29
|
Standard-3
|
1225.8
|
1955.6
|
37.3
|
3.42
|
2.85
|
5.29
|
3.64
|
2.07
|
2.13
|
0.82
|
0.95
|
2.73
|
-4.71
|
Mean
|
2795.7
|
5351.7
|
48.3
|
3.31
|
2.73
|
3.91
|
3.34
|
2.86
|
2.30
|
0.83
|
0.93
|
3.41
|
|
LSD (5%)
|
1079.80
|
|
|
1.07
|
|
1.12
|
|
1.28
|
|
0.16
|
|
1.19
|
|
CV (%)
|
16.7
|
|
|
17.71
|
|
20.81
|
|
29.76
|
|
14.79
|
|
20.59
|
|
Grain yield (GY), Anthesis-silking interval (ASI), Ear aspect (EA), HN = High-N, LN = Low-N, Number of ears per plant (EPP), Plant aspect (PA), Stay-green characteristics (SG), Base index (BI), YR = Yield reduction, CRIZEQ (P). |
Table 7
Performance of grain yield and selected yield traits of 15 quality protein maize hybrids (top 10 and bottom 5) and three checks evaluated under drought stress and well-watered conditions.
Hybrids
|
GY
|
YR
|
ASI
|
PA
|
EA
|
EPP
|
SG
|
BI
|
|
(kg/ ha)
|
(%)
|
(days)
|
(1–9 rating)
|
(1–9 rating)
|
|
|
|
|
DS
|
WW
|
|
DS
|
WW
|
DS
|
WW
|
DS
|
WW
|
DS
|
WW
|
DS
|
DS
|
P-24 × P-77
|
4020.2
|
5642.8
|
28.8
|
3.48
|
3.43
|
2.56
|
2.56
|
4.17
|
2.44
|
0.89
|
0.97
|
2.81
|
10.26
|
P-44 × P-77
|
3719.8
|
7404.1
|
49.8
|
3.34
|
2.64
|
3.25
|
2.85
|
3.84
|
2.31
|
0.71
|
0.97
|
4.32
|
5.13
|
P-14 × P-49
|
3658.8
|
5399.3
|
32.2
|
3.71
|
2.60
|
4.37
|
4.28
|
3.62
|
2.22
|
0.96
|
0.89
|
4.25
|
5.11
|
P-24 × P-40
|
3579.6
|
5550.3
|
35.5
|
3.38
|
2.86
|
3.95
|
3.10
|
5.25
|
2.54
|
0.85
|
0.99
|
3.71
|
3.90
|
P-14 × P-44
|
3491.5
|
6202.4
|
43.7
|
3.19
|
2.93
|
3.05
|
2.99
|
3.17
|
1.93
|
0.74
|
0.92
|
3.50
|
7.72
|
P-40 × P-46
|
3479.3
|
5508.4
|
36.8
|
2.86
|
2.86
|
4.10
|
3.20
|
3.77
|
2.73
|
0.73
|
0.97
|
3.75
|
5.88
|
P-46 × P-77
|
3469.7
|
5456.1
|
36.4
|
3.11
|
2.60
|
3.57
|
2.62
|
3.07
|
2.18
|
0.82
|
0.94
|
2.94
|
8.95
|
P-44 × P-49
|
3328.1
|
5651.6
|
41.1
|
3.75
|
2.78
|
4.81
|
3.23
|
3.09
|
2.26
|
0.71
|
1.03
|
4.54
|
1.55
|
P-44 × P-45
|
3232.2
|
5614.3
|
42.4
|
2.94
|
2.58
|
4.21
|
3.30
|
3.27
|
2.16
|
0.95
|
0.93
|
3.65
|
7.44
|
P-14 × P-77
|
3204.0
|
5532.8
|
42.1
|
3.91
|
3.41
|
3.68
|
3.28
|
3.96
|
2.16
|
0.74
|
0.88
|
3.11
|
3.55
|
P-40 × P-45
|
1598.5
|
5216.8
|
69.4
|
3.19
|
2.69
|
5.73
|
4.37
|
4.74
|
2.62
|
0.56
|
0.99
|
3.97
|
-6.36
|
P-40 × P-42
|
1363.0
|
4471.6
|
69.5
|
3.37
|
2.88
|
5.26
|
4.06
|
5.10
|
2.93
|
0.65
|
0.90
|
4.57
|
-7.83
|
P-42 × P-44
|
1330.8
|
6063.2
|
78.1
|
3.45
|
3.27
|
5.46
|
3.50
|
5.04
|
2.16
|
0.65
|
0.97
|
4.40
|
-8.05
|
P-5 × P-24
|
1075.9
|
2312.5
|
53.5
|
4.40
|
3.06
|
5.58
|
4.47
|
5.43
|
2.50
|
0.56
|
0.72
|
3.58
|
-11.45
|
P-5 × P-40
|
1072.3
|
4152.2
|
74.2
|
3.53
|
2.44
|
5.78
|
4.12
|
5.99
|
2.71
|
0.52
|
0.91
|
3.89
|
-11.1
|
Standard-1
|
2093.9
|
3863.9
|
45.8
|
3.43
|
4.08
|
5.68
|
4.54
|
4.26
|
2.56
|
0.69
|
0.88
|
3.26
|
-2.26
|
Standard-2
|
1509.1
|
4662.5
|
67.6
|
3.58
|
3.37
|
5.01
|
3.17
|
4.89
|
2.39
|
1.24
|
0.92
|
3.85
|
-1.09
|
Standard-3
|
1500.7
|
4520.7
|
66.8
|
4.15
|
2.82
|
5.44
|
4.03
|
5.31
|
2.05
|
0.75
|
1.01
|
4.45
|
-8.98
|
Mean
|
2582.9
|
5276.1
|
49.5
|
3.53
|
2.93
|
4.43
|
3.46
|
4.39
|
2.33
|
0.74
|
0.94
|
3.72
|
|
LSD (5%)
|
601.6
|
792.0
|
|
0.68
|
0.62
|
0.77
|
0.90
|
1.07
|
0.78
|
0.27
|
0.08
|
0.67
|
|
CV (%)
|
20.5
|
13.2
|
|
16.92
|
18.61
|
15.38
|
22.92
|
21.40
|
29.42
|
32.09
|
7.52
|
15.94
|
|
Grain yield (GY), Anthesis-silking interval (ASI), Drought stress (DS), Ear aspect (EA), Plant aspect (PA), Number of ears per plant (EPP), Stay-green characteristics (SG), Base index (BI), Well-watered (WW), YR = Yield reduction, CRIZEQ (P). |
3.6 Yield performance and stability of hybrids across test conditions
The significant genotype × environment interaction effect for GY under each conditions justified the need to assess GY pefromance and yield stability of the hybridsusing the genotype + genotype × environment (GGE) biplot procedure. The GGE biplot revealed that principal component 1 (PC1 = 74.6%) and principal component 2 (PC2 = 10.6%) explained 85.2% of the total variation in GY of the hybrids (Figs. 5 and 6). The “which-won-where” GGE biplot illustated by a polygon view was used to identify hybrids adapted to specific conditions. The polygon is divided intosectors where each sector representsa condition. In the polygon view, entries at the vertices were the highest yielding hybrids in the environment condition that fell within the sector. The distance between biplot origin and the entries was a measure of the yield difference among the hybrids under each and across conditions.The vertex entries identified within a sector of the ploygon were more responsive to thecondition than those identified within the polygon or close to the biplot origin. Based on these interpretations, six entries,63, 24, 72, 81, 2 and 32 were identified at the vertice of the polygon. In the polygin view, entry 63 (CRIZEQ-44 × CRIZEQ-77) was the vertex hybrid responsive to eight conditions (ENV-1, ENV-3, ENV-4, ENV-5, ENV-6, ENV-7, ENV-8 and ENV-10), while entry 24 (CRIZEQ-24 × CRIZEQ-25) located at anothervertex of the polygon was also responsive to two conditions (ENV-2 and ENV-9). On the other hand, entries 24, 81, 2 and 31 were vertex hybrids identified outside the range of the testing conditions used in this study, and thus, won in space. The hybrids foundwithin a sector of the polygon were less responsive to the testing conditions present (Fig. 5).
The “mean vr stability” GGE biplot was used to assess yield stability of the hybrids under each and across testing conditions. In the GGE biplot view, the vertical linepassing through the biplot origin is the average tester coordinate (ATC), separatingthe high yielding hybrids from the low yielding ones. The yield performance of each hybrid wasassessed based on its projection from ATC abcissa. The shorter the length of projections onto the ATC, the more stable it is, and the farther it is away from the ATC ordinate, the higher the yield performance. Based on this interpretation, entry 63(CRIZEQ-44 × CRIZEQ-77) was thehighest yielding and most stable hybrid across allconditions.
3.7 Stepwise multiple and sequential path analyses
Knowledge oninterrelationship of traits is crucial in the choice of important secondary traits for indirect selection for improved GY. In this study, the relationship among traits under low-N, DS and across conditions wasexamined following the protocol of Mohammadi et al. (2003) and illustrated with sequential path models. In this study, traits PASP, EPP, SD and PHwere identified as first order traits explaining77.0% of the total variation in GY under low-N condition (Fig. 7). Among these traits, EPP(0.348) and PH (0.190) manifested direct positive effect, whilePASP (-0.362) and SD (-0.368) also haddirect negative effect to the variations in GY. The second order traits EASP, AD, ASI, SL and EH contributed to GY through one or two of the first order traits. Among the second order traits, EASPhad indirect positive contribution to GY through PASP (0.538) and SD (0.004), while AD manifested indirect positive (0.843), and negative (-0.344) contribution through SD, and PASP, respectively. TheASI (0.276) had indirect positive effect through SD.The SL(-0.221) and EH (0.821) manifested indirect contribution to GY through PH. The third order traits compised of RLand SG. The RL had indirect contribution to GY through ASI, while SG had its contribution through SL (0.243) and ASI (-0.328) (Fig. 7).
Under DS condition, EA, AD, PA, and EPP were first order traits contributing 56% of the total variations in grain yield (Fig. 8). Among these traits, EPP (0.211) had direct positive contribution while EA (-0.346) had direct negative effect on GY. Four traits, PH, EH, ASI and SD manifested indirect effect through one or two of the first order traits, and were classified as second order traits. Among the second order traits, the PH had indirect negative effect through EA (-0.390) and PA (-0.500), while SD manifested indirect positive effect through EA (0.234) and PA (0.475). Also, ASI had indirect negative effect through AD (-0.351), while EH had indirect positive effect through EPP (0.425). The third order trait, SG, had indirect positive effect (0.001) through ASI (Fig. 8).
Averaged across low-N and DS conditions, PASP, SD, EPP and PH were identified as first order traits which explained 76.0% of the total variability in GY (Fig. 9). Among these first order traits, PASP and SD had direct negative effect contributions to GY, while EPP and PH with their positive path coefficients also manifested direct contribution to GY. Four traits, EASP, AD, EH and ASI, manifested indirect contribution through one, two or three of the first order traits and thus, were classified as second order traits. The trait, EASP, had indirect effect on GY through PASP and PH. Also, AD had indirect positive contribution through SD and PASP, while ASI manifested indirect positive effect through SD. The trait, EH, manifested indirect contribution to GY through all the first order traits, while ASI had indirect effect through SD. The third order trait, SG, partially contributed to GY through all the second order traits (Fig. 9).