Introduction
Maize (Zea mays L.) being the
highest yielding cereal crop in the world, attained significant importance for
countries like Pakistan (Ali et al. 2019, 2020). Maize accomplished a
greater role in rewarding the demand of generously increasing population which
has already exceeded food supplies (Sajjad et
al. 2016, 2020a, b). Maize has a wide range of uses and its grain contains
starch (72%), vitamins A and B (3 to 5%), proteins (10%), oil (4.8%), fiber
(5.8%), sugar (3.0%) and ash (1.7%) (Chaudhary 1983). Maize provides raw
material to the industry for the preparation of corn oil and starch, corn syrup
and flakes, dextrose, cosmetics, wax, alcohol and tanning material for leather
industry (Kumar et al. 2013). Ethanol
obtained from maize can be used as a biomass fuel. Stigmas from female corn
flowers, known as corn silk, can be used as herbal supplements. Maize is
largely used as the main source of calories in animal feeding and feed
formulation. Maize gives the highest conversion of a dry substance to meat,
milk and eggs compared to other cereal grains.
Recurrent selection is a
cyclical method of germplasm advancement which does not lead directly to the
release of cultivars in maize and other crops. In recurrent
selection, three basic steps are involved i.e., inter-mating,
assessment, and selection (Darrah et al. 2019; Sheikh et al. 2019). Recurrent selection is used for traits that are
polygenically inherited. Polygenic inheritance occurs when many genes,
each with a small effect, control the expression of a trait. Recurrent
selection is designed to improve the frequency of favorable
alleles in a population for quantitative traits in maize (Andorf et al. 2019) and further breeding
efforts are required to release a cultivar from an improved maize population
(Hallauer and Carena 2012; Kolawole
et al. 2017).
The
recurrent selection includes many types i.e., S0, S1,
S2, full-sib, half-sib, ear-to-row, reciprocal recurrent, and
reciprocal full-sib in maize (Sohail et
al. 2018; Khamkoh et al. 2019).
These procedures vary in the progeny which are evaluated through field testing
and are recombined to shape the next cycle. In the methods involving selfing, selfed progenies are
also evaluated. With S0 plant selection (vs. progeny for S1 and S2), selection must be
made before flowering and selected plants are recombined, allowing one cycle of
selection to be completed per season. The S0 plant selection is
considerably effective for simply inherited traits of maize (Noor et al. 2013).
The
S1 and S2 progeny recurrent selections are admirable
choices for acquiring improvement within maize populations, particularly
improving grain yield (Badu-Apraku et al.
2013). Selection in cycle-2 produced maximum grain yield with significant
hereditary gain and concluded that S1 recurrent selection was found
more effective for innate improvement in maize for yield traits (Khan et al. 2018; Chen et al. 2019). A significant
decrease was recorded for morphological traits while an increase in yield
traits with S1 recurrent selection in maize (Horne et al. 2016). Maize populations in selection cycle-2 produced maximum grain
yield with significant genetic gain and concluded that S1 recurrent
selection has been found effective for genetic improvement in maize (Sajjad et al. 2016).
Maize
breeders are mostly interested in generating heritable variations in the base
population for further selection (Ali et
al. 2017; ). However, the
observed variations in the population might be due to genotypic and
environmental factors and genotype by environment interactions. Genotypic
variance is heritable and durable while environmental variance is non-assignable
to the next generation and oscillates with the environment in maize populations
(Ali 2015; Annor et al. 2019; Cobb et al.
2019). Heritability has a key role in the inheritance of the traits and
also to partition the total variance into genetic and environmental components
in maize germplasm (Ullah et al. 2013; Rahman et al. 2015). Estimates of genetic variability and heritability are
of greater importance for the maize breeders and used as an indication of
selection pressure in the segregating populations, and to gauge that the
phenotypic change is heritable or not to the next generation (Ali et al. 2011a, b). High heritability
showed that the trait of interest was least affected by the environment and
will contribute to the overall improvement of the original population in maize
(Noor et al. 2013; Ishaq et al. 2014).
Previous
findings revealed that S1 progeny recurrent selection was the most
effective method to simultaneously improve earliness, morphological and yield
traits in maize (Sajjad et al. 2016;
Sajjad 2018). Therefore, the current study was
designed with the objectives to; a) assess the performance
of maize population under selfed progeny selection,
b) evaluate the responses of S1 and S2 lines in cycle-1
cycle-2, respectively and c) determine a better approach for improvement of
maize base population 'PSEV3' for earliness and grain yield traits.
Materials and Methods
Maize
source population
Maize base
population 'PSEV3' has derived from the cross between Azam (a white improved
white flint composite variety of medium maturity) and CHSW (a single cross
hybrid of white dent kernel with late maturity from CIMMYT,
Development
of maize breeding material
To develop
S1 lines, the source population PSEV3 was sown in spring season 2014
with rows and plants spacing of 75 and 25 cm, respectively at Cereal Crops
Research Institute (CCRI),
Evaluation
of maize S1 lines
Half of the
seed of 255 selected S1 progenies and control/base population (S0)
was evaluated in a 16 × 16 partially balanced square lattice design with two
replications during summer season 2014 at CCRI,
Recombination
of maize selected S1 progenies
Twenty-five
best S1 lines were selected based on grain yield and early maturity.
The half seed of the selected lines was sown (1:2 rows as male and female,
respectively) for recombination during spring season 2015. Seed in equal
quantity from all selected lines was bulked and grown as male. The female rows
were detasseled before anthesis to eliminate self-pollination and to facilitate
cross pollination. Ears from female rows were harvested, dried, shelled, and
preserved as C1 population for cycle comparison.
Development
of maize S2 lines
During the
same spring season 2015, remnant seed from the selected S1 lines was
sown in two rows having 3 m length with row and plant spacing of 75 and 25 cm,
respectively. Four hundred plants were selfed in the same way as mentioned
above. All the recommended inputs and cultural practices were equally applied
during the crop season.
Evaluation
of maize S2 progeny
One hundred
and sixty-nine S2 lines were evaluated in 13 × 13 partially balanced
square lattice design with two replications during summer 2015 at CCRI,
Nowshera. The same procedure was followed for evaluation of S2
progenies as mentioned earlier for S1 progenies. All the recommended
inputs and standard cultural practices were equally applied.
Recombination
of maize S2 progenies
Seventeen
selected S2 progenies were grown in isolation during spring season
2016 at CCRI,
Evaluation
of S1 and S2 lines
The
improved populations (S1 and S2 lines) in comparison with
the original population (PSEV3-S0) and selfed populations were
evaluated during the summer season 2017 at the CCRI,
Crop
husbandry
Maize is a
shallow-rooted crop, and it requires fine good tilth and well-prepared soil for
successful germination and growth of the crop. To get this, the field was
ploughed with deep plough then harrowed with planking each time to make the
soil loose, fine, levelled and pulverized. The stubbles of the previous crop
left in the field were also removed. A recommended fertilizer dose at the rate
of 200:90:90 NPK kg ha-1 was applied. Half
dose of nitrogen (N), full doses of phosphorus (P2O5),
and potash (K2SO4) were applied during land preparation
and just before planting in the form of urea, single super phosphate (SSP) and
sulphate of potash (SOP), respectively. The remaining half N was applied in the
form of Urea as side-dressing about one month after germination. Weeds were
controlled with Primextra Gold at 1.5 L ha-1 as a pre-emergence
application. The leftover weeds were manually controlled carrying out weeding
and earthing-up operations. Maize borer was controlled by using Confidor
(WP-60) at the rate of 50 g per 10 kg of maize seed through seed treatment
before planting. After one month of germination, Furadon (3%) granules at 20 kg
ha-1 were applied in the whirls. The crop was irrigated at the
proper interval when required, until one week before harvesting. All
the entries were equally treated in the field during the cropping season.
Data
recorded
Data were
recorded on ten competitive plants in each sub-plot for the traits i.e.,
days to silking, plant height (cm), ears per m2, ears per plant,
kernel rows per ear and grain yield (kg ha). To determine grain yield, grain
weight per plot was obtained and converted to kg per ha at 15% moisture content
using the following relationship (Carangal et
al. 1971).
%201806-1814_files/image002.png){kind=small}
Where MC:
Moisture content (%) in grains at harvest, FEW: Fresh ear weight (kg) at
harvest, Shelling coefficient: 0.80.
Statistical analysis
All the
recorded data on S1 and S2 progeny testing trials were
subjected to analysis of variance appropriate for lattice square design using
ANOVALAT procedure of MstatC program.
The
complete statistical model used was:
Yil(j): µ + ti + rj
+ (b/r)l(j) + eil(j)
Where,
Yil(j): the observation of the
genotype i (i = 1,…, v = k2), in the block l (l = 1,…, k) of the
replication j (j = 1,..., m)
µ: the constant common to all the observations
ti: the effect of the treatment I
rji: the effect of the replication j
(b/r)l(j): the effect of
the block l of the replication j
eil(j): the error associated to the
observation Yil(j).
where eil(j)~N(0,s), independent.
Expected
mean squares were calculated through a lattice square (partially balanced)
design for both cycles of selection.
Heritability (broad sense) estimation
Broad sense
heritability (h2bs) was estimated according to
Allard (1960) as under:
σ2e: M1
σ2g: (M2-M1)/r
h2(bs): σ2g /
(σ2e +σ2g)
Where σ2e: environmental variance, σ2g:
genetic variance, h2(bs):
heritability (broad sense).
Selection
differential
Selection
differential (S) was computed as:
S: µs - µ
Where µs: means for selected
S1 and S2 lines in cycles-1 and cycle-2, respectively, µ:
selfed populations (S1 and S2) of the first and second
selection cycles before selection
Estimation
of expected response
Expected
response to selection (Re) was estimated as follows:
Re: S × h2 (bs)
Where S:
selection differential, h2 (bs): heritability (broad sense).
Results
According
to the analysis of variance, the S1-unadjusted and adjusted lines
showed significant (P ≤ 0.01)
differences for all the studied traits in cycle-1 (Table 2). Similarly, the S2-unadjusted
and adjusted populations also revealed significant (P ≤ 0.01) variations for all the variables in cycle-2 (Table
3). The base population (S0), S1 and S2
selfed populations, and S1 and S2 selected families revealed greater genetic variability under the
existing environment. Such type of genetic variability is direly needed by
breeders which can be used for further improvement in maize through intensive
selection. However, some of the traits were recorded with low values of
coefficient of variation (CV%) (Table
2 and 3). By using Latin Square and Factorial Designs in the field
experiments, there is an advantage that the experimental error is reduced
because it removes row and column variations from the error which eventually
reduced the coefficient of variation (CV%) (Taye et al.
2002; Kozak et al. 2013).
Heritability
(broad sense) and selection differential were determined for all the traits in
both selection cycles to formulate the magnitude of genetic variability among S1
and S2 populations. For heritability (bs)
and selection differential, different trends were observed among the S1
and S2 populations for various traits in selection cycle-1 and
cycle-2. The trait-wise results are discussed herein.
Days to silking
Days to
silking reduced from 57.01 (base population - S0) to 52.85 days (S2
selected populations) and showed a 7.30% reduction with succeeding
selection cycles (Table 4). On average, in cycle-1 the minimum days to silking
were observed for S1 selected populations (54.00 days), followed by
S1 selfed families (55.43 days), while the maximum days were
utilized by base population - S0 (57.01 days). Similarly, in cycle-2
the least days to silking were recorded in S2 selected families
(52.85 days), followed by S2 selfed populations (53.85 days),
whereas the highest number of days was recorded in base population S0 (55.74
days). Overall, the S2 selected families took lesser days to silking
(52.85 days) as compared to S1 selected lines (54.00 days) which is
very encouraging and needs further exploitation. The highest and same values of
broad-sense heritability (0.83) were observed in both cycles of selection
(Table 5). The highest negative value of selection differential was recorded in
cycle-2 (-1.89 days) as compared to cycle-1 (-1.43 days), with corresponding
negative expected responses (-0.83 and -1.19 days) in cycle-2 and cycle-1,
respectively and showed a reduction in days to silking in the succeeding selection
cycle (Table 6).
Plant height
Plant
height reduced from 157 (S0) to 149 cm (S2 selected
populations) and showed a reduction of 5.10% with succeeding selection
cycle (Table 4). In cycle-1, the maximum plant height was observed in S1
selected families (160 cm), followed by base population S0 (157 cm)
and S1 selfed population (149 cm). In cycle-2, the utmost plant
height was recorded in S2 selected families (149 cm), followed by
base population S0 (146 cm) and S2 selfed population (126
cm). By comparing the performance of the S1 and S2
selected families, the minimum plant height was observed in S2
selected (149 cm) as compared to S1 selected
populations (160 cm) with succeeding generation. However, in selfed
populations of both cycles the reduction in plant stature ranged
from 149 (S1 selfed) to 126 cm (S2 selfed). High
magnitudes of heritability (broad sense) were observed for plant height in both
selection cycles (Table 5). Heritability (bs) was the
highest in cycle-2 (0.86), followed by cycle-1 (0.83). Likewise, the selection
differential also showed an increasing trend in plant height with a succeeding
selection cycle (ranging from 11.00 to 23.00 cm) in cycle-1 and cycle-2,
respectively (Table 6). Similarly, comparatively larger predicted gains were observed
in cycle-2 (19.78 cm) as compared to cycle-1 (9.13 cm) for the said trait.
Ears per square meter
Ears per
square meter were increased by 8.25% from 4.85 (base population) to 5.25 (S2
selected population) while reduced by 3.75% with selfing from 4.53 to 4.36,
respectively in cycle-1 and cycle-2 populations (Table 4). Ears per square
meter varied with selection cycles; however, in cycle-1 the maximum ears per
square meter were recorded for S1 selected families (4.96), followed
by base population S0 (4.85) and S1 selfed population
(4.53). A similar trend was also observed in cycle-2, and the highest number of
ears per square meter was recorded in S2 selected families (5.25),
followed by base population S0 (4.80) and S2 selfed
population (4.36). On average, maximum ears per square meter
were noted in S2 selected lines (5.25) in cycle-2 as compared to S1 selected families
(4.96) in cycle-1. High broad-sense heritability values were recorded for ears
per square meter in both cycles (Table 5). However, comparatively
the highest heritability was recorded in cycle-1 (0.70) as compared to cycle-2
(0.60). The selection
differential was also increased in the S2 population (0.89) as
compared to S1 families (0.43) in both cycles (Table 6). The
moderate and positive genetic gains (0.53 and 0.31) were also observed for the
said trait in cycle-2 and cycle-1, respectively.
Table 1: Genotypes with various characteristics used in the study
|
Populations |
Source |
Type |
Kernel shape / Colour |
Plant height |
Maturity |
Pedigree |
|
PSEV3C0 |
CCRI |
OPP |
Flint white |
Medium |
Medium |
Cross between Azam
and CHSW (Single cross hybrid, white kernels with late maturity from CIMMYT) |
|
S1 lines |
CCRI |
SP |
Flint white |
Short |
Medium |
Derived from selfing PSEV3-C0 |
|
S2 lines |
CCRI |
SP |
Flint white |
Short |
Medium |
Derived from selfing S1 lines |
CCRI: Cereal Crops Research Institute (CCRI),
Nowshera, Pakistan, OPP: Open pollinated population,
SP: Selfed population, PSEV3-C0: Base
population (C0)
Table 2: Mean squares for earliness and yield traits in maize PSEV3 - S1
lines
|
Source of variation |
d.f. |
Days to Silking |
Plant height |
Ears m-2 |
Ears plant-1 |
Kernel rows ear-1 |
Grain yield |
|
Replications |
1 |
3.78 |
1591.54 |
7.75 |
0.109 |
0.03 |
142174.45 |
|
S1 lines-unadjusted |
255 |
7.98** |
418.38** |
1.15** |
0.036** |
1.26** |
2599062.37** |
|
S1 lines-adjusted |
255 |
7.62** |
410.59** |
1.15** |
0.036** |
- |
- |
|
Blocks |
30 |
3.51 |
88.41 |
0.47 |
0.016 |
0.11 |
214556.64 |
|
Error effective |
225 |
1.19 |
70.02 |
0.34 |
0.015 |
- |
- |
|
RCBD 255 |
255 |
1.39 |
70.56 |
0.35 |
0.015 |
0.16 |
218485.73 |
|
Intra - blocks |
225 |
1.10 |
68.18 |
0.33 |
0.014 |
0.16 |
219009.61 |
|
CV (%) |
|
1.91¥ |
5.63 |
12.93 |
12.986 |
3.02 |
11.40 |
¥ - In Latin Square design, the experimental error is reduced because
it removes row and column variations from the error which eventually reduced
the coefficient of variation (CV%) (Taye et al. 2002; Kozak et
al. 2013)
Table 3: Mean squares for earliness and yield traits in maize PSEV3 - S2
lines
|
Source of variation |
d.f. |
Days to Silking |
Plant height |
Ears m-2 |
Ears plant-1 |
Kernel rows ear-1 |
Grain yield |
|
Replications |
1 |
25.59 |
1.48 |
1.19 |
0.004 |
2.79 |
2844533.09 |
|
S2 lines-unadjusted |
168 |
10.18** |
756.61** |
1.23** |
0.033** |
3.87** |
4216193.00** |
|
S2 lines-adjusted |
168 |
9.93** |
655.84** |
1.22** |
0.032** |
- |
4121578.52** |
|
Blocks |
24 |
2.07 |
286.01 |
0.54 |
0.023 |
0.47 |
1351808.98 |
|
Error effective |
144 |
1.75 |
90.47 |
0.49 |
0.018 |
- |
641809.54 |
|
RCBD 168 |
168 |
1.76 |
111.24 |
0.49 |
0.019 |
0.53 |
702460.28 |
|
Intra - blocks |
144 |
1.71 |
82.11 |
0.48 |
0.018 |
0.54 |
594235.50 |
|
CV (%) |
|
2.38¥ |
7.57 |
16.01 |
14.357 |
5.98 |
22.62 |
¥ - In Latin Square design, the experimental error is reduced because
it removes row and column variations from the error which eventually reduced
the coefficient of variation (CV%) (Taye et al. 2002; Kozak et
al. 2013)
Table 4: Mean performance of maize base (PSEV3-S0 - as check), selfed (S1, S2), and selected (S1s,
S2s) populations for various traits
|
Traits |
Cycle-1 |
Cycle-2 |
||||
|
Base population (S0) |
Selfed population (S1) |
Selected families (S1s) |
Base population (S0) |
Selfed population (S2) |
Selected families (S2s) |
|
|
Days to silking
(days) |
57.01 |
55.43 |
54.00 |
55.74 |
53.85 |
52.85 |
|
Plant height (cm) |
157 |
149 |
160 |
146 |
126 |
149 |
|
Ears per m2 (#) |
4.85 |
4.53 |
4.96 |
4.80 |
4.36 |
5.25 |
|
Ears per plant (#) |
0.97 |
0.93 |
0.96 |
0.98 |
0.95 |
1.00 |
|
Kernel rows ear-1 (#) |
14.00 |
13.19 |
15.03 |
13.00 |
12.15 |
15.50 |
|
Grain yield (kg ha-1) |
5638 |
4102 |
6175 |
5525 |
3542 |
6217 |
Table 5: Genetic components of variance in maize PSEV3 - selfed
and selected populations for various traits
|
Traits |
Cycle-1 (S1 lines) |
Cycle-2 (S2 lines) |
||||||
|
σ2g |
Σ2e |
Σ2p |
h2 (bs) |
σ2g |
σ2e |
σ2p |
h2 (bs) |
|
|
Days to silking |
3.30 |
1.39 |
3.99 |
0.83 |
4.22 |
1.76 |
5.10 |
0.83 |
|
Plant height |
173.91 |
70.56 |
209.19 |
0.83 |
329.75 |
111.24 |
385.37 |
0.86 |
|
Ears per m2 |
0.40 |
0.35 |
0.58 |
0.70 |
0.37 |
0.49 |
0.62 |
0.60 |
|
Ears per plant |
0.01 |
0.02 |
0.02 |
0.58 |
0.01 |
0.019 |
0.02 |
0.56 |
|
Kernel rows ear-1 |
0.55 |
0.16 |
0.63 |
0.87 |
0.70 |
0.53 |
0.96 |
0.72 |
|
Grain yield |
1190288.32 |
218485.73 |
1299531.19 |
0.92 |
1756279.72 |
702460.28 |
2107509.86 |
0.83 |
σ2g: genotypic
variance, σ2e: environmental variance, σ2p:
phenotypic variance, h2 (bs):
broad sense heritability
Ears per plant
For ears
per plant, the S2 selfed families (0.95) as compared to
S1 selfed populations (0.93) increased by 2.15%,
while S2 selected population (1.00) as compared to base populations
(0.97) increased by 3.09% in both selection cycles (Table 4). In cycle-1, the
values for ears per plant were comparable in S0, S1
selfed and S1 selected populations, however, numerically the highest
number of ears per plant were recorded in the base population (0.97), followed
by S1 selected families (0.96) and S1 selfed population
(0.93). In cycle-2, the maximum ears per plant were recorded in S2
selected families (1.00), followed by base population (0.98) and S2
selfed population (0.95). By comparing the performance of selected families in
both cycles, the highest number of ears per plant was recorded in S2
selected (1.00) as compared to the S1 selected populations (0.96).
Moderate estimates of heritability (bs) were recorded
for ears per plant during both cycles of selection (Table 5). Comparatively the
heritability (broad sense) was highest during cycle-1 (0.58) as compared to
cycle-2 (0.56). For ears per plant, the selection differential value was the
highest in cycle-2 (0.05) as compared to cycle-1 (0.03), while the expected
response was also more in cycle-2 (0.03) than cycle-1 (0.02), respectively
(Table 6).
Table 6: Selection differential and expected response in maize S1 and S2
families for various traits
|
Traits |
Selection cycle-1 |
Selection cycle-2 |
||
|
Selection differential (S) |
Expected response (Re) |
Selection differential (S) |
Expected response (Re) |
|
|
Days to silking
(days) |
-1.43 |
-1.19 |
-1.89 |
-0.83 |
|
Plant height (cm) |
11.00 |
9.13 |
23.00 |
19.78 |
|
Ears per m2 (#) |
0.43 |
0.31 |
0.89 |
0.53 |
|
Ears per plant (#) |
0.03 |
0.02 |
0.05 |
0.03 |
|
Kernel rows ear-1 (#) |
1.84 |
1.60 |
3.35 |
2.41 |
|
Grain yield (kg ha-1) |
2073.00 |
1907.16 |
2675.00 |
2220.25 |
S: Selection differential, Re:
Expected response (h2*S)
Kernel rows per ear
Kernel rows
per ear were reduced by 7.88% with selfing from 13.19 (S1 selfed) to
12.15 (S2 selfed) in cycle-1 and cycle-2, respectively (Table 4).
The S2 selected populations (15.50) compared to the base population
- S0 (14.00) showed an increase of 10.71%. However, in cycle-1 the
maximum kernel rows per ear were
recorded for S1 selected families (15.03), followed by base
population S0 (14.00) and S1 selfed population (13.19).
Similarly, in cycle-2 the highest kernel rows per ear were also noted for S2
selected lines (15.50), followed by the base population S0 (13.00)
while minimum in S2 selfed population (12.15). In S2
selected families (15.50), the kernel rows per ear were increased as compared
to S1 selected families (15.03). High heritability (bs) values were recorded for kernel rows per ear in both
cycles of selection (Table 5). However, in cycle-1 the heritability (0.87) was
higher as compared to cycle-2 (0.72). By comparing the selection differential
in both cycles, the selection differential was significantly increased in
cycle-2 (3.35) as compared to cycle-1 (1.84) for kernel rows per ear (Table 6).
Similarly, positive and enhanced expected genetic gain values (2.41 and 1.60)
were noted in cycle-2 and cycle-1, respectively for the said trait.
Grain yield
Grain yield
reduced by 13.63% with selfing from 4102 (S1 selfed) to 3542 kg ha-1
(S2 selfed), while S2 selected populations (6217 kg ha-1)
compared to S0 (5638 kg ha-1) showed an increase of
10.27% in the succeeding selection cycles (Table 4). In cycle-1, the highest
grain yield was produced by S1 selected lines (6175 kg ha-1),
followed by base population S0 (5638 kg ha-1) and S1
selfed population (4102 kg ha-1). Similarly, in cycle-2 the highest
grain yield was obtained in S2 selected populations (6217 kg ha-1),
followed by base population S0 (5525 kg ha-1) and S2
selfed families (3542 kg ha-1). On average, the S2
selected families (6217 kg ha-1) revealed higher grain yield than S1
selected families (6175 kg ha-1) which showed improvement through
selection in succeeding generations. A higher magnitude of broad sense heritabilities
was observed for grain yield in both cycles of selection (Table 5). However,
comparatively, the maximum heritability was observed in cycle-1 (0.92) as
compared to cycle-2 (0.83). Likewise, the highest values of selection
differential and expected genetic gains were observed in both cycles of
selection, ranging from 2073.00 to 2675.00 kg ha-1 and 1907.16 to
2220.25 kg ha-1 in cycle-1 and cycle-2, respectively (Table 6).
Discussion
Recurrent selection has been contentedly used for
evaluating S1 lines through mean performance, heritability, and
genetic gain from selection and improvement of maize populations (Ali 2015; Kolawole
et al. 2017; Sajjad et al.
2020a, b). In the present study,
maize populations (S0, S1 and S2)
derived from the base population - PSEV3, revealed highly significant
differences for earliness, morphological, and yield traits in selection cycle-1
and cycle-2. These populations revealed a greater proportion of genetic
variation and prospective for recurrent selection to improve their agronomic
traits and grain yield. Based on these findings, it can be concluded that one
of the main objectives of recurrent selection has been achieved, i.e.,
the genetic variability within population has been maintained at adequate
level, while the variability between populations has enhanced over succeeding
cycles of selection which contributing stability in the breeding program.
Recurrent selection is enhancing the frequency of favorable genes through
regular cycles of selection along with sufficient genetic variation in maize
families for their improvement (Kolawole et al. 2017; ). In two
cycles of S1 recurrent selection, the population revealed positive
responses for earliness, ear traits, and other yield-related traits in maize (Khalil et al. 2010;
Khan et al. 2018). Past
findings revealed that after selection, the genetic variance within and between
the maize populations amplified to a significant level in the succeeding cycles
of recurrent selection (Berilli et al.
2011; Cunha et al. 2012). Significant
variations were observed among the S1 lines for flowering, plant and
ear height, ear length, kernel rows per ear, 1000-kernel weight, and grain
yield in maize (Ahmad et al. 2010;
Berilli et al. 2013; Annor et al.
2019). Significant improvement was persuaded in maize populations
through six cycles of S1 recurrent selection with increased grain
yield and lodging resistance (Chen et al.
2019).
Crop production is influenced by many factors including
phenological traits. In maize, the flowering time like tasseling, silking,
pollen shedding, and anthesis-silking interval contributes to physiological
maturity. In the present study, the S2 selected families took less
days to silking than S1 and S2 selfed and base
populations, and tending to earliness by showing improvement in early maturity.
Past studies revealed that tasseling and silking time had a positive impact on
grain yield followed by ear height and flag leaf area in maize (Ahmad et al. 2010). Maize populations revealed
significant differences for days to tasseling and silking; however, selected S1
lines showed less days to flowering compared to selfed populations (Khalil et al. 2010; Sohail et al. 2018; Sheikh et al.
2019). With marginal differences and the same number of days to tasseling,
silking, and pollen shedding were recorded in selected and base populations in
maize (Sajjad et al. 2016). However,
some studies reported that S1 selected lines took more days than
original population for earliness traits with positive selection differential
exercised in improving population mean in maize (Ali et al. 2011a, b).
In plant architectural traits (plant and ear height),
the least increase is manageable and to protect the crop from lodging. In
present studies, within each cycle of selection, the selected populations
showed increased plant height compared to selfed and base populations in cycle-1 and
cycle-2. On average, with succeeding cycles, the S2 selected
families had the lowest mean values for morphological traits compared to S1
which are desirable from breeding point of view for population improvement.
Significant differences were observed in maize S1 and S2
families for plant and ear height; however, on average the selected lines
attained least plant and ear height than selfed and original maize populations
(Khalil et al. 2010). For plant and
ear height, the mean values of the S1 selected populations were
higher than the population mean with positive values of selection differential
in maize (Ahmad et al. 2010).
Significant genetic variability was reported among cyclical populations for
morphological traits, and maize populations with moderate
plant and ear height were found desirable because of their less exposure to
lodging and ultimately contribute to good yield (Noor et al. 2013; Ullah et al. 2013;
Khamkoh et al. 2019).
For
yield traits (ears per m2,
ears per plant and kernel rows per ear) and grain yield, the increased mean
values were observed with significant improvement in S1 and S2
selected families followed by original and S1 selfed
populations in both cycles. However, a significant increase was shown by S2
selected families than S1 for yield traits. Present results further
revealed greater genetic variation within and between maize populations and the
genetic gain made through two cycles of selection resulting in significant
improvement in yield traits. This improvement might be due to accumulation of
favorable genes in the selected versions of maize populations. Recurrent
selection effectively enhanced the accretion of desirable genes for
quantitative traits with significant improvement in maize improved populations
(Kolawole et al. 2017). Selection in cycle-2 provoked maximum grain yield
with significant genetic gain and concluded that S1 recurrent
selection has been found efficient for genetic improvement in maize (Ali et al. 2018, 2019; Khan et al. 2018). Current results also
revealed that in improved population C2 the increased grain yield
was associated with early flowering and maturity. Past investigations also
reported significant response in maize S1 populations with a
positive association between earliness and yield traits in maize (Khamkoh et al. 2019). As compared
to selfed and base populations, the maize selected populations showed increased
values for yield related traits (Sajjad 2018; Sajjad et al. 2020a, b). In breeding programs, the appreciated progress
mainly depends upon precise identification of promising populations for the specific
environment and the precision with which the studies are conducted.
Genetic
variability and heritability among maize populations provides valuable
information and assists the breeder to predict the behavior of a trait in
succeeding generations (Ali et al. 2012a, b; Kashiani et al.
2014). Heritability (broad sense) values were moderate to
high in S1 and S2 selfed families for earliness,
morphological and yield traits. Present results highlighted the lower impact of
the environment compared to genetic variances in S1 and S2
selection cycles, indicating sufficient genetic variability and suggesting
further improvement. The highest genetic variability and heritability were
observed for earliness traits which assured prospects for future improvement in
S1 populations of maize (Khalil et al. 2010; Ishaq et al.
2014). High broad-sense heritability was recorded for plant and ear height and
yield components in maize selfed progenies (Noor et al. 2013; Ullah et al.
2013; Annor et
al. 2019). Past studies on S1 recurrent selection
reported high heritability and selection
response for plant and ear height, ear length, kernel rows per ear, and
grain yield in maize (Rahman et al. 2015; Cobb et al. 2019). However, some other studies reported reduced
values of genetic variances and heritability for earliness and yield traits in
advanced cycles of selection in maize (Badu-Apraku et al. 2013).
Selection differential is the
deviation of selected S1 and S2 families from their
selfed populations, and the change occurred in the population mean is the
response to selection. In present studies, the higher negative values of
selection differential were reported for days to silking in S2
selected populations than S1 lines which help in early maturity of
the improved maize populations. Selected S1 and S2 lines
were observed with smaller mean values than the population mean, resulting in
negative values of selection differential for earliness traits in maize (Ali et al. 2011a, b; Kolawole et al.
2017). For plant height and yield traits, the selection differential values
were positive and significantly high with less inbreeding depression which also
confirmed improvement in the selected populations through succeeding
generations. High heritability and index of variation estimates were exhibited
by maize S1 and S2 populations for yield traits (Sajjad et al. 2016; Sajjad 2018). High
heritability and selection response with significant improvement in grain yield
suggested that S1 recurrent selection was found quite effective in
improving maize populations (Ahmad et al.
2010; Sajjad et al. 2020a, b).
Therefore, the improvement in present maize selected populations through
succeeding generations was dependent both on the genetic variability and
heritability in the population which determines the extent of progress through
recurrent selection.
Conclusion
The S1 and S2 selected population
revealed significant decrease for days to silking while increase for yield
related traits. Heritability (broad sense) was moderate to high for all the
traits in both cycles of selection. The selection differential and expected
responses were significantly negative for days to silking while positive for
yield related traits in S2 selected populations as compared to S1
lines. Overall, the selfed progeny recurrent selection method was found more
effective in improving the maize base population 'PSEV3' for earliness and
yield traits.
Author Contributions
MS, NUK and SG designed and executed the study; SUK and SA
recorded and analyzed the data; SAK and NA made Tables and illustrations; IT
and ZB collected the review and drafted the manuscript; IH and SMK interpreted
the results and reviewed the contents. All authors improved the write-up by
reading and approved the final version of the manuscript.
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