Variation and Genetic Studies on Selected Sweet Corn Inbred Lines
Nine advanced tropical sweet corn (Zea mays L. saccharata) inbred lines were evaluated to determine their performance, to estimate broad-sense heritability of the traits measured and to determine phenotypic correlations among the traits. The inbred lines showed a wide range of performance for husked fresh ear yield and its components. Based on performance and earliness, inbred lines Bakti-1-S7, TSS Melaka-S5, Thailand-S6, MM x Indonesia-S4 and Manis Madu-S7 were found to be superior and could be short-listed for further testing for general and specific combining ability analysis towards development of hybrid varieties. Broad-sense heritability (h2B) estimates obtained from the variance components method were found to be high for number of days to silking (80.5%), plant height (79.9%), number of days to tasseling (66.9%) and ear height (63.7%), moderate for husked ear yield (56.7%), total soluble solid concentration (54.2%), number of kernel rows per ear (53.9%), ear diameter (46.7%), dehusked ear yield (43.1%), number of kernels per row (42.6%) and number of ear per hectare (34.5%), while lowest estimates for dehusked ear length (12.9%) and husked ear length (0.3%). Traits found to be highly correlated with husked ear yield were dehusked ear yield (0.97), number of kernel rows per ear (0.71), plant height (0.69), husked ear length (0.67), ear height (0.66), dehusked ear length (0.63), ear diameter (0.55), number of kernels per row (0.50) and number of ears per hectare (0.49). For selection purposes, it is therefore suggested that emphasis should be given on traits like number of kernel rows per ear, plant and ear height and ear length for yield improvement among the inbred lines.
In Malaysia there is an increasing trend in sweet corn production mainly due
to its high economic return (Saleh et al., 2001).
However, the local varieties, which are currently planted throughout the country,
are late in maturity, tall and less sweet. One of the most important approaches
to improve the performance of the populations is by development of inbred lines
which can produce high-yielding hybrid varieties. Inbred line development is
the main prerequisite for production of hybrid varieties. This process is achieved
through successive generations of inbreeding followed by repeated testing and
selection. Inbreeding is the process of mating between genetically related individuals.
The fundamental genetic change that inbreeding produces a loss of heterozygosity,
was well understood early in the development of the field of genetics (Hill
et al., 2006; Marquez-Sanchez, 1998; Wright,
Selfing, the most extreme form of inbreeding, reduces heterozygosity by 50%
each generation. The decrease in heterozygosity typically results in phenotypic
changes that we call inbreeding depression, the genetic basis of which has been
debated for almost a century. As a consequence of selfing, recessive genes,
which were earlier masked in the heterozygous forms, become homozygous. These
genes, if conferring to undesirable phenotypes, will result in the deterioration
of the succeeding generations. In cross-pollinated crops which do not have self-incompatibility
problems, like sweet corn, inbred lines for hybrid varieties are developed through
selfing. Extensive studies on inbreeding depression in sweet corn (Zea mays
L. saccharata) have indicated that selfing is important in inbred line
development because it leads to rapid gene homozygosity, whereby desirable dominant
genes can be accumulated while the undesirable ones are eliminated (Gallais,
2009; Saleh et al., 1993). Selected plants
are usually self-pollinated for several generations until homozygosity is reached.
Hallauer and Miranda (1988) reported that many undesirable
recessive genes are eliminated from families as a result of the inbreeding process,
then selection is applied within and between lines for the best individual plants
(Acquaah, 2007). Hallauer (1990)
added that corn inbred lines developed from improved source populations would
have greater vigour and grain yield as compared to those developed from unimproved
sources. Stoskopf et al. (1993) also cited that
inbred lines are developed after five to seven generations of selfing, during
which selection for characters of interest is also conducted. Heritability is
a measure of the degree to which the variance in distribution of a phenotype
is due to genetic causes (Ullrich, 2007; Sleper
and Poehlman, 2006). Ali et al. (2003) found
that broad-sense heritability estimated from their sweet corn population were
moderate to high for the traits measured. The highest estimate was for ear height
(99.8%), while the lowest was revealed by ear diameter (61.9%). Saleh
et al. (2002a) reported that broad-sense heritability estimates for
grain yield in maize were generally moderate. Correlation is the co-relationship
between two variables and a correlation coefficient is the measure of the degree
of association between two variables (Gepts, 2002; Wallace
et al., 1993; Mayo, 1987). Correlations may
occur in positive or negative values from zero to ±1.0. The closer the
coefficient is to either -1 or 1, the stronger is the correlation between variables
(Miles and Shevlin, 2001). Hence, correlation is an important
parameter to be estimated as it helps to determine relationship among traits
before selection is conducted (Kashiani et al., 2008).
According to Saleh et al. (2002b), number of
ears per hectare, plant height, ear weight, ear length and number of kernel
rows per ear were found to be most correlated with ear yield. However, days
to tasselling, days to silking, ear diameter and total soluble solids content
were not correlated with ear yield. The objectives of this study were to determine
the performance of nine inbred lines as potential hybrid parents, to estimate
broad-sense heritability of the traits measured on the nine inbred lines and
to determine phenotypic correlations among those traits.
MATERIALS AND METHODS
Location of Study
The study was conducted from March to May 2009, at filed 10, Faculty of
Agriculture, Universiti Putra Malaysia, Serdang, Selangor, located at 3°
40' North; 101° 42' East and 31 m above sea level.
This study was an advanced stage of a long-term tropical sweet corn inbred
line development programme conducted at Universiti Putra Malaysia where a series
of near-homozygous inbred lines were formed from various tropical origin source
The inbred lines were namely Bakti-1-S7, Manis Madu-S7, TSS Tin-S7, Mas Madu-S6, Thailand-S6, Indonesia-S6, TSS Melaka-S5, Manis Madu x Indonesia-S4 and SBY-S2.
Experimental Design and Plot Arrangement
The evaluation was carried out in a Randomized Complete Block Design (RCBD)
with three replications. Each replicate consisted of nine plots, each represented
by an inbred line. The planting density used was 0.75x0.25 m. Each plot consisted
of seven 12 m long plant rows, where only the three middle rows measuring 4
m in length were used as the harvest area. All experimental plots were subjected
to uniform agronomic practices.
Before planting, the soil was ploughed twice to a depth of 15 to 30 cm,
followed by rotorvation. The soil type in the evaluation plot was sandy loam
(56% sand, 23% silt and 21% clay). The soil pH was raised up to 6.5 by applying
ground magnesium limestone one month before planting. Planting was done manually,
at the rate of two seeds per point and later thinned to just one plant per point
at 10 days after planting. Fertilizer at the rate of 160 kg ha-1
N, 100 kg ha-1 P2O5 and 100 kg ha-1
K2O was given in split applications, where the balanced compound
fertilizer (15:15:15) was used at seven days after planting, followed by Urea
(46% N) at 20 and 35 days after planting. Weeds were controlled manually using
a pre-emergence herbicide, Lasso (2-chloro-2'-6'-diethyl-N-methoxymethyl) and
a post-emergence contact herbicide, Gramoxone (1,1, dimethyl-dimethyl-4, 4'-bipyridinium).
A sprinkler irrigation system was used to supply water to the plants when necessary.
Data were collected from both pre- and post-harvest characters including
days to tasselling, days to silking, plant height (cm), ear height (cm), number
of ears per hectare, fresh ear yield (kg ha-1), ear length (cm),
ear diameter (mm), number of kernels per row, number of kernel rows per ear
and Total Soluble Solids (TSS) concentration (%). Fresh ears were harvested
from each plot separately, at 21 days after plant tasselling. Three middle rows
of each plot measuring four meters long, giving an area of 9 m2,
were used as the harvest area for yield and number of ears per hectare estimations.
Among the plants in the harvest area, 12 plants were used as samples for the
measurements of individual plant and ear data.
Data collected were analyzed using SAS computer package. The Analysis of
Variance (ANOVA) was used to determine the significance of variation among inbred
lines. Subsequently, the Duncan New Multiple Range Test (DNMRT) was applied
for comparison of mean performance of the inbred lines.
Broad-sense heritability (h2B) for the traits was estimated
using the variance components method suggested by Becker (1984),
||Variance for inbred lines
||Mean squares due to inbred line
||Error mean squares
||No. of replications
Simple correlations among the characters measured were also analyzed using
the formula based on Gomez and Gomez (1984), as follows:
||Correlation between the traits X and Y
||Mean value of character X
||Mean value of character Y
RESULTS AND DISCUSSION
Results of the analysis of variance showed significant effects among the inbred
lines evaluated for all characters studied except husked and dehusked ear lengths.
Mean values for the performance of the inbred lines are shown in Table
1. This indicates that the inbred lines evaluated varied substantially in
many aspects and these differences could be exploited for specific purpose in
breeding programmes. From separation of mean values indicated by DNMRT (Table
1), highest fresh husked ear yields were revealed by inbred lines Bakti-1-S7,
TSS Melaka-S5, Thailand-S6 and Manis Madu-S7,
with yields of 8043, 7532, 6053 and 5298 kg ha-1, respectively. The
superiority of inbred lines Bakti-1-S7, TSS Melaka-S5,
Thailand-S6 and Manis Madu-S7 was also revealed by the
high number of productive ears per hectare they produced, amounting to 35556,
41481, 44444 and 35556, respectively. There is therefore, a strong indication
of the potential of these inbred lines for their utilisation towards production
of new hybrid varieties. Bakti-1-S7 was found to be significantly
taller than other inbred lines with (175.1 cm) and also tasseled and silked
significantly sooner than the others. Indonesia-S6 was the only inbred
line which tasseled about the same time as Bakti-1-S7. The lowest
Total Soluble Solids (TSS) concentration was obtained from MM x Ind-S4,
while no significant difference among other inbred lines were observed. Kashiani
et al. (2008) reported that the inbred lines extracted from the source
populations including Bakti-1, TSS Melaka, Thailand and Manis Madu performed
better than the rest in term of fresh ear yield, number of productive ears per
hectare and days to silking and tasseling.
Genotypic and phenotypic variances and broad-sense heritability (h2B)
estimates for yield and yield components measured among the nine inbred lines
evaluated are shown in Table 2. Days to silking was found
to be the most heritable trait in the inbred lines, with heritability of 80.5%,
followed by plant height (79.9%), number of days to tasselling (66.9%) and ear
||Mean values for traits measured on nine sweet corn inbred
followed by the same letter(s) in the same column are not significantly
different at p≤0.05 based on DNMRT|
||Genotypic variances (σ2G), phenotypic
variances (σ2P) and broad-sense heritability
estimates (h2B) for traits measured on nine sweet
corn inbred lines
This indicates that selection for these traits in these inbred lines would
be most effective for the expression of these traits in the succeeding generations.
Similar results on broad-sense heritability were reported by Kashiani
et al. (2008) and Saleh (2003) showing high
estimates for days to silking, plant height and ear height. Saleh
et al. (2002c) also reported high broad-sense heritability estimates
for plant height and number of days to tasselling on selected sweet corn synthetic
populations. Moderate broad-sense heritability estimates were obtained from
husked ear yield (56.7%), total soluble solids concentration (54.2%), number
of kernel rows per ear (53.9%), ear diameter (46.7%), dehusked ear yield (43.1%),
number of kernels per row (42.6%) and number of ear per hectare (34.5%), while
lowest estimates were revealed by husked ear length (12.9%) and dehusked ear
length (0.3%). Fresh ear yield, ear diameter and number of kernels per row were
reported to possess moderate heritability estimates (54.7, 37.0 and 36.0%, respectively)
in a study on sweet corn breeding population conducted by Nigussie
and Saleh (2007).
||Simple correlation coefficients among traits measured on nine
sweet corn inbred lines
*Significant at p = 0.01, p = 0.05, respectively, ns: Non-significant,
TSS: Total soluble solids 1: Husked ear yield, 2: Dehusked ear yield,
3: No. of ears ha-1, 4: Plant height, 5: Ear height, 6: Days
to tasselling, 7: Days to silking, 8: Husked ear length, 9: Dehusked ear
length, 10: Ear diameter, 11: No. of kernels/row, 12: No. of kernel rows/ear|
Results on simple correlation among characters are presented in Table
3. Husked ear yield was found to be highly positively correlated (p≤0.01)
with dehusked ear yield, plant height, ear height, husked ear length, dehusked
ear length, ear diameter, number of kernels per row and number of kernel rows
per ear (with correlation coefficients of 0.96, 0.69, 0.66, 0.67, 0.63, 0.55,
0.50 and 0.71, respectively), while positively correlated (p≤0.05) with number
of ears per hectare (r = 0.48). This indicates that high measurements of these
traits had direct positive contribution to husked ear yield. Therefore, for
selection purposes to improve the inbred lines, it is suggested that emphasis
should be given on these traits. Plant height was highly significantly correlated
with husked and dehusked ear yields, indicating that taller plants were better
yielding compared to the shorter ones. This might be attributed to the high
dry matter accumulation function carried out by the high number of leaves possessed
by tall plants. A similar finding has been reported by Kashiani
et al. (2008), Sujiprihati et al. (2003)
and Saleh et al. (2002b).
In conclusion, significant differences in performance indicated that lines varied among the inbred lines for the measured traits substantially in many aspects and these differences could be exploited for specific purposes in breeding programmes. This also gave an indication of positional of the inbred lines to be used as promising parents for potential hybrids in Malaysia. Genotyping of inbred lines can be performed as an extra tool to select superior inbred lines for further diallel analysis to reveal general and specific combing abilities.
The authors gratefully acknowledge the Ministry of Science, Technology and Innovation (MOSTI) for the financial support on the research under e-Science Fund (Project No. 05-01-04-SF0700).
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