Genetic Variability and Correlation among Yield and Yield Contributing Traits in Sweet Corn

A total of 50 hybrids was generated by Line x Tester mating design using ten lines and five testers of sweet corn. These hybrids along with parents and checks were evaluated at TNAU, Coimbatore during Kharif -2015. Genotypes were significantly different for all the traits under study. Green cob yield was found to have positive significant correlation with number of kernel rows per cob , plant height, green cob weight, total chlorophyll content, cob length, cob breadth, 100 seed weight, number of kernels per row indicating that indirect selection for yield through these traits will be effective. Green cob yield had no significant correlation with total sugar content of kernels indicating the scope of developing high yielding hybrids with high sugar content. Traits viz., tassel branching, green cob weight, number of kernels per row, green cob yield, 100 seed weight, reducing sugar, non reducing sugar, photosynthetic efficiency, stomatal conductance and total chlorophyll content recorded high genetic advance as per cent of mean coupled with high heritability thus providing maximum opportunity for improvement and genetic gain through selection of these traits.

Sweet corn (Zea mays L. saccharata) which is bred through recessive gene mutation of field corn also known as sugar corn, is one of the most popular vegetable in countries like USA and Canada. Sweet corn is introduced to India from United States. Due to its sweet taste and tenderness, cultivation of sweet corn is the first choice of the farmers nowadays for green fodder and green cobs.
Genetic progress depends on the existence of genetic variability (Allard 2000). The magnitude of genetic variability for different traits and the extent of heritability of desirable traits play a vital role in deciding the success of any crop improvement programme. The estimates of variability for yield and yield contributing characters and their heritable components are more important in any crop-breeding programme. Heritability estimates play a role in deciding the selection intensity, while genetic advance is helpful in predicting the gain under selection.
Association studies reveal the degree and direction of association between two or more variables. Knowledge of the strength and type of association is an important prerequisite for the formulation of breeding procedures (Breese and Haywards, 1972). A knowledge of the association between yield and its component traits helps in improving the efficiency of selection.
Therefore, keeping in view the significance of variability and association studies in breeding programme , present investigation was undertaken in sweet corn. *Corresponding author email: chithuragul@gmail.com

Materials and Methods
Fifteen elite sweet corn inbreds were raised at Department of Millets, Tamil Nadu Agricultural University, Coimbatore during Summer, 2015. All the 15 inbreds were crossed in Line x Tester (10 lines and 5 testers) fashion to obtain 50 cross combinations. Evaluation of single cross hybrids, parents and checks (Sugar-75 and Madhuri) was done in Kharif 2015 at Millet Breeding Station, Tamil Nadu Agricultural University, Coimbatore under RBD with three replications. Biometrical observations were recorded on five tagged randomly selected plants in each replication for each genotype and the data recorded on these plants was used to arrive at the replication mean. The replication wise mean values were subjected to statistical analysis. The traits viz., days to 50% tasseling, days to silking, days to 50% silking, tassel length, tassel branches, plant height, cob placement height, green cob weight, cob length, cob breadth, number of kernel rows per cob, number of kernels per rows, green cob yield and hundred seed weight were recorded. The quality of sweet corn was ascertained by conducting biochemical tests. The immature kernels was collected at milky stage and stored at -20 0 C and this was used as the sample for estimation of quality traits. Total sugars were estimated using Anthrone method (Yemm and Willis, 1954), reducing sugar by Nelson Somogyi method (Somogyi 1952), non-reducing sugar by subtracting reducing sugar from total sugar. Physiological parameters viz., photosynthetic efficiency, stomatal conductance, transpiration rate and total chlorophyll content were recorded in parents and hybrids using PPS and SPAD meter at flowering stage of the crop.
The data generated was analysed for arriving at meaningful interpretations.

Results and Discussion
The estimates of σ 2 g, σ 2 p, genotypic (GCV) and phenotypic coefficients of variation (PCV), heritability and genetic advance as percent of mean (GAM) for different characters have been presented in Table 1.
In general, for all the traits studied, the phenotypic coefficient of variation (PCV) was higher than the genotypic coefficient of variation (GCV). The phenotypic coefficient of variation ranged from 5.42 (days to 50% tasseling) to 69.65 per cent (reducing sugars). The genotypic coefficient of variation ranged from 3.87 (Days to 50% tasseling) to 47.59 per cent (Reducing sugars). In the present study, though the phenotypic variances were greater than the genotypic variance for most of the traits, the differences between phenotypic and genotypic variance were minimum indicating the less influence of environmental factors (Sumathi et al., 2010).
The estimates of genetic advance as percent of mean (GAM) ranged from 5.61 to 83.21 per cent. High estimate of genetic advance as percent of mean (GAM) was noticed for reducing sugars (83.21 per cent), stomatal conductance ( 72.34 per cent), photosynthetic efficiency (64.14 per cent), non reducing sugars (51.06 per cent), total sugars (49.11 per cent), green cob weight (45.05 per cent), green cob yield (41.55 per cent), tassel branching (29.01 per cent), hundred seed weight (27.77 per cent) and number of kernels per row (27.34 percent). The results obtained are in accordance with the results of Azam et al.(2014) for yield/plant and ear height; Bharathi (2014) for cob yield, hundred grain weight, cob length, number of kernel rows per cob , number of kernels per row, tassel length, number of tassel branches and cob girth. Heritability estimates along with genetic advance are more helpful in predicting the gain under selection than heritability estimates alone. However, it is not necessary that a character showing high heritability will always exhibit high genetic advance (Johnson et al., 1955).Traits viz., days to 50% tasseling, days to silking, days to 50% silking, tassel length and cob length recorded low heritability with low genetic advance, Similar results of low heritability coupled with low genetic advance were reported by Singh and Singh (2011) for days to maturity. Characters viz., tassel branching, green cob weight, number of kernels per row, green cob yield, 100 seed weight, reducing sugar, non reducing sugar, photosynthetic efficiency, stomatal conductance and total chlorophyll content recorded high genetic advance as per cent of mean coupled with high heritability. DFT  DFS  PH  TL  TB  GW  CL  CB  KC  KR  HW  TS  RS  NRS  PE  SC  Hence, exercising selection on these traits can give maximum opportunity for improvement and genetic gain as expected. Similar findings were reported by Alake et al. (2008) for grain yield and kernel row per cob.

Association of characters
The genotypic correlation coefficient estimated between yield and its components including quality traits and physiological parameters and intercorrelation among the different yield attributes are furnished in Table 2.
Days to 50% silking (-0.253) recorded significant negative association with yield indicating that selection for early 50% silking will also be desirable to increase grain yield by decreasing anthesis silking interval (ASI) in sweet corn apart from yield contributing traits. Hybrids with high yield and early duration will be advantageous. Similar results are reported by Umakanth et al.(2000) and Nigussie and Zelleke (2001) for days to 50% silking. Plant height (0.511), green cob weight (0.491), cob length (0.418) and cob breadth (0.387) registered significant positive association with yield in this study too as indicated by Kashiani et al. (2010) for green cob weight. Number of kernel rows per cob (0.529) and number of kernels per cob (0.253) and 100 seed weight (0.333) had significant positive association with green cob yield and as earlier obtained by Thanga Hemavathy et al. (2008), Nemati et al. (2009) and Beiragi et al. (2011). Total chlorophyll content (0.448) registered significant positive association with yield. Similar result of positive correlation of chlorophyll content and yield was observed by Gekas et al.,(2013).
Hence, in breeding programmes, these traits can be used as selection indices in order to identify good inbreds for use in breeding programmes. There was no significant association existed between yield and total sugar content in kernels indicating the scope of development of high yielding hybrids with high sugar content. Ashish et al. (2010) in his studies also found the same relationship between yield and sugar content.