Heterosis performance in industrial and yield components of sweet corn

Heterosis and its components were evaluated in a diallel crossing system of sweet corn. In the 38 treatments, eigth parents, 28 hybrids and two controls were used, arranged in a randomized block design with three replications.The diallel analysis followed the methodology of Gardner and Eberhart (1966). The following traits were evaluated: male and female flowering, plant and ear height, ear index (number of ears/number of plants), oBrix, total ear weight, standard ear weight, industrial yield and total sugar content. There was genetic variability among genotypes, with significant differences except for the traits ear index, industrial yield and oBrix.Heterosis was found for most traits. The mean heterosis of hybrids compared with the parents was positive for most traits. There was a contribution of additive and dominance effects.The contribution of dominant genes was greatest to flowering, plant and ear height and standard ear weight.


INTRODUCTION
Sweet corn differs from field corn because it contains genes that modify the flavor, tenderness, texture, seed viability, and appearance of plants and ears (Tracy 1994).The world's area used for corn comprises 900 hectares, and in Brazil, 36 thousand hectares (Barbieri et al. 2005).About 66% of the production is in Goiás where yields reach 14 t ha -1 and the industrial yield is 33%, which may however vary according to the level of technology, seasons and requirements of each industry.
The sweet corn produced in Brazil is destined for industrial processing (Barbieri et al. 2005).The short supply of varieties on the market and lack of knowledge among consumers have resulted in a low demand for sweet corn.This crop can potentially increase, since sweet corn is traditionally produced and consumed corn in Brazil, which can facilitate the introduction of other forms of sweet corn consumption.
Due to the growing demand for sweet corn and the requirements of producers, industry and consumers, studies for more information about this crop are needed to select and identify genes that confer relevant agronomic and industrial traits for genetic breeding programs and the consumer market.A technique that helps choose the best parents based on their performance, for selection of promising hybrids, is the diallel mating system (Ramalho et al. 1993).Diallel crosses provide information on the type of predominant gene action, assess the heterotic potential and general and specific combining ability of genotypes A Assunção et al. (Hallauer and Miranda Filho 1981).Information obtained by diallel crossings are widely used in corn breeding programs.
Some studies with sweet corn in diallel crosses have been conducted.For the trait commercial yield of ears it was observed that the non-additive genetic effects exceeded the additive (Scapim et al., 1995).Other results show significance for general and specific combining ability for ear weight without straw, indicating the existence of variability for both additive and non-additive genetic effects.If the non-additive genetic effects prevail, there is a greater potential for exploitation of heterosis, and if additive effects are predominant, the success for improvement will be greater with the formation of synthetics (Teixeira et al. 2001).
The aim of this study was to evaluate eight sweet corn populations from crosses, and investigate the performance per se and of heterosis in industrial and yield components based on hybrid combinations.
The experiment was installed in June 2005, in the above experimental area, in a randomized block design with three replications, with a total of 38 treatments: eight parents (F 2 of crosses of each genotype), 28 generations F 1 (hybrids derived from crosses between all genotypes, taken two by two), and two commercial hybrids as controls (DO-04 and DAS-451).Each plot consisted of one 5-m row, with 0.75m between-row and 0.25m between-plant spacing.Three to four seeds per hole were sown at a depth of 3-4 cm.The plants were thinned to one seedling per hole 25-30 days after planting.
The following traits were evaluated: male flowering (MF), female flowering (FF), plant height (PH), ear height (first ear) (EH), ºBrix (soluble solids) by a refractometer (RT-30ATC 0-32°Brix), sugar (total sugar content) -determined in a laboratory according to the Lane-Eynon method; ear index (EI): total number of ears per plot divided by the number of plants per plot; total ear weight (TEW), standard ear weight (SEW): total weight of ears disregarding outliers; and industrial yield (Iy): the proportion of grains in relation to standard ear weight.The grain moisture content was corrected to 76%, which is considered ideal to harvest sweet corn.
For the analysis of variance the statistical program (SAS Institute 1997) was used.The means adjusted by least squares obtained by variance analysis were evaluated by the genetic model of diallel analysis proposed by Gardner and Eberhart (1966), using the GENES program (Cruz 1997).The general model is as follows: where: Y ij : value observed for the parent i or cross between i and j; overall mean of varieties; υ i : effect of the i th variety; υ j : effect of the j -th variety; h: effect of mean heterosis of all crosses, h i and h j : heterosis of the parents i and j, compared to h, respectively; s ij : effect of specific heterosis resulting from crosses between parents of the order i and j; ε ij : mean experimental error associated to the hybrid or parental means; θ: conditional coefficient, with values of θ = 0, where i = j and θ = 1 when i = j.The general combining ability of the parents (g i ) was also estimated by: g i = ½ (υ i i+ h i ).

RESULTS AND DISCUSSION
The mean of male flowering (MF) was 83.47 days after planting (DAP), and female flowering (FF) 86.28 DAP (Table 1).These values are higher than those found for sweet corn, but since the experiment was planted in June, with lower mean temperature, the growing cycle lasted longer.For sweet corn in this growing season, harvested 90 to 100 DAP, it is best to use cultivars of earlier cycles, mainly for irrigated crops, to reduce the time the crop stands in the field.
The values of mean heterosis (h) were -3.23 DAP (FF) and -2.95 DAP (MF) (Table 2).Significance indicates variability between the heterotic responses of the crosses.At least a few of the crosses differed from the hybrid mean.According to Vencovsky and Barriga (1973), based on this significance dominance was inferred and sufficient divergence of gene frequencies between genotypes in at least part of the loci with dominance.The negative h value indicated bidirectional dominance, with the occurrence of positive and negative heterosis.Since the mean of the crossings was lower than the mean of the parents, the situation is favorable for selection, since a reduction of the crop cycle is a breeding target.The means in F 1 were by 3.43% (MF) and 3.62% (FF) lower than for the parents (Table 3).
The effects of variety heterosis (h i ) ranged from -2.68 to DAP genotype AF-428 DAP to 1.32 for genotype DO-04 (FF), DAP and -1.88 for genotype AF-428 DAP to 1.56 for genotype HS2-2104 (MF) (Table 2).Gama et al. (1995) found significance for v i and total heterosis for MF in a diallel of 15 early common maize populations.In this case, h i ranged from -0.61 to 3.47 DAP.When h i is significant, not only additive effects (variety) should be considered, so the best genotypes are those with highest negative general combining ability, tending to reduce the cycle, which were: HS1-2004, Tropical and SWB-551, for MF and FF.Cruz et al. (2004) claimed that low estimates of positive or negative g i indicate genotypes with combinations that do not differ much from the overall mean of the crosses in the diallel system, indicating the importance of genes with predominantly additive effect.Working with maize populations in Hardy-Table 1. Summary of the analysis of variance of the diallel for the traits male flowering (MF, in days after planting -DAP), female flowering (FF, in DAP), ear index (IE), plant height (PH, in cm) and ear height (EH, in cm), according to the methodology of Gardner and Eberhart (1966) *,** Significantly superior to the error mean square by the F test, at 5% and 1% probability respectively.
Table 2. Estimates of the mean (μ), of the variety effect (υ i ), mean heterosis (h), variety heterosis (h i ), genotypic effect (g i ) and specific Table 3. Means of the parents (G), hybrid means (H) and magnitude of heterosis expressed in % (h) for the traits male flowering (MF, in DAP), female flowering (FF, in DAP), plant height (PH, in cm), ear height (EH, in cm), ear index (IE), soluble solids content (ºBrix), total sugar content (sugar, in %), total ear weight (TEW, in kg ha -1 ), standard ear weight (SEW, in kg ha -1 ), and industrial yield (Iy, in %) Heterosis performance in industrial and yield components of sweet corn in a recurrent selection program, or for the improvement of populations per se.The highest ºBrix values for the performance per se (υ i ) were observed for the genotypes AF-429 (1.23 °Brix) and DO-04 (1.33 °Brix) (Table 4).For total ear weight (TEW), which indicates the productivity, the mean was 12934.009kg ha -1 (Tableo6).Pereira et al. (2009) found the value of 11136.510okgha -1 for this same variable in sweet corn.There were significant differences between treatments, total heterosis and h, indicating the presence of heterosis.The mean expression, however, was the same for all populations, ie, the mean TEW was the same in all F 1 s, or any possible difference between these responses was undetectable, due to the non-significance of h i and s ij .The reason may be that the genotypes did not differ in their gene frequencies and means and that the gene frequency dispersion was the same, randomly distributed among populations in the different loci.The h contributed positively to TEW, with a parent yield of approximately 6798.416 kg ha -1 (Table 4), indicating that the mean yield of F 1 was higher, corresponding to 88.91% of the parental mean (Table 3).Lemos et al. (2002) worked with sweet corn lines and their hybrids, and found heterosis values in the crosses for TEW, which ranged from -26.43% to 117.51% of the parental mean.

Table 5 .
Estimates of the mean (μ), the variety effects (v i ), mean heterosis (h),variety heterosis (h i ), genotypic effect (g i ) and specific heterosis (s ij ) for the traits ear height (EH, in cm) and plant height (PH, in cm) HS2-2104; *** S ij =values of specific heterosis in the lower half of the table refer to the variable EH, and values in the upper half to PH.