Partial diallel and potential of super sweet corn inbred lines bt 2 to obtain hybrids

The aims of this study were to determine the potential of S4 super sweet corn inbred lines for hybrid synthesis, identify the predominant types of gene action and correlations among different traits, significant for breeding programs. The 81 hybrids obtained from a partial diallel 9x9 and three checks were evaluated. A complete randomized block design, with three replicates, and two sowing seasons was used. We could notice significant hybrid effects, general combining ability (GCA) of GI and GII groups and specific combining ability (SCA) in relation to evaluated traits, highlighting the existence of hybrids with superior performance and the expression of additive and non-additive effects. The inbred lines: L1, L3, L6 and L9 (GI) and L1’, L7’ and L9’ (GII) showed the best GCA and SCA estimates, being present in the nine selected hybrids with superior and competitive performance in relation to the checks. The estimated correlations indicate that, for a breeding program aiming to increase grain productivity, evaluating, at least, the dehusked ears, prioritizing genotypes with larger ear diameters and longer ear lengths is important.


Research
Hortic. bras., Brasília, v.37, n.3, July -September 2019 S weet corn is originated from recessive genetic mutations of common corn, which block starch synthesis, increasing endosperm sugar concentrations. Homozygous genotypes for shrunken (sh 1 and sh 2 ) or brittle genes (bt 1 and bt 2 ) present higher sugar content, being classified as super sweet, whereas other mutant genotypes are classified as sweet corn (Teixeira et al., 2013).
Super sweet corn is considered a vegetable and it is dedicated exclusively for human consumption. It has a high nutritional value, being consumed in natura or processed by vegetable canning industries in several countries (Kwiatkowski & Clemente, 2007;Teixeira et al., 2013;Luz et al., 2014). However, in Brazil, super sweet corn is basically consumed industrialized, commonly as canning corn grains, and the consumers themselves have no idea that these grains are a special type of corn.
Hybrids of super sweet corn inbred lines are cultivars which best meet canning industry demands, due to its uniformity and agronomic performance (Kwiatkowski & Clemente, 2007;Luz et al., 2014). In Brazil, these days, few cultivars of sweet and super sweet corn seeds are available to farmers, only 65 cultivars are registered by the Ministry of Agriculture, Livestock and Food Supply (Brazil, 2018). Therefore, developing new superior hybrid combinations is essential for expansion of super sweet corn production in Brazil (Santos et al., 2014).
Breeding programs of super sweet corn, which aim to obtain hybrids, develop many inbred lines, not all these lines will produce hybrids with high agronomic potential, though. Thus,
agronomic and genetic evaluations of these inbred lines based on their hybrid combination performance is extremely important (Kashiani et al., 2014). Diallel analysis is used as a tool which provides useful estimates to select more promising lines for synthesis of hybrids and to understand the magnitude of the effects which determine genetic traits (Cruz et al., 2004;Kwiatkowski et al., 2011;Worrajinda et al., 2013).
Estimates of combining ability of sweet corn inbred lines obtained using diallel analysis have been studied considering different agronomic traits, allowing selecting superior hybrid combinations and understanding additive and non-additive genetic effects when determining these traits (Kwiatkowski et al., 2011;Solomon et al., 2012;Rice & Tracy, 2013;Worrajinda et al., 2013).
Another important element for breeding is to understand correlation among agronomic traits of interest, since this knowledge can help out select more efficient selection processes, allowing selecting different traits simultaneously, increasing genetic gains in relation to low heritability traits (Entringer et al., 2014;Kashiani et al., 2014).
Given the above, the aims of this study were to determine potential of super sweet corn inbred lines which carry the gene brittle-2 for synthesis of hybrids, identify predominant types of gene actions in agronomic traits important for breeding and the association among these traits.

MATERIAL AND METHODS
Eighty one super sweet corn hybrids were produced, using partial dialled crosses of two groups of nine S 4 inbred lines, developed by Universidade Estadual de Londrina (UEL) Corn Breeding Program, homozygous for brittle-2 gene. These inbred lines were obtained from the backcross among elite common corn inbred lines with two super sweet corn populations, to introduce bt 2 gene, with later selffertilization. Thus, the inbred lines were separated into two groups, according to a previous knowledge of the performance of the elite crossbred lines and the source population for gene introduction.
The 81 hybrids and three checks (the synthetics ST0509A and ST2109B, developed at Farm School of UEL, and the hybrid Tropical Plus, from Syngenta Seeds), were evaluated at UEL, in the crop season 2013/2014, in two sowing dates (October 28, 2013 andNovember 28, 2013), both without artificial irrigation.
Climate data were collected at a local weather station. From the first sowing date until havest time, a total rainfall was 305 mm, regular rainfall distribution and maximum temperatures from 30 to 36 o C and minimum temperatures from 13.2 to 20.5 o C. Total rainfall of the second sowing date until harvest was 208 mm, being the last rain observed 6 days before flowering, with a total of only 5 mm, maximum temperatures from 30.8 to 38 o C and minimum temperatures from 17 to 20.5 o C.
The experimental arrangement was in complete randomized blocks, with three replicates and simple row plots, 4.00 m long, spacing 0.80 m between rows and 0.20 between plants, in two sowing dates.
Conventional soil preparation was carried out using plowing and harrowing, and agronomic standard procedures were carried out according to technical recommendations for the crop.
I n o r d e r t o a v o i d p o s s i b l e contaminations by common corn pollen, the experiments were isolated from other corn plantations. The check Tropical Plus, homozygous for the sh 2 gene, had its tassels removed before flowering, to avoid the conversion of tested hybrids into common corn.
Harvest was done manually as the ears reached kernel milky stage (green corn), when the grains of the ear of each plot presented 70 to 80% water content, considered a suitable content for in natura consumption and for canning (Kwiatkowski & Clemente, 2007).
The evaluated traits were a) days to flowering (DF, in days): considering as flowered plot when 50% of plants showed stigma-style measuring at least 1-cm length and one third of tassels releasing pollen; b) plant height (PH, in cm): average height of three plants of each plot, measured from ground level to the flag leaf insertion; c) ear height (EH, in cm): average ear height of three plants of each plot, measured from ground level to the superior ear insertion; d) husked ear yield (HEY, in t ha -1 ); e) dehusked ear yield (DEY, in t ha -1 ); f) grain yield at green corn stage (GY, in t ha -1 ); g) ear length (EL, in cm): average length of five ears of each plot; h) ear diameter (ED, in cm); average diameter of five ears of each plot; i) number of grain rows (NR): grain rows in five ears of each plot were counted; j) total soluble solids (TSS, in %): measured with a digital refractometer, using a sample of 0.3 mL juice extracted from the grain mixture.
In the first and second sowing dates, stand averages of 20.55 and 20.16 plants per plot were obtained, respectively, being yields corrected to an optimum stand of 20 plants per plot, using the methodology suggested by Vencovsky & Barriga (1992), and extrapolated to tons per hectare (t ha -1 ), with a stand of 62500 plants per hectare.
The analyses of variance were performed using program SAS (2002) (Statistical Analysis System) and Scott & Knott clustering test was performed using GENES program (Cruz, 2013).
Individual analyses of variance for each trait were done with the decomposition of treatment effects on effects of checks, hybrids and contrast hybrids vs checks, considering treatment effects as fixed. Degrees of freedom were decomposed through diallel analysis in general combining ability of groups GI and GII (GCA-I and GCA-II), and specific combining ability (SCA). Griffing method (1956), adapted to partial diallel crosses involving only F1 generations, was used to obtain effect estimates of GCA-I, GCA-II and SCA, using minimum square method (Cruz et al., 2004).
Genotype averages for different traits, taken two by two (X and Y), were used to estimate phenotypic correlations and, significance of Pearson correlation estimates were evaluated using t statistics, at 5% significance.

RESULTS AND DISCUSSION
The trials showed adequate experimental accuracy for most evaluated traits, when compared to other experiments with super sweet corn (Kwiatkowski et al., 2011;Santos et al., 2014) (Table 1). The average squares for treatments, hybrids and their unfolding in GCA-I, GCA-II and SCA, for the traits evaluated in two sowing dates, were significant, with exception of "days to flowering" in the second sowing date (Table 1). These results show different performances of the evaluated genotypes and that superior hybrid combinations exist, with inbred lines contributing differently for the performance of these hybrids, being possible to observe superior specific combinations of inbred lines, not just explained by their respective general combining abilities (Cruz et al., 2004).
Significant effects found in GCA and SCA for all traits, in two sowing dates, show that both additive and non-additive effects were important for genetic control of studied traits ( Table  1). Similar results for ear yield, grain yield, plant height and total soluble solids were obtained by Lemos et al. (2002); Bordallo et al. (2005), Solomon et al. (2012, Rice & Tracy (2013) and Suzukawa et al. (2018). However, for ear height and ear diameter, Solomon et al. (2012) reported that additive gene action had greater importance in relation to non-additive effects, and for TSS, the authors verified no significant difference for GCA and SCA in diallel crosses evaluated, whereas Yuwono et al. (2017) verified significant difference of SCA for TSS.
In the first sowing season, significant differences of checks for days to flowering, plant height and ear diameter were observed, but in the second sowing season were significant for dehusked ear yield, total soluble solids, days to flowering, ear height, ear diameter and number of grain rows, indicating that the checks did not show uniform behavior for these traits (Table 1).
Using Scott-Knott clustering average test, we verified that synthetics ST0509A and ST2109B showed potential for genetic breeding and being also competitive in relation to the hybrid Tropical Plus for productivity. This hybrid check exceeded ST0509A for husked ear yield only in the first sowing date and ST2109B for dehusked ear yield in the second sowing date (Table 2).
Contrasts between general averages of hybrids and averages of checks were significant for dehusked ear yield, grain yield, days to flowering, ear height and number of grain rows, in both sowing dates and, for total soluble solids, length and diameter of the ears, only in the second sowing date (Table 1). We could also verify superior average performance of hybrids in relation to the average of checks for these traits, with exception of number of grain rows ( Table 2).
Even without artificial irrigation and lack of rain in the second sowing season, which led to a sensitive genotype performance reduction, the averages obtained by the selected hybrids met the requirements presented by Pereira Filho et al. (2002) and Souza et al. (2013): husked ear yield above 12 t ha -1 ; minimum of 14 grain rows per *and ns= significant and non-significant at 0.05 probability using F test, respectively; HEY= husked ear yield (t ha -1 ); DEY= dehusked ear yield (t ha -1 ); GY= green corn grain yield (t ha -1 ); TSS= total soluble solids (%); DF= days to flowering; PH= plant height (cm); EH= ear height (cm); EL= ear length (cm); ED= ear diameter (cm); NR= number of grain rows. *significant at 0.05 probability by t test. Averages followed by same lowercase letters in the column belong to the same group by Scott-Knott test, at 0.05 probability. HEY= husked ear yield (t ha -1 ); DEY= dehusked ear yield (t ha -1 ); GY= green corn grain yield (t ha -1 ); TSS= total soluble solids (%); DF= days to flowering; PH= plant height (cm); EH= ear height (cm); EL= ear length (cm); ED= ear diameter (cm); NR= number of grain rows.
Partial diallel and potential of super sweet corn inbred lines bt 2 to obtain hybrids ear; length and diameter of ears above 15 and 3 cm, respectively. Husked ear yield, dehusked ear yield and grain yield at green corn stage presented a strong positive association among them. These yield traits also presented positive correlations with ear diameter, ear length and number of grain rows (Table 2). Kashiani & Table 3. Estimates of general combining ability of inbred lines of groups GI (ĝ i ) and GII (ĝ j ) for different traits, evaluated in two sowing seasons (S1 and S2). Londrina, UEL, 2013/2014.

PH (cm) EH (cm) EL (cm) ED (cm) NR S1
S2 S1 S2 S1 S2 S1 S2 S1 S2 General combining ability of inbred lines of group I (gi) ĝ L1 -1.8 -6.9 1. Saleh (2010), Entringer et al. (2014) and Nardino et al. (2016) also observed positive correlations between husked and dehusked ear yield and ear length, ear diameter and number of grain rows per ear. Kashiani & Saleh (2010) reported similar results: yield traits were negatively associated with days to flowering, showing that higher yield of super sweet corn was associated with greater values of earliness of these genotypes.
Grain yield is more strongly related to dehusked ear yield, with coefficients of determination of 69% and 81%, comparing with husked ear yield, 58% and 59%, in the first and second sowing dates, respectively (Table 2). Therefore, as the Brazilian market of super sweet corn intended mainly for the industrialization and production of canned green corn grains, the evaluations of yields should be done, at least, with dehusked ears since they present more coefficients of correlation, favoring the indirect selection for grain yield. However, in order to produce sweet corn aiming in natura market, evaluation of husked ear yield would be already sufficient.
The inbred lines with better estimates of general combining ability (ĝ i and ĝ j ) for yields were: L 1 , L 3 , L 6 and L 9 (GI) and L 1' , L 7' and L 9' (GII) ( Table 3). Among these inbred lines, favorable estimates of GCA were observed in L 3 for TSS; L 1 , L 6 , L 9 , L 7' and L 9' for flowering and height of plants; L 1 , L 6 , L 7' and L 9' for ear height; L 9 , L 1' and L 9 for ear length; L 1 , L 3 , L 6 , L 1' and L 7' for number of grain rows (Table  3). These favorable estimates of GCA highlighted higher accumulation of favorable alleles for these traits (Cruz et al., 2004;Kwiatkowski et al., 2011). In their study on diallels of super sweet corn inbred lines, Elayaraja et al. (2014) and Suzukawa et al. (2018) highlighted that it is difficult to obtain inbred lines with good general combining ability for productivity and total soluble solids, this trend being observed in this study, with an only exception of L 3 inbred line.
SCA (s ij ) can be estimated for a given trait through the model s ij = HS ij -(m+g i +g j ) using estimates of general average of diallel hybrids (m), average individual hybrid performance (HS ij ) ( Table 2) and GCA estimations of each inbred line (Table 3). The best estimates of SCA, in two sowing seasons, for yield were observed in eight selected hybrids, with an exception of HS 77' which presented negative values of SCA for yield in the first sowing season. Besides favorable estimates of SCA, for hybrid seed yield, it is important that at least one hybrid of inbred lines presents high estimates of favorable GCA for yield. Among selected hybrids, only HS 76' did not show any inbred lines containing favorable estimates of GCA for yield.