Heterotic patterns in sweet corn are weakly defined. Most sweet corn inbreds are descended from three open-pollinated cultivars: `Golden Bantam', Stowell's Evergreen', and `Country Gentleman'. Heterotic and phylogenetic relationships among these three cultivars and others are not clearly known. This investigation was designed to investigate the heterotic patterns among some historically important open-pollinated sweet corn cultivars: `Country Gentleman', `Golden Bantam', `Lindsey Meyer Blue', `Stowell's Evergreen', `Howling Mob', and `Pease Crosby'. The 15 possible hybrids from the diallel cross plus the 6 parents were grown in midspring and late summer plantings. Heterosis and combining ability effects were estimated for 13 traits. Hybrid × planting date interactions were significant for most of the main traits, hence, planting dates were analyzed separately. Average midparent heterosis for grain yield was 29.17% in the first planting date and 57.04% in the second planting. Midparent heterosis for yield and plant height were highest for hybrids with `Country Gentleman' as a parent. `Stowell's Evergreen' when crossed to `Pease Crosby', `Lindsey Meyer, and `Golden Bantam' exhibited high heterosis. The two late-maturity cultivars `Country Gentleman' and `Stowell's Evergreen' had higher general combining ability than the four early-maturity cultivars for most traits. Specific combining ability was seldom significant. Yield of `Country Gentleman' hybrids averaged over all crosses and planting dates was the highest. These data indicate a strong heterotic pattern—`Country Gentleman' × `Pease Crosby', `Golden Bantam', and `Lindsey Meyer Blue'—and a weaker one—`Stowell's Evergreen' × `Pease Crosby', `Golden Bantam', and `Lindsey Meyer Blue'.
Pedro Revilla and W.F. Tracy
Pedro Revilla and W.F. Tracy
Sweet corn is one of the most important vegetable crops in the United States, however the morphology and phylogeny of open-pollinated sweet corn cultivars has not been studied. Fifty eight open-pollinated sweet corn cultivars were characterized with thirty-four descriptors to provide information for breeders interested in broadening the genetic base of sweet corn. Principal component analysis and cluster analysis were performed to classify sweet corn cultivars based on morphology. Also, relationships among morphological variables in this set of cultivars were determined. The general ordination of cultivars followed an axis representing earliness, and plant, leaf, and tassel size, while ear and kernel attributes were less variable. The morphological variability among all of the widely used sweet corn cultivars, except `Country Gentleman', was not greater than the variability found among the `Golden Bantam' strains. Based on morphology, 52 of the cultivars could be considered as one race, which we propose be called `Northeastern Sweets'. These may be a subset of the race `Northern Flint'. Five of the remaining cultivars are from the north-central or southwestern United States and may represent races from those areas. The sixth cultivar is `Country Gentleman', a commercially important sweet corn cultivar. Due to the importance of `Country Gentleman' and the introgression of nonsweet germplasm into modern sweet corn, we believe that sweet corn should be defined based on its use as a vegetable and on the presence of one or more genes that increase sugar levels in the endosperm.
Bernardo Ordás, Rosa A. Malvar, Amando Ordás, and Pedro Revilla
Reciprocal effects in sugary × sugary enhancer hybrids of sweet corn (Zea mays L.) have been only reported for sugar content and in a very limited number of hybrids and have not been determined for agronomic traits. By evaluating 34 sugary × sugary enhancer hybrids with reciprocals in three environments, the main objectives of this work were, for agronomic and quality traits, to determine the presence or absence of reciprocal differences in the sugary × sugary enhancer hybrids, to study the interest of using sugary inbreds as seed parents of the sugary × sugary enhancer hybrids, and to determine if reciprocal differences are interacting with different genetic backgrounds and different environments. For agronomic traits as emergence, early vigor, and silking date, significant (P ≤ 0.05) reciprocal differences were found in many of the sugary × sugary enhancer hybrids, but for quality traits, significant (P ≤ 0.05) reciprocal differences were only found in a few hybrids. The sugary lines as seed parents of the crosses tended to have on average a favorable effect on agronomic traits, but this was only considerable in some environmental conditions. The difference between the sugary and sugary enhancer lines as seed parent of the crosses was strongly influenced by the genetic background.
Bernardo Ordás, Pedro Revilla, Pilar Soengas, Amando Ordás, and Rosa A. Malvar
The better emergence and seedling vigor of sweet corn (Zea mays L.) hybrids homozygous for the gene sugary1 (su1) make them more suitable for cultivation under European Atlantic conditions (cold, wet spring) than those homozygous for other traits. Elite sweet corn inbreds homozygous for both su1 and sugary enhancer1 (se1) could improve the table quality of su1 hybrids. The su1se1 inbreds for improving su1su1 hybrid performance can be chosen in several ways. The aim of this paper was to identify donors among su1se1 inbreds that might improve the quality of su1 hybrids. Eight su1se1 inbreds were crossed with eight su1 inbreds that were parents of fifteen su1 hybrids. Hybrids and inbreds were cultivated next to one another in two locations in northwestern Spain in 1999 and 2000. Several possible estimators for identifying su1se1 inbred donors with favorable alleles lacking in the su1 hybrid were determined. These estimators included the relative number of favorable alleles present in the donor but absent in the hybrid (μǴ), predicted three-way cross (PTC), minimum upper bound (UBND), net improvement (NI), probability of the net gain of favorable alleles when there is complete dominance (PNGg), probability of the net gain of favorable alleles when there is partial dominance or epistasis (PNGceg), and general combining ability (GCA). μǴ and NI were chosen for improving hybrid table quality. These estimators indicate that table quality and other traits of su1 hybrids can be improved by using germplasm from the su1se1 inbred lines. The best donor of quality for most of the hybrids was the inbred line IL731a.
Pedro Revilla, Pablo Velasco, María Isabel Vales, Rosa Ana Malvar, and Amando Ordás
Field corn (Zea mays L. var. mays) cultivar heterosis could improve sweet corn (Zea mays L. var. rugosa Bonaf) heterotic patterns. Two Spanish field corn (Su) and two sweet corn (su) heterotic patterns have been reported previously. The objective of this study was to determine which sweet × field corn crosses could be used to improve sweet corn heterotic groups. A diallel among three sweet corn cultivars (`Country Gentleman', `Golden Bantam', and `Stowell's Evergreen') that are representative of the variability among modern sweet corn cultivars, and three field corn synthetic cultivars [`EPS6(S)C3', `EPS7(S)C3', and `EPS10'] representing the heterotic patterns involving Spanish field corn, was evaluated for 2 years at two locations in northwestern Spain. Differences in heterosis effects (h jj') and average heterosis (h) were significant for all traits except grain moisture. Differences for cultivar heterosis (h j) and specific heterosis (s jj') were significant for grain yield, plant height, and kernel row number. `EPS6(S)C3' had lower s jj' for yield in crosses to `Golden Bantam' than to `Stowell's Evergreen', while `EPS7(S)C3' had higher s jj' in crosses to `Golden Bantam' than to `Stowell's Evergreen'. The best crosses to establish enhanced sweet corn heterotic patterns involving Spanish maize would be `Golden Bantam' × `EPS6(S)C3' and `Stowell's Evergreen' × `EPS7(S)C3'. New sugary 1 cultivars would require preliminary cycles of intrapopulational recurrent selection for agronomic performance and flavor prior initiating an interpopulational recurrent selection program to enhance heterosis.
Pablo Velasco, Rosa A. Malvar, Ana Butrón, Pedro Revilla, and Amando Ordás
Pink stem borer (Sesamia nonagrioides Lef.) is one of the most important insect pests of corn (Zea mays L.) in southern Europe. The objectives of this work were to determine the level of resistance in different sweet corn inbreds and to identify sources of resistance to ear feeding by the pink stem borer. Twenty-eight sweet corn (su1 and su1se1) inbreds and four resistant field corn (Su1Se1) inbreds were evaluated for ear resistance at different sowing dates, under two methods of artificial infestation. There were significant differences between infestation methods for ears with damaged grain, husks, cobs, and shanks. The inbred×infestation method interaction was significant for general appearance of the ear. The most resistant inbreds were identified by using mean comparisons and principal component analysis of ear damage traits. All inbreds were damaged. Hence, resistance was incomplete and in need of improvement. EP59, H3, I5125, IL767b, and V7726 were the most resistant sweet corn inbreds, which did not differ significantly from A635, the most resistant field corn inbred. General appearance of the ear appears to be a good indicator of pink stem borer resistance and can be used in preliminary evaluation. Variability exists in the resistance of these sweet inbreds to the pink stem borer and the use of field corn inbreds may not be necessary in the improvement of resistance, although further research is needed to determine if the sources differ in the pertinent genes conferring resistance.
Pedro Revilla, William F. Tracy, Pilar Soengas, Bernardo Ordás, Amando Ordás, and Rosa Ana Malvar
The genes sugary1 (su1) and shrunken2 (sh2) are commonly used to produce sweet and super-sweet corn (Zea mays L.), respectively. In this work we compare corn borer [european corn borer (ECB) (Ostrinia nubilalis Hbn.) and pink stem borer (PSB) (Sesamia nonagrioides Lef.)] susceptibility in seven pairs of su1 and sh2 near-isogenic sweet corn inbreds (101t, C23, C40, C68, Ia453, Ia5125, and P39) and the relationship between corn borer resistance and vegetative phase transition. The seven pairs of near-isogenic inbreds were evaluated under corn borer infestation during 3 years in northwestern Spain. Differences among inbreds were significant for most of the traits, although resistance was partial. Ia5125su1 and C40su1 were the most resistant inbreds. Differences between a few pairs of near-isogenic su1 and sh2 strains were significant for some vegetative phase change and corn borer damage-related traits. Generally su1 strains flowered earlier, had a shorter juvenile phase, fewer PSB, and more ECB larvae than sh2 strains. However su1 and sh2 strains did not differ significantly for most traits related to phase transition and corn borer damage; notably ear damage was not significantly different between su1 and sh2 strains. These results suggest that theoretical and practical results of sweet corn (sugary1) breeding for corn borer resistance could be capitalized for super-sweet corn (shrunken2) breeding.