Materials and Methods

Six sugary lines and seven sugary enhancer lines (Table 1) were used as the parental lines of 34 sugary × sugary enhancer hybrids with reciprocals (the hybrids are shown in Table 2). All but one of the sugary lines come from three historically important open-pollinated cultivars: Evergreen, Country Gentleman, and Golden Bantam, that have made the greatest contribution to modern sugary germplasm. The allele se1 was originally identify in IL667a (Gonzales et al., 1974) and quickly incorporated into many breeding program (Tracy, 2001). Except IL677a, no other original source for the se1 allele has been reported and so probably all existing inbred lines with the se1 allele derive from IL667a. Four of the sugary enhancer inbreds used in the present work are related to IL667a (Rhodes et al., 1982) and the two lines with unknown pedigree are probably related to IL677a also.

Table 1. Name, pedigree, and origin of the sugary and sugary enhancer parental inbred lines of the 34 sugary × sugary enhancer hybrids of sweet corn grown in three environments.

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Table 2. Monthly average temperature, mean of the minimum daily temperature, and rainfall during the maize growing cycle in Pontevedra and Pontecaldelas (Spain), the locations where the experiments were carried out.

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The hybrids were evaluated in a randomized complete block design with two replications during 1999 and 2000 at Pontevedra, Spain (Environments 1 and 2, respectively) and during 1999 at Pontecaldelas, Spain (Environment 3). Pontevedra (lat. 42°24′ N, long. 8°38′ W) is at 20 m and Pontecaldelas (lat. 42°23′ N, long. 8°32′ W) at 300 m above sea level with cool and wet springs and a short summer, conditions similar to those found in many areas of western Europe (Table 2). The soil is acid sandy loam and the agronomic handling was done according to the local practices. Each single-row experimental plot consisted of 20 hills with two kernels per hill. The rows were spaced 0.8 m apart and hills 0.21 m apart. All hills were thinned to one plant to achieve a final plant density of ≈60,000 plants/ha. All the seed for the experiment had been obtained the previous year in the same location.

Two traits related to stand establishment, namely emergence and early vigor, were measured on each plot. Emergence was measured as the percentage of emerged plants. Early vigor was measured on a subjective scale from 1 (very poor) to 9 (very good) at the five-leaf stage. Silking date was computed as the number of days from planting to 50% of plants showing silks. Plant height, ear length, ear diameter, row number, kernel depth, and kernel width were measured on five plants taken from competitive adult plants in each plot. Yield was measured in each plot and adjusted at a kernel moisture of 140 g·kg⁻¹ H₂O. Five trained panelists, who tested three ears per plot, determined table quality, which can be defined as the overall judgment on pericarp hardness, texture, and flavor. The soluble solid content of the same ears was measured with a hand refractometer.

A combined analysis of variance over environments was performed for each trait. For each hybrid, the difference between reciprocals was tested by a t test using the se from the combined analysis of variance. For each trait, we made the comparison “crosses with the sugary line as the parental seed” versus “crosses with the sugary enhancer as the parental seed” (for simplicity, we name this comparison sugary versus sugary enhancer). The same comparison was made for each subgroup of reciprocal hybrids with a common sugary inbred line. The components of the sum of squares for crosses resulting from these comparisons and for crosses × environment resulting from the interaction of these comparisons with environment were computed and tested by an F test. For each trait, Pearson's coefficient of correlation between the mean reciprocal difference of the hybrids of each inbred line (general reciprocal difference of the line) and the per se value of the inbred line was calculated to control if the well-performing parents are more adequate as the seed parent of the crosses. Finally, to study the relationship between the reciprocal effects of the different traits, Pearson's coefficients of correlation between the reciprocal effects of each pair of traits were calculated. All analyses were performed using SAS (Version 9.1; SAS Institute, Cary, NC).

Results and Discussion

Significant reciprocal differences were found in many of the hybrids for the agronomic traits emergence, early vigor, yield, and silking date (Table 3). The traits measured on the earlier stages of the plant development are expected to be more affected by reciprocal effects because both endosperm and pericarp play an important role by protecting the embryo and by feeding the seedling. As our results indicate, these effects can persist during all plant development, which is in accordance with Fakorede and Agbana (1983) and Ajala and Fakorede (1988) who found that early vigor is positively correlated with silking date and yield. Hansen and Baggett (1977) also reported significant reciprocal effects for a number of traits in sweet corn, but Tracy (1990) did not. It is important to stress that those experiments were carried out in one and two environments, respectively, whereas our study was carried out in three different environments and, unlike the other studies, it included not only su1 lines, but also su1se1 lines. For the other agronomic traits measured in this study, the reciprocal effects were not important because they were only significant in five or less of the hybrids (Table 3).

Table 3. Differences between reciprocal means (F₁ with the sugary inbred as seed parent minus F₁ with the sugary enhancer inbred as seed parent) for 12 characters of 34 sugary × sugary enhancer sweet corn hybrids grown in three environments.

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The comparison sugary versus sugary enhancer was significant for ear diameter and number of rows, but not significant for the rest of the agronomic traits (Table 4). The average difference was equal to 0.3 mm and 0.3 rows, respectively, the crosses with the sugary1 line as the seed parent having the higher value for both traits. There was a significant interaction between the comparison sugary versus sugary enhancer and environment for emergence and ear length. For emergence, in Environment 3, the average value of the crosses was 7% higher (significant at P ≤ 0.01) when the sugary line was the seed parent, but only 2% higher in the other environments (not significant at P ≤ 0.05). Also, for early vigor, the average value of the crosses was 0.25 higher (significant at P ≤ 0.1) when the sugary line was the seed parent in Environment 3, but only 0.10 higher in the other environments (not significant at P ≤ 0.1). For other traits such as ear length and yield, the difference between the sugary versus sugary enhancer line as the seed parent of the crosses was also significant at P ≤ 0.01 and P ≤ 0.1, respectively, in Environment 3. Therefore, our results suggest that for several agronomic traits, like for example emergence and early vigor, the sugary lines tend to be better than the sugary enhancer lines as the seed parent of the crosses, but the difference is only appreciable for some environmental conditions.

Table 4. Mean squares from the combined analyses of variance for 12 characters of 34 sugary × sugary enhancer hybrids of sweet corn with reciprocals grown in three environments.^z

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The reciprocal sugary × sugary enhancer hybrids differ in the endosperm (1 versus 2 se1 alleles) and in the pericarp (none versus 2 se1 alleles) genetic constitution, but not in the genotype of the embryo. Therefore, according to our results, an additional se1 allele in the genetic constitution of the endosperm and a sugary enhancer pericarp do not produce a considerable effect on emergence or early vigor, except for particular environmental conditions. However, large differences were found by Ordas et al. (2008) for emergence and early vigor between sugary and sugary enhancer hybrids. The difference between the pericarps of sugary and sugary enhancer hybrids is equal to that found between the pericarps of reciprocal sugary × sugary enhancer hybrids, but the endosperms of sugary and sugary enhancer hybrids differ by three se1 alleles, not by one like the reciprocal sugary × sugary enhancer hybrids. Furthermore, unlike reciprocal sugary × sugary enhancer hybrids, the sugary and sugary enhancer hybrids differ in the genetic constitution of the embryo (none versus two se1 alleles).

Although on average the hybrids with the sugary lines as the seed parent tended to have better emergence and early vigor than the hybrids with the sugary enhancer lines as the seed parent, the advantage of the sugary lines was strongly dependent on the particular inbred lines involved in the sugary × sugary enhancer crosses. Thus, for the hybrids with the sugary inbreds EP58, EP60, and I5125 as seed parents, the emergence was on average 15% higher (significant at P ≤ 0.01) than the emergence of the same hybrids with a sugary enhancer inbred as seed parent, but for the reciprocal hybrids with the sugary inbreds EP59 and V7726 as seed parents, the emergence was on average, ≈10% lower (significant at P ≤ 0.01). Similarly, for early vigor, the hybrids with the sugary inbreds EP58, EP60, EP62, and I5125 as seed parents had scores 0.7 higher (significant at P ≤ 0.05) than the same hybrids with a sugary enhancer inbred as the seed parent, but the reciprocal hybrids with the sugary inbred EP59 as the seed parent had scores 1.2 lower (significant at P ≤ 0.05).

The correlation coefficient between the performance and the general reciprocal difference of the inbreds was significant for early vigor (r = 0.72, P ≤ 0.01), but not significant for the rest of agronomic traits. These results agree with those of Mann and Pollmer (1981 and references therein) who found significant correlations between reciprocal effects and performance of the lines only in a limited number of agronomic traits. In conclusion, prediction of whether a line should be used as the male or the female parent based in the performance of the line seems to be possible only for early vigor, but even for this trait, the coefficient of determination is not too high (≈0.5) and so reciprocal crosses could be necessary to detect reciprocal differences.

There was a significant relationship between the reciprocal effects of different agronomic traits (Table 5). As expected, ear diameter was positively correlated with row number and kernel depth and yield was correlated to ear length. There were significant correlation coefficients between traits measured at early development stages (emergence and early vigor) and traits of later stages such as silking date or yield, which is in accordance with Revilla et al. (1999) who found a significant correlation between the reciprocal effects of early vigor and silking date.

Table 5. Correlation coefficients between the reciprocal effects of each pair of traits obtained from the evaluation of 34 sugary × sugary enhancer hybrids of sweet corn with reciprocals grown in three environments.

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For the quality traits table quality and soluble solids, the reciprocal effects were significant in three and one of the hybrids, respectively (Table 3), indicating that in the sugary × sugary enhancer hybrids, the reciprocal effects are, in general, not important for quality traits. The comparison sugary versus sugary enhancer was not significant for the quality traits (Table 4) and, therefore, which kind of line, either sugary or sugary enhancer, is the seed parent of the cross is irrelevant for quality traits.

Regarding the correlations between the reciprocal effects of agronomic and quality traits, what is remarkable is the small, but significant and positive, correlation of emergence and vigor with table quality. The hybrids with the lowest stand establishment are expected to be more negatively affected by stress conditions during the early stages of the development and those negative effects could persist until the adult phase and affect the plant ability to produce a high-quality kernel. Reciprocal cross differences have been reported in the case of sweet corn only for agronomic traits (Hansen and Baggett, 1977; Tracy, 1990) and, therefore, no data are available to compare with our results. From our results, we could conclude that if a line is selected as the seed parent because the hybrid has better emergence and vigor, the table quality of the hybrid will be also better or at least will not be worse.

The main conclusions of this work are first, reciprocal differences in sugary × sugary enhancer hybrids are common for some agronomic traits such as emergence and early vigor, but not for quality traits; second, the sugary lines as seed parents of the crosses tend to have on average a favorable effect on emergence, early vigor, and other agronomic traits, but this is only appreciable in some environmental conditions; third, the difference between the sugary and sugary enhancer lines as seed parents of the crosses is strongly influenced by the genetic background.