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a International Maize and Wheat Improvement Center (CIMMYT), P.O. Box MP163, Mount Pleasant, Harare, Zimbabwe
b Pioneer SARL, 24 Rue du Moulin, F-68740 Nambsheim, France
* Corresponding author (K.Pixley{at}cgiar.org)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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for grain yield and protein concentration in grain were largest (indicating least stability) for single-cross hybrids, followed by three-way, double-cross, and open-pollinated cultivars (OPCs), successively. The reverse trend was observed for endosperm modification score, suggesting that more homogeneous cultivars had greater stability for this trait. Additive main effects and multiplicative interactions (AMMI) analysis indicated that genotype x environment interaction effects for grain yield and endosperm modification score were different for hybrids than OPCs; certain environments favored either hybrids or OPCs. In conclusion (i) protein quality and endosperm modification score were always within expected values for QPM and (ii) tryptophan concentration in protein was the most stable trait, followed by protein concentration in grain, then endosperm modification score and finally grain yield.
Abbreviations: AMMI, additive main effects and multiplicative interactions model • CIMMYT, International Maize and Wheat Improvement Center • GEI, genotype x environment interaction effect • IPCA, Interaction principal component axis (from the AMMI analysis) • OPC, open-pollinated cultivar • QPM, quality protein (opaque-2) maize
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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QPM contains the opaque-2 mutation, which alters protein composition of the maize endosperm, resulting in increased concentrations of lysine and tryptophan (Mertz et al., 1964). Because of the 60 to 100% increase in concentration of these two essential amino acids, increased digestibility, and increased nitrogen uptake relative to normal-endosperm maize, the biological value (amount of N that is retained in the body) of QPM is about 80%, whereas that of normal maize is 40 to 57% (Bressani, 1992). QPM has about 90% the biological value of cow milk (National Research Council, 1988). There is considerable debate, mostly centered around the issue of whether malnutrition is most often caused by calorie or protein deficiency, about the potential impact of QPM on alleviating human malnutrition. There is ample evidence, however, that consumption of QPM may help alleviate human malnutrition problems in regions with maize-based diets (Bressani, 1992; Clark, 1978; Mertz, 1992; National Research Council, 1988). An alternative use of QPM is in feed rations for swine, poultry, or fish, where conventional sources of lysine, generally soybean meal or synthetic lysine, raise feed and hence production costs (Knabe et al., 1992; Lopez-Pereira, 1992).
Quality protein maize is the result of two decades of breeding work to overcome low yield, slow dry-down, ear rot, various other agronomic deficiencies, and the opaque endosperm phenotype associated with the original opaque-2 maize (Bjarnason and Vasal, 1992). Scientists at CIMMYT used backcross and recurrent selection techniques to convert several maize populations to opaque-2 and subsequently modify the undesirable traits associated with the mutation (Bjarnason and Vasal, 1992; Villegas et al., 1992; National Research Council, 1988). The number of genes involved in modifying the opaque phenotype of opaque-2 endosperm so that it is translucent and similar to that of normal maize is not known, but most reports indicate that inheritance is complex (e.g., Bjarnason and Vasal, 1992; Lopes and Larkins, 1996), and CIMMYT's experience is that several cycles of improvement are required to achieve satisfactory modification for most opaque-2 germplasm (Vasal et al., 1980). The result of this breeding work is QPM, which has superior protein quality, similar to opaque-2 maize, but resembles normal-endosperm maize both phenotypically and agronomically. The award of the 2000 World Food Prize to Drs. Villegas and Vasal recognized the magnitude of this achievement and has renewed global interest in QPM.
Recent research results have demonstrated the competitiveness for grain yield of QPM with the best normal maize cultivars in numerous tropical environments (Bjarnason and Vasal, 1992; Pixley and Bjarnason, 1993; Cordova and Pandey, 1999; Vergara et al., 2000). Little is known, however, about the stability of performance for QPM cultivars, particularly regarding endosperm modification.
The objective of this study was to compare grain yield, endosperm translucence (modification score), and protein quality traits, and determine whether single-cross, three-way, double-cross hybrids, and open-pollinated QPM cultivars differ in stability for these characters across diverse tropical environments.
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MATERIALS AND METHODS |
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Nine inbred lines (Table 1) were used to form 18 single-cross, 18 three-way, and 18 double-cross hybrids. The nine inbreds were the most elite white-endosperm, flint and dent, full-season, tropical QPM lines available at CIMMYT when the study was initiated in 1991. Five of the lines were from Population 62, but were derived from four different cycles of recurrent selection of this population. Similarly, the three inbreds from Population 63 originated from two different cycles of recurrent selection. The lines were selected because they were elite lines derived from different selection cycles of genetically broad-based populations, and because CIMMYT experience has shown that, for broad-based populations, intrapopulation interline hybrids can yield as much as interpopulation interline hybrids (Han et al., 1991; Vasal et al., 1992). The 54 experimental hybrids were random combinations among the nine parents, except that balanced or nearly balanced mating schemes were used such that each line was included in four single-cross, five to seven (average of six) three-way, and seven to nine (average of eight) double-cross hybrids. Balanced representation of the parent lines within each type of hybrid was necessary to allow valid comparisons among types of hybrids. Seed from reciprocal crosses were bulked for each hybrid.
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Field Methods
The 72 QPM cultivars were evaluated for grain yield at 13 environments, endosperm modification score at 10, and protein quality traits at nine sites from 1990 to 1993 (Table 2)
. Trials received management practices common (generally nonyield limiting) for each research station, except for the trial at Tlaltizapán during the 1991–1992 winter season, which was deliberately subjected to moderate drought stress. Trials were isolated from normal-endosperm maize by time of planting and by QPM border rows. The experimental design at each location was an alpha(0,1) lattice (Patterson et al., 1978), wherein each pair of treatments appeared together in a block once or not at all. Two replications were grown at each site. Plots were two 5-m rows spaced 75 cm apart and thinned to 53300 plants per hectare.
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Laboratory Methods
Protein content and quality were determined at the CIMMYT Cereal Quality Laboratory following procedures described by Villegas (1975) and Villegas et al. (1984). Briefly, whole-grain samples were finely ground, the resulting flour was defatted, and concentrations of nitrogen and tryptophan were colorimetrically determined for duplicate subsamples. Lysine concentration was not measured because the procedure was costlier than that for tryptophan and because lysine and tryptophan concentrations in the protein of o2 endosperm are significantly correlated (r = 0.85) (Hernandez and Bates, 1969). Grinding of whole-grain rather than endosperm tissue for analyses may have caused some bias due to pericarp pigments affecting colorimetric determinations (Villegas, 1975). This procedure has been used effectively to improve QPM germplasm at CIMMYT and seems justifiable in light of the cost savings relative to separation and analysis of endosperm tissue. Another advantage of analyzing whole-grain samples is that variability for embryo size, which can significantly affect protein quantity and quality of the grain (Bjarnason and Pollmer, 1972), may be exploited. Maize embryos from differing genetic backgrounds have relatively constant amino acid composition (Villegas, 1975), and we used only kernels of uniform size (avoiding those from the ends of the ears) in forming grain samples for protein analyses.
Endosperm modification score was assessed at the CIMMYT laboratory in Mexico by means of a random 100-kernel sample from 5 to 10 ears from each plot. Only full-size kernels, taken from the center of well-filled ears, were used. The kernels were sorted on a back-lit (candling) table and scored from 1 to 5, where 1 = completely modified (i.e., translucent, normal phenotype); 2 = 75% modified; 3 = 50% modified; 4 = 25% modified; and 5 = completely opaque. Bjarnason and Vasal (1992) published a photograph illustrating this scoring scheme. The endosperm modification score for a plot was the mean score for the 100-kernel sample.
Statistical Analyses
Data for individual locations were analyzed according to the lattice design and as randomized complete block experiments. Grain yields were adjusted using the covariate plant stand for Suwan, where stands were affected by water logging, and number of diseased seedlings for Tak Fa, where natural incidence of downy mildew (P. sorghi) caused barrenness of some plants. Across-location analyses used raw, lattice-adjusted, or covariance-adjusted means, according to which analysis resulted in greatest efficiency for each site. Cultivar effects were considered fixed and locations random in the analyses of variance.
Simple linear regression analyses were used to estimate Eberhart and Russel (1966) parameters for genotypic stability of cultivars across environments. Performance of the 10 check cultivars was averaged and used as an index of environmental potential for each trial site. Performance of each experimental cultivar was then regressed against this independent environmental index.
Pearson phenotypic correlation coefficients were calculated among protein concentration, protein quality traits, endosperm modification score and agronomic traits to elicit associations that might help explain environmental fluctuation (lack of stability) for these traits.
The additive main effects and multiplicative interactions (AMMI) model, which combines standard analysis of variance with principal component analysis (Zobel et al., 1988), was used to investigate the agronomic nature of genotype x environment interaction. The AMMI analyses were conducted using data from 9 to 13 sites (all sites with data for each trait), and for either all 62 QPM genotypes or for the four QPM cultivar types. Plots were prepared as described by Zobel et al. (1988). Pearson phenotypic correlation analysis was used to detect associations between agronomic traits and the first genotype x environment interaction principal component axis score (IPCA1) for grain yield, endosperm modification score, and protein concentration and quality measures.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Cultivar types did not differ for mean endosperm modification score (Tables 3 and 4), but statistically significant differences were measured among individual genotypes. Endosperm modification scores for the 62 experimental cultivars averaged across 10 locations ranged from 1.8 to 2.3 on the scale of 1 to 5. From a practical perspective, all of the experimental cultivars were on average (across sites) well modified, and generally similar in appearance to normal-endosperm maize.
Greater average protein concentration in grain for open-pollinated (94.7 g kg-1) relative to the hybrid cultivars (92.3–92.7 g kg-1) may have been an unintended result of emphasizing protein quality (tryptophan concentration in protein) during development of these CIMMYT QPM inbreds, coupled with the fact that whole-grain protein content and quality are generally negatively correlated with each other (Pixley and Bjarnason, 1993; also, see below). An alternative explanation is that the highest yielding cultivars, in this case hybrids, had lowest protein concentration because of the well-known negative association between yield and protein content of grain (e.g., Bhatia and Rabson, 1987). There were no large differences for tryptophan concentration in protein among cultivar types, although double-cross hybrids had slightly lower values for this trait than single-crosses and OPCs (P < 0.05).
Analyses of Variance
Variance of location, cultivar, and location x cultivar interaction effects was significant (P < 0.01) for all traits (Table 4). There was significant variation among cultivar types for all traits except endosperm modification score, and genotypes within each cultivar type varied for all traits except that three-way hybrids did not differ for grain yield, and open-pollinated cultivars did not differ for tryptophan concentration in protein. The lack of difference for tryptophan concentration in protein among open-pollinated cultivars may have been because only eight cultivars were evaluated or because genetic variation for this trait is generally small.
The location x cultivar variance was significant (P < 0.01) and accounted for 8% of total variation for grain yield, 35% for endosperm modification score, 23% for protein in grain, 23% for tryptophan in grain and 33% for tryptophan in protein (Table 4). Location x type of cultivar interaction, however, was significant only for yield and endosperm modification score. This indicated that cultivar types reacted similarly to changing environments for protein and protein quality traits, but at least one cultivar type responded differently than the others for grain yield and endosperm modification score.
The interactions of locations with cultivars within types were of particular interest to this study because comparison of these allowed inferences about the relative importance of location x cultivar interaction for each cultivar type. For grain yield, a clear trend of declining magnitude and probability of significance for variation of location x genotype interaction was observed between single-cross, three-way, double-cross, and open-pollinated cultivars (Tables 3 and 4). Location x genotype interaction was not significant for grain yield among open-pollinated cultivars, perhaps because their greater genetic heterogeneity provided "population buffering" (Allard and Bradshaw, 1964) against environmental variation. These results agree with theoretical expectations (Cockerham, 1961) and with experimental results for maize (Sprague and Federer, 1951; Eberhart and Russell, 1969; Weatherspoon, 1970) and sorghum (Patanothai and Atkins, 1974).
A pattern similar to that described above for grain yield (decreasing mean squares for interaction of locations x genotypes from single-cross to three-way, double-cross, and open-pollinated cultivars, successively), also indicated increased stability with increased genetic heterogeneity for protein concentration in grain (Tables 3 and 4). This pattern of increasing stability with increasing heterogeneity was observed also among hybrid types for tryptophan concentration in grain, but open-pollinated cultivars were intermediate between hybrid types for this trait. No pattern was observed for magnitude of genotype x location interaction within hybrid cultivar types for concentration of tryptophan in protein, and only OPCs had no significant variation for this interaction effect.
The trend in magnitude of location x genotype interaction mean squares was reversed for endosperm modification score in comparison to grain yield and protein concentration (Tables 3 and 4). Open-pollinated cultivars had the largest variance of interaction effects with environments, and can therefore be considered the least stable cultivar type for this trait. The OPCs may have been the least stable cultivar type for this trait because they represented less elite germplasm than the lines comprising the hybrids and might, for example, have fewer fixed (homozygous and homogeneous) favorable alleles for endosperm modification, be more susceptible to ear rots, have poorer husk cover or be sensitive to other site specific stresses that could affect kernel modification. Of 10 sites with endosperm modification data, five had no entries with a score of 2.5 or higher, three had three or fewer entries with a score of 2.5 (but below 3.0), Tak Fa had 12 entries with a score of 2.5, and Sete Lagoas had 20 entries with a score of 2.5 and two entries with a score of 3.0. No explanation was apparent for the higher endosperm modification score (more opaque phenotype) of several cultivars at Sete Lagoas. In general, however, endosperm modification score of all genotypes was within values expected for QPM.
Linear Regression Analysis of Stability
Slopes of linear regression (b) suggested that open-pollinated cultivars were least responsive of all cultivar types to environmental potential for grain yield and most responsive for endosperm modification score (Table 3). This can be interpreted as open-pollinated cultivars being the most stable for yield and least stable cultivar type for endosperm modification, which agrees with results from analyses of variance (Tables 3 and 4). Also consistent with results from analyses of variance, there were no apparent differences among cultivar types for b-values for protein content or quality of grain.
Despite theoretical objections that these parameters merely indicate appropriateness of the linear regression model for describing data (e.g., Lin et al., 1986), small sum of squares for deviations from regression (S2d) and large coefficient of determination (r2) have been widely cited as indicative of cultivar stability across environments (Eberhart and Russell, 1966, 1969; Patanothai and Atkins, 1974; Jensen and Smith, 1988). The average probability that S2d differs from zero (Table 3) quantifies the statistical significance of S2d values (Eberhart and Russell, 1966). For grain yield, trends in these linear regression parameters agreed with results from analysis of variance, indicating that open-pollinated cultivars were most stable, followed by double-cross, three-way, and finally single-cross hybrids (Tables 3 and 4). Results for protein and tryptophan content in grain also confirmed trends indicating a pattern of declining stability among cultivar types, particularly the hybrid types, with declining genetic heterogeneity. There was no clear trend for tryptophan content of protein, but r2 and S2d values hinted at greater stability of open-pollinated and double-cross relative to single-cross and three-way hybrids.
Small r2 values indicated that the linear regression model was not very suitable for analysis of endosperm modification score. This was likely because of the small range of environmental values for this trait, which indicated that endosperm modification score was quite consistent (i.e., stable) across environments. Nevertheless, a trend of increasing S2d suggested that open-pollinated cultivars were the least stable of the cultivar types, which agrees with results from analysis of variance.
It is important to note that, although several trends were apparent for stability parameters of genotypes within these cultivar types, on average, S2d values were not significantly different from zero for protein and protein quality traits. For endosperm modification score, S2d values were generally not significant or only significantly different from zero at P < 0.05. These results indicated that protein and tryptophan content of grain, tryptophan content of protein and endosperm modification were generally stable across environments.
Ideal QPM cultivars would respond to favorable environmental conditions and have above-average yield, protein content and protein quality at all sites (i.e., b = 1, high mean), while maintaining low endosperm modification score even at unfavorable sites (b < 1, low mean). Applying these criteria to the results summarized in Table 3, it seems that hybrids (especially double-crosses) were best for grain yield, hybrids (especially single- and three-way crosses) were best for endosperm modification, open-pollinated cultivars were best for protein content of grain, and all cultivar types were equally desirable for tryptophan content of protein.
Phenotypic Correlations among Traits
The negative association between grain yield and protein concentration in grain, and between tryptophan concentration in protein and protein concentration in grain (Table 5)
, are undesirable and very important for QPM breeders because they complicate simultaneous improvement of these traits. Conversely, lack of unfavorable correlation between endosperm modification score and other traits should facilitate QPM cultivar development efforts. We previously discussed similar results for another QPM study (Pixley and Bjarnason, 1993). Associations of agronomic traits with protein content and quality traits mostly reflected the well-known associations of grain yield with agronomic traits (e.g., severe ear rot reduces grain yield) and grain yield with protein traits (e.g., high yield is associated with lower protein content). It was important to document that endosperm modification score was not significantly associated with any of the agronomic traits.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication October 22, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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for grain yield, endosperm modification score, and protein traits for across-location analyses of a trial of QPM hybrids and open-pollinated cultivars.