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Published in Crop Sci. 44:1997-2005 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Combining Abilities of Quality Protein Maize Inbreds

S. Bhatnagar, F. J. Betrán* and L. W. Rooney

Maize Breeding and Genetics Program, Soil and Crop Sciences Dep., Texas A&M Univ., College Station, TX 77843-2474

* Corresponding author (javier-betran{at}tamu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Development and adoption of quality protein maize (Zea mays L.) (QPM) would increase the nutritional value of food and feed maize products. Breeding programs at the International Center for Maize and Wheat Improvement, Mexico (CIMMYT); Texas A&M University (TAMU); and University of Natal, South Africa (SA) have developed high-lysine inbreds. Information about how elite QPM inbreds of different origins combine and perform in hybrids will facilitate the selection of parents and breeding strategies for hybrid development. Our objectives were to estimate the general (GCA) and specific combining abilities (SCA) for grain yield and secondary traits among high-lysine inbreds from different sources and to identify potential heterotic relationships among them. Seven white (CML176, CML181, CML184, Bo59W, Tx807, Tx811, and TxX124) and nine yellow QPM inbreds (CML190, CML193, Tx802, Tx814, Tx818, Tx820, Do940y, TxX808, and TxX806) were evaluated in two separate diallel experiments in five southern U.S. environments. The QPM hybrids yielded less than commercial checks. Across environments, GCA effects were nonsignificant for grain yield but highly significant for agronomic and kernel-quality traits. On the basis of GCA effects, TAMU inbreds had earlier maturities, shorter plants, and less grain moisture content than more subtropical CIMMYT and SA inbreds. The best-yielding hybrids and highest SCA effects resulted from crosses among inbreds from different programs: TxX124 x CML176, Tx811 x CML181, and Bo59w x CML184 among the white hybrids, and Tx802 x Do940y among the yellow hybrids. The QPM inbreds developed in different programs could represent potential heterotic groups for use in hybrid development and introgression of germplasm.

Abbreviations: BV, biological value • CIMMYT, International Center for Maize and Wheat Improvement, Mexico • GCA, general combining ability • PH, plant height • QPM, quality protein maize • SA, University of Natal, South Africa • SCA, specific combining ability • TADD, tangential abrasive dehulling device • TAMU, Texas A&M University


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
GLOBALLY, MAIZE CONTRIBUTES 15% (representing more than 50 million t) of the protein and 20% of the calories derived from food crops in the world's diet (National Research Council, 1988). In many developing countries in Latin America, Africa, and Asia, maize is the staple food and sometimes the only source of protein in diet, especially in weaning food for babies. Normal maize, being deficient in amino acids lysine and tryptophan that are essential for monogastric animals and humans, is nutritionally poor with a biological value (BV) of 40 to 57% (Bressani, 1992). High-lysine maize with homozygous embryo and endosperm for mutant alleles o2 at the {alpha}-zeins regulatory gene opaque-2 shows about 60 to 100% increase in lysine and tryptophan and a higher BV (80% as compared with casein). Substituting normal maize with high-lysine maize on an equal-weight basis for growing pigs and sows can diminish the use of synthetic lysine in animal feeds to maintain proper amino acid balance (Asche et al., 1985; Burgoon et al., 1992; Knabe et al., 1992). In the USA, doubling lysine content in maize alone can add an estimated annual gross value of $360 million per year and can go up to $480 million per year if protein also is increased (Johnson et al., 2001).

The CIMMYT has developed QPM that has improved kernel quality characteristics over o2/o2 soft genotypes, by introducing modifier genes and selecting for a hard, vitreous endosperm in o2/o2 germplasm (Vasal, 2001). The CIMMYT QPM populations, pools, inbreds, and hybrids adapted to subtropical and tropical environments are widely used in the development of high-lysine maize in Brazil, China, Ghana, India, and several Latin American countries (Vasal, 2001). The maize breeding program at SA has developed high-lysine white (e.g., Bo46W and Bo59W) and yellow inbreds (e.g., Do940y and Ho4664) that produce hybrids competitive in yield with normal hybrids and tolerant to diseases (Gevers and Lake, 1992).

In the USA, more than 20 yr ago after the discovery of o2 mutant effects, breeding programs converted normal inbreds and populations to their opaque-2 soft counterparts (NTR1, NTR2, BSAA-o2, B73o2, SSSS-o2) (Mertz et al., 1964). After this initial effort, the interest in QPM or high-lysine maize decreased and it has since remained low. To our knowledge, only Crow's Hybrid Seed Company has continuously conducted a breeding program to improve high-lysine maize. Texas A&M University has also maintained a breeding program to develop QPM inbreds and hybrids with normal seed appearance, competitive yield, and adaptation to the southern USA (Betrán et al., 2003a, 2003b, 2003c).

There is an increasing number of elite exotic QPM inbreds being developed outside the USA. Therefore, characterization and selection for adaptation of these subtropical and tropical white and yellow QPM inbreds and a systematic introgression into temperate germplasm could enhance protein quality, increase genetic variability for quality, improve productivity, and be a source of valuable genes for abiotic and biotic stress resistance. Some of CIMMYT's tropical and subtropical germplasm with intermediate and early maturity has desirable kernel quality characteristics and can significantly enhance the nutritional value of temperate maize germplasm for both food and feed purposes (Vasal, 2001). Furthermore, QPM hybrids have been reported to be less susceptible to aflatoxin, a potent carcinogen that causes losses worth millions of dollars in the southern USA, than current commercial hybrids (Bhatnagar et al., 2003). Introgression of exotic germplasm into temperate adapted maize has been widely emphasized as a method to expand genetic diversity of maize germplasm in the USA (Goodman et al., 2000). Despite the nutritional quality advantages and improved abiotic and biotic stress tolerance of exotic QPM, very little effort has been made to characterize and introgress exotic QPM germplasm into temperate U.S. maize germplasm. Major reasons for underutilization of exotic germplasm, particularly QPM germplasm, are photoperiod sensitivity, poor standability, and low grain yield in comparison with temperate adapted germplasm (Bhatnagar et al., 2003). Before incorporating exotic QPM germplasm into temperate areas, an initial evaluation of exotic germplasm is useful to determine their breeding potential (Geadelmann, 1984). Diallels between elite exotic and temperate QPM inbreds can help determine heterotic relationships among exotic and temperate QPM inbreds, which are at present relatively unknown, and identify the best hybrids for both production and breeding purposes. Information on agronomic performance, combining abilities, and heterotic relationships of elite subtropical and tropical QPM parents developed at CIMMYT and in SA with temperate inbreds adapted to southern U.S. environments will facilitate their incorporation and introgression. Therefore, our objectives were to (i) estimate GCA effects for grain yield and agronomic traits of QPM inbreds originated in subtropical (CIMMYT and SA) and temperate (TAMU) breeding programs, and (ii) estimate SCA effects and identify best hybrid combinations and possible heterotic relationships among these inbreds.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Germplasm and Environments
Two separate diallel experiments for white and yellow QPM inbreds developed in three QPM breeding programs (CIMMYT, SA, and TAMU) were evaluated in the 1999 growing season (February to September) in five southern U.S. environments. Diallel crosses among the lines were made in 1998 summer at College Station, TX, and 1999 winter at Homestead, FL. Seeds from reciprocal crosses of the full diallel were bulked to form one set of hybrids because sufficient seed was not obtained for all the crosses. Twenty-one F1 crosses (Griffing's Method 4; Griffing, 1956) among seven white QPM inbreds (Table 1), two commercial checks (Pioneer Brand P32H39 and Asgrow RX901W), and five experimental checks were evaluated at College Station, Weslaco, Castroville, Halfway, and Dumas, TX. Thirty-six F1 crosses (Griffing's Method 4) among nine yellow QPM inbreds (Table 1), four commercial checks, including Pioneer Brand hybrids P3223, P3394, and P32Y65 and DeKalb hybrid DK668, and eight experimental checks were evaluated at College Station, Corpus Christi, Granger, Wharton, and Dumas, TX. The characteristics of the environments and mean grain yield for both white and yellow diallels are described in Table 2. Standard cultural and agronomic practices generally used at all locations were applied.


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  		Table 1. White and yellow maize inbreds involved in two diallel experiments evaluated in five southern U.S. environments during 1999.

 

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  		Table 2. Characteristics and mean grain yield for environments used to evaluate white and yellow quality protein maize (QPM) diallel hybrids and non-QPM checks in southern USA in 1999.

 
Field Measurements
Data were recorded on a plot basis for both white and yellow QPM diallel experiments on the following agronomic traits: grain yield (combine-harvested grain weight expressed in megagrams per hectare and standardized to 155 g kg–1 moisture content), silking date (number of days from planting until 50% of the plants showing silks), plant height (distance in centimeters from the ground to the top of tassel), ear height (distance in centimeters from the ground level to the main ear-bearing node), root lodging (percentage of plants leaning at an angle > 30% from the vertical), stalk lodging (percentage of plants with broken stalks at or below the main ear at maturity), grain moisture (grams of moisture per kilogram of grain at harvest), and test weight (recorded as grams per pint by standard equipment and converted to kilograms per cubic meter).

For white QPM hybrids, ear samples from competitive plants in a single replication per environment were collected at harvest time and used to measure the following kernel quality traits (Serna-Saldivar et al., 1991): 1000-kernel weight (in grams), endosperm hardness (recorded as percentage of kernel weight removed using a tangential abrasive dehulling device [TADD] to remove the pericarp uniformly using 45-g samples of whole kernels and dehulling in the TADD for 10 min); floaters (recorded as percentage kernels floating in a 1.275-g cc–1 sodium nitrate solution); pericarp removal (performed by cooking a 25-g sample in a steam kettle containing 1% lime for 20 min at boiling point, washing the samples, and staining them with eosin and methyl blue solutions to differentiate between pericarp [blue-green] and endosperm [light pink]). The samples were later rated on a scale of 1 to 5 for the extent of pericarp removal (1 = complete removal, and 5 = 100% pericarp retained). Endosperm hardness is related to the proportion of hard endosperm to soft endosperm and it is an important quality trait for the milling industry. Percentage floaters and pericarp removal are related to endosperm hardness and cooking time for production of masa used in making tortilla and tortilla chips (Serna-Saldivar et al., 2001).

Statistical Analyses
Both white and yellow diallel experiments were planted in two-row plots following an {alpha} lattice experimental design with two replications per environment. Individual ANOVAs per environment and across environments were conducted using PROC GLM (SAS Institute, 1997). Hybrids were considered fixed effects, and environments and replications random effects. Significances of hybrid, GCA, and SCA mean squares were estimated with F tests, using their interaction with the environment as an error term. General combining ability effects of the parents and SCA effects for the crosses, as well as their mean squares at each environment and across environments, were estimated following Griffing's Method 4 diallel analysis (Griffing, 1956) using a computer program originally written by Dr. S.G. Carmer (University of Illinois) and later modified and adapted by Dr. Hector Barreto at CIMMYT.

Biplots were constructed for both white and yellow diallel crosses using mean grain yield across locations to visualize relationships among parental inbreds in hybrid combinations and identify possible heterotic associations among them. Since parental inbred per se were not included in the diallel analysis, mean grain yield of inbreds in hybrids was used as inbred values for calculations. Biplots are commonly used to analyze two-way data where rows and columns represent different experimental units (e.g., genotypes and environments, Inbred A x Inbred B). In a diallel-cross data, both columns and rows represent the same parental inbreds, which are both an entry and a tester. Principal component scores (PC1 and PC2) were derived using PROC PRINCOMP (SAS Institute, 1997), following methods described by Yan and Hunt (2002), and used to construct the biplot. A polygon was drawn, connecting entries located furthest from the origin in each biplot. Subsequently, this polygon was divided into sectors by perpendiculars (A, B, C, and D) drawn from the origin to each side of the polygon. All testers and entries included in the same sector represent good hybrid combinations and potential heterotic groups for grain yield. The best hybrid in any sector is defined by the vertex entry and the tester that is located furthest from the origin. SCA effects between entries and testers in any sector can be visualized by projecting an entry onto the vector of the tester or its extension.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
White Hybrids
Significant differences among hybrids across environments were observed for all traits (Table 3). Mean grain yield across environments was 5.62 Mg ha–1 for the hybrids, 5.70 Mg ha–1 for QPM hybrids, and 6.27 Mg ha–1 for non-QPM checks. Significant differences among the QPM crosses and non-QPM checks were observed for days to flowering, plant and ear heights, and grain moisture. The QPM crosses, on average, flowered 5 d later (80.66 vs. 75.55 d), were taller (235.27 cm vs. 220.0 cm), had higher ear placement (98.92 cm vs. 80.38 cm), and higher grain moisture content (185.74 vs. 156.59) than non-QPM checks.


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  		Table 3. A combined ANOVA and means for grain yield and agronomic traits of white hybrids across five southern U.S. environments.{dagger}

 
Significant differences among diallel hybrids were observed for all traits except grain moisture (Table 3). Significant differences among GCA effects were observed for all agronomic traits except grain yield, whereas SCA effects were significant for grain yield and stalk lodging. Hybrids x environment effects were significant for all agronomic traits. The GCA x environment effect was significant (P < 0.05) for grain yield and highly significant for all other agronomic traits (P < 0.01) except stalk lodging. The SCA x environment effects were significant for silking date, plant height, ear height, and grain moisture. For quality traits, all GCA effects were significant whereas SCA effects were nonsignificant for all traits except test weight (Table 4).


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  		Table 4. A combined ANOVA and means for kernel traits of white hybrids across five U.S. environments.{dagger}

 
The GCA effects for grain yield showed significant variation between and within the three different groups of parental lines (CIMMYT, TAMU, and SA), in different environments (Table 5). Weslaco and Castroville were the only two environments with significant GCA effects. The highest GCA effect for grain yield across environments was observed for CML176 (0.21 Mg ha–1), but this was not significant. CML176 hybrids have shown high yield potential and low aflatoxin accumulation in Texas environments in previous studies (Betrán et al., 2002; Bhatnagar et al., 2003) and performed consistently well in trials conducted by CIMMYT across 29 locations in Latin America, Asia, and Africa (CIMMYT, 1999). CML176 and CML181 produced tall and late hybrids. CML176 has also an undesirable positive GCA effect for root lodging and an off-white grain color (data not shown). CML184 had negative GCA effects for days to flowering and plant and ear heights, which indicates that it is more suited for temperate environments than CML176 and CML181. Inbreds Tx807 and Tx811 had significant negative GCA effects for days to flowering, plant height, and ear height across environments indicating early maturity, shorter plants, and lower ear placements than exotic QPM lines. Inbred Bo59w had significant GCA effects for stalk lodging and plant height.


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  		Table 5. General combining ability (GCA) effects of seven white inbreds for grain yield (per environment and across environments), agronomic, and kernel traits across five southern U.S. environments.{dagger}

 
Inbred CML176 had the most desirable significant GCA effect for test weight, CML181 for 1000-kernel weight, TxX124 for endosperm hardness and floaters, and Tx807 for pericarp removal (Table 5). Inbred TxX124 had the best combination of GCA effects for quality traits, showing high GCA effects for test weight and 1000-kernel weight together with low GCA effects for endosperm hardness, floaters, and pericarp removal.

The top five performing crosses having high positive significant SCA effects and high grain yields were Tx811 x CML181 (SCA = 1.01, P < 0.01, 6.53 Mg ha–1), Tx807 x CML181 (0.87, P < 0.01, 6.30 Mg ha–1), Bo59W x CML184 (0.71, P < 0.01, 6.28 Mg ha–1), TxX124 x CML176 (0.69, P < 0.05, 6.82 Mg ha–1), and Bo59W x CML176 (0.41, P < 0.05, 6.18 Mg ha–1). The first two principal component axes in the biplot for mean grain yield of seven inbreds in entry x tester hybrids across environments explained 42.6 and 35.6% of the total variation, respectively (Fig. 1) . Entries Tx811, CML176, Bo59W, and CML181, which are located furthest from the origin, defined a polygon that was divided into four sectors by perpendiculars A, B, C, and D. In sector AB, the best hybrid combination was the vertex entry Tx811 x tester CML181. Another good hybrid in sector AB was Tx807 x CML181. In sector BC, the best hybrid was the vertex entry CML176 x Bo59W. Other good hybrids in this sector were CML176 x TxX124 and CML184 x Bo59W. Sectors CD and DA showed similar responses as observed for sectors AB and BC, respectively. Potential heterotic groups for the southern USA could involve crosses between TAMU inbreds (Tx811 and Tx807) and subtropical and tropical QPM inbreds (CML181, CML176, and CML184), and between CIMMYT lines CML176 and CML184, and SA inbred Bo59W.



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  		Fig. 1. Biplot for grain yield and putative heterotic relationships between seven white parental inbreds in hybrid combinations ({diamondsuit} entries and {blacksquare} testers) across five southern U.S. environments.

 
Yellow Hybrids
Significant differences among hybrids across environments were observed for all traits except root lodging (Table 6). Mean grain yield across environments was 6.44 Mg ha–1 for all hybrids and 6.25 Mg ha–1 for QPM hybrids, which was significantly lower than that for non-QPM crosses (8.18 Mg ha–1). The QPM hybrids across environments flowered approximately 3 d later (81.41 d vs. 78.55 d) and had higher grain moisture content (169.61 g kg–1 vs. 143.89 g kg–1) than non-QPM hybrids. Vasal et al. (1993) found similar responses of subtropical yellow QPM populations for days to flowering and ear height in U.S. environments.


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  		Table 6. A combined ANOVA and means for grain yield and agronomic traits of yellow hybrids across five southern U.S. environments.{dagger}

 
The GCA effects were significant for all traits except grain yield and root lodging. Highly significant differences for SCA effects were observed for grain yield, plant height, and test weight. The GCA x environment effects were highly significant for all traits, whereas the SCA x environment effects were significant for all traits except days to flowering, root lodging, and test weight.

The GCA effects for all yellow QPM inbreds varied significantly across environments (Table 7). Inbreds CML190 and CML193 had negative GCA effects for grain yield in most environments except CML190 at Granger, indicating their lack of adaptation to U.S. environments. Texas inbreds had positive GCA effects in most environments except Tx818, which showed significant negative GCA effects at Corpus Christi and Dumas. In general, Tx802, Tx820, and Tx814 showed high GCA effects for grain yield. Tx802 showed consistently high positive GCA effects for grain yield at most environments.


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  		Table 7. General combining ability (GCA) effects of nine yellow inbreds for grain yield (per environment and across) and agronomic traits across five southern U.S. environments.{dagger}

 
The CIMMYT inbreds showed variable GCA effects for agronomic traits across environments (Table 7). CML190 and Do940y had significant positive GCA effects for test weight and represent potential source germplasm to increase the test weight of TAMU inbreds such as Tx802 and Tx814 that had negative GCA effects for this trait. Texas inbreds, in general, had significant negative GCA effects for days to flowering (TxX808 and Tx820), plant height (Tx802 and TxX808), root lodging (Tx818 and Tx820), stalk lodging (Tx814), and grain moisture (TxX808). Do940y, a late-maturing inbred, had significant positive GCA effect for days to flowering.

The top three hybrid combinations having high positive SCA effects and high grain yields involved crosses between SA and TAMU inbreds [Do940y x Tx802 (0.88, P < 0.01, 7.51 Mg ha–1), Do940y x Tx818 (0.75, P < 0.01, 6.69 Mg ha–1), and Do940y x Tx820 (0.66, P < 0.01, 7.24 Mg ha–1)]. The first two principal component axes in the biplot for mean grain yield for the nine yellow inbreds in entry x tester hybrids across environments explained 47.3 and 22.4% of the total variation, respectively (Fig. 2) . Inbreds Do940y, Tx802, Tx820, Tx818, and CML193 defined a polygon that was divided into four sectors by the perpendiculars A, B, C, and D drawn to the sides of the polygon. In sector AB, the vertex entry Do940y showed a high positive response in hybrids with testers Tx802, Tx820, Tx818, and CML190. Sector BC showed similar relationships. The vertex entry CML193 in sector DA did not show any significantly high positive response with tester Tx814 that was located very close to the origin of the biplot. Inbred TxX808 in sector DA and TxX810 in sector AB, both derived from crosses between TAMU and SA inbreds, showed variable response with TAMU lines. Two potential heterotic groups were identified in the biplot for yellow inbreds. The first group included TAMU inbreds Tx802, Tx820, and Tx818, which combined well with SA inbred Do940y, and the second group included Do940y and CML190.



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  		Fig. 2. Biplot for grain yield and putative heterotic relationships between nine yellow parental inbreds in hybrid combinations ({diamondsuit} entries and {blacksquare} testers) across five southern U.S. environments.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Successful breeding approaches are a direct consequence of the gene action prevalent in the breeding population under consideration. The relative importance of additive vs. nonadditive effects for grain yield in diallel crosses is an indication of the type of gene action (Baker, 1978). In the two diallels reported here, the GCA effects for grain yield across environments were not significant, whereas SCA effects were highly significant (Tables 3 and 6). An opposite trend was observed for quality and agronomic traits, where GCA effects across environments were more important than SCA effects (Table 4). The type of more prevalent gene action for grain yield has been variable, depending on the parents and environments under consideration in other studies. San Vincente et al. (1998) reported greater relative importance of nonadditive genetic effects than additive genetic effects for grain yield in diallel crosses among improved tropical white endosperm populations. In contrast, additive genetic effects were prevalent in CIMMYT's lowland tropical late and subtropical QPM germplasm (Vasal et al., 1993) and subtropical and temperate intermediate maturity germplasm (Beck et al., 1991).

The genetic interpretation of a diallel with a reduced number of parental inbreds, such as the ones in this study, can be biased by the lack of independent distribution of genes in the parental lines (Baker, 1978). Therefore, combining abilities reported here could be biased by the correlation of gene frequencies and should be interpreted with caution. Despite this limitation, these diallels were useful to determine which QPM inbreds had the most desirable expression of relevant traits and to estimate the heterotic relationship among them. The biplot analysis helped visualize graphically the best hybrid combinations and the relationship among the parental inbreds. A potential constraint of the biplot method is that it may not explain all of the variation (Yan and Hunt, 2002). The amount of variation explained by the two principal components was >72% in both cases. In addition, the conclusions drawn from biplots were verified with the results from the conventional Griffing's analysis (Griffing, 1956).

Both the white and yellow lines used in these diallels varied in adaptation (Table 1). The CIMMYT and SA lines were mostly tropical and subtropical in adaptation, whereas TAMU lines were more temperate adapted. The testing environments ranged in latitude from 26°N to 35°N, representing a transition between subtropical and temperate areas of maize cultivation (Table 2). With increasing latitude and daylength, QPM hybrids of subtropical lines tended to be late maturing with more biomass, higher ear placement, and higher grain moisture content, as reflected by the GCA effects (Tables 5 and 7). Previous studies have shown that white QPM hybrids, in general, are more competitive for yield in subtropical environments as compared with temperate environments (Bhatnagar et al., 2003). Vasal et al. (1993), in a 10-parent diallel study of tropical white QPM germplasm conducted in three environments in Mexico and the USA, reported similar results. Overall, QPM hybrids yielded less than commercial checks. The gap in grain yield was greater in QPM yellow hybrids. Breeding efforts to enhance QPM hybrid performance in the USA should be devoted to increasing grain yield, standability, test weight, and 1000-kernel weight, and to reducing ear placement, plant height, maturity, and grain moisture.

The classification of inbreds into heterotic groups facilitates the exploitation of heterosis in maize, which can contribute to hybrid performance. Vasal et al. (1993) reported information on the combining ability and heterotic patterns of CIMMYT's subtropical QPM germplasm. Recently, CIMMYT started classifying QPM inbreds into heterotic groups (HG-A and HG-B) using two groups of testers (Cordova et al., 2003). Similar efforts have been undertaken in other breeding programs (e.g., TAMU). The biplot analysis in both white and yellow diallels suggests positive heterotic response between temperate and subtropical QPM inbreds that have been used as testers in these programs (Fig. 1 and 2). Therefore, inbreds from diverse backgrounds and adaptation can be used for hybrid identification and incorporation of exotic germplasm into temperate-adapted inbreds for southern U.S. environments. On the basis of these results, it seems plausible to characterize and classify QPM inbreds into heterotic groups and to determine the relationship among groups used in temperate and exotic QPM lines. In these diallels, the best hybrids were formed between parental inbreds originating from different breeding programs, which suggests that these inbreds can produce high-yielding hybrids. In future line recycling and in the development of source breeding populations, crosses among QPM lines from the same group may enhance the heterotic response as it has been observed in yellow dent maize.

The information obtained from these experiments can facilitate the identification of hybrids that combine quality traits, such as endosperm quality and disease resistance, from some inbreds with the adaptation and yield potential of other inbreds. For example, in the white hybrids, a breeding objective would be to combine endosperm hardness from TxX124, high test weight from CML176, low grain moisture from CML181, and reduced plant height and lodging, and early maturity from Tx807. In yellow hybrids, it would be desirable to combine high test weight from CML190, high yield from Tx802, and early maturity, low grain moisture, and low plant height from TxX808. A trait of particular interest in QPM is endosperm hardness, because it is associated with large flaking grits and low dry matter losses in alkaline processing of maize, and also with lower incidence of insect and pest damage and grain aflatoxin at maturity (Betrán et al., 2002). Several modifier genes with additive gene action are involved in endosperm hardness in the opaque 2 background of QPM (Wessel-Beaver et al., 1985; Vasal et al., 1993).

Superior QPM hybrids are extremely valuable for the white-maize food industry and yellow maize for feed in animal nutrition. In the USA, where almost 55% of corn produced is used as feed for swine and poultry, development of well-adapted QPM germplasm will have tremendous value for the feeding industry (Johnson et al., 2001). We concluded that the nutritional value of maize for both food and feed would be significantly enhanced by appropriate breeding strategies that emphasize the combination of desirable traits from exotic and temperate QPM lines.


   ACKNOWLEDGMENTS
 
Sincere thanks are extended to Dr. R.D. Waniska, the TAMU Crop Testing Program, and Dr. Wenwei Xu, Texas A&M University, for assistance in conducting quality analysis and field trials. Financial support for this work from the Texas Corn Producers Board is gratefully acknowledged.

Received for publication November 12, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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