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Crop Science 43:818-823 (2003)
© 2003 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Combining Ability of Maize Inbreds for Carotenoids and Tocopherols

C. O. Egesel, J. C. Wong, R. J. Lambert* and T. R. Rocheford

Department of Crop Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801

* Corresponding author (rjlamber{at}uiuc.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Carotenoids (provitamin A) and tocopherols (vitamin E) are lipid soluble antioxidants associated with decreased risk of several degenerative diseases. Both vitamins occur naturally in corn (Zea mays L.) grain. Corn grain is a major component in diets of humans and animals, and may have added value with increased levels of carotenoids and tocopherols. To estimate genetic variation among a group of inbreds and their breeding potential, we estimated combining abilities of four carotenoids and two tocopherols in a diallel of 45 corn hybrids. Ten inbreds were chosen that varied in endosperm color (yellow to orange), tocopherol levels and oil content. Plants were grown for two years (1998, 1999) at Urbana, IL, in an RCB design with three replications. High pressure liquid chromatography was used to assay for four carotenoids (lutein, zeaxanthin, ß-cryptoxanthin, and ß-carotene) and two tocopherols ({alpha}-tocopherol and {gamma}-tocopherol). Good agreement between year means for the six compounds was observed indicating minor environmental effects. Diallel analyses indicated, between 72 to 87% of the total sums of squares for hybrids for the six compounds was attributable to GCA effects. Significant SCA effects were found for several crosses. Estimates of GCA effects indicated A619 had high values for lutein, ß-cryptoxanthin, and total carotenoids. Inbred R84 was a poor genotype for xanthophyll pigments but a significant contributor for ß-carotene and {gamma}-tocopherol, and total tocopherol. These genetic stocks indicate corn hybrids can be developed with higher levels of provitamin A and vitamin E.

Abbreviations: GCA, general combining ability • SCA, specific combining ability


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
PROVITAMIN A (carotenoids) and vitamin E (tocopherols) are fat-soluble vitamins that occur in corn grain. Carotenoids are located in endosperm and tocopherols, a component of oil, in the germ (Weber, 1987b; Grams et al., 1970). Two classes of carotenoid pigments are carotenes and xanthophylls, which are responsible for the yellow and orange color of corn endosperm. Among the major food grains, only yellow corn contains significant levels of ß-carotene, a source of vitamin A (Buckner et al., 1990). Yellow corn is the main source of provitamin A in feed rations for swine and other animals (Brunson and Quackenbush, 1962). Increased levels of carotenoids and tocopherols in corn grain, because of their antioxidant activity, should increase the nutritive value of corn. In broiler production, skin and shank pigmentation of birds are important quality factors that are supplied by yellow corn in the ration. Egg-laying poultry require high pigment levels in their feed for the production of eggs with dark yellow yolks that are desirable for baking products. Commercial corn hybrids do not meet pigment requirements in the ration without supplementation. Producers use supplementary sources like alfalfa meal and processed marigold petals (Weber, 1987a; Grogan, 1964). These pigment sources have some disadvantages; alfalfa meal is high in fiber and low in energy; marigold petals are costly and lack nutrients so their use has decreased and replaced by synthetic yellow pigments. These considerations emphasize a need for increased levels of carotenoids in corn grain.

The presence of antioxidants in lipids improves their stability and delays oil rancidity (Chow and Draper, 1974; Weber, 1987a). Schaefer et al. (1995) and Williams (1997) found vitamin E supplementation in feed delayed discoloration and prolonged shelf life of packaged beef, poultry, and food products. Animals fed corn containing high levels of tocopherols could increase profits by reducing waste caused by discolored products. The presence of natural antioxidants, especially tocopherols, gives corn oil flavor stability during storage and cooking. Increased levels of tocopherols should enhance shelf life of corn oil (Weber, 1987b; Watson, 1988; White and Pollak, 1995).

Several studies have shown significant differences among corn inbreds for carotenoid and tocopherol levels. Blessin et al. (1963) reported ranges of 0.9 to 4.1 µg g-1 for carotenes and 18.6 to 48.0 µg g-1 for xanthophylls for 39 corn inbreds. Quackenbush et al. (1963) evaluated 125 inbreds and found provitamin A amounts ranged from a trace to 7.3 µg g-1, lutein from 2 to 33 µg g-1. Forgey (1974) evaluated 20 inbreds and found {alpha}-tocopherol ranged from 9.1 to 64.6 µg g-1 and {gamma}-tocopherol from 13.6 to 128.7 µg g-1. A study of 200 A632 x W64a F3 families showed genetic variation for carotenes and tocopherols in corn grain (Egesel, 1997), with a range in values for ß-carotene 0.13 to 2.9 µg g-1, for {alpha}-tocopherol 0.18 to 13.5 µg g-1, and for {gamma}-tocopherol 1.79 to 38.4 µg g-1. Using the same material, Wong (1999) reported broad sense heritability estimates for F3 family means of 33% for ß-carotene, 47% for ß-cryptoxanthin, 59% for zeaxanthin, 78% for lutein, 88% for {alpha}-tocopherol, and 87% for {gamma}-tocopherol. Galliher et al. (1985) obtained broadsense heritability estimates of 62% for {alpha}-tocopherol and 68% for {gamma}-tocopherol in S1 families of Illinois version of Stiff-Stalk synthetic (RSSSC).

In breeding for increased concentrations of carotenoids and tocopherols, it is important to know what effect male and female genotypes contribute to kernel content for these compounds when different genotypes are crossed. Several studies have shown the pollen parent affects carotenoid levels of F1 seed of reciprocal crosses (Mangelsdorf and Fraps, 1931; Johnson and Miller, 1938; Randolph and Hand, 1940; Grogan et al., 1963). Egesel (2001) evaluated a group of orange and yellow endosperm hybrids and their parents for endosperm dosage effects. He found a dosage effect for the four carotenoids and total carotenoids in seeds of a group of reciprocal crosses among hybrids. Carotenoid concentrations of F2 seeds of these hybrids was closer to the female parent although the pollen parent did effect carotenoid levels. However, the female effect on carotenoid concentration was much larger than the male effect. These results indicate open pollinated F2 seed produced among a group of hybrids that vary in carotenoid concentrations should give a good estimate of carotenoids because of this female effect. This effect probably is the result of two doses of the female alleles in the endosperm and only one dose from the pollen parent. Information on dosage effects on tocopherols is lacking in the literature. However xenia effects for total oil concentration could indirectly affect tocopherol seed levels. Results from data collected in this study showed oil concentration was associated with total tocopherols, which allowed for tocopherol differences among this group of hybrids varying in oil content.

Combining ability of a genotype is an important criterion in developing improved hybrids. Combining ability is useful in testing procedures where the objective is to compare hybrid performance of lines (Griffing, 1956). Aurand et al. (1947) found 10 yellow dent inbreds resulted in hybrids with high carotene levels. The corn lines varied widely in carotene content and determined the carotene content of the yellow dent hybrids.

There has been interest in carotenoids and tocopherols (Mangelsdorf and Fraps, 1931; Aurand et al., 1947; Blessin et al., 1963; Quackenbush et al., 1963; Grogan, 1964; Weber, 1987a; Galliher et al.1985), but to our knowledge recent breeding research has not been done. A major reason was likely a lack of rapid analytical procedures for these compounds. Additional information is needed on genetic variation of these compounds in corn germplasm, and combining ability of certain genotypes. The objectives of this study were to analyze genetic variation for carotenoid and tocopherol levels in the grain of 45 corn hybrids and estimate combining ability of the 10 parents.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
A diallel mating design was used to estimate general combining ability (GCA) and specific combining ability (SCA) effects. Model 1 was selected because the choice of parents was considered fixed (Griffing's method 4, model 1, 1956). The diallel consisted of 10 corn inbreds making 45 hybrids. Inbreds were chosen for their endosperm color, vitamin A and/or vitamin E levels, oil concentration, and adaptation (Table 1). The 45 treatments were arranged in a RCB design with three replications and grown in 1998 and 1999. A plot consisted of two rows 5.3 m in length and spaced 0.76 m apart. Each plot was thinned to 48 plants with a final plant density of approximately 69 000 plants ha-1. The experiments were conducted on the Crop Sciences Research and Education Center of the University of Illinois at Urbana, IL. Planting dates were 27 April 1998 and 3 May 1999. Harvest dates were 5 October 1998 and 26 September 1999. All plots were harvested with a plot combine and grain weight and moisture used to estimate grain yield of No. 2 yellow corn. A bulk sample (100 g) was saved from each plot for chemical analyses.


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  		Table 1. Pedigree and kernel phenotypes for 10 corn inbreds used in the diallel.

 
The HPLC method used for chemical analysis estimated from a single sample the levels of lutein, zeaxanthin, ß-cryptoxanthin, ß-carotene, {alpha}-tocopherol, and {gamma}-tocopherol. The carotenoid and tocopherol compounds assayed in this study are the primary ones found in corn grain. The extraction procedure for the carotenoids and tocopherols was a modified version of the procedure described by Weber (1987a) and Kurilich and Juvik (1999). Corn grains were ground in a M-2 Stein mill and ground samples stored at 8°C for several months before extraction. The extraction procedure used 600 mg of ground tissue, to which 6 mL of ethyl alcohol containing 0.1% (v/v) butylated hydroxytoluene (BHT) was added. Then samples were placed in a water bath (85°C) for 5 min. Next, 120 µL of 80% (w/v) potassium hydroxide was added and the mixture vortexed for 10 to 15 s before returning to the water bath for 10 min for saponification. Samples were vortexed once in the middle of the 10 min saponification period. The samples were then placed in an ice bath and 3 mL of hexane plus 3 mL of ice-cold deionized distilled water (ddH20) was added. Samples were vortexed and centrifuged for 10 min at 2700 rpm in a Beckman GS-6R centrifuge using a CH 3.8 rotor. A glass pipette was used to transfer the upper layer into a glass culture tube. The pellet was reextracted twice with 3 mL of hexane, washed with 3 mL of ddH20, vortexed, and centrifuged for 10 min before transfer to a glass tube. Tubes containing a final volume of the 7-mL extract were placed in a speed vac (Savant, plus SC110A, Savant universal vacuum system). In about 1 h, the hexane fraction was dried. The samples were dissolved in 200 µL of acetonitrile:methanol:methylene chloride (45:20:35 volumes) before loading them into the HPLC.

Carotenoid compounds are sensitive to light, heat, oxygen, and especially light and oxygen (Weber, 1987a). Therefore, sample preparation was done under safe light and stored in a freezer. Tubes were sealed when necessary to minimize degradation.

Quantification of the four carotenoids and two tocopherols was done with a Water Alliance HPLC System (Waters Chromatography, Milford, MA). The system consists of a separation module (Waters 2690), a photodiode array detector Waters 996, and a millennium 32 chromatography manager to run and manage the HPLC data. Samples were eluted through a C30, 5m, 4.6- by 100-mm column (YMC, Waters). The mobile phase consisted of acetonitrile:methanol:methylene chloride (75:20:5, by volume), containing 0.05% (w/v) triethylamine and 0.1% BHT, flow rate was 1.8 mL min-1. Carotenoids and tocohperols were measured at 450 and 295 nm, respectively (Darnoko et al., 2000). Standards for ß-carotene, {alpha}-tocopherol, and {gamma}-tocopherol were from Sigma Chemical (St. Louis, MO). Standards for ß-cryptoxanthin, and zeaxanthin were from Extrasynthese Corp. (Genay, France). Total carotenoid and tocopherols were obtained by adding the individual components for each type.

Statistical analysis of diallel data used Griffing's (1956) method 4, model 1 to partition the sum of squares for hybrids and years x hybrids into sources of variation due to GCA and SCA effects and their interactions with years (years x GCA and years x SCA). The statistical model used was Yijkl = µ + {alpha}l + bkl + {nu}ij + {alpha}{nu}ijl + eijkl, where Yijkl = observed trait for an experimental unit, µ = population mean, {alpha}l = year effect, bkl = replication within year effect, {nu}ij = F1 hybrid effect {nu}ij = gi + gj + sij, where gi = GCA effect for the ith parent, gj = GCA effect of jth parent, sij = SCA effect for ith F1 hybrid, {alpha}{nu}ijl = interaction between F1 hybrids and years {alpha}{nu}ijl = {alpha}gil + {alpha}gjl + {alpha}sijl, where {alpha}gil = interaction between GCA effect for the ith parent and years, {alpha}gil = interaction between GCA effect for the jth parent and years, {alpha}sijl = interaction between SCA effect for the ijth F1 hybrid and years, and eijkl = error effect. Data were measured on a plot basis, hybrids were fixed effects, with years and replications in years random effects. PROC GLM procedure of SAS statistical software (SAS Institute, 1999) was used for the analyses of variance. Mean squares for interactions of hybrids with years were used as the denominator for F tests for main effects of hybrids, GCA, and SCA (Hallauer and Miranda, 1988). GCA and SCA estimates were calculated by a SAS program written by Zhang and Kang (1997).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Significant effects were observed between years for all traits except {alpha}-tocopherols and grain yield (Table 2). Significant mean squares were observed for all traits for hybrids, and GCA effects. A large proportion of the sum of squares for hybrids was associated with GCA effects (varied from 72–87%) suggesting additive gene action was important for all traits. Significant SCA effects were observed for all traits except zeaxanthin suggesting dominant gene action. The interaction of years x hybrids was significant for all traits except {alpha}-tocopherol but years x GCA effects were significant for all traits.


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  		Table 2. Mean squares for four carotenoids, two tocopherols, total carotenoids and tocopherols, oil concentration, and grain yields for 45 corn hybrids grown in two years (1998–1999) at Urbana, IL.

 
The year means for the five carotenoid traits were higher in 1999 than 1998 (Table 3). Year means of {alpha}-tocopherol and oil were slightly higher in 1999, but lower in 1999 for {gamma}-tocopherol and total tocopherol. Average grain yields between the two years were similar. The range in values for all traits tended to be higher in 1999 than 1998. Zeaxanthin was the major carotenoid with a value greater than 50% of the total carotenoids agreeing with earlier data (Blessin et al., 1963; Wong, 1999). Data presented by Brunson and Quackenbush (1962) and Weber (1987b) however, showed lutein to be the major component of total carotenoids. The values presented in Table 3 for lutein are not in agreement with those reported above. These results could be due to the genetic materials analyzed. ß-Carotene was least plentiful of the four carotenoid compounds and breeding for higher concentrations may be needed because of its high antioxidant activity. In these materials, {gamma}-tocopherol makes up about 80% of the total tocopherols and was more variable between years than {alpha}-tocopherol. The levels of the compounds in the diallel were compared with those of five commercial hybrids by Egesel (2001). He found the means of these same 45 hybrids to be higher compared with those in commercial hybrids for seven of the eight compounds ranging from a 3% higher level for zeaxanthin to a 36% higher level for lutein. The mean for ß-carotene was about twice that of the commercial hybrids (0.70 vs. 1.50 mg g-1). When the upper end of the range in values for the eight compounds were compared, they were all higher than in the commercial hybrids. The upper range in grain yields was within that of commercial hybrids.


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  		Table 3. Year means (1998–1999) and ranges for four carotenoids and two tocopherols, total carotenoid and tocopherol content, oil concentration and grain yield for a diallel of 45 corn hybrids.

 
Parental means for the 10 inbreds for five carotenoids and three tocopherol compounds (Table 4) all had significant differences among parental means with total carotenoids having the largest range in values of 11.91 µg g-1 for carotenoids and for total tocopherols 98.65 µg g-1. Comparing parental means within each carotenoid compound showed no one inbred had high values for all carotenoids except A619. Inbreds with high means for individual carotenoids were RSC Hi-{alpha} for lutein, Mo17 for zeaxanthin, A632 for ß-cryptoxanthin, and A619 for ß-carotene. Parental means indicate the level of a specific carotenoid compound is genotype dependent which should allow for changing specific carotenoid levels with selection.


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  		Table 4. Parental means for 10 inbreds averaged over their nine hybrids for two years (1998–1999) for five carotenoids and three tocopherol compounds.

 
Inbreds with high total tocopherol means were R84, R335, and R416 all high oil inbreds (Table 4). A619 had high total tocopherol mean for inbreds with normal oil levels. Inbreds with high {gamma}-tocopherol means were R84, R335, and R416. R335, R416, A619, A632, and B73 had a high {alpha}-tocopherol levels. Inbreds with high parental means for total tocopherols also had the highest oil levels. Levy (1973) found an association between oil concentration and tocopherols among 14 corn hybrids. Inbred RSC Hi-{alpha} had the lowest {gamma}-tocopherol and total oil levels of the 10 inbreds and also the lowest total tocopherol level indicating a possible role of {gamma}-tocopherol in determining total tocopherol levels.

Parental means for grain yield (Table 4) varied from 9.2 to 6.1 Mg ha-1, which is lower than most commercial hybrids. However, the grain yield parental mean of Mo17 was the highest of the 10 inbreds and ranked second for total carotenoids and first for zeaxanthin. Correlation estimates of grain yield parental means with each of the five carotenoid compounds had significant associations only between grain yield and total carotenoids (+0.85**) and grain yield and zeaxanthin (+0.84**). These correlations indicate it may be possible to increase carotenoid levels in corn without reducing grain yields.

Forty-one percent of the GCA estimates had significant positive estimates for all 10 traits (Table 5). For lutein, lines A619 and RSC Hi-{alpha} had high positive GCA values, zeaxanthin had A632 and Mo17 with high positive GCA estimates; ß-cryptoxanthin had A632 with a high positive GCA estimate; ß-carotene had A619 with a high positive GCA effect; total carotenoids had A619 and Mo17 with high positive GCA values.


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  		Table 5. Specific combining ability estimates for 26 corn hybrids for four carotenoids and two oil traits for two years (1998–1999) at Urbana, IL.

 
About 26% of the SCA estimates (93 out of 360) were significant and 50% had positive SCA estimates (Table 5). Lutein had seven hybrids with positive SCA values and high oil inbreds were in the pedigrees of four hybrids. ß-Cryptoxanthin had five positive SCA estimates with R84 in four of the five hybrids. ß-Carotene had eight positive SCA estimates with R84 in four of the eight hybrids. Total carotenoids had only three positive SCA estimates with R84 in two of the three hybrids. Results show R84 contributes to SCA of its hybrids.

For tocopherols, high oil inbreds usually had positive GCA estimates (Table 6). R84 had a significantly higher positive GCA estimate for {gamma}-tocopherol than R335 or R416. {alpha}-Tocopherol had five positive GCA estimates with R335 being significantly higher than R416. R84 had a positive GCA estimate for {gamma}-tocopherol but a negative estimate for {alpha}-tocopherol. For total tocopherols, four inbreds had positive GCA values with R84, R335, and R416 all had high GCA estimates followed by A619. GCA estimates for oil concentration agreed with the GCA estimates for tocopherols with R84, R335, and R416 having positive GCA estimates for this trait. For grain yields, A619, A632, B73, Mo17, and W64a had positive GCA estimates with negative GCA estimates for the high oil lines (Table 6). This result indicates high oil lines contribute to tocopherol levels, but high oil lines did not contribute to yield. The grain yield data (Table 4) support this observation because the lowest grain yields involved hybrids that had one or two high oil parents.


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  		Table 6. General combining ability estimates of 10 corn inbreds, for four carotenoids, two tocopherols, oil concentration and grain yield evaluated in two years (1998–1999) at Urbana, IL.

 
SCA for {gamma}-tocopherol had four hybrids with positive SCA estimates with R84 x R335 being significantly higher than the other three hybrids. For {alpha}-tocopherol, five hybrids had positive SCA estimates with R84 x A632 being significantly higher than the other four. SCA effects for total tocopherols had four hybrids with positive estimates and R84 x R335 with the highest SCA value (Table 5). SCA for oil concentration had four hybrids with positive estimates with three having statistically equal values, B73 x A619, B73 x R416, and B73 x R84. All three hybrids had B73 in their pedigree and two had high oil inbreds. To enhance the tocopherol levels in corn, the data indicate crosses of high oil x normal inbreds, followed by selection, may enhance tocopherol levels in corn.

The scope of genetic stocks in this study indicates improvements can be made in the concentration of carotenoids and tocopherols in corn. The concentrations of ß-carotene and ß-cryptoxanthin, carotenoids with provitamin A activity in these inbreds, is too low to be a dietary source for this vitamin. However, the amount of genetic variability in this study indicates their concentraiton could be increased with selection. This genetic variability plus selection could result in increased levels of these vitamins in human and animal feeds.

Normal maize genotypes have more {gamma}-tocopherol, but {alpha}-tocopherol has greater antioxidant activity. Therefore, an effective increase in vitamin E concentration could be achieved by increasing total tocopherols or by increasing the {alpha}/total tocopherol ratio. Recent research on tocopherols has shown that it is possible to change the {alpha}/{gamma} tocopherol ratio in favor of the {alpha}-form by overexpression of {gamma}-tocopherol methyltransferase ({gamma}-TMT) gene (Shintani and Dellapenna, 1998). This approach may allow for much higher levels of vitamin E in corn.

Received for publication April 8, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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