Agronomy Publications Agronomy

2005 Breeding for grain quality traits L. M. Pollak Department of Agriculture

M. P. Scott State University, [email protected]

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This Article is brought to you for free and open access by the Agronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Maydica 50 (2005): 247-257

BREEDING FOR GRAIN QUALITY TRAITS

L.M. Pollak*, M.P. Scott

USDA-ARS, Corn Insects and Crop Genetics Research Unit, Ames, Iowa 50011, USA

Received February 9, 2005

ABSTRACT - Plant breeders have been extremely success- of vertically integrated grain utilization systems can ful at improving the yield of . Grain quality has re- capture the added value in an improved quality ceived less attention; however important advances have product. been made by breeders in this area as well. Maize with a The feasibility of breeding for grain quality in wide range of compositions and fractions within the major maize is best illustrated by the Long-Term grain components has resulted from breeders taking ad- Selection experiment for protein and oil. This ex- vantage of advances in biochemistry and genetics over the last fifty years. Breeding for grain quality provides end periment has been running for over a century (HOP- users with grain better suited to their needs. Maize with KINS, 1899, reviewed in DUDLEY and LAMBERT, 2004), improved amino acid balance allows animal feed to be and has resulted in germplasm representing the ex- produced at a lower cost. Maize with altered fatty acid tremes of protein and oil content known in maize. composition allows production of healthier vegetable oil. In addition, this material has provided valuable in- Maize with altered properties allows improvement sights into the physiology of metabolism in maize of many products that rely on starch based gels, films and (BELOW et al., 2004). adhesives. Although mutants that impact these traits have The focus of this review is on breeding for quali- been widely used, quantitative genetic approaches have al- ty traits that are determined by the constituents of so been successful when applied in long-term breeding three major components of grain (protein, oil, and programs. Two successful approaches involve elements of starch) including amino acid content, fatty acid con- both approaches, including the development of QPM based on the o2 mutant with selection for improved kernel tent, and starch quality. types and the development of high maize based on the ae mutation with selection for increased amylose. AMINO ACID CONTENT KEY WORDS: Amino acid balance; Fatty acid content; Amylose; ; Mutants; Starch quality. Why amino acid content is important and breeding objectives The majority of maize is used for animal feed. Ru- INTRODUCTION minant livestock depend on microbial populations in the rumen to make dietary protein available for me- Breeding for grain quality has become economi- tabolism. This affords ruminants flexibility in the cally beneficial because the development of niche amino acid composition of their diets. In contrast, markets for specialty grains and increasing numbers monogastric animals including humans require cer- tain amino acids, termed essential amino acids, in their diets. Maize is deficient in lysine, methionine Abbreviations: CIMMYT, International Wheat and Maize Im- and tryptophan relative to the dietary needs of these provement Center; DSC, Differential Scanning Calorimetery; organisms. Deficiencies in these amino acids result in GEM, Germplasm Enhancement of Maize project; NIR, Near-in- poor utilization of maize protein, and nitrogen that is frared reflectance spectroscopy; NITS, Near-infrared transmit- not used is excreted as waste. In animal feed, these tance spectroscopy; QPM, deficiencies are corrected by dietary supplementation * For correspondence (fax: +1 515 294 9359, e.mail: lmpol- with other protein sources or synthetic amino acids, [email protected]). but this adds to the cost of the diet. Thus, an impor- 248 L.M. POLLAK, M.P. SCOTT

tant goal for improving maize for monogastric ani- amino acids into essential amino acids and would mals is to increase the level of the limiting amino probably involve increasing the ratio of non-zein to acids, while for ruminant animals, increasing the zein proteins. This is an attractive prospect because quantity of protein available is more important. it would not require increased nitrogen fixation or The major storage proteins of maize grain require increased inputs or decreased yield. This ap- belong to the prolamin family and are called the proach should be feasible because the level of es- zeins. These proteins make up 40-60% of the total sential amino acids has been demonstrated to be a protein and are so abundant that their heritable trait (DOTY et al., 1946). A strategy to im- properties have a large impact on the properties of prove protein quality by reducing the level of zeins maize grain protein as a whole. Zeins lack the es- while increasing the concentration of tryptophan sential amino acids lysine and tryptophan, and this and lysine was proposed by FREY et al. (1949). The deficiency is reflected in the amino acid balance of feasibility of this approach was demonstrated with the grain. Thus, plant breeding strategies for im- one cycle of selection for tryptophan content which proving the amino acid balance often involve some resulted in an increase in the tryptophan content of type of modification of the zein content. the population (FREY et al., 1949). Similarly, two cy- cles of selection for lysine content were conducted Quantitative genetic approaches to improving in three maize populations resulting in improve- amino acid content ments to lysine content accompanied by much Quantitative genetic approaches treat amino acid smaller changes in protein content (ZUBER and HELM, levels as a multigenic trait with continuous varia- 1972). A third cycle of selection was conducted and tion, and incremental improvements are made by all cycles were evaluated in the same environment. recurrent selection. This is analogous to the way Two of the three populations studied had increased that total protein was manipulated in the Illinois lysine levels and total protein content following each Long-term Selection experiment (DUDLEY and LAM- cycle of selection (ZUBER, 1975). Lysine content was BERT, 2004). One approach to improving levels of reported to have a relatively high heritability, and essential amino acids is to select for total protein high lysine levels could be maintained in subse- content. Not surprisingly, a positive correlation ex- quent generations of breeding (CHOE et al., 1976). ists between essential amino acids and total protein While recurrent selection to increase amino acid content (MILLER et al., 1950); however, the correla- levels has been shown to be successful, there are at tion is greater between tryptophan and non-zein least two impediments to recurrent selection for in- protein than between tryptophan and total protein creasing amino acid levels: the lack of inexpensive, (FREY et al., 1949). Consistent with these observa- high-throughput assays and the time investment re- tions, several studies have found that the levels of quired to generate substantial improvements with one or more essential amino acids are negatively multiple cycles of selection. correlated with total protein content when ex- pressed as a percentage of the total protein (FLYNN Use of mutants in breeding et al., 1954; TELLO et al., 1965). An additional prob- for amino acid content lem with this approach is illustrated in the Illinois The poor amino acid balance of maize protein is Long Term Selection populations in which a high a direct consequence of the amino acid composition protein population yielded less grain per land area of the major seed storage proteins, the zeins. Several than did a low protein population (BELOW et al., mutants exist in which zein content is altered and 2004). This exemplifies a well known correlation therefore the amino acid balance of the kernel is al- between protein content and grain yield. While ni- so altered. Kernels carrying the mutation, opaque2, trogen fertilization is an effective way to increase have elevated levels of the essential amino acids ly- grain yield of the high protein varieties (BELOW et sine and tryptophan due to a reduced content of al., 2004), increasing total protein remains a costly zeins (MERTZ et al., 1964). From the time of this dis- approach to increasing essential amino acid levels. covery to the present, opaque2 and other mutants Selection for the levels of specific amino acids that improve the amino acid balance such as floury2 and thereby an increase essential amino acid content have been used extensively in breeding programs. without increasing total protein content proportion- However, kernels carrying these mutations tend to ally should be possible. Physiologically, this could have a number of pleiotropic effects that reduce occur by reallocation of nitrogen from non-essential their agronomic adaptability. They tend to be soft GRAIN QUALITY TRAITS 249

(LOESCH et al., 1977), which makes them susceptible content of the grain. In addition, oils with certain to mechanical damage (LAMBERT et al., 1969). In addi- fatty acid compositions are well suited to certain end tion, mutant plants have poor germination (LOESCH et uses. Although is relatively stable to oxida- al., 1978) and reduced grain yield (LAMBERT et al., tive changes during storage because of its native fat- 1969; SREERAMULU and BAUMAN, 1970). Thus, much of ty acid composition, new information about the rela- the effort in breeding programs based on these mu- tion of fatty acids in the diet to human health has tants is devoted to overcoming these adverse shifted oil consumption to more highly unsaturated pleiotropic effects. In a thirty year effort, breeders at oils such as canola, soybean, and sunflower oils and CIMMYT (the International Wheat and Maize Im- to monounsaturated oils such as olive oil. Aggressive provement Center) have successfully done this to breeding for the fatty acid compositions of some of produce Quality Protein Maize (QPM). QPM varieties these oils has improved them. There is sufficient evi- carry the opaque2 mutation and have elevated lysine dence to suggest that increased consumption (within and tryptophan levels. However, the kernels are limits of total fat ingested) of certain groups of fatty harder than normal opaque2 types and therefore acids (monounsaturated and polyunsaturated) may much better adapted agronomically. The develop- be beneficial (NATIONAL RESEARCH COUNCIL, 1989). If ment and characterization of QPM are based on a corn oil is going to compete with these oils in the tremendous effort by many researchers at the Uni- future, the issue of altering its fatty acid content versity of Illinois, Purdue University, the Istituto must also be addressed. Sperimentale per la Cerealicoltura, and other institu- Typical Corn Belt corn contains oil having about tions, in addition to CIMMYT, and have been thor- 10% palmitic, 2% stearic, 25% oleic, 62% linoleic and oughly reviewed (BJARNASON and VASAL, 1992; PRASAN- 1% linolenic acids, with a total of 12% saturated fatty NA et al., 2001; VASAL, 2001). The development of acids. In a study using exotic germplasm, fatty acid QPM is clearly one of the most important advances composition was used as a classification criterion for in breeding for grain quality. Accordingly, the 2000 Italian corn populations, and wide variability for World Food Prize was awarded to Dr. Surinder Vasal composition was found (CAMUSSI et al., 1980). Be- and Dr. Evangelina Villegas for their roles in devel- cause wide ranges of fatty acid compositions can be oping QPM. It is estimated that QPM was produced found in adapted (DUNLAP et al., 1995b) and exotic on more than 1 million hectares in 2003. germplasm (JELLUM, 1970; DUNLAP et al., 1995a), it A high methionine line of maize was identified might be possible to develop specialty corn oils, by screening for seedlings resistant to levels of ly- each with a unique fatty acid pattern, just as different sine plus threonine that are normally inhibitory to plant species produce oils with unique compositions. growth (PHILLIPS et al., 1981). This line was designat- Some compositional goals and their uses are: ed BSSS53 and was later released as B101 (HALLAUER 1. Low total saturates of 6% or less to compete and WRIGHT, 1995). The associated with the el- with highly polyunsaturated fats such as canola evated methionine trait, dzr 1 (originally designated and new soybean oils. Zpr10/(22); BENNER et al., 1989) functions by in- 2. High total saturates to provide oil “naturally” creasing RNA levels for a 10 kDa zein which is high hardened for margarine manufacturing. By mini- in methionine (KIRIHARA et al., 1988). BSSS53 was mizing processing, this oil would be low in un- used as the donor parent to develop back-cross de- healthy trans fatty acids. Researchers have pro- rived inbreds that were tested in hybrid combina- duced soybean oil with these properties. tions. Hybrids derived from the back-crossed in- 3. High oleic acid to provide an oil high in mo- breds produced 23-43% higher methionine than nounsaturated fatty acids to compete with control hybrids (OLSEN et al., 2003). canola, olive and new soybean oils. Genetics are known to have a significantly greater impact than environment on corn oil fatty FATTY ACID CONTENT acid composition. Until recently, however, little or no effort has been made to establish gene pools that Why fatty acid content is important carry favorable for altering levels of certain and breeding objectives fatty acids, even though little doubt exists that corn Oil provides a concentrated source of energy for may be bred for these traits (ALEXANDER and CREECH, animals and therefore there is interest in increasing 1977). Research suggests that substantial progress the oil content of maize grain to increase the caloric could be made rather quickly. Plant introductions 250 L.M. POLLAK, M.P. SCOTT

representing a diversity of geographic origins and modified chemically to improve its suitability for agronomic characteristics were studied for fatty acid these uses. If starch could be modified genetically to compositions by JELLUM (1970). He found extensive meet these demands, the cost of products requiring ranges of variation in fatty acid composition in the would be reduced. Thus, one breed- exotic materials that were much higher than the ing objective is to produce starch with novel physical ranges previously reported for corn oil. JELLUM devel- properties that would benefit certain end users. One oped an inbred line (GE 180) from one of these in- of the most important physical properties of starch is troductions (PI 175334) that had 18-19% stearic acid, its ability to form a gel when heated with water. compared to 2% in normal corn oil (JELLUM, 1984). Thus, creating starch with altered thermal properties JELLUM and WIDSTROM (1983) reported that the high would be a valuable modification (WHITE, 1994). stearic acid trait was caused by a single major reces- accumulates in endosperm tissue in sive gene with modifier in sister lines of GE the form of insoluble granules. It is composed of two 180. Agronomic characteristics of GE 180 are poor glucose polymers, amylopectin and amylose, which and it has not been used commercially. A discovery differ in their chain length and degree of branching. by ALEXANDER and JELLUM (1992) led to an interna- Amylopectin is more highly branched and normally tional patent by The Lubrizol Corporation for corn constitutes about 75% of the starch granule, while oil with high oleic acid content (45-57%). Additional amylose is mostly linear and constitutes 25% of the breeding produced progeny with seed containing granule. These two polymers have different physical 65% oleic acid in the oil. NAGEL (1999) developed in- properties so much of the effort of breeding for bred lines through mutagenesis with low saturated unique starch properties have involved changing the fatty acid content that led to an international patent relative proportions of these two polymers. by Mycogen Plant Science, Inc., WRIGHT (1995) re- covered a gene from mutagenized B73 that in- Industrial processes creased the oleic acid concentration from 27 to 52%. One important breeding objective is to increase Other fatty acids that have been studied geneti- the amount of extractable starch during wet-milling. cally are linoleic, oleic, palmitic, and linoleic ZEHR et al. (1995) did a survey of wet-milling prop- (PONELEIT and ALEXANDER, 1965; JELLUM, 1966; PONELEIT erties of 15 inbred lines and 20 related hybrids. and BAUMAN, 1970; DE LA ROCHE et al., 1971; WID- They found, using a laboratory-scale procedure STROM and JELLUM, 1975). PONELEIT and BAUMAN (1970) (ECKHOFF et al., 1996), that gene action for starch re- determined that progress could be made in selecting covery after wet-milling appeared to be additive. for both fatty acid quantity and quality using breed- They also found a positive correlation between ing systems that exploit additive genetic variance. starch yield and kernel composition of starch using The lines used in the previous studies were near-infrared reflectance spectroscopy (NIR). largely derived from adapted U.S. germplasm. DU- DIJKHUIZEN et al. (1998) also found positive correla- VICK et al. (2003) utilized germplasm introgressed tions between starch composition determined by with genes from Tripsacum, a wild relative of corn, NIR and starch yield after a laboratory-scale wet- to alter fatty acid content in corn lines. Lines were milling procedure using families derived from a developed that had elevated levels of oleic acid and cross between the Illinois High Protein and the Illi- others had either elevated or lowered saturated fatty nois Low Protein corn populations. acids. The lines with elevated saturated fatty acid SINGH et al. (2001) evaluated wet-milling proper- composition had better oxidative stability than tradi- ties for 49 Latin American accessions used in the tional corn oil (SHEN et al., 1999). Germplasm Enhancement of Maize project (GEM) and found that they wet-milled poorly compared to the commercial check hybrids. In general, the exot- STARCH QUALITY ic populations had higher protein and lower starch composition levels than the hybrids, and poor Why starch quality is important starch-gluten separation in milling led to starch with and breeding objectives greater than acceptable protein content. In examin- Corn starch is used for many purposes including ing samples of two hybrids harvested at various food pastes and sweeteners, moisture absorbing stages of maturity, JENNINGS et al. (2002b) found that products, fuel ethanol production, and as a thicken- starch yield after a laboratory-scale wet-milling pro- ing agent in many products. Starch is frequently cedure was not affected by hybrid or maturity. GRAIN QUALITY TRAITS 251

However, DIEN et al. (2002) found that percent applications requiring films, and biodegradable of total starch available for conversion into ethanol packaging foam. VINEYARD and BEAR (1952) reported did vary significantly among hybrids when they un- the discovery of a mutant amylose extender (ae) al- derwent the dry-grind ethanol process, indicating lele, which increases the ratio of amylose to amy- that ethanol yield is not exclusively dependent on lopectin in starch, but does not change the total starch composition. A study of ethanol yields in 91 amount of starch in the grain. commercial hybrids showed a variability of 23% in FERGASON (1994, 2001) also reviewed the breed- ethanol yields (SINGH et al., 2004). Other factors that ing of high-amylose corn, which, compared to affected ethanol yield in their study were kernel breeding waxy corn, is more difficult. Mutant ae al- hardness with increased density, resulting in higher leles do not confer 100% amylose starch, but rather ethanol yields, and planting location. a range of amylose percentages depending on the Several seed companies have classified their hy- genetic background and environment. An unknown brids for highly fermentable starch using proprietary number of genetic modifiers interact with ae, con- NIR calibrations. However, there has been little tributing to the range of values of amylose found breeding for industrial processes in either the public when inbred lines are converted. In addition, the or private sector (SIMON, 2004). relative amount of amylose in breeding lines and populations must be determined by chemical analy- Waxy starch sis, thus the breeding program must be integrated Waxy starch refers to starch produced by vari- with a wet laboratory or have access to a source of eties carrying a mutant of the Waxy1 (Wx1) accurate amylose determinations. This adds a signif- gene. The Waxy1 gene is required for the biosyn- icant expense to the breeding program. Because en- thesis of amylose, and mutant alleles of wx1 pro- vironment does have a major influence on amylose duce waxy starch that is 100% amylopectin. Films levels, the separation of genetic versus environmen- made from waxy starch tend to be clear and waxy tal effects is important in order to make genetic starch gels are resistant to re-crystallization. Waxy progress. In spite of these limitations, selection and starch is also used in many adhesive applications. breeding have increased the amylose content of Maize with a wx1 mutant allele was discovered high amylose hybrids from approximately 50% to in in the early 1900’s (COLLINS, 1909), but did well over 70% (BOYER and HANNAH, 1994). not become commercially important until World Advances in breeding for amylose content can War II when starch, which had similar prop- come from new breeding methods, cheaper and erties, became unavailable. FERGASON (1994, 2001) more rapid methods of analysis, or new modifier has reviewed the breeding of waxy maize, which genes. CAMPBELL et al. (1997, 1999) investigated has largely been done in the private sector. Breed- near-infrared transmittance spectroscopy (NITS) ing of waxy maize is relatively straightforward be- with calibration for amylose content. Their calibra- cause waxy starch in kernels and pollen grains tion had limited precision and thus could not re- stains reddish-brown when treated with potassium place chemical analysis, but can be used by grain iodide while normal starch stains blue. The back- handlers needing to detect severe contamination, or cross breeding method has been used extensively by breeders doing initial screening of large sample to convert elite lines of the best commercial hybrids sizes. The NITS calibration did not discriminate well to the waxy phenotype. Because this method en- among with overlapping amylose con- sures that waxy hybrids will always be inferior to tents, for example in comparison of ae single to ae newer commercial hybrids, effort has been placed double mutants (CAMPBELL et al., 2000). In a study to upon developing waxy breeding populations as the compare NITS and a relatively rapid and inexpen- numbers of waxy converted inbreds increased. This sive wet chemistry method with the standard io- has led to the development of waxy hybrids with dine-binding wet chemistry method, CAMPBELL et al. small or insignificant yield differences compared to (2002) found the relatively rapid wet chemistry their non-waxy counterparts. method to be useful for identifying genotypes with high apparent amylose concentrations. Another im- High amylose starch portant finding of this study was that exotic In contrast to waxy starch gels, high amylose germplasm might be an important source of new starch gels are opaque and have a higher strength, modifying factors. Previously, ROBUTTI et al. (2000) making high amylose useful in confections, had found higher amylose contents in both high 252 L.M. POLLAK, M.P. SCOTT

starch content and low starch content races when strated a dosage effect at the su2 on starch examining 239 accessions of 12 Argentinean lan- structure and function. Previous investigations draces. The values were generally higher than showed no effect on amylose content by gene found in U.S. maize. dosage associated with the su2 allele of corn (KRAMER and WHISTLER, 1949); however, by evaluat- Other mutants ing functional characteristics such as DSC values, The relationship between the major structural viscosities, gel strengths, and structural characteris- features of starches (amylose and amylopectin) and tics such as x-ray diffraction, a dosage effect was their function is fairly well understood, but effects observed. Research examining the presence of of the fine structural characteristics on function are modifying factors in exotic germplasm on starch far more difficult to decipher. The application of dif- mutant effects showed wider ranges of DSC values ferential scanning calorimetry (DSC) to starch gela- in two su2 exotic populations than in su2 with a tinization with improved gel permeation chromatog- Corn Belt background (CAMPBELL et al., 1995b). Simi- raphy techniques to study starch fine structure has lar to the case with ae starch, this indicated that ge- aided in this pursuit. netic modifiers could be used to alter thermal prop- There is increasing difficulty in obtaining regula- erties and possible functional properties of su2 tory approval of chemically modified starches for starch. the food industry (SANDERS et al., 1990), so there is By increasing the size of starch particles, the effi- great potential for using novel starches from new ciency of wet-milling may increase because less corn lines. The production of corn starches with starch would be lost in the various steps in the naturally occurring modifications does not require process. In addition, particle size affects efficiency expensive processing for modifications, and so of starch modification and has a role in applications could provide additional profits over chemically such as degradable plastic films, powders, and fat modified starches. Also, because of consumer de- substitutes. The soft starch (h) gene, first described mands, there is a premium price paid by the indus- by MUMM (1929), causes the starch particles in the try for ingredients that are “natural”. In some cases, granule to be loosely packed. Several studies have the food industry may sacrifice quality to put all shown that the gene also causes the granules to be “natural” ingredients on the label thereby severely larger (BROWN et al., 1971; WANG et al., 1993c; WIL- limiting the range of functionalities of the available SON et al., 2000a,b). WILSON et al. (2000a) found the natural starches. trait to be predominantly controlled by additive but Several known endosperm mutants have large also by some dominance effects, suggesting that the effects on quality of corn starch (INOUCHI et al., allele could be used for developing hybrids that 1984; BROCKETT et al., 1988; SANDERS et al., 1990; wet-milled more efficiently. WILSON et al. (2000a) WANG et al., 1992, 1993a,b,c). Endosperm mutant along with GUTIÉRREZ et al. (2002), found variation genes also can be combined in multiple sets of two in starch granule size in normal inbreds and their or more genes that cause genetic interactions result- h/h conversions, while CAMPBELL et al. (1996) found ing in starches with unusual characteristics (BOYER variation in granule size in exotic germplasm. and HANNAH, 1994). These genetic interactions have been exploited commercially for high-value starches Quantitative and environmental factors as described in several patents (WURZBURG and FER- Factors other than those introduced through mu- GASON, 1984; ZALLIE et al., 1986; FRIEDMAN et al., tant genes also have been shown to affect starch 1988a,b,c,d,e,f; FRIEDMAN et al., 1989a,b). Production properties. KRUEGER et al. (1987) examined the effect of these starches was later abandoned because of of inbred line differences on the thermal properties the deleterious effect of the mutants on agronomic of normal cornstarch. With thermal properties as an productivity. indicator of starch primary chemical structure, they The sugary-2 allele (su2), first described by found significant variations among the maize lines. EYSTER (1934), has many effects on starch quality in- CAMPBELL et al. (1995a) examined thermal properties cluding higher amylose content and lower gela- of 26 adapted and exotic inbreds and also found tinization temperature content as measured by dif- highly significant differences, along with highly sig- ferential scanning calorimetry (INOUCHI et al., 1991), nificant inbred by year interactions. They also found resulting in a patent on the use of su2 starches correlations between thermal properties and starch (WHITE et al., 1994). CAMPBELL et al. (1994) demon- viscosity and gel strength, indicating that thermal GRAIN QUALITY TRAITS 253

properties measured by DSC may also be predictive mature grain (JENNINGS et al., 2002a). Variations in of other functional characteristics. This finding was DSC properties of inbred maize starches based on supported by results of SEETHARAMAN et al. (2001). LI kernel maturity were likely due to fine structural et al. (1994) further demonstrated wide genetic vari- differences during development (NG et al., 1997a). ability in the DSC thermal properties of starches CAMPBELL et al. (1994) examined thermal effects of from 35 tropical corn populations. WHITE et al. starch on inbreds and their F1 progeny at four (1990) demonstrated variability in thermal behavior planting dates, and found some differences in ther- among five open-pollinated populations of geneti- mal traits but not for amylose content or size of the cally variable maize and significant differences starch granules. Other studies illustrating the large among plants within the same population, indicat- environmental effect on starch quality were done by ing that genetic variability for thermal behavior of NG et al. (1997b), KRIEGER et al. (1988), and JI et al. the starches, and likely for starch structure, may ex- (2004). ist within populations. Other studies have shown significant variation in thermal and functional prop- erties of starches from Argentinean land races SUMMARY (SEETHARAMAN et al., 2001), exotic populations used in GEM (SINGH et al., 2001), and breeding materials The major components of maize are protein, oil from GEM (POLLAK, 2003). and starch. Breeders have successfully manipulated SINGH et al. (2001) found that gel strengths of these components for more than a century, result- starches recovered from GEM exotic populations ing in maize with a wide range of compositions. were greater than that from the commercial hybrids Over the last fifty years, advances in biochemistry used as checks. In another study comparing nine and genetics have allowed manipulation of fractions exotic inbreds with nine Corn Belt inbreds, howev- within these main grain components and these ef- er, the Corn Belt inbreds had wider ranges for most forts have resulted in grain that is better suited to a thermal traits than the exotic inbreds (POLLAK and variety of end uses. Amino acid balance, fatty acid WHITE, 1997). There seemed to be no consistent composition and starch physical properties are im- trend in comparing values of the inbreds with those portant targets for modification because they impact of reciprocal hybrids within sets, although there the value of grain for animal feed, human health was a trend toward reciprocal differences. and industrial applications. The most widely used Several studies describe the development of in- approach has been to use single gene mutants to breds from GEM exotic by adapted breeding crosses give step-wise improvements in these traits. Quanti- selected for unusual starch properties (JI et al., tative genetic approaches have been successful 2003a,b, 2004). The lines generally had low onset when applied in long-term breeding programs. The temperature of gelatinization and gelatinization over most successful approaches involve the use of re- a wide temperature range, explained by greater current selection and mutants together, for example, numbers of shorter branch chains of amylopectin the development of QPM based on the o2 mutant and different starch granule size distributions than with selection for improved kernel types and the those of normal starch. Some lines had two inde- development of high amylose maize based on the pendent gelatinization transitions, caused by two ae mutation with selection for increased amylose. separate types of granules within the endosperm. Quantification of gelatinization properties over gen- Outlook for the future erations of inbred development showed a signifi- The 50th anniversary of Maydica comes at a time cant interaction with breeding success for of transition in grain quality trait breeding. New tar- these traits. Exotic background had a large influence gets are developing as new uses for maize are de- on the inbreeding needed to genetically fix the un- veloped. The fastest growing use of maize is fuel usual gelatinization properties, and there was a ethanol production, which will benefit from in- genotype by environment interaction effect. creased levels of fermentable carbohydrates in Maturity of maize at harvest can affect starch grain. While mutants have been the basis of breed- quality. Two hybrids harvested at three stages of ing for grain quality traits for the past 50 years, rela- development showed that gelatinization started at a tively few new mutants with utility in breeding pro- lower temperature and occurred at a narrower grams have been identified in recent years in spite range of temperature for immature grain than for of efforts to produce new mutants by mutagenesis 254 L.M. POLLAK, M.P. SCOTT

(YAMIN et al., 1999). Technologies are emerging to dosperm of inbred IA 5125. Starch/Staerke 40: fill this niche, however. Biotechnology allows trans- 175-177. genic approaches that result in single-gene step- BROWN R.P., R.G. CREECH, L.J. JOHNSON, 1971 Genetic control of wise gains in quality traits in the same way that mu- starch granule morphology and physical structure in devel- tants do. Like traditional mutants, engineered genes oping maize . Crop Sci. 11: 297-302. require back-crossing to develop agronomically ac- CAMPBELL M.R., P.J. WHITE, L.M. POLLAK, 1994 Dosage effect at ceptable lines. And like traditional mutants, engi- the sugary-2 locus on maize starch structure and function. Cereal Chem. 71: 464-468. neered genes may have detrimental pleiotropic ef- fects that will have to be overcome, for example, CAMPBELL M.R., L.M. POLLAK, P.J. WHITE, 1995a Genetic variation for starch thermal and functional properties among nonmu- maize engineered to have reduced zein content has tant maize inbreds. Cereal Chem. 72: 281-286. opaque phenotype kernels (HUANG et al., 2004). CAMPBELL M.R., P.J. WHITE, L.M. POLLAK, 1995b Properties of sug- Thus, in the next 50 years, quality trait breeding will ary-2 maize starch: influence of exotic background. Cereal likely involve a shift from using natural or induced Chem. 72: 389-392. mutations to using transgenic approaches. Quantita- CAMPBELL M.R., J. LI, T.G. BERKE, D.V. GLOVER, 1996 Variation of tive genetic approaches will remain crucial to the starch granule size in tropical maize germplasm. Cereal success of these programs, and long-term invest- Chem. 73: 536-538. ments in recurrent selection for quality traits should CAMPBELL M.R., T.J. BRUMM, D.V. GLOVER, 1997 Whole grain amy- be encouraged. lose analysis in maize using near-infrared transmittance spec- troscopy. Cereal Chem. 74: 300-303.

ACKNOWLEDGEMENTS - This paper is a joint contribution from CAMPBELL M.R., S.R. MANNIS, H.A. PORT, A.M. ZIMMERMAN, D.V. the Corn Insects and Crop Genetics Research Unit, USDA-ARS, GLOVER, 1999 Prediction of starch amylose content versus and project no. 3781 of the Iowa Agriculture and Home Econom- total grain amylose content in corn by near-infrared transmit- ics Experiment Station, Ames, IA 50011. Names are necessary to tance spectroscopy. Cereal Chem. 76: 552-557. report factually on the available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use CAMPBELL M.R., J. SYKES, D.V. GLOVER, 2000 Classification of sin- of the name by the USDA implies no approval of the product to gle- and double-mutant corn endosperm genotypes by near- the exclusion of others that may be suitable. infrared transmittance spectroscopy. Cereal Chem. 77: 774- 778.

CAMPBELL M.R., H. YEAGER, N. ABDUBEK, L.M. POLLAK, D.V. GLOVER, 2002 Comparison of methods for amylose screening among amylose-extender (ae) maize starches from exotic back- REFERENCES grounds. Cereal Chem. 79: 317-321.

CAMUSSI A., M.D. JELLUM, E. OTTAVIANO, 1980 Numerical taxono- ALEXANDER D.E., R.G. CREECH, 1977 Breeding special industrial my of Italian maize populations: fatty acid composition and and nutritional types. pp. 363-390. In: G.F. Sprague (Ed.), morphological traits. Maydica 25: 149-165. Corn and Corn Improvement. American Society of Agrono- my, Madison, WI. CHOE B.H., M.S. ZUBER, G.F. KRAUSE, E.S. HILDERBRAND, 1976 In- heritance of high lysine in maize. Crop Sci. 16: 34-38. ALEXANDER W.L., M.D. JELLUM, 1992 Corn oil. International Patent WO 92/01367. COLLINS G.N., 1909 A new type of Indian corn from China. US- DA Bur. Plant Ind. Bull. 161: 7-30. BELOW F.E., J.R. SEEBAUER, M. URIBELARREA, M.C. SCHNEERMAN, S.P. MOOSE, 2004 Physiological changes accompanying long- DE LA ROCHE I.A., D.E. ALEXANDER, E.J. WEBER, 1971 Inheritance term selection for grain protein in maize. pp. 133-149. In: J. of oleic and linoleic acids in Zea mays L. Crop Sci. 11: 856- Janick (Ed.), Plant Breeding Reviews, Part 1: Long-term Se- 859. lection: Maize. vol. 24. John Wiley & Sons, Inc., Hoboken, DIEN B.S., R.J. BOTHAST, L.B. ITEN, L. BARRIOS, S.R. ECKHOFF, 2002 NJ. Fate of Bt protein and influence of corn hybrid on ethanol BENNER M.S., R.L. PHILLIPS, J.A. KIRIHARA, J.W. MESSING, 1989 Ge- production. Cereal Chem. 79: 582-585. netic analysis of methionine-rich storage protein accumula- DIJKHUIZEN A., J.W. DUDLEY, T.R. ROCHEFORD, A.E. HAKEN, S.R. ECK- tion in maize. Theor. Appl. Genet. 78: 761-767. HOFF, 1998 Near-infrared reflectance correlated to 100-g BJARNASON M., S.K. VASAL, 1992 Breeding of Quality Protein wet-milling analysis in maize. Cereal Chem. 75: 266-270. Maize (QPM). pp. 181-216. In: J. Janick (Ed.), Plant Breeding DOTY D.M., M.S. BERGDOLL, H.A. NASH, A.M. BRUNSON, 1946 Reviews, vol. 9. Timber Press, Portland, OR. Amino acids in corn grain from several single cross hybrids. BOYER C.D., L.C. HANNAH, 1994 Kernel mutants of corn. pp. 1- Cereal Chem. 23: 199-209. 28. In: A.R. Hallauer (Ed.), Specialty Corns. CRC Press, Boca DUDLEY J.W., R.J. LAMBERT, 2004 One hundred years of selection Raton, FL. for oil and protein in corn. pp. 79-110. In: J. Janick (Ed.), BROCKETT E., D. THOMPSON, T. DAVIS, C. BOYER, 1988 Gelatiniza- Plant Breeding Reviews, Part 1: Long-term Selection: Maize, tion characteristics of starch from du, wx, ae, and ae wx en- vol. 24. John Wiley & Sons, Inc., Hoboken, NJ. GRAIN QUALITY TRAITS 255

DUNLAP F.G., P.J. WHITE, L.M. POLLAK, 1995a Fatty acid composi- HALLAUER A.R., D. WRIGHT, 1995 Registration of B101 maize tion of oil from exotic corn breeding materials. J. Amer. Oil germplasm. Crop Sci. 35: 1238-1239. Chem. Soc. 72: 989-993. HOPKINS C.G., 1899 Improvement in the chemical composition of DUNLAP F.G., P.J. WHITE, L.M. POLLAK, T.J. BRUMM, 1995b Fatty the . Illinois Agric. Exp. Station Bull. 55: 205-240. acid composition of oil from adapted, elite corn breeding HUANG S., W.R. ADAMS, Q. ZHOU, K.P. MALLOY, D.A. VOYLES, J. AN- materials. J. Amer. Oil Chem. Soc. 72: 981-987. THONY, A.L. KRIZ, M.H. LUETHY, 2004 Improving nutritional DUVICK S.A., L.M. POLLAK, P.J. WHITE, 2003 Altered fatty-acid, quality of maize proteins by expressing sense and antisense protein, oil and starch corn lines and method for producing zein genes. J. Agric. Food Chem. 52: 1958-1964. same. USA Patent 6,639,132. INOUCHI N., D.V. GLOVER, Y. SUZIMOTO, H. FUWA, 1984 Develop- ECKHOFF S.R., S.K. SINGH, B.E. ZEHR, K.D. RAUSCH, E.J. FOX, A.K. mental changes in starch properties of several endosperm MISTRY, A.E. HAKEN, Y.X. NIU, S.H. ZOU, P. BURIAK, M.E. TUM- mutants of maize. Starch/Staerke 36: 8-12. BELSON, P. KEELING, 1996 A 100-g laboratory corn wet-milling INOUCHI N., D.V. GLOVER, Y. SUGIMOTO, H. FUWA, 1991 DSC char- procedure. Cereal Chem. 73: 54-57. acteristics of single-, double-, and triple mutants and their EYSTER W.H., 1934 Genetics of Zea mays. Bibliogr. Genet. 11: normal counterparts in the inbred Oh43 maize (Zea mays L.) 187. background. Starch/Stärke 43: 468.

FERGASON V., 1994 High amylose and waxy corns. pp. 55-77. In: JELLUM M.D., 1966 Fatty acid composition of corn oil of parental A.R. Hallauer (Ed.), Specialty Corns. CRC Press, Boca Raton, inbreds and reciprocal crosses. J. Hered. 57: 243-244. FL. JELLUM M.D., 1970 Plant introductions of maize as a source of FERGASON V., 2001 High amylose and waxy corns. pp. 63-84. In: oil with unusual fatty acid composition. J. Agr. Food Chem. A.R. Hallauer (Ed.), Specialty Corns. Second ed. CRC Press, 18: 365-370. Boca Raton, FL. JELLUM M.D., 1984 Registration of high stearic acid maize GE180 FLYNN L.M., M.S. ZUBER, D.H. LEWEKE, R.B. GRAINGER, A.G. HOGAN, germplasm. Crop Sci. 24: 829-830. 1954 Relation between protein content of corn and concen- JELLUM M.D., N.W. WIDSTROM, 1983 Inheritance of stearic acid in tration of amino acids and nicotinic acid. Cereal Chem. 31: germ oil of the maize kernel. J. Hered. 74: 383-384. 217-228. JENNINGS S., D. MYERS, L. JOHNSON, L.M. POLLAK, 2002a Effects of FREY K.J., B. BRIMHALL, G.F. SPRAGUE, 1949 The effects of selection maturity on corn starch properties. Cereal Chem. 79: 703- upon protein quality in the corn kernel. Agron. J. 41: 399-403. 706. FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, JENNINGS S.D., D.J. MYERS, L.A. JOHNSON, L.M. POLLAK, 2002b Ef- 1988a Food stuffs containing starch of a dull sugary-2 fects of maturity on grain quality and wet-milling properties genotype. USA Patent 4,792,458. of two selected corn hybrids. Cereal Chem. 79: 697-702. FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, JI Y., K. SEETHARAMAN, K. WONG, L.M. POLLAK, S. DUVICK, J. JANE, 1988b Food stuffs containing starch of an amylose extender P.J. WHITE, 2003a Thermal and structural properties of un- dull genotype. USA Patent 4,790,997. usual starches from developmental lines. Carbohydrate Poly- FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, mers 51: 439-450. 1988c Foodstuffs containing starch of a dull waxy geno- JI Y., K. WONG, J. HASJIM, L.M. POLLAK, S. DUVICK, J. JANE, P.J. type. USA Patent 4,789,557. WHITE, 2003b Structure and function of starch from ad- FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, vanced generations of new corn lines. Carbohydrate Poly- 1988d Starch of the duh genotype and products produced mers 54: 305-319. therefrom. USA Patent 4,774,328. JI Y., L.M. POLLAK, S. DUVICK, K. SEETHARAMAN, P.M. DIXON, P.J. FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, WHITE, 2004 Gelatinization properties of starches from three 1988e Starch of the wxsh1 genotype and products pro- successive generations of six exotic corn lines grown in two duced therefrom. USA Patent 4,767,849. locations. Cereal Chem. 81: 59-64.

FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, KIRIHARA J.A., J.P. HUNSPERGER, W.C. MAHONEY, J.W. MESSING, 1988 1988f Starch of wxfl1 genotype and products produced Differential expression of a gene for a methionine-rich stor- therefrom. USA Patent 4,789,738. age protein in maize. Mol. Gen. Genet. 211: 477-484.

FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, KRAMER H.H., R.L. WHISTLER, 1949 Quantitative effects of certain 1989a Foodstuffs containing starch of a waxy shrunken-2 genes on amylose content of corn endosperm starch. Agron. genotype. USA Patent 4,801,470. J. 41: 409.

FRIEDMAN R.B., D.J. GOTTNEID, E.J. FARON, F.J. PUSTEK, F.R. KATZ, KRUEGER B.R., C.A. KNUTSON, G.E. INGLETT, C.E. WALTER, 1987 A 1989b Foodstuffs containing starch of an amylose extender differential scanning calorimetry study on the effect of an- sugary-2 genotype. USA Patent 4,798,735. nealing on gelatinization behavior of corn starch. J. Food Sci. 52: 715-718. GUTIÉRREZ O.A., M.R. CAMPBELL, D.V. GLOVER, 2002 Starch particle volume in single- and double-mutant maize endosperm KRIEGER K.M., L.M. POLLAK, T.J. BRUMM, P.J. WHITE, 1988 Effects genotypes involving the soft starch (h) gene. Crop Sci. 42: of pollination method and growing location on starch ther- 355-359. mal properties of corn hybrids. Cereal Chem. 75: 656-659. 256 L.M. POLLAK, M.P. SCOTT

LAMBERT R.J., D.E. ALEXANDER, J.W. DUDLEY, 1969 Relative perfor- SANDERS E.G., D.B. THOMPSON, C.D. BOYERS, 1990 Thermal be- mance of normal and modified protein (opaque2) maize hy- havior during gelatinization and amylopectin fine structure brids. Crop Sci. 9: 187-190. for selected maize genotypes as expressed in four inbred lines. Cereal Chem. 67: 594. LI J., T.G. BERKE, D.V. GLOVER, 1994 Variation for thermal prop- erties of starch in tropical maize germplasm. Cereal Chem. SEETHARAMAN K., A. TZIOTIS, F. BORRAS, P.J. WHITE, M. FERRER, J. 71: 87-90. ROBUTTI, 2001 Thermal and functional characterization of starch from Argentinean corn. Cereal Chem. 78: 379-386. LOESCH P.J., R.L. GRINDELAND, E.G. HAMMOND, A.V. PAEZ, 1977 Evaluation of kernel hardness in normal and high-lysine SHEN N., S. DUVICK, P. WHITE, L. POLLAK, 1999 Oxidative stability maize (Zea mays L.). Maydica 22: 197-212. and AromaScan analyses of corn oils with altered fatty acid content. J. Amer. Oil Chem. Soc. 76: 1425-1429. LOESCH P.J., W.J. WISER, G.D. BOOTH, 1978 Emergence compar- isons between opaque and normal segregates in two maize SIMON K., 2004 Fuel farming The Corn and Soybean Digest. 67: synthetics. Crop Sci. 18: 802-805. 16-17.

MERTZ E.T., L.S. BATES, J. NELSON, O.E., 1964 Mutant gene that SINGH S.K., L.A. JOHNSON, P.J. WHITE, J.-L. JANE, L.M. POLLAK, 2001 changes protein composition and increases lysine content of Thermal properties and paste and gel behaviors of starches maize endosperm. Science 16: 279-280. recovered from accessions used in the Germplasm Enhance- ment of Maize project. Cereal Chem. 78: 315-321. MILLER R.C., L.W. AURAND, W.R. FLACH, 1950 Amino acids in high and low protein corn. Science 112: 57-58. SINGH V., G.S. MURTHY, J.V. GRAEBER, M.E. TUMBLESON, 2004 Grain quality issues related to corn dry grind processing Interna- MUMM W.J., 1929 A factor for soft starch in . Anat. Rec. tional Quality Grains Conference. pp. 2-7. 44: 279. SREERAMULU C., L.F. BAUMAN, 1970 Yield components and protein NAGEL B., 1999 Corn products with low saturated fatty acid con- quality of opaque-2 and normal diallels of maize. Crop Sci. tent and methods for their production. International Patent 10: 262-265. WO 99/65294. TELLO F., A. ALVAREZ-TOSTADO, G. ALVARADO, 1965 A study on the NG K.-Y., S.A. DUVICK, P.J. WHITE, 1997a Thermal properties of improvement of the essential amino acid balance of corn starch from selected maize (Zea mays L.) mutants during de- protein. Cereal Chem. 42: 368-384. velopment. Cereal Chem. 74: 288-292. VASAL S.K., 2001 High quality protein corn. pp. 85-129. In: A.R. NG K.-Y., L.M. POLLAK, S.A. DUVICK, P.J. WHITE, 1997b Thermal Hallauer (Ed.), Specialty Corns. Second ed. CRC Press, Boca properties of sixty-two exotic maize (Zea mays L.) lines Raton, FL. grown in two locations. Cereal Chem. 74: 837-841. VINEYARD M.L., R.P. BEAR, 1952 Amylose content. Maize Gen. NATIONAL RESEARCH COUNCIL, NATIONAL ACADEMY OF SCIENCES, 1989 Coop. Newsletter 26: 5. Diet and Health: Implications for Reducing Chronic Disease WANG Y.-J., P. WHITE, L. POLLAK, 1992 Thermal and gelling prop- Risk. erties of maize mutants from the Oh43 inbred line. Cereal OLSEN M.S., T.L. KRONE, R.L. PHILLIPS, 2003 BSSS53 as a Donor Chem. 69: 328-334. Source for Increased Whole-Kernel Methionine in Maize: Se- WANG Y.-J., P. WHITE, L. POLLAK, 1993a Physicochemical proper- lection and Evaluation of High-Methionine Inbreds and Hy- ties of starches from mutant genotypes of the Oh43 inbred brids. Crop Sci. 43: 1634-1642. line. Cereal Chem. 70: 199-203. PHILLIPS R.L., P.R. MORRIS, F. WOLD, B.G. GENGENBACH, 1981 WANG Y.-J., P. WHITE, L. POLLAK, J. JANE, 1993b Amylopectin and Seedling screening for lysine-plus-threonine feedback resis- intermediate materials in starches from mutant genotypes of tant maize. Crop Sci. 21: 601-607. the Oh43 inbred line. Cereal Chem. 70: 521-525.

POLLAK L.M., 2003 The history and success of the public-private WANG Y.-J., P. WHITE, L. POLLAK, J. JANE, 1993c Characterization project on Germplasm Enhancement of Maize (GEM). Adv. of starch structures of 17 maize endosperm mutant geno- Agron. 78: 45-87. types with Oh43 inbred line background. Cereal Chem. 70: POLLAK L.M., P.J. WHITE, 1997 Thermal starch properties in corn 171-178. belt and exotic corn inbred lines and their crosses. Cereal WHITE P.J., 1994 Properties of corn starch. pp. 29-54. In: A.R. Chem. 74: 412-416. Hallauer (Ed.), Specialty Corns. CRC Press, Boca Raton, FL.

PONELEIT C.G., D.E. ALEXANDER, 1965 Inheritance of linoleic and WHITE P.J., I. ABBAS, L. POLLAK, L. JOHNSON, 1990 Intra- and inter- oleic acids in maize. Science 147: 1585-1586. population variability of thermal properties of maize starch. Cereal Chem. 67: 70-73. PONELEIT C.G., L.F. BAUMAN, 1970 Diallel analyses of fatty acids in corn (Zea mays L.) oil. Crop Sci. 10: 338-341. WHITE P.J., L. POLLAK, L. JOHNSON, 1994 Starch-thickened acidic foodstuffs and method of preparation. USA Patent 5,356,655. PRASANNA B.M., S.K. VASAL, B. KASSAHUN, N.N. SINGH, 2001 Quali- ty protein maize. Current Science. 81: 1308-1319. WIDSTROM N.W., M.D. JELLUM, 1975 Inheritance of kernel fatty acid composition among six maize inbreds. Crop Sci. 15: 44- ROBUTTI J., F. BORRAS, M. FERRER, M. PERCIBALDI, C.A. KNUTSON, 46. 2000 Evaluation of quality factors in Argentine maize races. Cereal Chem. 77: 24-26. WILSON J.A., D.V. GLOVER, W.E. NYQUIST, 2000a Effect of dosage GRAIN QUALITY TRAITS 257

at the soft starch (h) locus on maize starch granule volume. ZALLIE J., R. TREMBLE, H. BELL, 1986 Bread containing wxsu2 Plant Breed. 119: 177-178. genotype starch as an antistalent. USA Patent 4,615,888.

WILSON J.A., D.V. GLOVER, W.E. NYQUIST, 2000b Genetic effects of ZEHR B.E., S.R. ECKHOFF, S.K. SINGH, P.L. KEELING, 1995 Compari- the soft starch (h) and background loci on volume of starch son of wet-milling properties among maize inbred lines and granules in five maize inbreds. Plant Breed. 119: 173-176. their hybrids. Cereal Chem. 72: 491-497.

WRIGHT A., 1995 A gene conditioning high oleic maize oil, ZUBER M.S., 1975 Protein quality improvement in maize. pp. OLC1. Maydica 40: 85-88. 166-184. In: H.D. Loden (Ed.), Thirtieth Annual Corn and Research Conference. H.D. Loden, D. Wilkinson, WURZBURG O.B., V.L. FERGASON, 1984 Starch thickness character- Chicago, IL. ized by improved low-temperature stability. USA Patent 4,428,972. ZUBER M.S., J.L. HELM, 1972 Approaches to improving protein quality in maize without the use of specific mutants. pp. 241- YAMIN F.F., M. LEE, L.M. POLLAK, P.J. WHITE, 1999 Thermal prop- 252. In: L.F. Bauman, P.L. Crane, D.V. Glover, E.T. Mertz, erties of starch in corn varieties isolated after chemical muta- D.W. Thomas (Eds.), High-quality protein maize. Dowden, genesis of inbred line B73. Cereal Chem. 76: 175-181. Hutchinson & Ross, Inc., Stroudsburg, PA.