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Progress in nutritional improvement of maize and triticale

E.M. Villegas,* B.O. Eggum,** S.K. Vasal,* and M.M. Kohli*

*International Maize and Wheat Improvement Center (CIMMYT), Mexico, D.F., Mexico
** National Institute of Animal Science, Animal Physiology and Chemistry, Copenhagen, Denmark

Introduction

Considerable interest exists around the world in upgrading the quality of protein in maize and other major cereal crops to improve their nutritional value. In some crops such as maize, barley, and sorghum, the quality of protein can be enhanced genetically by manipulation of known mutant genes, while in other crops the search for such mutant genes is still under way. Man has developed a new cereal crop named triticale. This cereal is produced by cross-breeding wheat ( Triticum) and rye (Secale). Under certain ecological conditions its yield out-performs that of wheat or rye. Although the cross of wheat and rye was demonstrated over 100 years ago, triticale remained much of a laboratory curiosity until the techniques of embryo culture and doubling of the number of chromosomes by colchicine treatment were developed in the 1940s. These techniques intensified and made the production of hexaploid triticales (durum wheat x rye) possible. The protein nitrogen content of triticale falls between the protein content of its two immediate parents. This appears to be also true of the proportion of essential amino acids present, the lysine content in triticale protein being generally higher than in wheat but lower than in rye.

Maize

In maize only half of the actual protein content present in the endosperm is of importance from the nutritional standpoint. This is so because roughly 50 per cent of the protein of maize endosperm is constituted by zein fraction, which is practically devoid of lysine in its amino acid profile. The mutant genes that affect the quality of protein in maize can reduce the synthesis of zein in protein, thereby resulting in an increased proportion of other protein fractions that have good levels of lysine and tryptophan. This alteration in the proportion of different protein fractions in maize endosperm is thus responsible for giving a boost to protein quality in maize.

Breeding for improved protein quality of maize endosperm through the use of different mutant genes has been under way for the past 13 years. Though several genes are known to almost double the levels of lysine and tryptophan in protein, only the Opaque-2 gene has been used extensively to convert normal maize genotypes to Opaque-2.

It may be of interest to point out some of the developments and the progress that have taken place since the biochemical effects of the Opaque-2 gene were first discovered.1 Historically, the years 1963 - 1964 generated considerable interest among maize breeders around the world to develop maize materials with superior protein quality in the endosperm. Straight Opaque-2 versions of normal, open-pollinated varieties and the parental inbred lines involved in hybrid combination were obtained in the first six to seven years of intensive research efforts. Some of these materials moved into commercial production in the early 1970s in different countries, but by and large they have failed to give a performance comparable to their normal counterpart maize materials. As soon as some of the Opaque-2 materials came into commercial production in some countries, the problems associated with Opaque-2 maize became more evident. Some of the problems that need to be highlighted include: (a) reduced kernel weight, (b) unacceptable kernel appearance, (c) greater vulnerability to ear rot organisms, (d) more infestation by weevils during storage, and (e) slower drying of grain following physiological maturity.2 Though minor problems still exist, a major breakthrough in remedying some of these problems has already been made. The period 1972 - 1977 may be considered to be of tremendous significance in the breeding of high-quality protein maize for the following reasons:

- Attempts to solve problems associated with quality protein maize were initiated.
- Basic information on the effects of the Opaque-2 gene was gathered in greater depth.
- Genetic and biochemical information on Opaque-2 modifiers in altering undesirable effects of the Opaque-2 gene has been accumulated.
- Interaction of the Opaque-2 gene with other endosperm mutants has been studied to solve problems confronting Opaque-2 maize.
-The relative importance of different problems affecting Opaque-2 maize was further assessed.
- Refinement in analytical techniques and new methods to detect the presence of protein quality were devised. Protein quality laboratories have been established in many national programmes to support breeding programmes to develop quality protein maize materials, and personnel training has been provided.
- Biological tests on acceptable types of Opaque-2 materials have been continued and have produced encouraging results demonstrating that protein quality was being maintained and was of superior biological value.
- Seed increase and commercial production of Opaque-2 materials started in some countries during this period.

Breeding of High-quality Maize

In breeding Opaque-2 materials with acceptable characteristics, one must consider, in the first place, the kinds of materials that are accepted in different countries. Depending on the need, therefore, the following points should be stressed: a. In areas where soft Opaque-2 corn is acceptable (e.g., the Andean region), the major emphasis in the programme should be on increased yield and greater resistance to ear rot organisms. b. Where hard flints and dents are preferred, emphasis should be on development of hard endosperm Opaque-2 corn comparable in performance to normal corn.

Development of Broad-based Hard Endosperm Opaque-2 Source Populations and Other Opaque-2 Materials at Advanced Stages of Development

Intensive research efforts have been under way for the last four years to develop hard endosperm Opaque-2 corns with modifiers accumulated from a wide range of maize materials being grown in different areas of the world, including ClMMYT's experiment stations in Mexico. The major emphasis in all such materials has been to increase kernel vitreosity while maintaining the same protein quality as that of soft Opaque-2 materials. In the initial cycles, all emphasis was placed on improving protein quality. At present, when kernel appearance has reached the point of acceptance, it is proposed to exert more pressure on the stability of the Opaque-2 modifiers responsible for changing soft endosperm to hard endosperm. Now the conversion of all tropical and temperate gene pools to hard endosperm Opaque-2 is being undertaken in the UNDP-CIMMYT Global Research Project. A special project is also being conducted to convert highland gene pools to Opaque-2, and a sugary-2/Opaque-2 conversion programme is in a very early stage.

The Protein Content and Quality of Opaque-2-Converted Materials

The ears selected in each generation of Opaque-2-converted materials undergo further selection for best vitreous segregates that are available in each ear. In general, ten seeds from each family are analyzed for protein and tryptophan content by the micro-Kjeldahl and Hernandez and Bates procedures, respectively.3 The families that do not meet the minimum acceptable quality levels are eliminated before pollination. This means that pollinations are restricted among selected families; for example, the mean values of several families analyzed in each population from different cycles for both protein content and quality are given in Table 1. It can be seen from this table that mean values for protein and tryptophan in protein of hard endosperm Opaque-2 versions are, in general, fairly good in advanced materials. Lysine is determined in selected materials following tryptophan determination.

In some materials where the breeder is interested in the quality of the whole kernel, dye-binding capacity (DBC) analysis is performed.4 The quality of protein in such materials is indicated by the quality index (QI) value, which is calculated by dividing the DBC value by the percentage of protein in the whole grain. Values above 3.5 represent good-quality protein. This QI correlates well with the percentage of lysine in protein. Table 2 shows that most of the materials analyzed by this method had good quality protein.

TABLE 1. Mean Values for Protein and Tryptophan in Endosperm of Quality Protein Hard Endosperm Opaque-2 Materials

Pedigree Origin No. of families Mean values
Protein (%) Tryptophan
in protein (%)
Mezcla tropical blanca HE 0-2 PR-76B825 42 7.9 0.78
Ant. x Ver.181 HE, 0-2 806 178 8.6 0.82
Mix. 1-Col. Gpo.1 x Eto HE 0-2 827 21 7.7 0.82
Mezcla amarilla HE, 0-2 828 29 7.7 0.80
Amarillo cristalino HE 0-2 807 188 8.5 0.77
Tuxpe˝o caribe HE 0-2 808 144 8.0 0.86
Eto blanco HE 0-2 837 - 8.1 0.77
Ant. x Rep. Dominicana PR-76B Lote 99 319 7.8 0.80
La Posta HE 0-2 809 85 7.8 0.91

 

TABLE 2. Mean Values for Protein and Quality Index in Some Hard Endosperm
Opaque-2 Materials (Whole kernel analyses)

Pedigree Origin No. of families Mean values
Protein (%) Quality index
Mezcla tropical blanca HE 0-2 PR-76B 803 134 9.8 4.0
Mix. 1-Col. Gpo. 1 x Eto HE 0-2 802 58 9.3 3.9
Mezcla amarilla HE 0-2 805 162 9.8 3.6
Amarillo dentado HE 0-2 801 96 7.8 4.1
White 0-2 BU pool PR-76B Lote 91 443 7.0 3.7
Yellow 0-2 BU pool Lote 92 430 9.3 4.7

Triticale

CIMMYT started working on triticale during 1964 - 1965 in collaboration with the University of Manitoba, Canada. During the 1970s this work was intensified with the financial assistance of the Canadian International Development Agency (CIDA), covering all aspects of triticale improvement. Currently, this new crop is being considered as an alternative source of human food and animal feed in many countries of the world.

Yield and Adaptation

Original triticale material received by CIMMYT from Canada had problems of floret infertility, excessive height, and lateness (photo - sensitivity) that caused low fields. In 1968 a triticale line was observed to have high fertility that could be inherited in the segregating populations. This line was named Armadillo and has been used extensively in the breeding programme. Good floret fertility combined with the dwarfness of Norin 10 gene (from wheat) has improved the yields of newer advanced lines considerably. The overall improvement in yield potential is indicated in Figure 1, which compares the yields of the top varieties of triticale and bread wheat grown in the Yaqui Valley, Sonora, Mexico. However, under marginal conditions, where wheat culture is very poor, triticale has shown a still larger increase in terms of yield. This favors introduction of triticale to these non-wheat-adapted areas first.

The transfer of photo-insensitivity from Mexican wheats to triticale has improved the adaptation of triticale in latitudes within 30░ N and 30░ S. At higher latitudes, where photo-sensitivity and/or vernalization is required, newer germ plasm derived from winter x spring triticales seems to be more adaptable. The results obtained from the International Triticale Yield Nursery (ITYN) identify the following areas where triticale is highly successful: (a) acid soils with aluminum, iron, or manganese toxicity-e.g., Brazil, Mexico, Ethiopia; (b) highland and mountainous regions with low temperatures-e.g., India, Pakistan, Nepal, Kenya, and the Andean region of South America; (c) areas endemic for the diseases of wheat-e.g., Kenya (stem rust), Andean region (stripe rust), North Africa (leaf blotch), etc.

FIG. 1. Yields of the Best Wheat and Triticale Strains

TABLE 3. Progress on the Development of High-Yielding Triticale Strains at CIMMYT

Advanced trials in Mexico ITYN
Year Identity Yield
kg/ha
Test wt.
kg/hi
Year Ave. yield
kg/ha
No. of
locations
1968 - 69 Bronco X224 2,356 64.4 1969 -70 2,579 39
1969 - 70 Arm T909 3,100 65.8 1970 -71 3,272 17
1970 - 71 Badger PM122 4,492 68.5 1971 -72 3,274 34
1971 - 72 Arm X208-14Y 5,490 65.4 1972 -73 3,716 25
1972 - 73 Cinammon 5,550 66.8 1973 -74 4,437 47
1973 - 74 Maya II-Arm X2802 6,300 70.0 1974 -75 4,746 45
1974 - 75 Yoreme 7,000 71.0 1975 -76 4,483 60
1975 - 76 Beagle 7,500 68.0      
1976 - 77 Mapache 8,000 72.0      

Progress on the development of high-yielding and widely adapted triticale strains at CIMMYT is summarized in Table 3

Triticale is grown commercially in Hungary, the USSR, China, Spain, South Africa, Argentina, Mexico, Canada, and the USA. Expanded production of the crop is expected to continue initially in the areas mentioned above.

TABLE 4. Triticale Lines of Test Weight above 72 kg/hi and Yield above 7,000 kg/ha, Y-76/77

Variety
No.
Cross and pedigree Test
wt.
Yield
kg/in
19 4 CML-PATO SEL 495 76.9 7,050
3 13
IAX1648-1N-4M-6Y-3M
1Y-0M
75.5 7,067
* M1AX2148-5N-2M-3Y-2M-0Y 76.3 7,223
7 21 IRA-COQUENA X14595-2Y -2Y-2M-0Y 75.9 7,067
10 28 IRA2-M2A2 X 11308-B-2M3Y-2Y-4M-0Y 77.5 7.050
2 23 KLA-IAX8814-D-3Y-1M-1Y-0Y 75.3 7,188
4 16 M2A-IRA-X11923-30M-1Y-0Y 76.0 7,096

*Average of two or more tests.

Grain Type

A major problem in the improvement of triticale is that of overcoming the tendency of the seed to shrivel at maturity. The degree of seed shrivelling is reflected in its low test-weight and thereby total grain yield, with less flour extracted during milling.

Although triticale lines tend to produce fair to good test-weights under optimum crop conditions, adverse growth conditions reduce weight sharply. During the 1970s a constant effort has been made to improve the grain type in triticales. As a result, newer strains in 1977 had much better kernel density and test-weight. Table 4 shows seven advanced lines with a yield over 7 tons/ha and a test-weight of over 75 kg/hi. In addition, there is a total of 87 lines with yields over 6,500 kg/ha and test-weights ranging from 70 to 77 kg/hi.

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