Genetic variation in total carotene, iron, and zinc contents of maize and cassava genotypes

Genetic variation in total carotene, iron, and zinc
contents of maize and cassava genotypes
B. Maziya-Dixon, J. G. Kling, A. Menkir, and A. Dixon
Abstract
Introduction
Deficiencies of vitamin A, iron, and zinc are widespread
Maize and cassava provide a large proportion of the
in sub-Saharan Africa, where the diets are mainly plant-
daily intake of energy and other nutrients, including
based and the intakes of animal products are low. The micronutrients, for poor populations in many areas
overall objective of this investigation was to determine of sub-Saharan Africa that have limited access to
the extent of genetic variation of these micronutrients in
animal foods [1]. Cassava plays an important role in
16 yellow-seeded improved maize varieties, 109 maize food security by providing a stable food base in areas
inbred lines (60 from mid-altitude and 49 from lowland/
prone to drought and during periods of civil distur-
savannah agroecologies), and 162 cassava clones resist-
bances because of its flexibility in time of planting
ant to the cassava mosaic disease. The yellow-seeded and harvesting and its tolerance to poor soil and pest
improved maize varieties were analysed for physical and
or disease problems. Root crops are often considered
chemical characteristics and total carotene content; the to be primarily sources of low-cost energy, but not
maize inbred lines and cassava clones were analysed important sources of other nutrients. However, protein
for iron and zinc content. The results showed statisti-
content and essential vitamins and minerals vary
cally significant and large genotypic differences in total considerably among roots and tubers [2]. Developing
carotene content among the 16 yellow-seeded improved,
cultivars of maize and cassava with high available
open-pollinated maize varieties. The total carotene con-
micronutrient content could significantly improve the
tent ranged from 143 to 278 µg/g. Significant genotypic health and nutritional status of the poor, especially
variation was also observed for iron and zinc concentra-
women and children. Because consumers in some areas
tions in maize inbred lines and cassava storage roots. of West Africa prefer maize with yellow endosperm
Iron concentration ranged from 15 to 159 ppm for mid-
and add palm oil during the processing of cassava
altitude and from 14 to 134 ppm for lowland maize to gari (roasted cassava granules) to impart a yellow
inbred lines; zinc concentration ranged from 12 to 96 colour, the problem of vitamin A deficiency can be
ppm for mid-altitude inbreds and from 24 to 96 ppm addressed through consumption of yellow-coloured
for lowland inbred lines. For cassava storage roots, the genotypes of both maize and cassava.
range was 4 to 95 ppm for iron and 4 to 18 ppm for zinc.
A breeding programme was initiated at the Inter-
A strong and positive relationship was observed between
national Institute of Tropical Agriculture (IITA) in
iron and zinc concentrations for both mid-altitude and 1998 to develop maize varieties and cassava clones
lowland maize inbred lines, but this relationship was with high levels of provitamin A carotenoids, iron,
weak for the cassava clones. The potential exists for and zinc. Selection of maize and cassava for improved
improving carotene, iron, and zinc contents in maize and
micronutrient content is dependent on knowledge of
cassava genotypes through plant-breeding.
the extent of genetic variation expressed in a given
environment. A second requirement for selection of
superior genotypes with high micronutrient content
is that the trait is maintained across different environ-
ments. The objective of the research reported here
was to determine the extent of genetic variation for
The authors are affiliated with the International Institute of
total carotenoid, iron, and zinc in maize and cassava
Tropical Agriculture in Ibadan, Nigeria.
genotypes.
Food and Nutrition Bulletin, vol. 21, no. 4 © 2000, The United Nations University.
419

---

420
B. Maziya-Dixon et al.
Materials and methods
TABLE 1. Total carotenoid and fat contents of 16 adapted
improved maize varieties
Maize genotypes

Carotenoids
Genotype Fat
(%)
(µg/g)
Sixteen yellow-seeded, improved, open-pollinated
IK 91 TZL COMP3-Y-C1
5.2
143
maize varieties were grown in a replicated field trial at
TZB-SR-SGY 4.1
159
the IITA research farm, Ibadan, Nigeria, under uniform
TZSR-Y-1 C4
4.8
164
conditions during the first rainy season (March to
TZESR-Y-1 3.6
172
August) in 1998. The genotypes were selected for
TZUTSR-W-SGY 4.3
173
potential variability in carotenoid content based on
TZEE-Y 3.7
179
kernel colour. Within each variety, bulk pollen was
AK 9331-DMR-SR
3.9
186
used to avoid pollen contamination from other varie-
SUWAN-2-SR 3.8
194
ties. In addition, 109 elite inbred lines developed at
EV 8728-SR
3.1
204
IITA for the mid-altitude and lowland agroecologies
ACR 91 SUWAN-1-SR
5.2
210
of West and Central Africa were grown in a field trial
MAKA-SR 5.2
212
POOL 26 SEGUA
3.8
216
at Ibadan under irrigation during the 1995 dry season
AK94-DMR-ESR-Y 5.6
222
(November to April). The trial was not replicated.
AK 9528-DMR
4.6
228
The inbred lines were planted in December 1995 and
DMR-LSRY 4.1
266
harvested in May 1996. Seeds of inbred lines were pro-
STR-SYN-Y 7.1
278
duced by self-pollination in order to avoid contamina-
tion. At maturity, the maize was hand harvested, and
Mean 4.55
200
ears with poor seed sets were discarded. The ears were
LSD 0.33
3.41

artificially dried at 40°C until the moisture was reduced
to a range of 11% to 12%. The ears were hand shelled
The maize kernel contains two fat-soluble vitamins:
to minimize physical damage and were stored at 10°C
provitamin A carotenoids and vitamin E. Caroten-
and brought up to 25°C before analysis. Samples of oids are found mainly in yellow maize genotypes, in
the inbred lines were bulked and kept in cold storage amounts that can be genetically controlled, whereas
(4°C) until needed for analysis. They were brought up
white maize varieties have little or no carotenoid
to 25°C before analysis.
content. Most of the carotenoids are found in the
hard endosperm, and only small amounts are found
Cassava genotypes
in the germ [4]. Weber [5] analysed 15 yellow corn
inbred lines and found total carotenoid concentrations
A total of 142 improved cassava clones and 20 African
ranging from 30 to 77 µg/g. The carotene content frac-
landraces of cassava selected for various agroecologies
tion constituted half of the total carotenoids in some
of sub-Saharan Africa and resistant to cassava mosaic
inbred lines. In others the carotene content exceeded
disease were grown in a replicated field trial at the the xanthophyll levels, indicating that inbred lines
IITA research farm at Ibadan, Nigeria, in 1997 and could be selected for high percentages of carotenes
harvested 12 months after planting. Random samples
or xanthophylls. When the distribution of caroten-
of five storage roots of each clone were peeled, shred-
oids in hand-dissected corn kernel fractions (horny
ded, and dried in an oven at 40°C until they attained endosperm, floury endosperm, and germ) was ana-
constant weight. Samples were ground to pass through
lysed in four inbred lines [5], 74% to 86% of the
a 1.0-mm sieve and stored in an airtight container carotenoids were found in the horny endosperm, 11%
before analysis for iron and zinc content.
to 20% in the floury endosperm, and only 1% to 4%
in the germ.
Our results show that genotypic variation for total
Results and discussions
carotenoid content exists among varieties. Further
studies on ?-carotene content and its conversion to
Table 1 shows the total carotenoid and fat contents of
vitamin A are needed. In addition, exotic germplasms
the 16 improved maize varieties. Significant differences
need to be screened to identify suitable sources of
among varieties were observed for total carotenoids yellow genes that can be used in a breeding pro-
and fat content. The fat content ranged from 3% to gramme.
7%, and the total carotenoid content varied from 143
Statistically significant (p < .01) genotypic dif-
to 278 µg/g, with an overall mean of 200 µg/g. Our ferences in iron and zinc concentrations between
results are in agreement with those of Carballido et mid-altitude and lowland inbred lines of maize were
al. [3], who reported a range of 82 to 280 µg/g for
observed. The iron content varied from 15 to 159
total carotenoids in hybrid corn grown in different ppm, whereas the zinc content ranged from 12 to 96
regions.
ppm for mid-altitude inbred lines; the iron and zinc

---

Genetic variation in total carotene, iron, and zinc
421
concentrations of the lowland inbred lines ranged from
for production and consumption. A positive relation-
14 to 134 ppm and from 24 to 96 ppm, respectively ship was found between iron and zinc concentration,
(table 2). There appeared to be more genetic variation
suggesting that selecting for high iron concentration
for iron than for zinc. The frequency distributions for
in maize grain and cassava roots will not have a nega-
lowland and mid-altitude inbred lines indicate that a tive effect on zinc or vice versa. Therefore, it can be
tremendous potential for developing inbred lines high
concluded that a potential exists for developing maize
in iron and zinc concentrations exists. A strong and genotypes and cassava clones high in both iron and
positive relationship was observed between iron and zinc.
zinc concentrations for the mid-altitude maize inbred
Although the present results are encouraging, this
lines (r = 0.88, p < .01) and for the lowland inbred study should be considered preliminary, because more
lines (r = 0.62, p < .01). An in-depth study involving in-depth studies are under way to identify the best
replicated field trials will be undertaken to verify the sources of micronutrients in African landraces and
observed results.
improved and introduced germplasm of both maize
Significant differences (p < .01, r2 = 0.70) among cas-
and cassava. Determination of the genetic relationships
sava clones were observed for iron and zinc concentra-
for micronutrient content will optimize the use of
tions. Iron concentration varied from 4 to 49 ppm and
these genetic resources in the breeding programme.
zinc concentration from 4 to 18 ppm (table 2). For Research will be undertaken to determine the mode
iron, 15% of the clones were more than two standard
of inheritance of micronutrient concentrations in
deviations above the overall mean. For zinc, 12% of the
maize and cassava. Such information is important to
clones were more than two standard deviations above
combine increased levels of provitamin A, iron, and
the overall mean. However, a very weak relationship zinc in a single genotype or clone through hybridiza-
(r = 0.18, p < .01) was found between iron and zinc tion and introgression of the germplasm sources and
contents in cassava.
selection.
There is also the need to develop a simple visual
method that can be used by breeders in selecting
Looking ahead
for high micronutrient content. Information is also
needed on the expression and stability of high con-
This investigation shows that genotypic variation exists
centrations of carotene, iron, and zinc under different
for total carotenoid content among maize genotypes environmental conditions and their stability under
and for iron and zinc content among maize and cas-
different processing and storage methods. In addition,
sava genotypes. Although some of the yellow-seeded the bioavailability of these micronutrients in promis-
improved maize varieties have high concentrations of
ing genotypes will be investigated using animal models
total carotenoids, it will be necessary to determine the
and chemical methods. Antinutritional and other
proportion of carotenoids with provitamin A activity
factors, which have been implicated as promoters or
and their bioavailability and stability during process-
enhancers of iron and zinc absorption, will also be
ing and storage before such varieties are promoted investigated.
TABLE 2. Simple descriptive statistics (mean, standard deviation, minimum, and maximum)
for iron and zinc concentrations (ppm) of maize and cassava
Crop Mineral
n
Mean SD Min Max
60 maize mid-altitude
Iron
60
39.84
37.05
14.70
159.43
inbred
lines
Zinc

60 24.63 17.93 11.65
95.62
49 maize lowland
Iron
49
87.30
30.72
13.60
133.82
inbred
lines
Zinc

49 49.82 17.40 23.50
94.94
2 cassava clones
Iron
162
18.76
6.10
3.50
48.85
(storage roots)
Zinc
162
9.86
2.66
4.32
17.97

---

422
B. Maziya-Dixon et al.
References
1. Allen LH. The Nutrition CRSP: What is marginal
maize. Anal Bromatol 1971;23:291.
malnutrition and does it affect human function? Nutr
4. Food and Agriculture Organization. Maize in human
Rev 1993;51:255–67.
nutrition. Food and Nutrition Series No. 25. Rome:
2. Horton D. Underground crops. Long-term trends in
FAO, 1992.
production of roots and tubers. Morrilton, Ark, USA:
5. Weber EJ. Carotenoids and tocols of corn grain deter-
Winrock International, 1988.
mined by HPLC. Journal of the American Oil Chemists
3. Carballido A, Lorento V, Centeno M. Composition of
Society 1987;47:337–9.

---