Crop Science 42:1806-1811 (2002)
© 2002 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Inheritance of Medium Stearic Acid Content in the Seed Oil of a Sunflower Mutant CAS-4
Begoña Pérez-Vicha,
Rafael Garcésb and
Jose María Fernández-Martínez*,a
a Instituto de Agricultura Sostenible (CSIC), Apartado 4084, E-14080 Córdoba, Spain
b Instituto de la Grasa (CSIC), Apartado 1078, E-41080 Sevilla, Spain
* Corresponding author (cs9femaj{at}uco.es)
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ABSTRACT
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The sunflower (Helianthus annuus L.) line CAS-4 obtained by mutagenesis has an increased stearic acid (C18:0) content in its seed oil (130 g kg-1), which is desired for some food products. The objectives of this study were to determine (i) the inheritance of medium C18:0 content in CAS-4, (ii) the relationship between CAS-4 and the high C18:0 mutant CAS-3, and (iii) if recombinants with higher C18:0 content than CAS-3 could be obtained. CAS-4 was reciprocally crossed with its original parental line RDF-1-532 (80 g kg-1 C18:0), HA-89 (standard low 50 g kg-1 C18:0), and CAS-3 (250 g kg-1 C18:0). The fatty acid content of the F1, F2, BC1F1 to both parents, and F3 seeds was analyzed by gas-liquid chromatography (GLC). Alleles controlling low C18:0 exhibited partial dominance to those for medium C18:0, and these were partially dominant to those for high C18:0. Segregation patterns fit a two-loci model for the HA-89 x CAS-4 cross and one-locus model for the RDF-1-532 x CAS-4 and the CAS-3 x CAS-4 crosses. We concluded that medium C18:0 content of CAS-4 was controlled by alleles at the Es1 and Es2 loci previously identified in CAS-3, or at tightly linked loci. The CAS-4 alleles at the Es2 locus are those present in CAS-3 (es2es2), whereas the alleles at the Es1 locus are different from those of CAS-3 (es1es1) and have been designated es1bes1b. The proposed genotype (C18:0 content) of CAS-4 is es1bes1bes2es2. The continuous distribution observed in the HA-89 crosses with CAS-4 indicated that minor genes and environmental effects also are important in the expression of C18:0 content in CAS-4.
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INTRODUCTION
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INCREASE OF THE STEARIC ACID (C18:0) content in sunflower seed oil could enhance the oil quality for specific edible uses (Álvarez-Ortega et al., 1997). Increased levels of saturated fatty acids are desired for the food industry for the development of solid or semisolid fats without harmful chemical processes such as hydrogenation or transesterification (Kritchevsky et al., 1995; Ascherio and Willet, 1997). Moreover, compared with other saturated fatty acids, C18:0 is preferred because its neutral effect on serum lipoprotein cholesterol (Pearson, 1994). The use of induced mutagenesis permitted the development of different sunflower mutant lines with a higher C18:0 content than that found in standard commercial sunflower oil (C18:0 content of about 50 g kg-1) (Osorio et al., 1995). Two of them, CAS-3 and CAS-4, showed a five- and a three-fold increase, respectively, in their seed oil C18:0 content (Osorio et al., 1995).
The genetic control of the high C18:0 trait has been studied in the mutant CAS-3 (C18:0 of about 250 g kg-1) (Prez-Vich et al., 1999). The C18:0 inheritance pattern in crosses between CAS-3 and its original parental line, RDF-1-532, fit a one locus (designated Es1) model with two alleles (Es1, es1) and partial dominance of the allele controlling low C18:0 levels over that controlling high C18:0 content. Crosses between CAS-3 and a sunflower inbred line with standard low C18:0 content, HA-89, completed this genetic model. The segregation patterns in this cross indicated the presence of a second independent locus (designated Es2) with two alleles (Es2, es2). The Es1 locus had a greater effect on the C18:0 levels than the Es2 locus. The proposed genotypes for the three lines were CAS-3 = es1es1es2es2; RDF-1-532 = Es1Es1es2es2; and HA-89 = Es1Es1Es2Es2.
The genetic control of increased C18:0 content of other mutants has not been studied. The identification of loci for increased C18:0 content other than Es1 and Es2 could enable a further increase of stearic acid content in sunflower seed oil. The objectives of this study were to determine (i) the inheritance of medium C18:0 content in CAS-4, (ii) the relationship between CAS-4 and the high C18:0 mutant CAS-3, and (iii) if recombinants with higher C18:0 content than CAS-3 could be obtained.
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MATERIALS AND METHODS
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The lines used in this study (Table 1)
were the high C18:0 mutant CAS-3 (250 g kg-1 C18:0) and the medium C18:0 mutant CAS-4 (131 g kg-1 C18:0), obtained independently by Osorio et al. (1995) after chemical mutagenesis, their parental line RDF-1-532 (80 g kg-1 C18:0) (Osorio et al., 1995), and the inbred line HA-89 (46 g kg-1 C18:0), which has standard fatty acid profile and widely used for the development of commercial hybrids (Fernández-Martínez et al., 1993). Fatty acid methyl esters, used to determine fatty acid composition, were obtained as described by Garcés and Mancha (1993), then analyzed on a gas-liquid chromatograph with a 2-m-long column packed with 3% SP-2310/2% SP-2300 on Chromosorb WAW (Supelco Inc., Bellefonte, PA). The oven, injector, and flame ionization detector were held at 190, 275, and 250°C, respectively.
Half-seeds of the mutant line CAS-4 and the inbred line HA-89 were germinated, and after 15 d in a growth chamber [25/15°C (day/night) with 16-h day length], transplanted into pots, and grown under greenhouse conditions [35/15°C (day/night) with 16-h day length] in November 1994. At flowering, each head was covered with a paper bag to avoid contamination with external pollen. Plants of CAS-4 were reciprocally crossed with plants of HA-89. Crossing was done by emasculating florets of the female parent followed by pollination of their stigmas with pollen from the male parent. The parents and F1 half-seeds from each of the 10 crosses obtained were planted at the experimental farm of the Instituto de Agricultura Sostenible (sandy loam, deep alluvial, Typic Xerofluvent) at Córdoba (southern Spain) in Spring 1995. F1 plants were self-pollinated and backcrossed to both parents to obtain the F2 and BC1F1 seed, respectively. Reciprocal crosses between the two parents were repeated to obtain reciprocal F1 seeds in the same environment as the F2 and BC1F1 seed. Half-seeds of CAS-4 and HA-89, and 88 random F2 half-seeds were grown and F3 seeds were obtained from each F2 plant (F2:3 line).
Individual F1 and parent seeds obtained in the field in 1995 were analyzed for fatty acid composition by GLC. An evaluation of the fatty acid composition at the F1 plant level was made by averaging the GLC analyses of the F2 seeds from each F1 individual plant. The F2 generation was evaluated through the analysis of the fatty acid composition of 605 F2 half-seeds from reciprocal F1 plants. One hundred eighty-seven BC1F1 (F1/HA-89) seeds and 132 BC1F1 (F1/CAS-4) seeds were also analyzed by GLC. A progeny test was carried out through the analysis of 12 individual F3 half-seeds from each of 88 F2 plants. A 24-seed bulk of parent plants was also analyzed for fatty acid composition.
The mutant line CAS-4 was reciprocally crossed with its parental line RDF-1-532 in December 1995 under greenhouse conditions. F1 half-seeds were analyzed by GLC. A total of 12 F1 reciprocal plants were transplanted into the field in spring 1996. F1 plants were self-pollinated to obtain the F2 seed, and reciprocal crosses between the two parents were repeated to have F1 seeds in the same environmental conditions as the F2 seed. Two hundred thirty-four F2 half-seeds were analyzed by GLC.
Reciprocal crosses between the mutant lines CAS-3 and CAS-4 were made under greenhouse conditions in December 1994. F1 half-seeds were analyzed by GLC. A total of 11 F1 reciprocal plants were transplanted into pots and grown in a mesh cage in July 1995. F1 plants were self-pollinated and backcrossed to both parents, and reciprocal crosses between the two parents were also made. Seventy-eight BC1F1 (F1/CAS-3), 57 BC1F1 (F1/CAS-4), and 676 F2 half-seeds were analyzed by GLC. A total of 17 F2 half-seeds, representing all the classes for C18:0 concentration detected in the F2 generation, were selected, germinated, and transplanted into the field in spring 1996 to obtain the F3 seed. For the evaluation of the F3 generation, GLC analyses on 12 to 48 F3 half-seeds from each of the 17 F2 plants were done.
The distributions for BC1F1 and F2 seeds were divided into phenotypic classes on the basis of the C18:0 content of the parents grown in the same environment. The classes consisted of phenotypes with C18:0 values (i) equal to the parent with the least C18:0 content, (ii) equal to the parent with the greatest C18:0 content, and (iii) intermediate to the parents. Because of the existence of maternal effects in the cross between HA-89 and CAS-4, the C18:0 distribution in the BC1F1 (F1/HA-89) from this cross was divided into classes on the basis of the C18:0 content of the F1 seeds from the cross HA-89 x CAS-4, obtained in the same environment. Similarly, the C18:0 distribution in the BC1F1 (F1/CAS-4) from this cross was divided into classes on the basis of the C18:0 content of the F1 seeds from the cross CAS-4 x HA-89. For the HA-89 cross to CAS-4, F2:3 lines were classified into three categories on the basis of the pattern of within-line segregation: nonsegregating HA-89, with all the F3 seeds having a C18:0 content similar to HA-89; segregating, with at least one F3 seed having a C18:0 content intermediate to the parents; and nonsegregating CAS-4, with all the F3 seeds having a C18:0 content similar to CAS-4. A single-gene model (F2 genetic ratio of 3:1) was used to evaluate the CAS-4 cross with CAS-3, and the CAS-4 cross with RDF-1-532, and a two-gene model (F2 genetic ratio of 1:14:1) was assumed for crosses between HA-89 and CAS-4. Goodness of fit to tested ratios was measured by the chi-square statistic.
A continuous distribution for C18:0 was observed in the CAS-4 cross with HA-89; therefore, heritability estimates for C18:0 content were calculated by two methods. The formula provided by Mahmud and Kramer (1951) was used to compute heritabitily estimates on an F2 seed and F2:3 line (F3 seeds from each F2 plant averaged) basis:
where h2 = heritability estimate, 
2F2 = phenotypic variance among F2 seeds or F2:3 lines, 2P1 and 2P2 = phenotypic variances among seeds or plants of the parents. Heritability in standard units for F2 seeds was computed as the phenotypic correlation between the F2 seeds and the F2:3 lines (F3 seeds from each F2 plant averaged) (Frey and Horner, 1957).
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RESULTS AND DISCUSSION
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The C18:0 content of F1 seeds was significantly different from that of both parents and lower than the midparent values in all the crosses evaluated (Table 2)
. Mean C18:0 content in the reciprocal F1 seeds was 80 g kg-1 for crosses between HA-89 and CAS-4, 92 g kg-1 for crosses between RDF-1-532 and CAS-4, and 147 g kg-1 for crosses between CAS-3 and CAS-4, while the midparent values were 90 g kg-1, 101 g kg-1, and 172 g kg-1, respectively. These results indicated that alleles controlling low C18:0 content in HA-89 and in RDF-1-532 are partially dominant to those determining medium C18:0 in CAS-4, and that alleles controlling medium C18:0 content in CAS-4 exhibited partial dominance to those for high C18:0 content in CAS-3.
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Table 2. Mean stearic acid (C18:0) content at the seed or plant level of the sunflower parents HA-89 and CAS-4, RDF-1-532 and CAS-4, or CAS-4 and CAS-3, and of their reciprocal F1s.
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There was a significant difference in the C18:0 mean value between reciprocal F1 half seeds from the crossing of CAS-4 with both HA-89 and RDF-1-532 (Table 2). However, these differences were smaller than those between the F1 and each of the parents, indicating that the control of C18:0 content was mainly embryonic with a slight maternal effect. These differences between reciprocal F1 half seeds were not observed between reciprocal F1 plants (F2 seeds averaged) (Table 2), indicating the absence of cytoplasmic effects. No significant differences between C18:0 means of reciprocal F1 seeds were observed in crosses between CAS-3 and CAS-4 and is further evidence for the absence of maternal or cytoplasmic effects (Table 2).
The C18:0 values of the F2 progeny from reciprocal crosses between CAS-3 and CAS-4 were distributed in a bimodal pattern (Fig. 1) . Since C18:0 values in F1 seeds overlapped the range of variation of CAS-4 (Fig. 1), the first class was assigned to the combined category low-intermediate (C18:0 < 200 g kg-1), corresponding to the combined range of CAS-4 and the F1, and the second class to the high category (C18:0 
200 g kg-1), corresponding to the range observed for CAS-3. In the six F2 families evaluated, the C18:0 content fit a 3:1 (low-intermediate:high) ratio (Table 3) that would be expected for segregation of alleles at a single locus. There were some F2 seeds with a C18:0 content slightly lower than the seeds of CAS-4, or higher than those of CAS-3 (Fig. 1).
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Fig. 1. Distribution of stearic acid (C18:0) content in individual seeds of the sunflower parental lines CAS-4 and CAS-3, in F1 and F2 seeds from their cross, and in BC1F1 (F1/CAS-3) and BC1F1 (F1/CAS-4) seeds.
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Table 3. Number of seeds having different stearic acid (C18:0 content and chi-square analyses in the F2 and BC1F1 seeds from crosses between the sunflower mutant lines CAS-3 and CAS-4.
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The F3 generation of the cross CAS-3 x CAS-4 was evaluated to get an accurate characterization of the genotype of F2 half-seeds. Some F3 families were homogeneous for the phenotype of CAS-3 (Group III) or CAS-4 (Group I), whereas others segregated for C18:0 content (Group II) (Fig. 2
and Table 4)
. The evaluation of the segregating F3 families revealed in all cases a 3:1 (C18:0 < 200 g kg-1: C18:0 200 g kg-1) ratio, with no clearly transgressive F3 seeds (Table 4), which confirmed the segregation of one gene. Therefore, the few transgressive C18:0 values observed in the F2 probably resulted from environmental effects or minor genes. Bubeck et al. (1989) and Fehr et al. (1991) also reported the occurrence of F2 transgressive values in their studies on soybean [Glycine max (L.) Merr.] mutants, which were not observed in the next generation. This effect was also not attributed to major loci. The monogenic segregation for C18:0 content in the CAS-4 cross to CAS-3 was also confirmed with the analysis of the BC1F1 to both parents, which fit the expected 1:1 ratios (Table 3).
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Fig. 2. Distribution of stearic acid (C18:0) content in individual seeds of the pooled F3 families from groups I to III from the cross between the sunflower mutant lines CAS-4 and CAS-3.
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Table 4. Number of seeds having different stearic acid (C18:0) content and chi-square analyses in the F3 half-seeds from crosses between the sunflower mutant lines CAS-3 and CAS-4.
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The C18:0 content of the F2 seeds from crosses between the standard line HA-89 and CAS-4 showed a continuous distribution, ranging from 38 g kg-1 to 125 g kg-1 (Fig. 3)
. The C18:0 values in F2 seeds of the CAS-4 cross with RDF-1-532 ranged from 65 g kg-1 to 135 g kg-1. On the basis of the C18:0 values found in the parental lines grown under the same environment, the limit between a low (equal to HA-89) and an intermediate class was established as a C18:0 value of 60 g kg-1, which was the highest C18:0 content found among seeds of HA-89. The limit between an intermediate and a high class (equal to CAS-4) was defined as a C18:0 content of 105 g kg-1, which was the lowest C18:0 value found among seeds of CAS-4. F2 seeds segregated for C18:0 content following a 1:14:1 (C18:0 < 60 g kg-1: 60 g kg-1 
C18:0 < 105 g kg-1: C18:0 > 105 g kg-1) ratio for crosses between HA-89 and CAS-4 (Table 5)
, that would be expected for segregation of alleles at two independent loci. For crosses between RDF-1-532 and CAS-4, F2 seeds segregated for C18:0 content following a 3:1 (C18:0 < 105 g kg-1: C18:0 > 105 g kg-1) ratio (Table 5) that would be expected for segregation of alleles at one locus.
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Fig. 3. Distribution of stearic acid (C18:0) content in individual seeds of the sunflower parental lines HA-89 and CAS-4, in F1 and F2 seeds from their cross, and in BC1F1 (F1/HA-89) and BC1F1 (F1/CAS-4) seeds.
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Table 5. Number of seeds having different stearic acid (C18:0) content and chi-square analyses in the F2 and BC1F1 seeds from crosses between the sunflower lines HA-89 and CAS-4, and in the F2 seeds from crosses between RDF-1-532 and CAS-4.
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Of the 88 progeny-tested F2 plants from the CAS-4 cross with HA-89, seven were homozygous for the C18:0 content of HA-89, with homogeneous F3 half-seeds with a C18:0 content lower than 60 g kg-1, and two were homozygous for the C18:0 content of CAS-4, with homogeneous F3 half-seeds having C18:0 levels higher than 105 g kg-1. Seventy-nine F2 plants were segregating, with at least one F3 half-seed with a C18:0 content between 60 g kg-1 and 105 g kg-1. The observed genotypic frequency of the F2:3 lines (7:79:2) satisfactorily fit a 1:14:1 ratio (
2 = 2.67, P = 0.26), which confirmed the segregation of two genes.
Backcrosses to both parents were made for the cross between HA-89 and CAS-4. Because of the small C18:0 range in the BC1F1 generations and the existence of a partial maternal effect in this cross, the C18:0 distribution for the BC1F1 (F1/HA-89) seeds was divided into classes on the basis of the C18:0 content of the F1 seeds from the cross HA-89 x CAS-4, and that for the BC1F1 (F1/CAS-4) on the basis of the C18:0 content of the F1 seeds from the cross CAS-4 x HA-89 (Table 5). The segregation for C18:0 content in the BC1F1 seeds satisfactorily fit a C18:0 ratio of 3:1 in backcrosses to HA-89, and a ratio of 1:3 in backcrosses to CAS-4 for these classes based on F1 values (Table 5).
The segregation of crosses between HA-89 and CAS-4 indicated that two major genes with partial dominance of low C18:0 over medium C18:0 content are involved in the genetic control of the C18:0 levels in CAS-4. These genes were tentatively designated Esx and Esy (CAS-4 genotype: esxesxesyesy). Only one of these genes was segregating in crosses between RDF-1-532 and CAS-4, which suggested that the RDF-1-532 line already carried one of the partially recessive alleles involved in the control of the medium C18:0 trait in CAS-4. Pérez-Vich et al. (1999) also reported two major genes (Es1 and Es2) segregating in crosses between HA-89 and the high C18:0 mutant CAS-3, while only one of these segregated in the mating between CAS-3 and RDF-1-532.
Segregation from crosses between CAS-3 and CAS-4 lacked transgressive C18:0 values, which indicated that these lines had alleles for C18:0 content at the same loci, or tightly linked loci. Moreover, this segregation fit a one locus model, suggesting that both mutants differed in alleles at one locus, and shared the same recessive allele at the second locus. We hypothesized that the common allele was es2, since it has been demonstrated that this allele was already present in the original parental line of both CAS-3 and CAS-4 mutants (RDF-1-532; Pérez-Vich et al., 1999). As a result, the esy allele in CAS-4 mentioned above was identified as es2, and the esx allele in CAS-4 was named es1b, to distinguish it from the es1 in CAS-3 (Pérez-Vich et al., 1999). Therefore the proposed genotypes for the four lines used for this study are: HA-89 = Es1Es1Es2Es2, RDF-1-532 = Es1Es1es2es2, CAS-3 = es1es1es2es2, and CAS-4 = es1bes1bes2es2.
Although major genes are involved in the genetic control of the medium C18:0 trait in CAS-4, the continuous distribution of this fatty acid observed in the CAS-4 crosses with HA-89 (Fig. 3) suggested that minor genes and environmental effects also are important in the expression of this character. The segregation of this cross was analyzed as a quantitative character. Heritability estimates were 0.75 on a seed basis and 0.86 on a line basis. These different values were attributed to greater environmental variance among individual seeds than among lines. Phenotypic correlation between the C18:0 content of F2 seeds and F2:3 lines, equivalent to heritability in standard units (Frey and Horner, 1957) was 0.64. These heritability estimates were large enough to justify selection among F2 seeds. A similar behavior has been described for the low linolenic acid (C18:3) character in the soybean mutant line A5. Rennie and Tanner (1991) identified one major gene controlling the low C18:3 trait in A5 when crossed with another low C18:3 line. However, Graef et al. (1988) and Fehr et al. (1992) found that segregation for this trait from crosses between A5 and high-yielding soybean cultivars was continuous, which was attributed to the existence of minor genes affecting the genetic control of the low C18:3 trait (Fehr et al., 1992).
The third objective of this research was to obtain recombinants with higher levels of C18:0 in their seed oil than those present in the mutant parental lines, CAS-3 and CAS-4. This phenotype was not recovered because medium C18:0 content in CAS-4 seems to be controlled by alleles at the same two loci identified in the high C18:0 mutant CAS-3, or at tightly linked loci. However, the evidence for the role of minor genes in the genetic control of C18:0 indicate that through adequate strategies of selection, it might be possible to increase the levels of this fatty acid over the values currently available in CAS-3, as has been demonstrated in soybean. For example, Horejsi et al. (1994) obtained a further reduction in palmitic acid (C16:0) content in soybean by recovering plants carrying major alleles for reduced C16:0, and also favorable minor genes for this fatty acid in a backcrossing program.
Received for publication November 14, 2001.
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TOP
ABSTRACT
INTRODUCTION
