During 1993 and 1994, 105 different pedigrees of maize were evaluated for their protein content in the endosperm. A sample constituted of 30 to 60 grains was taken from each pedigree. The pericarp and the embryo were removed from each individual grain, then the endosperm was defatted, air-dried and milled to 60 mesh. Proceeding in this way, samples of each pedigree were prepared and the protein content for each was determined following the classic procedures of Kjeldahl method (A.O.A.C., 1965 and 1981).
The endosperm texture of these maizes is variable: some of them are flint, others are opaque2 and many of them are waxy. Thirty-seven pedigrees were selected as they had more than 10% protein content in the endosperm (see Table 3). These materials will be employed in future breeding plans.
The pedigrees analyzed could be classified as follows: a) inbred lines (Sn); b) F1 hybrids; c) F2 hybrids; d) First Backcross (BC1); e) populations; f) first sib generation (Sl1); g) first self-pollinated generation (S1) and h) second self-pollinated generation (S2).
The different F1 and F2 hybrids, as well as the BC1's, were obtained through different types of crossings such as:
INBRED LINE X INBRED LINE
INBRED LINE X POPULATION and its reciprocals
POPULATION X POPULATION
The data obtained from the several analyses confirm the hypothesis developed by Magoja (Com. Inv. UNLZ 3(6), 1980) in which a model useful to explain the inheritance mechanism for protein content was presented. This hypothesis was stated supposing crossing between inbred lines, but our results demonstrate that it is also suitable for crossing between heterozygotes.
Let a/a/a or a/a/A be the genotypes for low protein content and A/A/A or A/A/a the genotypes for high protein content. Table 1 presents 8 different cases of crossing between high and low protein content maizes, differing in their genotype (homozygous or heterozygous condition).
An analysis of data presented in Table 2 for Pedigree 3063 lets us conclude that the lowest protein content is overdominant. Nevertheless, data presented in the same table for Pedigrees 3062, 3065 and 3075 seem to confirm that high protein content is dominant, at least in some degree. Similar inheritance mechanisms for protein content were also reported by Genter et al. (Agron. J. 49:283-285, 1957).
Table 1. Crossings and genotype structure supposing a: low protein content
and A: high protein content.
|4||a/a/A||A/A/a||a/a/a;; a/a/A; A/Aa; A/A/A|
|8||A/A/a||a/a/A||A/A/A; A/Aa; a/aA; a/a/a|
Table 2. Protein content (in percent) for different types of crossings
according to the model presented in Table 1.
|1||ZN6 (9.7%)||H38 (10.2%)||93-3063 (8.8%)|
|3||SCV2 (8.1%)||ZN6 (9.7%)||93-3062 (10.4%)|
|4||SCV1 x A255 (9.3%)||SCV1 (10.8%)||93-3074 (10.1%)|
|4||SCV1 x A255 (9.3%)||FW (11.4%)||93-3075 (13.8%)|
|5||H38 (10.2%)||ZN6 (9.7%)||93-3065 (11.1%)|
|6||CM1 (12.3%)||WEM (8.7%)||93-3066 (11.4%)|
|8||SCV1 (10.8%)||SCV1 x A255 (9.3%)||93-3073 (10.7%)|
Observing Table 2 in its whole and according to the genotypes presented in Table 1, it may be concluded that, in addition to the dominance or overdominance observed, the genotype of the female parent of the crossing strongly conditions the protein content of the results observed for the reciprocal hybrids 3063 and 3065.
In many experiments of selection, negative correlations between yield (bu/a) and endosperm protein content were observed. For example, when high yielding plants are selected, the protein content is reduced and this fact could explain the generalized low protein content of commercial hybrids. Jugenheimer (Corn, J. Wiley & Sons, NY, 1976) analyzed in 145 maize inbreds the previously mentioned correlation and found a value of r = -0.522.
According to the previous paragraph in complete agreement with Bathia (Euphytica 24:789-794, 1975) it would not be advantageous to select separately by yield or protein content. The most effective way of selection would be considering simultaneously both traits (yield and protein content), which means to evaluate yield in protein content/grown surface (Kg protein/acre).
Table 3. Percentage of protein content and endosperm texture of the
37 pedigrees selected from the whole analyzed.
|Generation||Pedigree||Protein Content %||Endosperm Texture|
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