Breeders who are developing white hybrids for food and milling purposes strive for pure white colors in the kernels, because off-white colors can result in inferior products after processing. While some of these effects have not been studied genetically as far as I am aware (and are low-expression traits such as smoky gray or yellow coloring in the pericarp and endosperm), there are major factors affecting the colors whose phenotypic effects can be defined, and a summary may be useful. Problems arise especially when yellow endosperm-red cob inbreds from elite yellow dents are converged with white endosperm-white cob sources. The accompanying chart is an attempt to systematize the major factors that are involved, more to emphasize what is expected and what we do not yet know than to finalize a key for these expressions. Any comments or inputs from cooperators who have worked with these expressions in ways I may have overlooked would be appreciated.
The assumptions of the chart are that standard U.S. cornbelt genotypes are being used--that is, dominant for A1, A2, etc., but recessive for c1, r, and pl. The constitution c1 r pl determines colorless aleurone and embryo tissue (in the light or in the dark), and defines by pl that pigmentation in any parts of the growing plant (e.g., coleoptiles, plant base, anthers, etc.) will be light-dependent--i.e., sun-red. Pigment in these parts of the plant will be present if an active allele is present either at the B locus (B, the so-called "booster", or B-b, barred glumes) or at the R locus (r-r). Presence of pigment is regularly dominant to absence. At the B locus, barred alleles (represented by B-b in the chart) and null alleles (b) are common; stronger alleles from exotic sources will have the same effects as B-b, magnified in degree. Inbred 33-16 is B-b P-ww r-r pl; A619 is b P-ww r-r(w) pl; B37 is b P-wr r-r pl; K55 is b P-ww r-r pl; Mo17 is b P-wr r-r(w) pl; Mo20W is B-b P-ww r-r(w) pl. Some strains may carry dominant modifiers of plant color such as C2-Idf, which results in dosage-dependent reduction of some or all pigmentation. A very few breeding lines carry Pl, which will result in pigmentation that is sun-independent and stronger in expression. The effects of the salmon silk factor, sm, are interactive with P-wr and r-r, altering red silk color to salmon or brown depending upon genotype; these interactions are excluded pending clarification of the details. The designations r-r(s) and r-r(m) are intended to designate strong versus medium alleles, strong alleles including the "cherry" types; the expression in the pericarp of a cherry-red "stain" in areas exposed to light is one of the problem pigmentations. As indicated in the chart, reliably colorless pericarp is expected only in the b P-ww r-g constitution (no detectable pigment in any plant parts). Pearly white corns (including inbred K55) nonetheless often carry r-r and show red coleoptiles, leaf tips, anthers, etc.; either the r-r and the background genotype do not result in red "stain" color, or else light exposure never occurs on these ears.
In MNL 57:33-34, my note about expression of P-wr in the tassel has brought inquiries about the exact localization of the faint, yellow brown color in the hyaline margins of the tassel glumes. The accompanying drawing is marked to show the edges where the pigment is recognizable. I would be interested to know whether this pigmentation is found to be reliable as an indicator of cob color.
E. H. Coe, Jr.
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