In 1927 Demerec (J. Hered.18:421) described a factor (Md) that determines pigment in cells surrounding the vascular bundles that themselves surround the 'pit' (pith). The pigment controlled by Md is independent of cob and pericarp pigment, and, according to Demerec, is not changed in the presence of homozygous a a. We have several stocks segregating for this type of pigment distribution, but we also have stocks that are pigmented in the pith itself, the most common of which are those carrying a 'strong' P-RR allele, which consistently have pigment in the cob pith as well as in the rest of the cob and pericarp. Such P alleles also determine tassel pith pigment, and plants can be characterized by breaking the tassel to expose the pith. P-WR and P-WW plants do not have this pith pigment, but another P allele which determines a 'grainy' pericarp and cob pigment consistently has strong pith pigment.
As well as stocks with P-determined pith pigments, we have stocks segregating for a factor determining strong anthocyanin pigment in the pith. Pl is required (we have not eliminated the possibility that a Pl variant might be responsible, although we think not), and also required is an R-g allele that 'responds' to a3 by producing plant anthocyanins (i.e., Stadler's Group D or Group 4 R alleles). Expression is best when the plant is heterozygous or homozygous a3, but we have at least one 'R-g' allele, that in A3 stocks responds to the presence of 'Pth' by producing not only pith pigment, but red anthers and pericarp pigment also. No other R or B alleles allow the pith pigment to be expressed, not even R-ch', r-ch, or B which together with Pl, can produce strong purple pigments in other cob tissues.
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