3. The Action of R and B. No anthocyanin pigment is produced in maize except in the presence of suitable alleles of A1, A2, and either R or B. For certain tissues B will serve as well or better than R; for others R is essential regardless of the presence of B. In those tissues which may be colored by the action of either R or B, the essential step in anthocyanin synthesis which is accomplished by R must be accomplished also by B, or it must be made unnecessary by some alternative step accomplished by B.
The effects of varying R action are shown by the phenotypes of the various R alleles and a similar comparison may be made for B by comparing it with the weakened B alleles described by Emerson in 1921. Several additional B alleles intermediate in action between B and b have been picked up in exotic strains and in dent corn varieties. Their study is not quite as convenient as that of the R alleles, but is facilitated by the use of Anderson's chromosome 2 inversion to intensify the linkage with seedling markers. The B alleles, like the R alleles, differ in the occurrence and the intensity of the pigmentation of various organs, and in their major plant color effect they may be arranged in a single sequence of increasing strength on the assumption of different thresholds of response in different tissues. The order of response of the different tissues is however quite different from that found for the R alleles. The standard B used produces rather strong pigmentation of the seedling leaf sheath, coleoptile and mesocotyl, and deep pigmentation of the mature sheath, blade culm, tassel, and cob. With successively weaker B alleles, blade color is restricted to the midrib and soon disappears, sheath color becomes weakened first in the lowermost sheaths and last in the middle sheaths. Glume color diminishes first at the tip region of the glume, and with successive steps is limited more and more closely to the base of the glume. In the weakest allele distinguishable from b, plant color is limited to a narrow transverse line at the base of the glume and to scattered streaks of color on the culms and sheaths of the middle internodes of the plant. The pigmentation of mesocotyl, coleoptile, and seedling sheath disappears early in this sequence, and most of the alleles give wholly colorless seedlings.
The response of R and B genotypes to sugar feeding of excised tissues (News Letter 1942: 31; Amer. Jour. Bot., 29: 17s) is sharply different. Sib plants of rch b and B rg (with A1, A2, Pl) are about equally colored in coleoptile and seedling leaf sheath. In later growth the latter becomes much more deeply colored in leaf sheath and blade. Excised leaf sections of the rch plants, in seedling or later stages, produce anthocyanin abundantly with externally supplied glucose, the amount of anthocyanin varying with the glucose concentration. Seedling leaf sections of the B plants produce no anthocyanin, regardless of the glucose concentration, and leaf sections taken at a stage when anthocyanin is being produced in the leaf show no effect of added sugar upon the rate of anthocyanin production. The presence of B in addition to rch does not increase the rate of anthocyanin production by the excised leaf sections, and the addition of B to weaker alleles of R, which produce anthocyanin at a lower rate than rch, does not increase their response to added glucose.
L. J. Stadler