Resumo:
Somatic mutations usually found in Coffea arabica L. result in most cases from doubling or reduction in chromosome number. Gene instability in somatic tissues was reported for the na na and xc xc alleles which control, respectively, habit of growth and leaf size, and the color of the ripened fruits. No case of somatic instability of genes responsible for the color of leaves has been reported previously. Young leaves of coffee plants are usually green or of a dark or light bronze color ; mature leaves are green. Leaves of plants of the variety purpurascens (Coffea arabica L. var. purpurascens Cramer), however, present dark purple young leaves and slightly purple mature leaves. Purpurascens behaves as recessive to green. Among the progeny of a backcross (green x purpurascens) x purpurascens, a seedling was found that possessed normal green leaves except for two opposite branches arising from the same node. One of the branches was entirely purpurascens ; normal green and purpurascens areas could be seen in the mature leaves of the other. Since this plant had originally a non purpurascens phenotype, it should be a heterozygous Pr pr or a homozygous pr pr possessing in a heterozygous condition a factor that suppresses the expression of pr. This factor is now under investigation. The appearence of purpurascens tissues in the plant mentioned above is considered as due to a mutation of the Pr allele or of the other factor already mentioned. It is also recognized that chromosome Isos or a deficiency at one of the two loci could have produced the same result.
