[Physiological genetics of quantitative characters in Arabidopsis thaliana (L.) Heynh. : Part 1: segregation and biosynthesis of pigments in chlorophyll-b defect mutants].

In biometric studies on the genetics of quantitative characters the problem of regulation of the activity of genes is rarely considered. Mutants of Arabidopsis thaliana (L.) Heynh. (Cruciferae), defective for chlorophyll b, permit a direct biochemical and physiological determination of their quantitative gene effects in the biosynthetic pathway of the plastid pigments. In agreement with other authors, the detectable genotypes are found to be based on multiple alleles (ch (+), ch (1) and ch (2)) at the ch locus. This evidence was obtained by test crosses of two mutants with the wild type and by a phaenotypic classification of the progeny on the basis of preparatory thin layer chromatography, paper chromatography tests of single plants, and quantitative spectrophotometry of the lines. In ch (1)/ch (1) the synthesis of chlorophyll b is completely blocked, in ch (2)/ch (2) only about 10 percent of the wild-type pigment is present, and in the heterozygote ch (2)/ch (1) a complementation effect is observed (i. e. superdominance of ch (2) over ch (1)) resulting in still more pigment production. In the segregating generations the variances for chlorophyll and carotinoids of all the genotypes are higher than in the parents and therefore suggest a possible polygenic background for pigment development. However, the physiological effect of the genetic background is slight as compared with the effect of the major gene ch. Preliminary experiments show that in tissue extracts of wild type leaves (ch (+)/ch (+)) there is an effective agent which enables irradiated chloroplasts of the defective ch (1)/ch (1) and ch (2)/ch (2) genotypes to synthesize chlorophyll b in vitro. Further information on the mechanism, extent and the time of action of the active agent should be obtained by varying the conditions of irradiation and by refining the tests for pigment precursors.