[Physiological genetics of quantitative characters in Arabidopsis thaliana (L.) Heynh. : Part 2: Modification of primary and secondary genetic effects of single-gene chlorophyll-b mutants by long-wave irradiation].

An analysis of the quantitative molecular effects of genes requires information on the norm of reaction ("Reaktionsnorm") and on its variability in the genotype tested. 1) Experiments were carried out in an automatically controlled growth chamber to determine the optimal conditions for the norm of reaction of the plants to be tested. Genotypically determined variances between the characters studied and differences of means between test samples were most frequently significant when the growth conditions were those of an intermittent 16 hour day. 2) Changes in the transmitted radiation to wavelengths of above 500 nm or 600 nm respectively, while keeping the total radiant energy input constant, led to certain characteristic modifications in three of the mutants and in the wild type of Arabidopsis thaliana (L.) Heynh. Four ontogenetic stages were affected. The mutants are monogenic defect mutants for synthesis of chlorophyll b; two different gene loci are involved. 3) Treatment with radiation of wavelengths above 600 nm (filtered red light) results in marked developmental disorders, after 100 days (three times the normal growth period) leaf rosettes had not yet developed. Moreover, leaf pigment and dry matter production of the wild type decreased by about 20 to 30 percent as compared with production in unfiltered normal light. 4) Filtered yellow light of wavelengths above 500 nm decreases the biosynthesis of chlorophyll a and of the carotinoids and the net assimilation rate, but does not influence chlorophyll b production, leaf area index and apparent use of radiant energy. Each of the genotypes, however, shows a different reaction to the same radiation treatment. Furthermore, each shows characteristic differences in response to altered quality of radiation. The wild type and the mutant ch (2)/ch (2) show certain similarities which are not shared by the two other genotypes ch (1)/ch (1) and ch 3/ch 3. The latter two are characterized by more serious plastid defects. 5) The ch-locus has a primary effect on biosynthesis of chlorophyll b and secondary physiological effects on production of other leaf pigments and dry matter. When chlorophyll b is absent, there is no increase, but rather a reduction of the amount of chlorophyll a; however, there is no quantitative correlation between production of the two chlorophylls. The primary genetic effect of the two mutant loci is discussed, in accordance with Shibata's model of chlorophyll transformations. The secondary pleiotropic gene effects on dry matter production are probably due to additional binding of radiant energy by chlorophyll b and by the carotenoids. Complex interactions between the effects of the genes, and between gene effects and environmental effects must also be considered.