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l h e nonallelic sweetclover (Melilotus alba Desr.) mutants U371 (ch70/ch10 genotype) and U372 (ch7l/ch77 genotype) are derived from the U389 (+/+ genotype) parenta1 strain. Crowth of the U389 strain at a temperature of 17 or 26'C results in plants normally green in appearance. l h e U371 and U372 mutant plants grown at 26'C are slightly to moderately chlorophyll (Chl) deficient and have decreased Chl b/a ratios. Crowth of the mutants at 17'C results in plants severely deficient in Chl a, with markedly reduced levels of carotenoids except for violaxanthin, and with negligible amounts of Chl b or apoproteins for the light-harvesting complex of photosystem 11. If mutant plants grown at 17°C are transferred to 26"C, during the next 20 d the amount of Chl per fresh weight will increase 5-fold and both the Chl b/a ratio and the expression of the light-harvesting complex apoproteins will progressively increase. Studies of the U371 mutant during the temperature-induced greening demonstrate progressive changes in chloroplast ultrastructure and leaf carbon isotope fractionation that parallel the increases in Chl. Changes observed in the leaf carbon isotope fractionation in the mutant suggest that, in addition to the already known effects of various abiotic factors, strudural and metabolic interna1 factors can also influence whether the limitation in C02 fixation is at the leve1 of diffusion or carboxylation. Such temperature-initiated progressive greening in these and similar mutants may make them useful tools to elucidate not only the biosynthesis and assembly of the photosynthetic apparatus, but also physiological phenomena such as the influence of light-driven energy produdion on the overall carbon isotope fractionation during photosynthesis.

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