The influence of polyploidization on chloroplast coupling factor 1 (CF1) was examined in leaves and leaf protoplasts from isogenic diploid-tetraploid (DDC 2X-4X) and tetraploid-octoploid (IC 4X-8X) sets of alfalfa (Medicago sativa L.). Alfalfa CF1 was purified to homogeneity and was found to contain five subunits with molecular weights of 57,900, 54,300, 38,700, 23,100, and 15,200. Rocket immunoelectrophoresis with anti-spinach CF1 gamma immunoglobulins was used to quantify CF1 from protoplasts and leaves. In the DDC 2X-4X set, fresh weight per cel and cellular content of CF1, chlorophyll (Chl), and DNA doubled with ploidy. Ratios of CF1 to Chl of leaf protoplasts and leaves were similar between diploids and tetraploids. In the IC 4X-8X set, octoploid protoplasts were 90% higher in Chl than were comparable tetraploids, whereas octoploids were 50 to 60% higher than tetraploids in fresh weight per cell and cellular content of CF1 and DNA. Concentrations of CF1 and Chi in leaves and ratios of CF1 to DNA in protoplasts were similar across ploidy levels of both isogenic sets. Therefore, cellular content of CF1 increases proportionately with the amount of DNA per cell or gene dosage.Nuclear polyploidization has been suggested to be an important component in the evolution of many species of flowering plants (10,19). Increases in nuclear ploidy have been reported both to enhance and to decrease photosynthetic activities in higher plants (4,14,25). Such contradictory observations can result from genetic limitations of the plant material under investigation. Genetic differences between allopolyploid genotypes (4, 15) and inbreeding depression associated with colchicine-induced polyploids (2, 25) may confound the consequences ofploidy effects. To minimize these limitations in the study of ploidy effects, isogenic diploidtetraploid and tetraploid-octoploid alfalfa (Medicago sativa L.) populations, to which heterozygosity was restored, were developed.In these isogenic alfalfa populations, no significant differences between ploidy levels were observed in net CO2 exchange per mg Chl of leaves (24) Chloroplast thylakoid-bound CF 1, which constitutes about 10%