Evidence that the amount of chloroplast DNA exceeds that of nuclear DNA in mature leaves.

Cell-free homogenates containing intact chloroplasts and nuclei were allowed to settle for up to 1 h before the top 2 ml of the 5-ml homogenate was withdrawn. Whereas less than 18% of the chloroplasts moved from the top to the bottom portions, the ratio of nuclei to chloroplasts in the top portion changed from approximately 1/200 to 1/900. The total numbers of chloroplasts and nuclei were counted in the homogenate before settling and in the top 2 ml and bottom 3 m1 after settling. The total DNA content of the homogenate and the top and bottom portions after settling was determined by the diphenylamine colorimetric assay. By simultaneous equations, the absolute amount of DNA in chloroplasts and nuclei was determined. The results are consistent with previous observations of chloroplast DNA by fluorescence microscopy which indicated that the amount of chloroplast DNA per chloroplast is a function of chloroplast size. In addition, the results show that the amount of chloroplast DNA per average chloroplast in large leaves is 0.14 times 10(-12) g, a magnitude higher than previous reports in the literature, and that large leaves contain about twice as much chloroplast DNA as nuclear DNA.