1. The variety 'Mrs. Parker' of Pelargonium zonale (fig. 1) is a periclinal chimera of the constitution white-over-green (LI and L II: white, L III: green). Reciprocal crosses with the green variety 'Trautlieb' demonstrate a biparental, extranuclear inheritance of the character green.- white. The F1 consists of green, green-white variegated and white seedlings (table 1). 2. In green-white variegated F1-plants "mixed cells" (fig. 2) have been found containing two genetically different types of plastids: green plastids (from 'Trautlieb') and white plastids (from 'Mrs. Parker'). The white cells of 'Mrs. Parker' represent a white plastid mutant (= plastom mutant); its genetic designation is "extranuclear: alba-1", symbol en: alba-1. 3. Leaf material for biochemical studies was obtained from pure white and entirely green shoots of variegated F1 hybrids (fig. 3). The ultrastructure of the white plastids was studied by electron microscopy. 4. From normal green cells of Pelargonium zonale four bands of high molecular weight ribosomal RNA can be isolated: 25 S and 18 S RNA of the cytoplasmic ribosomes and 23 S and 16 S RNA of plastid ribosomes (fig. 5a). 5. The mutation of the plastid DNA in the plastids of 'Mrs. Parker' causes an altered RNA pattern: The 23 S and 16 S RNA of the plastid ribosomes cannot be detected in polyacrylamid gels (whereas 25 S and 18 S RNA are present) (fig. 5b). 6. In cells of white leaves numerous plastids are present. They are smaller than normal chloroplasts and have a double-layered envelope. However, the formation of normal internal membrane structures is blocked. 7. In mutated plastids of 'Mrs. Parker' ribosomes cannot be detected in electron micrographs (fig. 6). 8. From these findings we conclude that protein synthesis cannot be performed in mutated plastids. The multiplication of the plastids - and presumably also the replication of plastid DNA - is not impaired by the deficiency in plastid protein synthesis. 9. These results indicate that protein synthesis within the plastids is necessary for full development and differentiation of the chloroplasts, although an essential part of the plastidal proteins are synthesized on cytoplasmic ribosomes.

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