Plastiden-metamorphose und chromoplasten bei Chrysosplenium

Abstract

Summary 1. The leaves of the Golden Saxifrage, Chrysosplenium alternifolium and Chr. oppositifolium , are deeply green, whereas the bracts and the small leaflets of the calyx (a corolla is lacking) appear (greenish-)yellow during flowering time. Correspondingly, leaf cells contain chloroplasts, and bract cells chloro-chromoplasts. The sepals contain chromoplasts, especially in their epidermal cells. 2. The sepals exhibit remarkable changes in color. They are green when the flower buds break open, and yellow during anthesis. During seed development they enlarge and turn green again. A reversible plastid metamorphosis is responsible for these color changes. 3. The fine structure of leaf and calyx plastids, and metamorphosis of plastids in calyx leaflets in particular, has been investigated (mainly in Chr. alternifolium) by conventional electron microscopy. The pictures have been analyzed by morphometric methods (for results, compare fig. 6). 4. Mesophyll chloroplasts of green leaves are uneven-shaped. The numerous grana are oriented in different planes and comprise up to 30 thylakoids in one granum (figs. 11 a, b). Starch grains are always present. Osmiophilic plastoglobules are numerous but rather small (Fig. 9). 5. The sepals contain, in every state of their development, typical chromoplasts in their upper epidermis. These chromoplasts are small and flat (figs 3, 4 and 8). They contain, in a central layer, a few thylakoids and rudimentary grana with but a few thylakoids per granum (fig. 7, see also table 1). The periphery of the chromoplasts is filled with large-size, osmiophilic plastoglobules. They are mostly transformed into polyhedral crystals in the open flower (fig. 4). After anthesis, these carotenoid-containing crystals partly enlarge. Simultaneously, new spherical and often less osmiophilic plastoglobules are formed in a more central region of the chromoplasts (fig. 8). On the whole, metamorphosis of plastids is “monotropic” in the upper sepal epidermis. 6. Macroscopic color changes of the sepals are due to a reversible chloroplast-to-chromoplast transformation in the mesophyll. At the outset of anthesis, the plastids of these cells are typical lens-shaped, starch-containing chloroplasts (fig. 2). During anthesis, chromoplasts differentiate by enlargement of plastoglobules and concomitant reduction of the thylakoid and grana system as well as of stroma matrix material and starch (fig. 5). These chromoplasts are considerably smaller than the chloroplasts from which they originate (fig. 1). Later, during seed formation, these mesophyll chromoplasts of the sepals revert to chloroplasts by reforming larger grana und starch grains, and by reducing the size of the chlotenoid-carrying plastoglobules. Hence, these plastids behave quite differently from the epidermal plastids of the very same leaflet, and thus present a good example of a “non-monotropic”, reversible transformation. 7. Regarding the other components of sepal cells, no conspicuous changes have been observed. 8. These results are discussed in relation to some data of the chromoplast literature, and a contemporary classification of different chromoplast fine structure types is given.