Root cell ultrastructure of Zea mays embryo during early stages of germination.

The ultrastructure of root cells of the germinating corn embryo has been studied during the first 72 hours of soaking. The most spectacular ultrastructural modifications occur in the nucleus. In the dry seed, the chromatin is heavily condensed and complete dispersion occurs during the first 8 hr of germination. The nucleolus appears as a compact structure in the dormant embryo, and as a uniform granular structure after 3 hr. At the 8th hour, large nucleolar vacuoles appear filled with material structurally similar to chromatin. Later on, the nucleolus is composed of a central, fibrillo-granular region surrounded by a thin, peripheral, granular region and fewer nucleolar vacuoles are found.In a previous autoradiographic study (Deltour, 1970), it was shown that the onset of RNA synthesis in these cells occurs 4 hr after soaking. From that time to the 8th hour, uridine-(3)H is incorporated exclusively into the chromatin. Incorporation of radioactive uridine into the nucleolus begins only after the 8th hour.It is interesting that the onset of RNA synthesis in the chromatin occurs simultaneously with the dispersion of this cell component, and that the appearance of vacuoles in the nucleolus is correlated with the beginning if uridine incorporation into this organelle.The following ultrastructural changes take place in the cytoplasm; (a) the lamellae system of proplastids increases slightly; (b) phytoferritin granules present in the proplastids of the dry seed disappear very rapidly; (c) polysomes appear 72 hr after soaking; (d) the spherosomes which are essentially localized in the vicinity of the wall in the dormant embryo become uniformly distributed throughout the cytoplasm at the 72nd hr.