Senescence in the nongreening region of the rice (Oryza sativa) coleoptile

The coleoptile of rice (Oryza sativa L. cv. Nippon-bare) emerges from the i mbibed seed on day 2 after sowing and ceases its growth on day 3. In cross section, the cells near the outer epidermis turn into green between days 2 and 3, while those near the inner epidermis remain colorless. In this study , the complete process of the development in the nongreening cells in the c oleoptile was examined by fluorescence and electron microscopy. Embryonic m orphology on day 0 was rapidly converted into the differentiated greening o r nongreening cells between days 1 and 2. Senescence in the inner, nongreen ing region first appeared on day 4 in the third or fourth cell layer from t he inner epidermis and then spread towards both the inner and the outer epi dermis, and the inner cells collapsed completely before the outer cells sen esced. Cells adjacent to the inner epidermis, which senesced slowly, follow ed a sequence of events during development: (1) degradation of plastid DNA; (2) dispersal of nuclear chromatin, differentiation of plastids into amylo plasts, degradation of mitochondrial DNA; (3) degradation of the starch in amyloplasts; (4) disorganization of plastids; (5) condensation of the nucle us, shrinkage of mitochondria; (6) complete loss of cellular components, di stortion of cell walls. In the interior cells, the early events including d egeneration of plastid DNP, and mitochondrial DNA occurred in parallel with those in the cells adjacent to the inner epidermis, yet rapid collapse of all the cellular components proceeded between days 3 and 5, and nuclear con densation could not be detected.