A Comparison Of The Death Induced By Fungal Invasion Or Toxic Chemicals In Cowpea Epidermal Cells. I. Cell Death Induced By Heavy Metal Salts
Death was induced in cowpea leaf epidermal cells by the application of the metal salts CuCl2, CuSO4, and HgCl2. When unfixed, salt-treated cells were observed by light microscopy, their degeneration and death were seen to follow a series of stages: (1) cessation of cytoplasmic streaming, (2) morphological changes in the cytoplasm such as the formation of large vesicles and the appearance of particles in the vacuole, and (3) protoplast collapse. The relative timing of these stages after salt application was affected by the nature and concentration of the chemical used. The application of fluorescein diacetate and plasmolysing sucrose solutions before or after the addition of the copper salts suggested that the semipermeability of the tonoplast was lost during stage 2 and that of the protoplast boundary at stage 3. Following the cessation of cytoplasmic streaming, the changes observed in unfixed cells occurred rapidly and correlated well with the ultrastructural changes observed in the same cells after fixation. First, microtubules appeared to decrease in abundance. Then coiled polyribosomes, endoplasmic reticulum, and Golgi bodies rapidly became undetectable, mitochondrial cristae became dilated, and the cytoplasm consisted of aggregates of ribosome-containing material interspersed with vesicles. However, the study also showed that the first observable sign of degeneration seen in unfixed cells (i.e., the cessation of cytoplasmic streaming) appears to have no clearly diagnostic feature detectable by electron microscopy.