EFFECTS OF HEAT-SHOCK AND THERMOADAPTATION ON THE ULTRASTRUCTURE OF COWPEA (VIGNA-UNGUICULATA) CELLS

Suspension culture cells of cowpea (Vigna unguiculata) were examined using transmission electron microscopy to characterize morphological changes associated with hyperthermal stress. Cultures maintained at 26-degrees-C (unadapted cells) and 38-degrees-C (thermoadapted cells) were examined before and after exposure to elevated temperatures of 34-degrees-C and 45-degrees-C, respectively. Observations indicate that while there were significant ultrastructural differences between unadapted and thermoadapted cells, the following structural modifications in response to heat stress were observed in cells of both cultures: (a) almost a complete loss of polyribosomes, rough ER, and dictyosomes, (b) migration of intracellular waste material (presumably proteinaceous in composition) into the cell vacuole, (c) swelling of the nucleolus with assumed accumulation of preribosomal RNP granules, and (d) retraction of the tonoplast from the cytoplasm into the vacuoles of some cells. Heat shock granules (two size classes) were observed in the cytoplasm of stressed thermoadapted cells along with hollow-cored granules within the leucoplasts. Apart from a few minor differences, the morphological modifications that were made in apparent response to hyperthermal stress were remarkably similar in both cultures.