Ultrastructural changes of the leaf cells in response to heat stress in thermo-resistant cultivar and
thermo-sensitive cultivar of Brassica oleracea var. capitata L. were investigated. No ultrastructural change
was shown in mesophyll cell of the thermo-resistant cultivar. All membrane-bound structure remained intact.
However, the cell ultrastructure of thermosensitive cultivar showed significant changes. One of the major
effects of heat stress was the disruption of membrane structure. Chloroplasts and nucleus were extremely
sensitive to heat stress. Chloroplasts rounded off in different extents and their outer membranes disappeared.
Structural alteration of the thylakoid membrane were visualized. Large amount of plastoglobulis appeared
within the chloroplast. Mitochondria were far more resistant to heat stress than chloroplasts. There was no
distinct changes of mitochondria structure. Nucleus suffered from serious damage as heat caused disruption
of nuclear envelope and condensation of nucleoplasm, showing, eventually, numerous fibrillar granulous
material and irregularly shaped hollow spaces presented within the nucleus.