Ultrastructural study of different types of callus from leaf expiants ofAesculus hippocastanum L.

Summary The cell ultrastructure in three types of callus obtained from leaf explants ofAesculus hippocastanum L. has been studied. Remarkable differences have been shown between the cells of the forerunner E₁ callus and those of the callus arising from it, according to the culture conditions.The peculiar characteristics of E₁ are the scarcity of intercellular spaces and the occurrence of autophagic vacuoles in the cells.An embryogenic friable callus (E₂) is formed in time when E₁ is maintained on solid culture medium. The E₂ cells show cytological features typical of a higher metabolic level and contain starch. Diffused middle lamella digestion leads to the detachment of small embryogenic cell aggregates consisting of vacuolated parenchymatous-like cells and small meristematic cells which may be regarded as embryoids initials.Shaking E₁ in the same liquid medium and subsequent culture on solid medium lead to the differentiation of a non-embryogenic callus (NE), whose cells are very large and highly vacuolated, devoid of starch and with organelle-rich cytoplasm. The NE callus shows a high degree of growth, but does not attain embryogenic competence in time.Abbreviations c cell - cr crystal - cw cell wall - d dictyosome - er endoplasmic reticulum - m mitochondrion - mb microbody - n nucleus - p plastid - s starch - v vacuole