Cytological comparison of leaves and stems of Prunus Avium L. shoots cultured on a solid medium with agar or gelrite

An axillary proliferating clone of Prunus avium L. was subcul- tured every four weeks on solid MS medium with agar as the gelling agent. Vitrification (hyperhydricity) of shoots was induced in one four week cycle with the same medium except that agar was replaced by gel- rite. During culture on the vitrifying medium, the water content of the shoots progressively increased with a parallel decrease in chlorophyll content. Cytological differences between the leaves and stems of the vitrified and normal shoots were detected by light and electron (both transmission and scanning) microscopy. Leaves of vitrified shoots were characterized by lower number of chloroplasts in the palisade parenchyma and by a defective cuticle. The stems of vitrified shoots had a less developed and lignified xylem tissue, lacked sclerenchy- matic areas and showed hypertrophy of the cortical parenchyma. More intense vacuolar activity with evagina- tions of the chloroplast envelope into the vacuole was noted in cells of vitrified leaves