Effect of the Cytokinin Type in the Culture Medium on the Ultrastructure of Leaf Chloroplasts and Photosynthetic Pigment Content of In Vitro Apple (Malus x domestica Borkh.) Shoots

Thidiazuron, 6-benzylaminopurine riboside, and meta-topolin are cytokinins often used in apple tissue cultures. Three different CK-containing Murashige and Skoog media were used during the experiments: medium without CK or media containing 4.5 μM thidiazuron, 4.5 μM 6-benzylaminopurine riboside, or 4.5 μM meta-topolin, respectively. Comparative ultrastructural studies across cytokinin types and apple cultivars were lacking. We studied the changes in photosynthetic pigment content of the leaves with absorption spectroscopy and chloroplast structure with light and transmission electron microscopy. At the light microscopy level, large changes were detected in the length and length-to-width ratios of the chloroplasts in the spongy and palisade mesophyll cell sections in 6-benzylaminopurine riboside- and meta-topolin-treated leaves of the McIntosh scion. In the chloroplasts of the McIntosh plants treated with 6-benzylaminopurine riboside and meta-topolin, and Húsvéti rozmaring leaves treated with meta-topolin, the diameter of grana increased. In both cultivars, thidiazuron caused the height of grana to increase. Thidiazuron and 6-benzylaminopurine riboside influenced leaf anatomy both in the Húsvéti rozmaring and McIntosh cultivars. 6-benzylaminopurine riboside and thidiazuron treatments reduced the content of photosynthetic pigments in the in vitro leaves of both cultivars. In contrast, meta-topolin treatment had no significant effect on the chlorophyll content as compared to the control. Differences were observed not only among the effects of cytokinins, but even between the two apple scions examined. In in vitro apple shoot cultures, TOP maintained chloroplast integrity and pigment content, whereas TDZ exerted stress-like effects.