Different expression of cytodifferentiation in relation to growth regulators of Pyrus communis L.: Histological and Ultrastructural observations

In the context of an investigation in the pear cultivar “Mirandone” on its in vitro response in relation to several growth regulators and/or their combinations, three principal different types of calli were obtained. An initial callus type was induced on MS medium with 0.88 µM 2,4-D and successively maintained in this medium for over two years: this callus line when transferred after five months to the same basal medium containing a combination of 2.68 µM NAA and 8.88 µM BA formed two callus lines with characteristics different from the original callus, and also from each other. The callus grown in 2,4-D was characterized, principally by a white colour (“White Callus”) a low degree of compactness, parenchymatous cells without chloroplasts and with few organelles. In comparison the other two callus lines compact in structure were composed of cells with cytoplasm rich in organelles, abundant starch grains and other reserves and therefore with high metabolic activity. One of these callus type appeared completely green in colour (“Green Callus”) indicative of the presence of chloroplasts and chlorophyll pigments. The other callus type (“Red Callus”) was prevalently red in colour since in most cells anthocyanin pigments accumulated. Each callus line maintained the respective phenotype for more than two years of culture. No morphogenic event was observed in any callus. The most peculiar findings were some different characteristics of the Green Callus and the Red Callus, although kept exactly in the same cultural conditions: The results found in this in vitro material studied by light and electron microscopy are discussed in relation to callus cell organization and differentiation (“microenvironment”) and different possible states of competence of the cells to respond to the external signals (“macroenvironment”).