Anatomical and biochemical changes during adventitious rooting of apple rootstocks MM 106 cultured in vitro.

Adventitious rooting in microcuttings of Malus rootstocks MM106 was studied as regards their histological and biochemical aspects. Microcuttings from shoots raised in Murashige and Skoog's (1962) medium were transferred into a rooting medium containing IBA in the dark, then fixed 0, 3, 5, 7 and 10 days after. Some cambial zone and adjacent phloem cells became dense cytoplasm, nuclei with prominent nucleoli and the first cell divisions were observed at day 3. Meristemoids became individualized, consisting of densely staining cells (with enlarged nucleoli) formed outside the xylem by day 5. Identifiable root primordia with a conical shape and several cell layers were present at day 7. Roots with organized tissue system emerged from the stem 10 days after the root induction treatment. From these histological observations, it can be established that the rooting induction stage ended before day 3. The initiation stage, with the first histological modifications to the formation of meristemoids, would correspond to the transient increase of our biochemical marker (peroxidase activity) until day 5. The best rooting percentage obtained with cultures in the presence of auxin during 5 days confirms this hypothesis. The expression of rooting can then take place.