‘Delicious’ apple (Malus domestica Borkh.) and several of its strains, which have been difficult to root in vitro, were successfully propagated with rooting percentages up to 100%. The combination of treatments used to achieve this result included placing the shoots on rooting medium in the dark at 30°C for the first week of the rooting stage, then moving them to a regime of 16 hr light-8 hr dark at 25°C. The rooting medium contained half strength Murashige and Skoog salts plus 1.2 μM thiamine HCl, 0.56 mM myo-inositol, 1 mM phloroglucinol (PG), 1.4 μM indolebutyric acid (IBA), 1.3 μM gibberellic acid (GA3), 87.6 mM sucrose, and 7 g l−1 Difco Bacto agar. Dark treatment applied during the proliferation stage (etiolation) was less effective than one applied at the beginning of the rooting stage. The optimum length of dark treatment during rooting was 4 to 7 days. Increasing the temperature from 25°C to 30°C improved rooting of ‘Delicious’, ‘Royal Red Delicious’, and ‘Vermont Spur Delicious’ in the absence of PG but generally had less effect in the presence of PG. Further increase in temperature to 35°C stimulated rooting of ‘Royal Red Delicious’ but reduced rooting of ‘Vermont Spur Delicious’. Transfer of the cuttings to auxin-free medium after 1 week had no effect on percentage rooting and increased the number of roots per cutting for only 1 of 4 cultivars tested and then only in the presence of PG. In general PG stimulated rooting of ‘Delicious’ and its strains, but had no effect on ‘Golden Delicious’.
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