A PROTOCOL FOR ROOTING AND GROWING APPLE ROOTSTOCK MICROSHOOTS

Microshoots of apple (Malus domestica Borkh.) rootstocks G. 65, G.30 a nd G.11 were transferred from stage II axillary shoot cultures to stag e III rooting media containing 10 levels of indole-butyric acid (IBA) for four weeks to determine optimal conditions for rooting. Microshoot s were inverted or left in an upright position. Rootstocks and microsh oot position affected rooting and survival; the highest rooting was 30 % for G.65 inverted with 2 mg.L-1 IBA, 100% for G.30 upright with 3 mg .L-1 IBA, and 100% for G.11 inverted with 1 or 2 mg.L-1 IBA. No single set of conditions provided consistent rooting in vitro, and plants we re not always of high quality. In a second experiment, microshoots wer e rooted and acclimatized ex vitro in a custom-built fog chamber to as sess the effect of two carbon dioxide levels (450 or 1350 mu mol.mol(- 1)) and three light levels (30, 50 or 100 mu mol.m(-2).s(-1)). All roo tstocks rooted equally well ex vitro, but survival varied. Plantlets h ad the highest dry mass, leaf area and growth rates under high light c onditions compared to medium light or low light conditions. Supple men tal carbon dioxide had no consistent effect. If plants survived fog ch amber conditions, subsequent survival in the fog tunnel and greenhouse was 100%. Thus, these rootstocks can be rooted successfully in the fo g chamber at high light conditions, which yielded high quality plants. A third experiment was conducted to evaluate the effect of chilling f or four weeks at 3.3 degrees C, or spraying with gibberellic acid 3 (G A(3)) on post-rooting dormancy, a problem which frequently occurs with recently rooted apple microshoots. Chilled plants had greater dry mas s than control or GA(3) plants, and the GA(3) effect was short-lived. The ex vitro procedure suggested by these experiments could reduce the time associated with rootstock micropropagation to at most 6 months.